Alkene

An alkene is a class of unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond.^[1] The general molecular formula for simple acyclic alkenes is $\ce{C_nH_{2n}}$\ceCn​H2n​, where $n$n is an integer greater than or equal to 2, distinguishing them from saturated alkanes by their reduced hydrogen content.^[2] This double bond consists of a sigma bond and a pi bond formed from the overlap of sp²-hybridized carbon orbitals, conferring greater reactivity compared to alkanes due to the electron-rich pi bond that readily undergoes addition reactions.^[3] Alkenes are named by replacing the -ane suffix of the corresponding alkane with -ene, with the position of the double bond indicated by the lowest possible number.^[4] Physically, alkenes exhibit properties similar to those of alkanes, including insolubility in water and increasing boiling points with molecular mass, though alkenes generally have slightly lower boiling points than the corresponding alkanes due to the double bond.^[1] Chemically, they are more reactive, participating in electrophilic additions such as hydrogenation, halogenation, and hydration, which are foundational to organic synthesis.^[5] In nature, alkenes occur in essential oils, pheromones, and plant volatiles, contributing to biological signaling and defense mechanisms.^[6] Industrially, alkenes serve as critical feedstocks in the petrochemical sector, with ethylene ($\ce{C2H4}$\ceC2H4) being the most produced organic chemical worldwide—as of 2024, with a global production capacity of approximately 225 million metric tons per year—used primarily as a monomer for polyethylene plastics and other polymers.^[7][8] Other alkenes like propylene and butadiene enable the manufacture of polypropylene, synthetic rubbers, and a range of products including detergents, pharmaceuticals, and fuels.^[8] Their versatility in polymerization and functionalization reactions underscores their economic significance, with global production exceeding hundreds of millions of tons annually.

Nomenclature and Isomerism

IUPAC Naming Conventions

Alkenes are defined as acyclic or cyclic hydrocarbons that contain one or more carbon-carbon double bonds, belonging to the class of unsaturated hydrocarbons.^[9] According to IUPAC recommendations, the systematic naming of alkenes involves replacing the "-ane" ending of the corresponding alkane with "-ene" to indicate the presence of a double bond.^[10] The parent chain is selected as the longest continuous carbon chain that includes the maximum number of double bonds, ensuring the structure captures the principal functional group.^[9] Numbering of the chain begins from the end that assigns the lowest possible locant to the double bond; if multiple double bonds are present, the numbering prioritizes the lowest set of locants for all such bonds.^[10] For a single double bond, the suffix is "-ene" with the position indicated by a numerical prefix, such as in propene (CH₃-CH=CH₂), where the double bond is between carbons 1 and 2, but the name omits the locant "1" as it is unambiguous for the shortest chain.^[9] In cases of multiple double bonds, suffixes like "-diene," "-triene," or "-polyene" are used, with locants preceding each, as in buta-1,3-diene for CH₂=CH-CH=CH₂.^[10] Substituents on the parent chain are named as prefixes in alphabetical order, each preceded by its locant, and the chain is numbered to give the lowest possible numbers to substituents if the double bond locants are equivalent.^[9] For example, 2-methylpropene refers to (CH₃)₂C=CH₂, where the methyl group is at position 2 on the propene chain.^[10] Another illustration is 3-methylbut-1-ene for CH₂=CH-CH(CH₃)-CH₃, demonstrating the prioritization of low numbering for the double bond over substituents.^[9] For cyclic alkenes, known as cycloalkenes, the naming follows similar rules but treats the ring as the parent structure, with the double bond receiving locant 1 and indicated by "-ene."^[10] Cyclohexene, for instance, is C₆H₁₀ with the double bond between carbons 1 and 2 in the ring.^[9] In polyunsaturated cyclic compounds, multiple locants are used, such as cyclohexa-1,3-diene for the benzene precursor with two double bonds.^[10] These naming conventions were formally adopted by IUPAC in the 1979 edition of Nomenclature of Organic Chemistry, providing a standardized system for organic compounds, with minor updates in 1993 for clarity and in 2013 to introduce Preferred IUPAC Names (PINs) for regulatory and indexing purposes.^[11] The 2013 revisions maintained core principles while refining rules for complex structures, ensuring consistency across international chemical literature. The online edition was updated to version 3 in December 2023, incorporating corrections and refinements.^[12][9]

Geometric Isomerism

Geometric isomerism in alkenes arises primarily from the restricted rotation about the carbon-carbon double bond. Each carbon atom in the double bond is sp² hybridized, forming a sigma bond through end-to-end overlap of sp² hybrid orbitals and a pi bond through sideways overlap of unhybridized p orbitals. The pi bond's electron density lies above and below the plane of the molecule, creating a barrier to rotation that requires breaking the pi bond to interconvert isomers; this results in distinct geometric configurations when each carbon of the double bond bears two different substituents.^[13][14] For disubstituted alkenes in which each carbon of the double bond is attached to two different substituents, such as a hydrogen and an alkyl group on each (e.g., 2-butene), the cis-trans nomenclature is used. In the cis isomer, the two higher-priority groups (typically the non-hydrogen substituents) are on the same side of the double bond, while in the trans isomer, they are on opposite sides. A classic example is 2-butene: cis-2-butene has the two methyl groups on the same side, whereas trans-2-butene has them on opposite sides. This nomenclature is straightforward for such symmetric cases but becomes ambiguous for trisubstituted or tetrasubstituted alkenes, where not all substituents are identical.^[15] To address more complex alkenes, the E/Z system, based on the Cahn-Ingold-Prelog (CIP) priority rules, provides an unambiguous designation. Under CIP rules, substituents on each carbon of the double bond are ranked by priority: the atom directly attached with the highest atomic number receives the highest priority; if atomic numbers are tied, the attached atoms are compared by atomic number in order of decreasing value, moving outward along the chain until a difference is found; multiple bonds are treated as duplicated atoms for priority purposes (e.g., a -CH=CH₂ group is considered as if the carbon is attached to two carbons and two hydrogens). The configuration is Z (from German zusammen, meaning together) if the two highest-priority groups are on the same side of the double bond, and E (from entgegen, meaning opposite) if they are on opposite sides. For instance, in (E)-2-bromo-2-butene, the bromine (higher priority than methyl on one carbon) and the ethyl group (higher priority than methyl on the other carbon) are on opposite sides, despite the methyl groups appearing cis-like. This system ensures consistent naming even for highly substituted or asymmetric alkenes.^[16][15][17] In general, trans (or E) isomers exhibit greater thermodynamic stability than their cis (or Z) counterparts due to minimized steric hindrance; the larger substituents are positioned farther apart, reducing repulsive interactions. For example, trans-2-butene is approximately 1 kcal/mol more stable than cis-2-butene, as measured by heats of hydrogenation. This stability trend holds across most disubstituted and higher alkenes, influencing synthetic selectivity and reaction outcomes. A special consideration applies to cyclic alkenes, particularly in bridged bicyclic systems, governed by Bredt's rule. This empirical rule, formulated in 1924, prohibits the formation of a double bond at a bridgehead carbon in small-ring systems (typically where the largest ring is smaller than eight members) because the required trans configuration for planarity around the double bond cannot be accommodated without excessive strain; the pi bond's overlap would be distorted, leading to instability. For example, bicyclo[2.2.1]hept-1-ene (norbornene with a bridgehead double bond) is highly strained and not isolable under standard conditions. Larger systems, such as bicyclo[3.3.1]non-1-ene, may tolerate bridgehead double bonds if the geometry allows sufficient orbital overlap.^[18]

Structure and Bonding

Covalent Bonding

In alkenes, the carbon-carbon double bond (C=C) is composed of one sigma (σ) bond and one pi (π) bond. The σ bond arises from the head-on overlap of sp² hybrid orbitals on each carbon atom, providing strong directional bonding along the internuclear axis. The π bond forms from the sideways overlap of parallel, unhybridized 2p orbitals perpendicular to the σ bond axis, creating a region of high electron density above and below the molecular plane.^[19] Each carbon atom involved in the C=C bond undergoes sp² hybridization, where one 2s orbital and two 2p orbitals mix to form three equivalent sp² hybrid orbitals in a trigonal planar arrangement with bond angles of approximately 120°. The remaining unhybridized 2p orbital on each carbon is oriented perpendicular to this plane, enabling the p-p overlap necessary for π bonding. This hybridization contrasts with the sp³ hybridization in alkanes, where all four orbitals are used for σ bonds, and sp hybridization in alkynes, which allocates two orbitals to a linear σ framework and leaves two p orbitals for additional π bonds.^[20][21] The C=C bond length is approximately 1.34 Å, significantly shorter than the 1.54 Å C-C single bond in alkanes due to the increased electron density from the π bond pulling the nuclei closer together. The total bond dissociation energy for the C=C double bond (gas-phase homolytic cleavage at 298 K) is about 731 kJ/mol, comprising approximately 370 kJ/mol from the σ bond and 361 kJ/mol from the π bond; these values reflect the stronger σ overlap in sp²-hybridized carbons compared to sp³ (347 kJ/mol for C-C in alkanes), making the total slightly greater than twice the alkane single bond energy. In comparison, the triple bond in alkynes has a length of about 1.20 Å and energy of 835 kJ/mol, owing to one σ bond and two π bonds that enhance orbital overlap efficiency. The σ and π molecular orbitals can be visualized through orbital diagrams: the σ bonding orbital results from constructive in-phase overlap of sp² lobes, while the π bonding orbital forms from the parallel overlap of p lobes, with each having corresponding antibonding counterparts (σ* and π*) of higher energy. These orbitals underscore the double bond's restricted rotation and reactivity, as the π bond's overlap is sensitive to torsional strain.^[19]

Molecular Geometry

The molecular geometry of alkenes is characterized by the planar arrangement around the carbon-carbon double bond, arising from the sp² hybridization of the involved carbon atoms. Each sp²-hybridized carbon forms three sigma bonds in a trigonal planar configuration, with bond angles of approximately 120°.^[22] This planarity stems from the overlap of one s and two p orbitals to form the sp² hybrids, leaving a pure p orbital on each carbon for pi bond formation, as briefly referenced in the sigma-pi bonding model.^[23] The double bond imposes significant conformational rigidity due to restricted rotation, in contrast to the free rotation possible around single C-C bonds in alkanes. Rotation around the double bond would require breaking the pi bond, which demands substantial energy (typically around 264 kJ/mol), preventing interconversion at room temperature and leading to stable geometric isomers.^[1] In ethene (C₂H₄), the simplest alkene, both carbon atoms exhibit perfect trigonal planar geometry with H-C-H angles of 117.5° and all atoms lying in one plane.^[24] Propene (C₃H₆), with one methyl substituent, maintains trigonal planar arrangement at the double-bonded carbons, though the sp³-hybridized methyl carbon deviates slightly from planarity.^[25] In cis and trans configurations of disubstituted alkenes, such as 2-butene, the fixed geometry results in differing steric interactions; the cis isomer experiences greater nonbonded repulsion between substituents on the same side of the double bond, leading to higher energy compared to the trans form.^[26] This strain is primarily steric rather than torsional, as the planar structure eliminates eclipsing interactions typical of single bonds. Exceptions occur in strained cyclic systems like cyclopropene, where the three-membered ring distorts the ideal geometry, compressing the C-C=C angle to about 50.8° and increasing angular strain to approximately 54 kcal/mol, rendering it highly reactive.^[27][28]

Theoretical Modeling

Theoretical modeling of alkenes relies on quantum mechanical approaches to elucidate their electronic structure and reactivity. Molecular orbital (MO) theory provides a foundational framework for understanding pi bonding in alkenes, where the sideways overlap of p_z orbitals on adjacent sp²-hybridized carbon atoms forms a bonding π orbital, designated as the highest occupied molecular orbital (HOMO), and an antibonding π* orbital, the lowest unoccupied molecular orbital (LUMO). This π bonding stabilizes the molecule, with the HOMO-LUMO energy gap influencing reactivity; for isolated alkenes like ethene, the gap is relatively large (approximately 10 eV), rendering them less reactive toward electrophiles compared to conjugated systems where delocalization narrows the gap and facilitates reactions such as cycloadditions.^[29][30] Density functional theory (DFT) has become a cornerstone for computing alkene bond energies and geometries, offering a balance of accuracy and computational efficiency through software packages like Gaussian 16 (released post-2010). DFT calculations, particularly using hybrid functionals such as B3LYP, predict the C=C bond dissociation energy in ethene at around 728 kJ/mol (gas-phase homolytic cleavage at 298 K), closely aligning with experimental values and enabling analysis of strain effects in substituted alkenes. For geometries, DFT reproduces structural parameters with high fidelity; in ethene, B3LYP/6-31G* yields a C=C bond length of 1.339 Å (experimental: 1.337 Å, error <0.002 Å) and H-C-C angle of 121.4° (experimental: 121.3°, error <0.1°), demonstrating the method's reliability for predicting planar sp² configurations and aiding in the design of alkene-based materials.^[31][32][33] As of 2025, advances in AI-assisted modeling have enhanced predictions of alkene reactivity in catalysis, integrating machine learning with DFT to forecast outcomes like regioselectivity in hydroformylation of terminal alkenes. For instance, stacking ensemble models trained on limited experimental data achieve high precision (R² > 0.95) in estimating linear-to-branched product ratios under varying conditions such as temperature and pressure, accelerating catalyst optimization without exhaustive quantum simulations. These hybrid approaches reduce computational costs while maintaining accuracy for complex catalytic cycles involving alkenes.^[34][35] In drug design, theoretical modeling plays a pivotal role for alkene-containing molecules, where DFT and QM/MM methods simulate pi interactions with biological targets to predict binding affinities and metabolic stability. For pharmaceuticals like statins featuring alkene moieties, these computations reveal how double-bond geometry influences receptor docking, with errors in energy predictions below 5 kcal/mol, guiding the rational modification of leads to improve efficacy and reduce side effects.^[36][37]

Physical Properties

Thermodynamic Properties

Alkenes display boiling and melting points that generally parallel those of alkanes with equivalent carbon chain lengths, owing to comparable intermolecular van der Waals forces dominated by London dispersion interactions. As the carbon chain length increases, both boiling and melting points rise progressively; for instance, ethene boils at -104 °C and 1-heptene at 94 °C, reflecting the enhanced surface area and molecular weight that strengthen these forces.^[38] Geometric isomerism influences these phase transition temperatures, with cis-alkenes exhibiting higher boiling points than trans isomers due to induced dipole moments that promote stronger dipole-dipole attractions, a phenomenon detailed further in the discussion of geometric isomerism. In contrast, trans-alkenes typically possess higher melting points, as their linear symmetry enables more efficient crystal lattice packing; for example, cis-2-butene melts at -138.3 °C compared to -105.5 °C for trans-2-butene.^[39] The nonpolar nature of alkenes renders them insoluble in water but highly soluble in nonpolar organic solvents such as benzene, chloroform, and hexane, mirroring the solubility behavior of alkanes. However, increasing degrees of unsaturation can slightly diminish solubility in purely nonpolar solvents, as the pi electrons in double bonds confer minor polarity that reduces compatibility with apolar media.^[39] Thermodynamically, alkenes possess higher energy content than their saturated alkane counterparts, primarily due to the relative instability of the carbon-carbon double bond. The standard heat of combustion for ethene, for example, is -1411 kJ/mol, releasing substantial energy upon complete oxidation to CO₂ and H₂O and underscoring the exothermic relief of pi bond strain.^[40] Standard enthalpies of formation further quantify this; propene (C₃H₆) has a value of +20.4 kJ/mol in the gas phase, positive relative to elemental carbon and hydrogen, indicating endothermic formation from precursors.^[41] Density trends in alkenes align with those of hydrocarbons broadly, remaining below that of water (typically 0.6–0.8 g/cm³ for liquid alkenes at 20 °C) and increasing with chain length due to greater molecular mass per unit volume. Compared to alkanes, alkenes show marginally higher densities for the same carbon count, attributable to the shorter C=C bond length (1.34 Å versus 1.54 Å for C-C), which allows closer molecular approach and packing. For representative cases, liquid 1-butene has a density of 0.595 g/cm³, exceeding n-butane's 0.579 g/cm³.^[39] Vapor pressure of alkenes decreases with increasing chain length, consistent with rising boiling points and reduced volatility; shorter alkenes like ethene exhibit high vapor pressures (around 40 atm at 20 °C), while longer ones such as 1-decene approach 0.01 atm under similar conditions. This trend facilitates industrial handling, with lower alkenes being gaseous and higher ones liquid at ambient temperatures.^[39]

Spectroscopic Properties

Infrared (IR) spectroscopy is a primary method for identifying the presence of alkene functional groups through characteristic absorption bands associated with C=C and =C-H vibrations. The C=C stretching vibration typically appears as a weak to medium band between 1620 and 1680 cm⁻¹, with intensity varying by substitution pattern: terminal alkenes (R-CH=CH₂) show stronger absorptions due to asymmetry, while symmetrically substituted alkenes like trans-RCH=CHR exhibit weaker or absent bands because of symmetry-forbidden transitions.^[42][43] The =C-H stretching vibrations occur at 3000-3100 cm⁻¹, slightly higher than alkane C-H stretches, and are often medium in intensity, providing evidence of sp²-hybridized hydrogens.^[44] Nuclear magnetic resonance (NMR) spectroscopy provides detailed structural information on alkenes, particularly through chemical shifts and coupling patterns of vinylic protons and carbons. In ¹H NMR, vinylic protons resonate at 4.5-6.5 ppm due to the deshielding effect of the sp²-hybridized carbon, with complex splitting arising from vicinal couplings: trans couplings (J_trans) are larger (12-18 Hz) than cis couplings (J_cis, 6-12 Hz), allowing distinction of geometric isomers via the Karplus relationship.^[45][46] In ¹³C NMR, the sp²-hybridized carbons of alkenes appear in the 110-150 ppm range, shifted downfield from sp³ carbons due to reduced electron density around the double bond.^[47] Interpretation rules emphasize integrating peak areas to count vinylic hydrogens (typically 1-3 per alkene) and using coupling constants to assign configurations, with double bond geometry influencing slight shift variations (e.g., cis protons slightly more deshielded than trans).^[45] For 1-butene (CH₂=CH-CH₂-CH₃), the ¹H NMR spectrum shows three vinylic signals: the terminal CH₂ protons at ~4.9-5.0 ppm (two geminal doublets of doublets, J_gem ≈ 1-2 Hz), the =CH- proton at ~5.8 ppm (multiplet from couplings to both CH₂ protons and the adjacent CH₂), and allylic CH₂ at ~2.1 ppm (influenced by long-range coupling); integration confirms two protons for the terminal methylene and one for the internal vinylic proton.^[48] The IR spectrum of 1-butene features a medium C=C stretch at ~1640 cm⁻¹ and =C-H stretches near 3080 cm⁻¹, with out-of-plane bending at 990 and 910 cm⁻¹ confirming the terminal monosubstituted alkene.^[49] Ultraviolet-visible (UV-Vis) spectroscopy detects alkenes through π → π* transitions, with isolated double bonds absorbing below 200 nm; for ethene, λ_max is approximately 175 nm (ε ≈ 15,000 M⁻¹ cm⁻¹).^[50] Conjugated systems shift absorption to longer wavelengths due to extended π-orbitals: for example, 1,3-butadiene has λ_max at 217 nm (ε ≈ 21,000 M⁻¹ cm⁻¹), enabling characterization of polyenes.^[51] In 1-butene, the UV spectrum mirrors ethene with λ_max ~175 nm and a shoulder at 162 nm, indicating no conjugation.^[52] Mass spectrometry (MS) of alkenes reveals fragmentation patterns dominated by allylic cleavage, where the molecular ion (often weak) breaks at the bond adjacent to the double bond to form resonance-stabilized allylic carbocations.^[53] Common fragments include m/z 41 (C₃H₅⁺, allyl ion) as a base peak in many linear alkenes, with additional losses of alkyl groups (e.g., 15 for CH₃, 29 for C₂H₅) producing even-electron ions; McLafferty rearrangement may occur if a γ-hydrogen is available, yielding an m/z 54 (C₄H₆⁺•) peak.^[54] For 1-butene (M⁺ at m/z 56), the spectrum shows intense peaks at m/z 41 (99%), 39 (60%), and 27 (from further fragmentation), highlighting allylic C-H₂-CH₃ cleavage.^[52]

Reactions

Electrophilic Addition

Electrophilic addition reactions are a primary mode of reactivity for alkenes, where the electron-rich π bond of the carbon-carbon double bond acts as a nucleophile to attack an electrophile, leading to the formation of new σ bonds. This process typically proceeds in two steps: first, the electrophile adds to one of the sp²-hybridized carbons, generating a carbocation intermediate (or a bridged ion in some cases), and second, a nucleophile adds to the electrophile-bound carbon, resulting in overall addition across the double bond. The regioselectivity of these additions often follows Markovnikov's rule, which states that in the addition of an unsymmetrical reagent like HX to an unsymmetrical alkene, the hydrogen adds to the carbon with more hydrogens, yielding the more stable carbocation intermediate.^[55][56] In catalytic hydrogenation, alkenes react with hydrogen gas in the presence of a metal catalyst such as palladium on carbon (Pd/C), reducing the double bond to a single bond with syn stereochemistry, meaning both hydrogens add from the same face of the alkene. This reaction is widely used for saturation and proceeds via surface adsorption of H₂ and the alkene onto the catalyst, followed by stepwise addition of hydrogen atoms. For example, ethene undergoes hydrogenation to form ethane: $$\ce{CH2=CH2 + H2 ->[Pd/C] CH3-CH3}$$\ceCH2=CH2+H2−>[Pd/C]CH3−CH3 The syn addition preserves stereochemistry in cyclic alkenes, producing cis products from disubstituted alkenes.^[57][58] Halogenation involves the addition of halogens like bromine (Br₂) to alkenes, forming vicinal dihalides through an anti addition mechanism mediated by a three-membered halonium ion intermediate. The π electrons of the alkene attack the polarized halogen molecule, displacing a halide ion and forming a cyclic bromonium ion, which is then opened by backside attack from the halide nucleophile, ensuring trans stereochemistry. This reaction occurs readily in inert solvents and is often used for qualitative tests of unsaturation due to the decolorization of Br₂. A representative example is the addition to ethene, yielding 1,2-dibromoethane: $$\ce{CH2=CH2 + Br2 -> CH2Br-CH2Br}$$\ceCH2=CH2+Br2−>CH2Br−CH2Br The bridged intermediate prevents carbocation rearrangements, making this addition stereospecific.^[59] Hydrohalogenation entails the addition of hydrogen halides (HX, where X = Cl, Br, or I) to alkenes, following Markovnikov regioselectivity under ionic conditions, where the electrophilic proton adds first to form the more stable carbocation, followed by halide capture. However, in the presence of peroxides, HBr undergoes a free-radical mechanism, leading to anti-Markovnikov addition, where the bromine attaches to the less substituted carbon. This peroxide effect is unique to HBr due to the bond dissociation energies involved and does not apply to HCl or HI. For propene, the ionic addition yields 2-bromopropane: $$\ce{CH3-CH=CH2 + HBr -> CH3-CHBr-CH3}$$\ceCH3−CH=CH2+HBr−>CH3−CHBr−CH3 Rearrangements can occur if the initial carbocation is unstable, migrating groups to form more stable ions.^[55] Acid-catalyzed hydration adds water across the alkene double bond to form alcohols, proceeding via protonation of the double bond to generate a carbocation intermediate, followed by nucleophilic attack by water and deprotonation, again adhering to Markovnikov's rule. The reaction requires a strong acid like sulfuric acid and is reversible, with equilibrium favoring the alkene under basic conditions. Carbocation rearrangements, such as hydride or alkyl shifts, are common in unsymmetrical alkenes, leading to the more stable alcohol product. For instance, propene hydrates to 2-propanol: $$\ce{CH3-CH=CH2 + H2O ->[H2SO4] CH3-CH(OH)-CH3}$$\ceCH3−CH=CH2+H2O−>[H2SO4]CH3−CH(OH)−CH3 This method is industrially significant but limited by side reactions in sensitive substrates.^[60][61]

Pericyclic and Cycloaddition Reactions

Pericyclic reactions of alkenes are concerted processes that occur through cyclic transition states, preserving stereochemistry and often governed by orbital symmetry considerations. These reactions include cycloadditions where the π-bond of the alkene participates in forming new σ-bonds without intermediates, distinguishing them from stepwise mechanisms. The Woodward-Hoffmann rules provide a framework for predicting the feasibility of such reactions under thermal or photochemical conditions by analyzing the symmetry of frontier molecular orbitals. For cycloadditions involving alkenes, suprafacial approaches are typically allowed, ensuring stereospecific outcomes.^[62] The Diels-Alder reaction exemplifies a [4+2] cycloaddition, where a conjugated diene reacts with an alkene (dienophile) to form a cyclohexene ring. Discovered in 1928, this thermal, concerted process involves the diene in s-cis conformation adding to the alkene's π-bond, with the reaction proceeding suprafacially on both components to maintain stereochemistry. Electron-withdrawing groups on the dienophile accelerate the reaction by lowering the LUMO energy, enhancing orbital overlap.^[63] The stereospecificity ensures that cis substituents on the dienophile remain cis in the product, while trans remain trans, as confirmed by experimental studies on substituted norbornenes.^[64] The endo rule, formulated in 1934, predicts preferential formation of the endo adduct, where the dienophile's substituents align toward the diene in the transition state, due to secondary orbital interactions stabilizing the endo geometry. Under photochemical conditions, [2+2] cycloadditions between two alkenes become symmetry-allowed, forming cyclobutanes via diradical intermediates. These reactions typically require UV irradiation to excite one alkene to its triplet state, enabling biradical formation and bond closure, as opposed to the thermally forbidden concerted pathway.^[62] For instance, irradiation at 370 nm promotes cycloaddition of styrenes with N-alkyl maleimides in dichloromethane, yielding cyclobutane products in up to 67% yield without catalysts, with the mechanism involving triplet energy transfer and stereospecific addition.^[65] Such photochemical variants are valuable for synthesizing strained rings, though regioselectivity depends on substituent electronics. Ozonolysis represents an oxidative [3+2] cycloaddition of ozone to the alkene π-bond, leading to cleavage of the double bond. The initial step forms a primary ozonide (molozonide), an unstable 1,2,3-trioxolane that rapidly decomposes via the Criegee mechanism into a carbonyl compound and a zwitterionic carbonyl oxide intermediate.^[66] This intermediate then undergoes a second [3+2] cycloaddition with another carbonyl to yield the stable 1,2,4-trioxolane (ozonide). Reductive workup with zinc/acetic acid or dimethyl sulfide cleaves the ozonide to aldehydes, while oxidative workup with hydrogen peroxide yields ketones or carboxylic acids from aldehydes, providing a versatile method for determining alkene structures.^[67] The process is stereospecific, with syn addition preserving the alkene's geometry in intermediate formation. Epoxidation of alkenes with peracids, such as m-chloroperbenzoic acid (mCPBA), proceeds via a concerted, stereospecific oxygen transfer to form epoxides. Known as the Prilezhaev reaction, it involves a spiro transition state where the electrophilic peroxy oxygen adds syn to the π-bond, retaining the alkene's cis or trans configuration in the product—cis-alkenes yield cis-epoxides, and trans yield trans.^[68] The reaction occurs in nonaqueous solvents like chloroform at room temperature, with mCPBA providing clean transfer of one oxygen atom, leaving m-chlorobenzoic acid as byproduct.^[69] This stereospecificity arises from the concerted nature, avoiding carbocation intermediates, and makes epoxides useful precursors for further transformations.

Polymerization and Metathesis

Alkenes undergo chain-growth polymerization, a key process for producing important industrial polymers. Free radical polymerization of ethene, for instance, proceeds through three main stages: initiation, propagation, and termination. In initiation, an organic peroxide decomposes under heat to generate free radicals, which then add to the double bond of ethene, forming a new radical species.^[70] Propagation involves the growing radical chain repeatedly adding to additional ethene molecules, extending the chain via attack on the π-bond and radical relocation to the terminal carbon.^[70] Termination occurs when two radicals combine or disproportionate, yielding stable polyethylene chains of varying lengths.^[70] This method, conducted at high pressures (up to 3000 atm) and temperatures (150–300 °C), produces low-density polyethylene with branching due to chain transfer.^[71] For stereoregular polymers, Ziegler-Natta catalysis enables precise control over tacticity. These heterogeneous catalysts, typically TiCl₄ supported on MgCl₂ with AlR₃ cocatalysts (R = alkyl), polymerize propylene to isotactic polypropylene, where methyl groups align on the same side of the chain.^[72] The mechanism involves coordination of propylene to a Ti center with an empty orbital, followed by migratory insertion into a Ti-alkyl bond, favoring cis-1,2-insertion for isotacticity due to steric constraints from ligands.^[72] Propagation continues with sequential insertions, while termination can occur via β-hydride elimination.^[72] This approach, recognized with the 1963 Nobel Prize, yields high-molecular-weight, linear polymers essential for materials like packaging films.^[72] Olefin metathesis represents another transformative reaction for alkenes, involving the redistribution of alkylidene groups across double bonds. Catalyzed by transition metal complexes, it proceeds via metal carbene intermediates that form a metallacyclobutane with an alkene, leading to exchange and reformation of new alkenes.^[73] The 2005 Nobel Prize in Chemistry highlighted advancements by Yves Chauvin, Robert H. Grubbs, and Richard R. Schrock, particularly Grubbs' ruthenium-based catalysts (e.g., Grubbs' first and second generation), which are air-stable and functional-group tolerant.^[73] Cross-metathesis, for example, exchanges substituents between terminal alkenes like 1-hexene and 1-octene to form internal alkenes and ethene, useful in synthesizing fine chemicals.^[73] Ring-closing metathesis cyclizes dienes, such as converting a linear chain with terminal alkenes into a macrocycle like civetone (a musk compound), releasing ethene as a byproduct.^[73] Ring-opening metathesis polymerization (ROMP) extends metathesis to cyclic alkenes, producing polymers with controlled microstructures. Strained rings like norbornene or cyclopentene undergo ring-opening via metal carbene catalysts (e.g., Grubbs' Ru or Schrock's Mo), forming a propagating carbene that adds monomers, yielding unsaturated polymers like polydicyclopentadiene used in durable coatings.^[74] The reaction is driven by relief of ring strain, with the metallacyclobutane intermediate facilitating chain growth and double-bond retention in the backbone.^[74] On an industrial scale, polybutadiene—a key elastomer—is produced via coordination polymerization of 1,3-butadiene using Ziegler-Natta-type catalysts like Ni or Co complexes with AlR₃, yielding high-cis (96%) content for tire applications.^[75] Olefin metathesis also plays a role in polybutadiene processing, such as cross-metathesis for telechelic oligomers or depolymerization to recover monomers, enhancing sustainability through metal carbene-mediated degradation.^[76] Global production was approximately 4.5 million tons in 2024, underscoring these methods' impact.^[77]

Substitution and Rearrangement Reactions

Allylic bromination represents a prominent substitution reaction in alkenes, where a bromine atom replaces a hydrogen at the allylic position—the carbon adjacent to the double bond—via a free-radical mechanism. This process, known as the Wohl-Ziegler reaction, employs N-bromosuccinimide (NBS) as the brominating agent, typically in the presence of a radical initiator such as azobisisobutyronitrile (AIBN) or light, and a solvent like carbon tetrachloride.^[78] The reaction proceeds through the in situ generation of a low concentration of molecular bromine from NBS, which initiates radical abstraction of the allylic hydrogen, forming a resonance-stabilized allylic radical intermediate that then reacts with Br2 to yield the allylic bromide while preserving the double bond's position or allowing for allylic rearrangement.^[79] For instance, cyclohexene undergoes allylic bromination to produce 3-bromocyclohexene in high yield under these conditions, highlighting the selectivity for the allylic site over direct addition to the double bond.^[78] Beyond bromination, allylic oxidation and other substitutions enable the introduction of functional groups at the allylic position while maintaining the alkene functionality. Copper-aluminum hydroperoxo complexes catalyze the allylic oxidation of alkenes using tert-butyl hydroperoxide and a carboxylic acid, selectively forming allylic esters through a mechanism involving allylic C-H abstraction and subsequent trapping by the carboxylate. This method is particularly effective for internal alkenes, providing regioselective access to oxygenated products without disrupting the double bond, as demonstrated in the conversion of 1-decene to its 1-decen-3-yl acetate derivative. Similarly, palladium-catalyzed allylic amination with aliphatic amines under blue light irradiation achieves enantioselective C-H activation at the allylic site, yielding allylic amines with high stereocontrol via a Pd(0/I/II) cycle.^[80] Skeletal rearrangements in alkenes involve the migration or repositioning of the double bond to form more thermodynamically stable isomers, often catalyzed by acids or metal complexes. Acid-catalyzed isomerization protonates the double bond to generate a carbocation intermediate, which undergoes hydride or alkyl shifts to relocate the double bond toward a more substituted position, followed by deprotonation.^[81] A classic example is the conversion of 1-butene to 2-butene over silica-alumina catalysts, where the terminal alkene isomerizes to the more stable internal isomer with high selectivity under mild conditions.^[82] Certain variants of olefin metathesis, such as those employing ruthenium catalysts, can also induce skeletal rearrangements by facilitating double-bond migration through carbene intermediates, though these are typically localized compared to extensive chain exchanges.^[81] The Cope rearrangement exemplifies a pericyclic rearrangement in alkenes, specifically a thermal [3,3]-sigmatropic shift in 1,5-dienes that interconverts two allyl units in a concerted, suprafacial manner via a six-membered transition state.^[83] First reported in 1940, this reaction is stereospecific and proceeds without intermediates, often favoring the formation of more stable products under heating, as seen in the isomerization of 1,5-hexadiene to itself or substituted variants like 3-methyl-1,5-heptadiene to 1,5-heptadiene with a methyl branch.^[83] The process is reversible and entropy-driven, with activation energies around 40-50 kcal/mol for unsubstituted systems, making it a key tool for synthesizing complex diene architectures.^[84]

Synthesis

Industrial Methods

The industrial production of alkenes, particularly commodity ones like ethene and propene, has evolved significantly since the mid-20th century. Following World War II, the petrochemical industry shifted from coal-derived feedstocks—such as those used in Fischer-Tropsch synthesis during the war—to abundant and cheaper petroleum and natural gas sources, enabling scalable cracking processes that underpin modern plastics and chemicals manufacturing.^[85] Steam cracking remains the dominant method for producing ethene, accounting for the vast majority of global capacity, which exceeded 220 million metric tons in 2023.^[86] In this process, hydrocarbon feedstocks like ethane or naphtha are mixed with steam (typically in a 3:1 volume ratio for ethane) and heated in tubular furnaces to 750–950°C at near-atmospheric pressure for short residence times (0.1–1 second), promoting thermal pyrolysis into smaller molecules while steam dilutes the mixture to suppress coke formation on furnace tubes. The cracked gases are rapidly quenched to 200–300°C to halt reactions, compressed, and separated via distillation and absorption: water is removed first, followed by recovery of ethene at cryogenic temperatures (-140°C), yielding primarily ethene alongside byproducts like hydrogen, methane, and propene. Yields vary by feedstock; ethane cracking achieves ~75–80% ethene selectivity, with typical yields of ~35-55 mol% ethene (e.g., 37.7 mol% in modeled conditions), while naphtha yields ~25–30% ethene and ~15% propene.^[87][88] Catalytic dehydrogenation is a key on-purpose route for propene, converting propane to propene via endothermic removal of hydrogen over metal oxide catalysts at elevated temperatures. Industrial implementations include the UOP Oleflex process, which uses a Pt-Sn/Al₂O₃ catalyst in a continuous moving-bed reactor at 550–620°C and low pressure (0.3–1 atm), achieving 80–88% propene selectivity and near-complete conversion per pass with integrated regeneration. The Lummus Catofin process employs a CrOₓ/Al₂O₃ catalyst in fixed-bed swing reactors at 600–700°C, offering 48–65% conversion and ~85% selectivity but requiring periodic catalyst cycling to manage deactivation from coking. These processes operate at 500–600°C under platinum or chromium-based catalysis to balance activity and selectivity while minimizing side reactions like cracking.^[89] Fluid catalytic cracking (FCC) in petroleum refineries contributes significantly to C₃–C₄ alkene production as a byproduct of converting heavy gas oils into gasoline and diesel. In FCC units, preheated vacuum gas oil contacts a fluidized zeolite catalyst (e.g., Y-type faujasite) at 500–550°C and slight overpressure, where acid-catalyzed carbenium ion mechanisms break C–C bonds, yielding ~15–20 wt% light olefins relative to the total products: typically 4–6 wt% propene and 3–5 wt% butenes from the feed, enhanced in high-severity variants designed for olefin maximization. This makes FCC the second-largest source of propene after steam cracking, integrating seamlessly with refinery operations.^[90] These methods are energy-intensive, with steam cracking consuming ~20–30 GJ per metric ton of ethene, primarily for furnace heating and compression, though process integration recycles fuel gases for partial offset. Byproducts include valuable hydrogen—estimated at 3.5 million metric tons annually from U.S. steam crackers alone—which is often purified via pressure swing adsorption for use in hydrotreating or as clean fuel, improving overall efficiency by 15–90% in life-cycle emissions compared to standalone hydrogen production.^[91]

Elimination-Based Synthesis

Elimination-based synthesis of alkenes primarily involves the removal of small molecules, such as hydrogen halides or water, from saturated precursors like alkyl halides or alcohols, yielding unsaturated products through unimolecular (E1) or bimolecular (E2) mechanisms.^[92] These reactions are fundamental in organic synthesis, enabling the controlled formation of carbon-carbon double bonds under basic or acidic conditions.^[93] The E2 elimination, a concerted one-step process, is base-promoted and commonly employs strong bases like alcoholic potassium hydroxide (KOH) on alkyl halides.^[94] In this mechanism, the base abstracts a β-hydrogen while the leaving group (e.g., halide) departs simultaneously, requiring an anti-periplanar geometry between the hydrogen and leaving group for optimal orbital overlap in the transition state.^[95] Regioselectivity follows Zaitsev's rule, favoring the more substituted (thermodynamically stable) alkene as the major product, as the transition state resembles the product with partial double-bond character.^[93] In contrast, the E1 elimination proceeds via a two-step carbocation intermediate under acidic or weakly basic conditions, where the leaving group first ionizes to form a carbocation, followed by deprotonation of an adjacent hydrogen. This mechanism is prone to rearrangements, such as hydride or alkyl shifts, to generate more stable carbocations (e.g., secondary to tertiary), which can alter the alkene regiochemistry.^[96] Like E2, E1 also adheres to Zaitsev's rule, but the carbocation's stability influences product distribution.^[92] Dehydration of alcohols represents another key E1-type elimination, catalyzed by concentrated sulfuric acid (H₂SO₄) at elevated temperatures, converting the hydroxyl group into a good leaving group via protonation.^[97] For secondary and tertiary alcohols, the mechanism involves rapid protonation, loss of water to form a carbocation, and subsequent deprotonation, with tertiary alcohols reacting fastest due to inherent carbocation stability.^[94] Primary alcohols, however, often require harsher conditions and may proceed via an E2-like pathway to avoid unstable primary carbocations, though rearrangements can still occur.^[98] A classic example is the dehydrohalogenation of 2-bromobutane with alcoholic KOH, which via E2 yields predominantly trans-2-butene (following Zaitsev's rule) over cis-2-butene and 1-butene, with the trans isomer favored due to lower steric strain.^[92] Similarly, dehydration of ethanol with H₂SO₄ at 170°C produces ethene through protonation of the hydroxyl, E2-like elimination of water, and regeneration of the acid, serving as a simple laboratory preparation of this alkene.^[97] In cyclic systems, such as cyclohexyl halides, E2 elimination demands anti-periplanar alignment, restricting reactivity to trans-diaxial conformations in chair forms; thus, cis-2-methylcyclohexyl tosylate undergoes slower elimination compared to its trans isomer, highlighting stereochemical control over reaction rates.^[95] This conformational requirement ensures stereospecificity, often yielding endocyclic alkenes without rearrangement in rigid rings.

Functional Group Transformations

Functional group transformations provide key methods for synthesizing alkenes from carbonyl compounds and alkynes, enabling precise control over carbon-carbon double bond formation and stereochemistry in complex molecules. These reactions are particularly valuable in organic synthesis, as they allow the conversion of readily available precursors like aldehydes, ketones, and triple bonds into alkenes without relying on elimination from saturated systems. Seminal developments in this area include phosphorus- and silicon-based olefinations of carbonyls, titanium-mediated couplings, and selective reductions of alkynes, each offering distinct advantages in terms of yield, selectivity, and functional group tolerance. The Wittig reaction, discovered by Georg Wittig in 1954, represents a cornerstone of alkene synthesis from carbonyls. It involves the nucleophilic attack of a phosphonium ylide (typically generated from a phosphonium salt and a base) on an aldehyde or ketone, forming a betaine intermediate that cyclizes to an oxaphosphetane and subsequently collapses to the alkene and triphenylphosphine oxide. The reaction is broadly applicable to aliphatic, aromatic, and functionalized carbonyls, proceeding under mild conditions in aprotic solvents like ethers or dimethyl sulfoxide. Stereoselectivity is governed by the ylide substitution: non-stabilized ylides (e.g., from alkylphosphonium salts) favor (Z)-alkenes through a concerted, irreversible oxaphosphetane formation under salt-free conditions, while stabilized ylides (conjugated with electron-withdrawing groups like esters) produce (E)-alkenes via a reversible betaine pathway that equilibrates to the more stable stereoisomer. For example, the reaction of benzylidenetriphenylphosphorane with benzaldehyde yields stilbene with tunable E/Z ratios depending on conditions. This method's versatility has made it indispensable in total syntheses, though byproduct management remains a practical consideration.^[99][100] A related approach, the Peterson olefination, serves as a silicon-based variant of the Wittig reaction, first reported by David J. Peterson in 1968. Here, an α-silyl carbanion (prepared from a silylmethyl metal reagent, such as (trimethylsilyl)methyllithium, and a carbonyl) adds to form a β-hydroxysilane intermediate. This adduct undergoes syn elimination of silanol under acidic (for E-selectivity) or basic (for Z-selectivity) conditions to yield the alkene, with stereochemistry controlled by the elimination pathway and reaction temperature. The Peterson method excels in cases where phosphorus byproducts are undesirable or when silyl groups provide handling advantages, such as in the synthesis of allylsilanes, and it accommodates sensitive functionalities like esters without interference. For instance, the olefination of cyclohexanone with (phenylthio)(trimethylsilyl)methyllithium followed by acid treatment affords 1-methylenecyclohexane in high yield. Its mechanistic similarity to the Wittig reaction yet distinct elimination step allows complementary stereocontrol, making it a valuable tool in stereoselective synthesis. The McMurry coupling, introduced by J. E. McMurry in 1974, enables the direct formation of alkenes from two carbonyl molecules via titanium-mediated reductive dimerization. Low-valent titanium species, generated in situ from titanium(IV) chloride and a reductant like zinc or magnesium in THF, promote single-electron transfer to the carbonyls, forming organotitanium intermediates that couple to the alkene while extruding titanium oxides. This reaction is particularly suited for symmetrical or intramolecular alkenes from ketones and aldehydes, including hindered or aromatic systems, and tolerates a range of functional groups such as halides and nitro compounds under the reductive conditions. Stereoselectivity often favors (E)-alkenes for acyclic cases due to thermodynamic control, though cyclic substrates can yield mixtures resolvable by conditions like added pyridine. A representative example is the coupling of two molecules of acetophenone to (E)-1,2-diphenylethene in over 80% yield. The method's intramolecular variant has been pivotal in constructing medium-sized rings for natural product synthesis, despite challenges like over-reduction in some cases. Alkenes are also accessed from alkynes through selective partial reductions that cleave the triple bond to a double bond while preserving stereochemistry. The Lindlar reduction, developed by Herbert Lindlar in 1952, employs a poisoned palladium catalyst (5% Pd on calcium carbonate, deactivated with lead acetate and quinoline) under atmospheric hydrogen pressure to achieve syn addition, yielding cis-alkenes exclusively from internal alkynes. This heterogeneous catalysis operates at room temperature in solvents like ethanol or hexane, stopping at the alkene stage due to the poison's inhibition of further hydrogenation, and is highly effective for non-terminal alkynes with yields often exceeding 90%. For example, 2-butyne is converted to (Z)-2-butene quantitatively. The method's mildness makes it ideal for polyfunctional molecules, such as in vitamin A synthesis. Complementarily, the dissolving metal reduction using sodium (or lithium) in liquid ammonia at -33°C effects anti addition via a vinyl radical anion mechanism, producing trans-alkenes from internal alkynes. This homogeneous process, which involves electron transfer followed by protonation from ammonia, achieves high stereoselectivity (>95% trans) and is compatible with isolated double bonds but deprotonates terminal alkynes. An illustrative case is the reduction of 2-hexyne to (E)-2-hexene in 85-95% yield. These orthogonal reductions from alkynes provide stereodefined alkenes essential for target-oriented synthesis.^[101] Additional transformations from alkynes include partial hydrogenation variants and hydration-rearrangement sequences, though the latter typically involves initial addition to form enols that can be manipulated to alkenes in specific contexts, such as base-catalyzed isomerization in allylic systems. However, partial hydrogenation remains the dominant route for direct alkene formation, with the aforementioned methods offering robust control.

Modern and Sustainable Approaches

Recent advancements in alkene synthesis emphasize eco-friendly strategies that minimize energy consumption, waste generation, and reliance on fossil feedstocks, leveraging biological, photonic, and electrochemical principles to achieve high selectivity under mild conditions. These methods address limitations of conventional processes by incorporating renewable energy sources and catalysts, enabling scalable production with reduced environmental footprint. As of 2025, global ethylene capacity has grown to approximately 230 million metric tons, with ongoing additions in Asia and the Middle East.^[102] Biocatalytic approaches utilize engineered enzymes to facilitate direct formation of alkenes from alcohols via dehydration, offering stereoselectivity and operation in aqueous media at ambient temperatures. For instance, linalool dehydratase/isomerase (LinD) has been repurposed to catalyze the dehydration of α-allyl methyl alcohols, yielding diverse alkene products with tunable regioselectivity by modulating substrate structure and reaction conditions. Engineered variants of fatty acid dehydratases, such as those derived from oleate hydratases, enable asymmetric hydration in reverse for alkene synthesis, though primary focus remains on dehydration for net alkene output, achieving up to 90% enantiomeric excess in chiral alkene formation from prochiral alcohols. These enzymes, often cofactor-independent, demonstrate robustness in continuous bioprocesses, with yields exceeding 80% for terminal alkenes like those derived from bio-based feedstocks.^[103][104] Photocatalytic methods harness visible light to drive dehydrogenation of alkanes to alkenes, bypassing high-temperature cracking and utilizing abundant solar energy. A notable example involves single-atom Ni grafted on Pd/TiO₂ catalysts (T-Ni₀.₆Pd₀.₂₄) for non-oxidative dehydrogenation of ethane to ethene under simulated solar irradiation at ambient pressure and temperature (328 K), achieving near-unity selectivity (99.3–100%) and production rates up to 9.5 mmol·g⁻¹·h⁻¹ in flow reactors, with minimal coke formation and stable performance over 10 hours. This approach co-produces hydrogen as a valuable byproduct, enhancing overall process efficiency, and demonstrates an apparent quantum efficiency of 22.3% at 350 nm, significantly lowering energy demands compared to thermal methods. Metal complexes, such as those based on iridium or ruthenium, further enable dehydrogenative coupling of styrenes to allylic compounds under visible light, with selectivities over 90% and reduced oxidant use.^[105][106] Electrochemical synthesis provides a pathway for alkene production from renewable inputs like CO₂ or alcohols, powered by electricity from intermittent sources. Advances in Cu-based catalysts have boosted CO₂ reduction to ethene, with surface-modified CuO electrodes (e.g., thiol-functionalized) attaining 79.5% Faradaic efficiency (FE) at -1.2 V vs. RHE and partial current densities exceeding 200 mA·cm⁻², favoring C-C dimerization via stabilized *CO intermediates. For higher alkenes, Cu nanocrystals with (100) and (111) facets enable propylene synthesis from CO₂ with 1.4% FE and 5.46 mA·cm⁻² partial current density at -0.65 V vs. RHE, a 65-fold enhancement over prior benchmarks through facet-specific adsorption. Direct dehydrogenative coupling of alcohols to alkenes uses Ni or Pt electrodes in flow cells, yielding styrenes with >85% selectivity at low overpotentials (<0.5 V), minimizing side reactions. These processes operate in neutral or alkaline electrolytes, integrating with anion-exchange membranes for pure-water feeds to avoid corrosive KOH.^[107][108] Integration of these methods with continuous flow chemistry enhances scalability and safety for green alkene production, allowing precise control over residence times and heat/mass transfer. Flow reactors facilitate photocatalytic dehydrogenation, as seen in microstructured TiO₂-Pd systems converting propane to propene with 95% selectivity under UV/visible light, reducing byproduct formation by 50% versus batch setups. Similarly, electrochemical flow cells with Cu gas-diffusion electrodes achieve stable ethene production from CO₂ at industrial-relevant currents (>300 mA·cm⁻²) over 100 hours, with modular designs enabling on-site integration with renewables. Biocatalytic flow processes, using immobilized dehydratases, yield alkenes from bio-alcohols at throughputs of 1–10 g·L⁻¹·h⁻¹, minimizing enzyme deactivation through compartmentalization.^[109] Sustainability is quantified through metrics like atom economy (AE) and E-factor, revealing superior performance of these approaches over traditional steam cracking. Steam cracking of ethane to ethene exhibits low AE (<40%) due to multiple byproducts (e.g., methane, hydrogen) and high E-factor (10–50 kg waste/kg product), driven by energy-intensive pyrolysis at 800–900°C. In contrast, electrocatalytic CO₂ reduction to ethene achieves AE >90% by incorporating all reactant carbons into the product, with E-factors as low as 1–5 when using renewable electricity, though anion effects and membrane losses can elevate it to 10. Photocatalytic dehydrogenation approaches AE of 86% for ethane to ethene (C₂H₆ → C₂H₄ + H₂), with E-factors near 0 in optimized flow systems due to byproduct valorization. Biocatalytic dehydrations reach AE >95% and E-factors <1, leveraging water as the sole byproduct in aqueous media. These metrics underscore a 5–10-fold reduction in waste intensity, supporting circular economy principles.^[110][111]

Applications

Industrial and Material Uses

Alkenes serve as fundamental feedstocks in the petrochemical industry, primarily for the production of polymers, synthetic rubbers, and various chemicals essential to manufacturing. Ethene, the most produced alkene, is predominantly used to manufacture polyethylene, a versatile plastic available in low-density (LDPE) and high-density (HDPE) forms. LDPE is commonly employed in flexible packaging, films, and coatings due to its pliability and moisture resistance, while HDPE finds applications in rigid containers, pipes, and bottles owing to its strength and durability.^[112][113] Additionally, ethene is a key precursor to chloroethene (vinyl chloride), which polymerizes to form polyvinyl chloride (PVC), widely used in construction materials, pipes, and flooring for its corrosion resistance and cost-effectiveness.^[114][115] Propene ranks as the second most important alkene industrially, serving as the primary monomer for polypropylene, a thermoplastic valued for its toughness and used in automotive parts, textiles, and packaging. It also enables the synthesis of acrylonitrile, a building block for acrylic fibers and resins in textiles and adhesives, as well as propylene oxide, which is converted into polyurethanes for foams, coatings, and elastomers.^[116][117] These derivatives highlight propene's role in diverse sectors, from consumer goods to industrial coatings.^[118] Butadiene, a conjugated diene alkene, is critical for synthetic rubber production, particularly styrene-butadiene rubber (SBR), which constitutes a major portion of tire manufacturing and conveyor belts due to its elasticity and abrasion resistance. Approximately 60% of butadiene consumption goes toward rubber and latex production. It is also used to produce adiponitrile, an intermediate for nylon-6,6 fibers and engineering plastics in textiles and automotive components.^[119][120] Globally, alkenes production exceeds 400 million metric tons annually, with major contributors like ethene, propene, and butadiene driving a market valued at over USD 300 billion in 2024, projected to grow amid rising demand for plastics and chemicals.^[121] This scale underscores their economic significance, with ethene alone accounting for around 200 million tons of capacity.^[122] Recent advancements have expanded alkene applications into biofuels and advanced composites. In biofuels, microbial engineering enables biobased production of alkenes like bio-ethene from renewable feedstocks, offering drop-in alternatives to fossil-derived fuels with reduced carbon footprints. For advanced composites, alkenes such as propene-derived polypropylene reinforce lightweight materials in aerospace and automotive sectors, enhancing strength-to-weight ratios through polymer matrix integration.^{[123][124][125]}

Biological and Medicinal Applications

Alkenes play a crucial role in pharmaceutical compounds, where their structural features contribute to biological activity. For instance, tamoxifen, a selective estrogen receptor modulator (SERM) used in breast cancer treatment, incorporates a styryl group—a diarylethylene moiety that facilitates binding to estrogen receptor alpha (ERα).^[126] This alkene-containing structure enables tamoxifen to act as an antagonist in breast tissue by competitively inhibiting estrogen binding to ERα, thereby blocking estrogen-dependent cell proliferation.^[127] The mechanism involves conformational changes in the receptor upon tamoxifen binding, recruiting corepressor proteins to suppress gene transcription associated with tumor growth.^[127] Olefin metathesis has emerged as a powerful tool in the synthesis of complex pharmaceuticals during the 2010s, enabling efficient construction of carbon-carbon double bonds in drug scaffolds. This reaction, often employing ruthenium-based catalysts, has been applied in the production of inhibitors like Relacatib, a cathepsin K antagonist for osteoporosis treatment, where ring-closing metathesis forms a key macrocyclic precursor.^[128] Similarly, ring-closing metathesis was utilized in the synthesis of AMG 176, a BCL-XL inhibitor for cancer therapy, highlighting the method's scalability for late-stage drug development.^[129] These applications underscore olefin metathesis's versatility in assembling intricate molecular architectures with high stereoselectivity, as documented in pharmaceutical patent literature from the period.^[130] In medicinal catalysis, metal-alkene complexes leverage the Dewar-Chatt-Duncanson model to describe their bonding interactions, which are essential for therapeutic applications. This model posits a synergistic bond where the alkene donates π-electrons to the metal's empty orbital (σ-donation) while receiving back-donation from the metal's filled d-orbitals into the alkene's π* orbital, elongating the C=C bond and activating it for reactions.^[131] In Wilkinson's catalyst (RhCl(PPh₃)₃), the rhodium center forms such complexes with alkenes during hydrogenation, facilitating oxidative addition of H₂ and subsequent syn-addition to the coordinated double bond, a process critical for stereoselective reductions in drug synthesis.^[132] These π-acceptor ligand properties enhance catalytic efficiency in pharmaceutical processes, such as asymmetric hydrogenations for chiral intermediates.^[132] Alkenes are integral to agrochemicals, particularly in pyrethroid insecticides, where they contribute to the molecules' potency and selectivity. Pyrethroids like permethrin feature an alkene moiety in their ester side chain, which introduces cis/trans isomerism that influences insecticidal activity by modulating binding to voltage-gated sodium channels in neuronal membranes.^[133] This structural element disrupts nerve impulse transmission in target insects while minimizing mammalian toxicity, making pyrethroids widely used in crop protection and public health applications.^[133] Emerging applications of alkenes in targeted therapies as of 2025 involve variants of click chemistry, such as thiol-ene reactions, for precise bioconjugation in drug delivery systems. Thiol-ene click chemistry enables rapid, metal-free coupling of thiols to electron-deficient alkenes like maleimides, forming stable thioether linkages under mild aqueous conditions suitable for biomolecules.^[134] This approach has been employed to functionalize hydrogels for localized cancer treatment, where thiol-ene cross-linking incorporates dual-drug payloads (e.g., doxorubicin and paclitaxel) for controlled release at tumor sites, enhancing therapeutic efficacy through site-specific activation.^[135] Recent advances highlight its role in conjugating alkenes to antibodies or nanoparticles for targeted delivery, improving pharmacokinetics and reducing off-target effects in precision medicine.^[134]

Occurrence and Environmental Aspects

Natural Occurrence and Biosynthesis

Alkenes occur naturally in various biological systems, with ethylene (C₂H₄) serving as a prominent example in plants, where it functions as a gaseous hormone regulating processes such as fruit ripening, senescence, and stress responses.^[136] Produced endogenously in plant tissues, ethylene acts as a ripening signal, triggering climacteric fruit maturation by coordinating gene expression and enzymatic activities.^[137] Global natural emissions of ethylene are estimated at 18–45 million tonnes per year, with approximately 74% released from vegetation (plants), underscoring its ecological significance in agricultural and forest ecosystems.^[136] Terpenes and isoprenoids represent a vast class of alkene-containing natural products, derived from isoprene units and ubiquitous in plant essential oils, resins, and latex. Monoterpenes, consisting of two isoprene units (C₁₀), include limonene, a cyclic alkene abundant in citrus fruit peels, where it contributes to aroma and acts as a defense compound against herbivores.^[138] Sesquiterpenes (C₁₅), formed from three isoprene units, are key components of essential oils from plants like lavender and sandalwood, providing volatile alkenes that deter pathogens and attract pollinators.^[139] These compounds exemplify the structural diversity of natural alkenes, often featuring conjugated double bonds that enhance their reactivity and biological roles. The biosynthesis of alkenes in terpenoids and related compounds proceeds primarily through two acetate-derived pathways in plants: the mevalonate (MVA) pathway in the cytosol and the 2-methyl-D-erythritol 4-phosphate (MEP) pathway in plastids. The MVA pathway generates isopentenyl diphosphate (IPP) from acetyl-CoA via mevalonic acid intermediates, supplying precursors for sesquiterpenes, triterpenes, and steroids, while the MEP pathway produces IPP from glyceraldehyde 3-phosphate and pyruvate, fueling monoterpenes, diterpenes, and carotenoids.^[140] In steroid biosynthesis, two farnesyl pyrophosphate molecules condense to form presqualene diphosphate, which is reduced to squalene—a linear polyene alkene—followed by cyclization to lanosterol, an intermediate bearing multiple double bonds that are subsequently modified to yield cholesterol and other steroids.^[141] These enzymatic cascades highlight the precision of alkene formation in natural product assembly. Marine environments contribute significantly to natural alkene diversity, with brown algae (Phaeophyta) producing C8 and C11 alkenes such as ectocarpene and dictyopterene as pheromones that mediate gamete attraction and sexual reproduction.^[142] These volatile polyenes, released by female gametes, induce directed movement in male counterparts, facilitating fertilization in turbulent waters.^[143] From an evolutionary perspective, alkenes in terpenoids have played pivotal roles in plant signaling and defense, enabling adaptations to environmental pressures over millions of years. Terpenes function as semiochemicals for interspecies communication, attracting beneficial insects or repelling herbivores through volatile emissions that signal predation risks.^[144] Their double bonds facilitate rapid oxidation and conjugation, enhancing toxicity against pathogens and contributing to the chemical arms race in plant-herbivore interactions.^[145] This versatility underscores alkenes' ancient origins as modular building blocks for survival strategies in terrestrial and aquatic ecosystems.^[146]

Environmental Impact and Sustainability

Alkene production, particularly through steam cracking of hydrocarbons, releases volatile organic compounds (VOCs) such as ethylene and propylene, which contribute significantly to atmospheric pollution. These emissions from cracking processes total around 4,000 tons of VOCs annually in major production regions like the Netherlands, with ethylene accounting for approximately 36% due to its proportional share in olefin output. Ethylene, a key alkene, acts as a tracer for anthropogenic VOC emissions and is highly reactive in the troposphere, primarily undergoing oxidation with hydroxyl (OH) radicals to form peroxy radicals that drive ozone and smog formation in urban environments. This reactivity enhances ground-level ozone production, exacerbating photochemical smog, especially when combined with nitrogen oxides from industrial sources. Alkenes exhibit varying degrees of toxicity, often acting as mild irritants to the respiratory tract, eyes, and skin upon acute exposure. For instance, ethylene causes irritation at concentrations above 1,000 ppm, potentially leading to coughing and pulmonary edema in severe cases, while propylene is similarly irritating but with lower potency. In aquatic systems, alkenes like ethylene show moderate toxicity, with LC50 values for Daphnia magna ranging from 53 to 153 mg/L over 48 hours, though bioaccumulation is minimal due to low bioconcentration factors (e.g., BCF of 2.4 for related compounds). Mammalian LD50 values indicate low acute oral toxicity, exceeding 5,000 mg/kg for both ethylene and propylene in rats, underscoring their relatively low systemic risk compared to other hydrocarbons. The carbon footprint of fossil-based alkene production via steam cracking is substantial, emitting 1.2 to 1.5 metric tons of CO2 per ton of ethylene, driven primarily by the energy-intensive pyrolysis of feedstocks like naphtha or ethane. This process accounts for nearly 1% of global CO2 emissions, with annual totals exceeding 260 million metric tons from ethylene alone. To mitigate these impacts, sustainability initiatives include shifting to bio-based feedstocks, such as dehydrating ethanol derived from corn or sugarcane to produce ethylene; production capacity is scaling, with the global bio-based ethylene market projected to reach USD 609 million in 2025, supported by innovations like Braskem's ethanol-derived polyethylene applications. Additionally, chemical recycling of polyolefins—polymers from alkenes like ethylene and propylene—via pyrolysis or depolymerization recovers monomers, reducing waste and emissions by up to 80% compared to virgin production in pilot scales. Regulatory frameworks have tightened post-2020 to curb alkene-related emissions from chemical plants. The U.S. EPA's 2024 National Emission Standards for Hazardous Air Pollutants for Ethylene Production require facilities to reduce VOC and hazardous air pollutant emissions through improved leak detection, storage controls, and flaring efficiency, targeting over 90% reduction in ethylene oxide (a derivative) and associated alkenes without startup/shutdown exemptions. These rules apply to approximately 200 U.S. plants, aiming to cut annual emissions by thousands of tons while aligning with broader Clean Air Act VOC limits to protect air quality.