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Higher alkane
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Tetracosane is a representative higher alkane

Higher alkanes are alkanes with a high number of carbon atoms. It is common jargon.[1] One definition says higher alkanes are alkanes having nine or more carbon atoms. Thus, according to this definition, nonane is the lightest higher alkane.[2] As pure substances, higher alkanes are rarely significant, but they are major components of useful lubricants and fuels.[3]

Synthesis

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The preparation of specific long-chain hydrocarbons typically involves manipulations of long chain precursors or the coupling of two medium-chain components. For the first case, fatty acids can be a source of higher alkanes via decarboxylation reaction. Such processes have been investigated as a route to biodiesel.[4]

Fatty acid esters and fatty acid nitriles react with long chain Grignard reagents to give, after suitable workup, long-chain ketones. The Wolff-Kishner Reaction provides a way to remove the ketone functionality, giving long-chain hydrocarbons.[1]

Even-numbered, long-chain hydrocarbons can also be synthesized through electrolysis[5] and the Wurtz reactions of alkyl bromides.

Occurrence

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Higher alkanes can also be isolated and purified from natural or synthetic mixtures. Coal tar is a traditional source of mixtures of long-chain hydrocarbons.[3] Careful fractionation, first using urea clathrates to remove branched hydrocarbons, and then distillation, produces pure n-hydrocarbons from petroleum.[6]

Regarding synthetic sources, the Fischer-Tropsch process (or FT process) produces a mixture of hydrocarbons by the hydrogenation of carbon monoxide. The products obtained are liquid hydrocarbons and waxy solids, mostly n-paraffins. The liquid fraction ranges from C6 to C20, while the solid fraction consists of hydrocarbons above C21.[7]

Bioactivity

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Some branched higher alkanes are insect pheromones. 9- and 7-methyltricosanes are active for ladybird beetles (Adalia bipunctata).[8] The emerald ash borer (Agrilus planipennis Fairmaire) responds to 9-methylpentacosane.[9] Female Asian long-horned beetles Anoplophora glabripennis, which are very damaging, secrete 2-methyldocosane.[10]

Reactions

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Higher alkanes in general are relatively inert, just like low molecular weight alkanes they can react with oxygen and start a combustion reaction. They can undergo cracking in the presence of alumina or silica catalysts, forming lower alkanes and alkenes.

Uses

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Alkanes from nonane to hexadecane (those alkanes with nine to sixteen carbon atoms) are liquids of higher viscosity, which are less suitable for use in gasoline. They form instead the major part of diesel, kerosene, and aviation fuel. Diesel fuels are characterised by their cetane number, cetane being an older name for hexadecane. However the higher melting points of these alkanes can cause problems at low temperatures and in polar regions, where the fuel becomes too thick to flow correctly. Mixtures of the normal alkanes are used as boiling point standards for simulated distillation by gas chromatography.[11]

Alkanes from hexadecane upwards form the most important components of fuel oil and lubricating oil. In latter function they work at the same time as anti-corrosive agents, as their hydrophobic nature means that water cannot reach the metal surface. Many solid alkanes find use as paraffin wax, used for lubrication, electrical insulation, and candles. Paraffin wax should not be confused with beeswax, which consists primarily of esters.

Alkanes with a chain length of approximately 30 or more carbon atoms are found in bitumen (asphalt), used (for example) in road surfacing. However, the higher alkanes have little value and are usually split into lower alkanes by cracking.

Names

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Some alkanes have non-IUPAC trivial names:

Properties

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See also: List of straight-chain alkanes

Nonane is the lightest alkane to have a flash point above 25 °C, and is classified as flammable under the US National Library of Medicine. [13]

The properties listed here refer to the straight-chain alkanes (or: n-alkanes).

Nonane to hexadecane

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This group of n-alkanes is generally liquid under standard conditions.[3]

Nonane Decane Undecane Dodecane Tridecane Tetradecane Pentadecane Hexadecane
Formula C9H20 C10H22 C11H24 C12H26 C13H28 C14H30 C15H32 C16H34
CAS number [111-84-2] [124-18-5] [1120-21-4] [112-40-3] [629-50-5] [629-59-4] [629-62-9] [544-76-3]
Molar mass (g/mol) 128.26 142.29 156.31 170.34 184.37 198.39 212.42 226.45
Melting point (°C) −53.5 −29.7 −25.6 −9.6 −5.4 5.9 9.9 18.2
Boiling point (°C) 150.8 174.1 195.9 216.3 235.4 253.5 270.6 286.8
Density (g/ml at 20 °C) 0.71763 0.73005 0.74024 0.74869 0.75622 0.76275 0.76830 0.77344
Viscosity (cP at 20 °C) 0.7139 0.9256 1.185 1.503 1.880 2.335 2.863 3.474
Flash point (°C) 31 46 60 71 79 99 132 135
Autoignition temperature (°C) 205 210 205 235 201
Explosive limits 0.9–2.9% 0.8–2.6% 0.45–6.5%

Heptadecane to tetracosane

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From this group on, the n-alkanes are generally solid at standard conditions.

Heptadecane Octadecane Nonadecane Eicosane Heneicosane Docosane Tricosane Tetracosane
Formula C17H36 C18H38 C19H40 C20H42 C21H44 C22H46 C23H48 C24H50
CAS number [629-78-7] [593-45-3] [629-92-5] [112-95-8] [629-94-7] [629-97-0] [638-67-5] [646-31-1]
Molar mass (g/mol) 240.47 254.50 268.53 282.55 296.58 310.61 324.63 338.66
Melting point (°C) 21 28–30 32–34 36.7 40.5 42 48–50 52
Boiling point (°C) 302 317 330 342.7 356.5 224 at 2 kPaa 380 391.3
Density (g/ml) 0.777 0.777 0.786 0.7886 0.792 0.778 0.797 0.797
Flash point (°C) 148 166 168 176

a [Should be quoted on a consistent basis]

Pentacosane to triacontane

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Pentacosane Hexacosane Heptacosane Octacosane Nonacosane Triacontane
Formula C25H52 C26H54 C27H56 C28H58 C29H60 C30H62
CAS number [629-99-2] [630-01-3] [593-49-7] [630-02-4] [630-03-5] [638-68-6]
Molar mass (g/mol) 352.69 366.71 380.74 394.77 408.80 422.82
Melting point (°C) 54 56.4 59.5 64.5 63.7 65.8
Boiling point (°C) 401 412.2 422 431.6 440.8 449.7
Density (g/ml) 0.801 0.778 0.780 0.807 0.808 0.810

Hentriacontane to hexatriacontane

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Hentriacontane Dotriacontane Tritriacontane Tetratriacontane Pentatriacontane Hexatriacontane
Formula C31H64 C32H66 C33H68 C34H70 C35H72 C36H74
CAS number [630-04-6] [544-85-4] [630-05-7] [14167-59-0] [630-07-9] [630-06-8]
Molar mass (g/mol) 436.85 450.88 464.90 478.93 492.96 506.98
Melting point (°C) 67.9 69 70–72 72.6 75 74–76
Boiling point (°C) 458 467 474 285.4 at 0.4 kPa 490 265 at 130 Pa
Density (g/ml) 0.781 at 68 °C[14] 0.812 0.811 0.812 0.813 0.814

Heptatriacontane to dotetracontane

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Heptatriacontane Octatriacontane Nonatriacontane Tetracontane Hentetracontane Dotetracontane
Formula C37H76 C38H78 C39H80 C40H82 C41H84 C42H86
CAS number [7194-84-5] [7194-85-6] [7194-86-7] [4181-95-7] [7194-87-8] [7098-20-6]
Molar mass (g/mol) 520.99 535.03 549.05 563.08 577.11 591.13
Melting point (°C) 77 79 78 84 83 86
Boiling point (°C) 504.14 510.93 517.51 523.88 530.75 536.07
Density (g/ml) 0.815 0.816 0.817 0.817 0.818 0.819

Tritetracontane to octatetracontane

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Tritetracontane Tetratetracontane Pentatetracontane Hexatetracontane Heptatetracontane Octatetracontane
Formula C43H88 C44H90 C45H92 C46H94 C47H96 C48H98
CAS number [7098-21-7] [7098-22-8] [7098-23-9] [7098-24-0] [7098-25-1] [7098-26-2]
Molar mass (g/mol) 605.15 619.18 633.21 647.23 661.26 675.29
Boiling point (°C) 541.91 547.57 553.1 558.42 563.6 568.68
Density (g/ml) 0.82 0.82 0.821 0.822 0.822 0.823

Nonatetracontane to tetrapentacontane

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Nonatetracontane Pentacontane Henpentacontane Dopentacontane Tripentacontane Tetrapentacontane
Formula C49H100 C50H102 C51H104 C52H106 C53H108 C54H110
CAS number [7098-27-3] [6596-40-3] [7667-76-7] [7719-79-1] [7719-80-4] [5856-66-6]
Molar mass (g/mol) 689.32 703.34 717.37 731.39 745.42 759.45
Boiling point (°C) 573.6 578.4 583 587.6 592 596.38
Density (g/ml) 0.823 0.824 0.824 0.825 0.825 0.826

Pentapentacontane to hexacontane

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Pentapentacontane Hexapentacontane Heptapentacontane Octapentacontane Nonapentacontane Hexacontane
Formula C55H112 C56H114 C57H116 C58H118 C59H120 C60H122
CAS number [5846-40-2] [7719-82-6] [5856-67-7] [7667-78-9] [7667-79-0] [7667-80-3]
Molar mass (g/mol) 773.48 787.50 801.53 815.58 829.59 843.6
Boiling point (°C) 600.6 604.7 ? 612.6 ? 620.2
Density (g/ml) 0.826 0.826 ? 0.827 ? 0.827

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Higher alkane

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Higher alkanes are acyclic saturated hydrocarbons consisting of five or more carbon atoms linked by single bonds, following the general molecular formula \ceCnH2n+2\ce{C_nH_{2n+2}} where n≥5n \geq 5, exemplified by compounds such as pentane (\ceC5H12\ce{C5H12}) and hexane (\ceC6H14\ce{C6H14}). Unlike lower alkanes (methane through butane), which are gases at room temperature, higher alkanes are typically liquids or solids, with their physical state depending on chain length and branching. These compounds exhibit significant conformational flexibility due to free rotation around C-C bonds, favoring staggered conformations—particularly anti arrangements—to minimize steric repulsion and achieve low-energy chain structures. Chemically, higher alkanes are nonpolar and relatively inert, showing low reactivity toward most but undergoing to produce and , as well as under UV light. Their boiling and melting points increase with molecular weight due to enhanced London dispersion forces, while branching reduces these points by decreasing molecular surface area; for instance, n-pentane boils at 36.1°C, compared to 9.5°C for . Higher alkanes are primary constituents of , where they form the liquid fraction after , serving as key components in fuels like (predominantly C5–C12 alkanes) and diesel. They are also used as solvents, lubricants, and waxes, with straight-chain variants providing higher in fuels but branched isomers preferred to reduce . Branched higher alkanes, such as isooctane, exhibit superior antiknock properties, making them essential in high-performance formulations.

Definition and Nomenclature

Scope and Classification

Higher alkanes are acyclic saturated hydrocarbons consisting of straight-chain or branched carbon skeletons with five or more carbon atoms, commencing with (\ceC5H12\ce{C5H12}). These compounds adhere to the general \ceCnH2n+2\ce{C_nH_{2n+2}} for acyclic structures, where n≥5n \geq 5, and represent a subset of the broader family distinguished by their extended chain lengths. Higher alkanes differ from lower alkanes (C₁ to C₄) primarily in their physical states and profiles, transitioning from gases to higher-boiling liquids and waxy solids at ambient conditions. For instance, exhibits a of 31°C, while n-octane has a of 13°C. Under standard classifications such as OSHA, liquids with s below 37.8°C are flammable, so both remain flammable, but the higher s of longer-chain higher alkanes reduce volatility and alter flammability risks compared to shorter chains, making them suitable for applications like lubricants. Higher alkanes are classified into straight-chain (normal or n-alkanes) and branched (isoalkanes) variants, with linear higher alkanes receiving primary emphasis due to their in natural deposits and simpler structural analysis. Straight-chain examples include (\ceC11H24\ce{C11H24}) and longer homologs, while branched forms like isodecane feature alkyl substitutions that influence packing and properties. The term "higher" is informal originating in early 20th-century chemistry to describe longer-chain alkanes in fractions beyond the low-boiling gases and light liquids.

Naming Systems

Higher alkanes, defined as those with five or more carbon atoms, follow the International Union of Pure and Applied Chemistry (IUPAC) nomenclature system for systematic naming, which ensures unique and descriptive identifiers based on molecular structure. For unbranched (straight-chain) higher alkanes, the name consists of a prefix derived from Greek or Latin numerical roots indicating the number of carbon atoms, followed by the suffix "-ane." This system extends the pattern established for lower alkanes, such as pentane (\ceC5H12\ce{C5H12}) and hexane (\ceC6H14\ce{C6H14}), to higher members like nonane (\ceC9H20\ce{C9H20}), decane (\ceC10H22\ce{C10H22}), undecane (\ceC11H24\ce{C11H24}), dodecane (\ceC12H26\ce{C12H26}), and eicosane (\ceC20H42\ce{C20H42}). For chains exceeding 20 carbons, prefixes combine numerical multipliers with base terms, such as heneicosane (\ceC21H44\ce{C21H44}), docosane (\ceC22H46\ce{C22H46}), and triacontane (\ceC30H62\ce{C30H62}); tetracosane (\ceC24H50\ce{C24H50}), for instance, uses "tetra-" (four) combined with "cosane" from eicosane (twenty). These names adhere to IUPAC recommendations, prioritizing simplicity and consistency for chains up to 100 carbons or more in theoretical contexts. For branched higher alkanes, IUPAC rules require identifying the longest continuous carbon chain as the parent structure, naming it as an unbranched alkane, and then denoting branches (substituents) as alkyl groups prefixed with their position numbers. The chain is numbered from the end that assigns the lowest possible locants to substituents; if locant sets are equivalent, the lowest number goes to the substituent that comes first in alphabetical order. Multiple identical substituents use multiplicative prefixes like "di-," "tri-," or "tetra-," listed alphabetically without regard to multipliers (e.g., "ethyl" before "methyl"). An example is 2-methylnonane (\ceCH3CH(CH3)(CH2)6CH3\ce{CH3CH(CH3)(CH2)6CH3}), where the nine-carbon chain is nonane, and the methyl group at position 2 receives the lowest number. Complex branches may require naming as substituted alkyl groups, but the core principles ensure unambiguous identification even for intricate structures. Certain higher alkanes retain historical or common (trivial) names, particularly those with industrial or biological significance, alongside their IUPAC designations. For instance, hexadecane (\ceC16H34\ce{C16H34}), the straight-chain IUPAC name, is commonly called cetane in the context of ignition quality, where it serves as the reference standard for the scale (assigned a value of 100). Similarly, hentriacontane (\ceC31H64\ce{C31H64}) is the IUPAC name for the straight-chain alkane prominent in natural waxes, such as comprising 8-9% of . These trivial names persist in specialized literature but are not recommended for general use under IUPAC guidelines. Naming very long-chain higher alkanes (C₃₀ and above) presents challenges due to the exponential increase in possible isomers, which complicates exhaustive enumeration while still requiring systematic application of IUPAC rules for specific structures. For \ceC30H62\ce{C30H62}, there are over 4 billion constitutional isomers, necessitating precise identification of the longest (often 30 carbons) and careful assignment to distinguish branches amid vast structural diversity. Straight-chain names remain straightforward using extended numerical prefixes (e.g., pentatriacontane for \ceC35H72\ce{C35H72}), but branched variants demand rigorous adherence to lowest- and alphabetical rules to avoid ambiguity in databases or synthesis reports. In practice, computational tools often aid in generating and verifying these names for ultra-long chains encountered in or isolation.

Physical Properties

As the carbon chain length in higher alkanes increases, their boiling points rise progressively, typically by approximately 20-30 °C for each additional CH₂ group, owing to the strengthening of van der Waals (London dispersion) forces between the elongated nonpolar molecules. For instance, (C₉H₂₀) has a of 151 °C, while eicosane (C₂₀H₄₂) reaches 343 °C, illustrating how these intermolecular attractions require more energy to overcome in longer chains. Melting points of higher alkanes also generally increase with chain length due to enhanced molecular interactions in the solid state, but they exhibit an alternation known as the odd-even effect, where melting points show a pattern attributed to differences in crystal packing efficiency between even- and odd-numbered s. This results in varying packing densities, with even chains often aligning more efficiently in certain ranges. For example, (C₁₈H₃₈, even) melts at 28 °C, compared to nonadecane (C₁₉H₄₀, odd) at 32 °C. Density and viscosity both show gradual increases with increasing chain length in higher alkanes, reflecting greater and stronger intermolecular forces that enhance cohesion and resistance to flow. higher alkanes have densities ranging from about 0.7 to 0.8 g/cm³, with longer chains approaching the upper limit as the hydrocarbon structure becomes more compact relative to volume. rises notably, impacting applications like , as longer chains entangle more readily under shear. Solubility trends in higher alkanes highlight their nonpolar nature: solubility in water decreases sharply with chain length due to heightened hydrophobicity, as the larger hydrophobic surface area disrupts water's hydrogen-bonding network more effectively. Conversely, solubility in nonpolar solvents remains high and increases slightly with chain length, facilitating dissolution via compatible van der Waals interactions. At (around 25 °C), higher alkanes transition from liquids for chains of C₉ to C₁₇ to solids for C₁₈ and beyond, driven by rising melting points that exceed ambient conditions in longer members. Associated flash points also elevate with chain length, exceeding 60 °C typically for C₁₁ and higher, reducing volatility and fire risk compared to shorter alkanes.

Variations by Chain Length

Higher alkanes exhibit distinct physical property variations depending on their carbon chain length, transitioning from liquids to solids with increasing molecular weight. In the C9-C16 range, these compounds are predominantly colorless liquids at , characterized by low s and moderate s. For instance, n-nonane (C9H20) has a of -53.5 °C, a of 150.8 °C, and a of 0.718 g/cm³ at 20 °C. At the upper end of this range, n-hexadecane (C16H34) shows a of 18.2 °C and a of 286.5 °C, serving as the reference standard ( 100) for evaluating ignition quality in the cetane scale. The C18-C24 range marks a transition to waxy solids, with melting points rising above ambient temperatures and s exceeding 300 °C, accompanied by increasing viscosities that reflect enhanced intermolecular van der Waals forces. N-Octadecane (C18H38), for example, melts at 28 °C and boils at 317 °C. Further along, n-tetracosane (C24H50) has a melting point of 51.3 °C and a of 407.8 °C, contributing to the waxy consistency observed in mixtures of these alkanes. For the C25-C36 range, higher alkanes form solid paraffins with well-defined crystalline structures, displaying between 50 °C and 80 °C. N-Pentacosane (C25H52) exemplifies this with a melting point of 53.3 °C. At the higher end, n-hexatriacontane (C36H74) melts at 75.5 °C. A notable feature in this series is the odd-even alternation in melting points, observed across longer n-alkanes due to differences in packing efficiency. Beyond C37, these compounds are high-melting solids with s approaching or exceeding 100 °C, suitable for applications requiring thermal stability. N-Heptatriacontane (C37H76) has a of 77.7 °C, while n-hexacontane (C60H122) reaches approximately 100 °C. In this regime, refractive indices typically range from 1.42 to 1.45, decreasing slightly with chain length, and surface tensions fall between 25 and 30 mN/m at elevated temperatures near , reflecting their nonpolar, hydrophobic nature. Branching in higher alkanes disrupts the linear packing in the solid state, generally lowering s compared to their straight-chain s. For example, isononane (a branched C9 ) has a of -99.5 °C, significantly below that of n-nonane at -53.5 °C, due to reduced from irregular molecular shapes.

Chemical Properties

Inertness and Stability

Higher alkanes exhibit significant chemical inertness due to the strength of their carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds, with average bond dissociation energies of approximately 350 kJ/mol for C-C and 410 kJ/mol for C-H. These robust bonds render the molecules unreactive under ambient conditions toward common such as acids, bases, or mild oxidants, as the high energy barrier for bond cleavage prevents facile reaction initiation./Alkanes/Properties_of_Alkanes/Chemical_Properties_of_Alkanes) This stability arises from the nonpolar, saturated nature of alkanes, lacking sites for nucleophilic or electrophilic attack that are present in compounds with functional groups like alkenes or alcohols. Thermally, higher alkanes maintain stability up to temperatures around 400–500°C in their pure form, beyond which or cracking begins to occur, typically requiring 450–750°C under industrial conditions to break C-C bonds. They show resistance to , as the absence of polar bonds precludes water-mediated cleavage, and to photolysis under standard visible or near-UV light, where insufficient energy is available to disrupt the strong bonds without specialized high-energy . In terms of oxidative stability, higher alkanes undergo slow autooxidation in air at room temperature, forming hydroperoxides as initial products, but this process is markedly accelerated by trace metals or exposure to ultraviolet light, which initiate radical chain reactions. Compared to lower alkanes, the reactivity in autooxidation for linear higher alkanes increases with chain length up to about C12 and then plateaus, owing to the increasing proportion of more reactive secondary hydrogens, though overall rates remain low without catalysts. This inherent resistance underscores their utility in applications requiring durability, contrasting sharply with the polar reactivity of unsaturated or oxygenated hydrocarbons.

Reactivity Patterns

Higher alkanes undergo complete in the presence of sufficient oxygen, producing and as the primary products. The general balanced for the combustion of an alkane with the formula \ceCnH2n+2\ce{C_nH_{2n+2}} is: \ceCnH2n+2+3n+12O2−>nCO2+(n+1)H2O\ce{C_nH_{2n+2} + \frac{3n+1}{2} O2 -> n CO2 + (n+1) H2O} This releases significant energy, with the molar increasing as the carbon chain lengthens due to the additional C-C and C-H bonds. For example, n-nonane (\ceC9H20\ce{C9H20}) has a standard of combustion of approximately -6125 kJ/mol (or 6.125 MJ/mol), while for longer chains like n-hexadecane (\ceC16H34\ce{C16H34}), it rises to about -10700 kJ/mol (10.7 MJ/mol). Another key reactivity pattern involves cracking, where higher alkanes are thermally or catalytically decomposed into smaller, more useful hydrocarbons such as lower alkanes and alkenes. cracking occurs at high temperatures (450–750°C) and pressures (up to 70 atm), breaking C-C bonds via free radical mechanisms to yield mixtures rich in alkenes. Catalytic cracking, using zeolites at around 500°C and lower pressures, favors the production of branched alkanes and aromatics, converting heavy fractions like C16+ hydrocarbons from into gasoline-range products (C5–C10). Free-radical halogenation represents a where higher alkanes react with or under UV light or heat, replacing a with a . This process proceeds via a chain mechanism involving , , and termination steps, with reactivity depending on the type of C-H bond. For chlorination at , the relative rates of hydrogen abstraction are approximately 1:3.8:5 for primary:secondary:tertiary hydrogens, while bromination is far more selective at 1:82:1600. In longer-chain alkanes, overall selectivity decreases because the abundance of secondary hydrogens leads to a greater of isomeric monohalo products./06:_Alkanes_and_Alkenes/6.03:_Properties_of_Alkanes) Higher s, being fully saturated, do not undergo as they lack sites for addition. Sulfonation, typically via sulfochlorination or sulfoxidation with SO2 and O2, can introduce groups to form alkane sulfonates, but this is generally limited to mid-range chains (C8–C18) for applications like detergents, where longer chains become less soluble and practical.

Synthesis Methods

Laboratory Synthesis

Laboratory synthesis of higher alkanes typically involves controlled carbon-carbon bond-forming reactions starting from simpler alkyl halides, carbonyl compounds, or carboxylic acids, enabling the preparation of specific chain lengths in small quantities for research purposes. These methods prioritize selectivity and yield over scalability, often requiring conditions and careful handling of reactive intermediates. Common approaches include reactions and reductions that extend or dimerize carbon chains, producing alkanes with 6 or more carbon atoms. The facilitates the synthesis of symmetric higher alkanes by coupling two in the presence of sodium metal under dry ether conditions. In this process, sodium donates electrons to form alkyl radicals or organosodium intermediates that combine to yield the dimerized alkane, as represented by the equation:
2\ceRX+2Na−>R−R+2NaX2 \ce{RX + 2 Na -> R-R + 2 NaX}
where R is an and X is a (typically or for better reactivity). This method is particularly suitable for preparing alkanes in the C9 to C20 range, such as from octyl , though it suffers from side products like elimination when using secondary or tertiary halides. Grignard coupling provides a versatile route to higher alkanes by first forming s from alkyl s and magnesium, followed by copper-catalyzed dimerization or cross-coupling. The (RMgX) is generated in :
\ceRX+Mg−>[ether]RMgX\ce{RX + Mg ->[ether] RMgX}
Subsequent treatment with a copper(I) salt, such as , forms a dialkylcuprate intermediate that couples with another alkyl (R'X) to produce R-R' alkanes:
2\ceRMgX+CuI−>R2CuMgX+MgXI2 \ce{RMgX + CuI -> R2CuMgX + MgXI}
\ceR2CuMgX+R′X−>R−R′+RCu+MgXI\ce{R2CuMgX + R'X -> R-R' + RCu + MgXI}
For symmetric higher alkanes, using identical R groups yields chains like from hexylmagnesium bromide and hexyl bromide, with yields improved by additives like 1-phenylpropyne to suppress side reactions. This approach is favored in laboratories for its compatibility with primary alkyl groups and tolerance. The Wolff-Kishner reduction enables chain extension in higher alkanes by converting to methylene groups, effectively reducing C=O to CH2 without affecting other functionalities. The process involves forming a intermediate from the and , followed by base-catalyzed decomposition under high temperature:
\ceR2C=O+H2N−NH2−>R2C=NNH2\ce{R2C=O + H2N-NH2 -> R2C=NNH2}
\ceR2C=NNH2+KOH−>[heat]R2CH2+N2\ce{R2C=NNH2 + KOH ->[heat] R2CH2 + N2}
For example, applying this to a dialkyl like decyl methyl yields (C12H26), allowing precise elongation of existing chains derived from natural or synthetic precursors. This method is especially useful for complex or branched higher alkanes, as it proceeds under basic conditions that avoid acidic side reactions. Decarboxylation via offers an electrochemical method to couple two ions, generating symmetric higher alkanes through anodic oxidation and radical dimerization. In this reaction, salts of carboxylic acids are electrolyzed in aqueous or alcoholic solution, producing the alkane at the alongside CO2:
2\ceRCOO−−>[anode]R−R+2CO22 \ce{RCOO^- ->[anode] R-R + 2 CO2}
For instance, electrolysis of sodium undecanoate yields eicosane (C20H42), making it ideal for long-chain higher alkanes from derivatives. The reaction requires a and controlled current to minimize over-oxidation, with yields typically 30-50% for primary alkyl chains.

Industrial Processes

Higher alkanes are predominantly produced on an industrial scale through refining, beginning with the of crude oil in atmospheric and columns. This process separates crude into various fractions based on boiling points, yielding the cut (primarily C9–C16 alkanes) at 150–275°C and heavier vacuum residue fractions containing waxes (C20+ alkanes) above 500°C. To obtain purer straight-chain higher alkane streams, particularly for lubricants and specialty waxes, dewaxing is applied to these distillates; selectively forms crystalline adducts with n-paraffins (e.g., from 400–500°C distillates), enabling their separation via filtration and recovery of high-purity n-alkane products with yields exceeding 90% for C20–C40 chains. A key synthetic route for higher alkanes is the , which catalytically hydrogenates and (syngas) to produce a spectrum of hydrocarbons, with conditions tuned for C9–C30 fractions suitable for diesel, lubricants, and waxes. The overall reaction is given by n CO+(2n+1) H2→CnH2n+2+n H2On \, \mathrm{CO} + (2n+1) \, \mathrm{H_2} \rightarrow \mathrm{C_nH_{2n+2}} + n \, \mathrm{H_2O} Iron-based catalysts operate at 220–350°C for syngas with low H₂/CO ratios (e.g., from ), while catalysts at 220–270°C favor longer-chain n-alkanes from natural gas-derived syngas (H₂/CO ≈ 2), achieving chain growth probabilities (α) of 0.85–0.95 for wax-like products later hydrocracked. Industrial implementations, such as Shell's Pearl GTL facility in (140,000 barrels/day capacity), demonstrate scalability, with catalysts yielding up to 70% C10+ hydrocarbons in low-temperature reactors. Branched higher alkanes, essential for high-octane , are manufactured via the process in refineries, where light olefins (C3–C4 from ) react with under strong . (at ~5–10°C) or (at ~30°C) catalysts promote mechanisms, forming C7–C9 branched alkanes like with research octane numbers of 94–99 and minimal byproducts when maintaining /olefin ratios above 10:1. This process consumes ~1.05 volumes of and olefins per volume of alkylate produced, contributing significantly to clean blending stocks that meet low-sulfur regulations. Post-2020 advancements emphasize sustainable bio-based production of C10+ alkanes from plant oils, addressing dependency through catalytic upgrading of triglycerides and fatty acids. Hydrodeoxygenation (HDO) removes oxygen functionalities using sulfides like MoS₂ catalysts under 9 MPa H₂ at 310–350°C, converting unsaturated fatty acids (e.g., from castor or ) to linear alkanes with yields up to 97 wt%, via initial of C=C bonds followed by /decarbonylation. Complementary routes employ to cleave plant oil double bonds into shorter alkenes, followed by HDO or over Pd/C or Ni catalysts, yielding branched and iso-alkanes for lubricants with carbon efficiencies >80% from feedstocks like . These methods, scaled in pilot plants, reduce by 50–70% compared to routes while targeting C15–C30 chains for drop-in fuels. Recent advances as of 2025 include microbial production of higher alkanes through engineered metabolic pathways in , yeasts, and , converting renewable feedstocks like sugars or CO₂ into C10+ alkanes. These bioprocesses leverage and to achieve titers up to several g/L, offering a carbon-neutral alternative for fuels and chemicals, with ongoing scaling in bioreactors.

Natural Occurrence

Geological Sources

Higher alkanes, particularly those with nine or more carbon atoms (C9+), are predominantly obtained from abiotic geological formations, with serving as the primary source. Crude oil, formed over millions of years from ancient under heat and pressure in sedimentary basins, contains higher alkanes as a major component, typically comprising 20-50% of its total content depending on the crude's geological origin and maturity. These alkanes are unevenly distributed across fractions: the diesel-range fraction ( 200-350°C, corresponding to C10-C20) is enriched in straight-chain and branched higher alkanes, making up 20-30% of crude oil volume, while the residuum or residue fraction ( >500°C, >C20) includes heavier higher alkanes alongside asphaltenes and resins, accounting for 30-50% of the oil. Extraction of higher alkanes from begins with and production from reservoirs in formations such as sandstones and carbonates, followed by initial separation at the . processes like atmospheric and concentrate these alkanes in middle and heavy distillates, enabling their isolation for further use. Industrial separation of straight-chain (n-) higher alkanes from branched and cyclic impurities in petroleum mixtures relies on clathrate formation, where urea crystals selectively encapsulate linear alkanes in hexagonal channels, allowing efficient purification with recoveries exceeding 90% for C10-C20 n-alkanes. This method, developed in the mid-20th century, remains a cornerstone of production and base stock refining. Minor geological sources of C9-C15 higher alkanes include and condensates. , a byproduct of coal during coke production or , yields 5-15% aliphatic hydrocarbons in this range through at 800-1000°C, though aromatics like dominate the composition. condensates, recovered from associated gas in reservoirs via pressure reduction and cooling, contain 10-20% higher alkanes (C9-C15) amid lighter C5-C8 components, processed through to isolate these fractions. Global of crude , which harbor substantial higher alkane content, stood at approximately 1.57 trillion barrels at the end of , with estimates near 1.7 trillion barrels into 2025 based on ongoing assessments. These reserves are distributed across major basins like the and Permian, but extraction rates have declined post-2020, averaging 2-5% annual drops in mature fields due to the accelerating shift toward renewables, reduced investment in new projects, and slower demand growth projected at 1.1 million barrels per day through 2025.

Biological Sources

Higher alkanes, particularly straight-chain n-alkanes with 25 to 35 carbon atoms, are prominent components of cuticular waxes in , serving primarily to provide a hydrophobic barrier that repels and prevents non-stomatal . These waxes are synthesized and deposited on the aerial surfaces of leaves, stems, and fruits by epidermal cells, where odd-numbered chains such as C29 and C31 predominate, enhancing surface hydrophobicity and protecting against and entry. In , a microbial-plant-derived material produced by honeybees from floral and sources, higher alkanes like C27, C29, and C31 constitute major fractions, contributing to its waterproofing properties in comb construction. In animals, higher alkanes are less abundant but play roles in insect cuticles and chemical communication. Insect exoskeletons, particularly in and bees, incorporate C23 to C27 n-alkanes as part of cuticular hydrocarbons that form a protective layer against and facilitate nestmate recognition through pheromonal cues. For instance, in species like exsecta ants, these mid-chain alkanes blend with branched hydrocarbons to modulate intraspecific aggression and social behavior. In mammals, higher alkanes occur in trace amounts within tissues such as adipose and neural structures, likely derived from dietary plant sources rather than , with concentrations significantly lower than in or plants. The of higher alkanes in and microbes follows a conserved pathway involving elongation and . In higher , de novo occurs in plastids to produce C16-C18 chains, which are then elongated in the to very-long-chain fatty acids (VLCFAs, C20-C36) using elongases; these VLCFAs are reduced to aldehydes by reductases (e.g., CER3 in ) and subsequently decarbonylated to n-alkanes by aldehyde-deformylating oxygenases. This process, localized primarily in epidermal cell endomembranes, favors odd-numbered alkanes due to the step removing one carbon atom. In microbes like , a similar reductase (AAR) and aldehyde-deformylating oxygenase (ADO) pathway operates in membranes, producing mid-chain alkanes (C13-C17) as byproducts of . Recent advances in have enabled microbial engineering for higher alkane production as sustainable biofuels. Engineered strains, expressing cyanobacterial AAR and ADO genes alongside pathway optimizations, convert glucose to C10-C20 alkanes, achieving titers up to 425 mg/L in shake-flask cultures through model-assisted . Post-2022 studies have further improved yields via CRISPR-based pathway tuning and cofactor balancing, reaching approximately 1 g/L in fed-batch fermentations for medium-chain alkanes, highlighting potential for scalable production.

Applications

Fuels and Lubricants

Higher alkanes, particularly those in the C9-C20 range, form the primary components of diesel and fuels derived from crude oil , with boiling points typically between 163°C and 357°C. In diesel, these straight-chain and branched alkanes contribute to efficient , while primarily consists of C9-C16 alkanes, enabling its use as a lighter distillate . The ignition performance of diesel is quantified by the , a scale where n-hexadecane (C16H34) is assigned a rating of 100 due to its optimal autoignition properties, contrasting with lower-rated reference compounds like alpha-methylnaphthalene ( 0). In fuels, such as Jet A and Jet A-1, higher s in the C9-C16 range predominate, blended to achieve low freezing points—maximum -40°C for Jet A and -47°C for Jet A-1—to prevent solidification at high altitudes. These mixtures ensure thermal stability and energy density suitable for turbine engines, with iso-s enhancing low-temperature fluidity. Higher alkanes also serve as key constituents in lubricants, where mineral base oils containing predominantly C15-C30 saturated hydrocarbons provide the necessary and stability for industrial and automotive applications. These paraffinic components yield a high , typically above 90, allowing consistent performance across temperature extremes. Synthetic lubricants, including polyalphaolefins (PAOs), are produced by oligomerizing C10-C12 alpha-olefins followed by to form branched structures, offering superior oxidative stability and low volatility compared to conventional oils. Transport fuels incorporating higher alkanes accounted for approximately 50% of global demand in 2023, totaling around 50 million barrels per day out of 102.7 million barrels per day overall. However, the post-2023 shift toward biofuels, with projected to rise 23% by 2028 through expanded production of renewable diesel and sustainable fuels, is gradually reducing reliance on petroleum-derived higher alkanes.

Waxes and Materials

Higher alkanes, particularly straight-chain n-alkanes with 20 to 40 carbon atoms, constitute the primary components of paraffin waxes, which are derived from . These waxes exhibit a range of 46–65°C, allowing them to transition from solid to liquid states at moderate temperatures suitable for various applications. In production, paraffin waxes provide a and moldable structure due to their low reactivity and consistent burning properties. For , they serve as moisture barriers in materials like and coatings, enhancing durability and preventing contamination. In , such as lip balms and ointments, they act as emollients, forming protective films on the skin while facilitating smooth application. Bitumen, a complex mixture used in asphalt for road construction, incorporates higher alkanes with 30 or more carbon atoms as part of its saturated fraction, which comprises approximately 10% of the overall composition. These long-chain alkanes contribute to the viscoelastic binding properties of asphalt, enabling it to adhere aggregates firmly under traffic loads and varying temperatures. In road paving, this binding action ensures pavement integrity, resisting cracking and deformation while providing to the underlying layers. The presence of these alkanes enhances the material's flexibility and longevity in applications. Higher alkanes, often processed into polyethylene waxes, function as additives in formulations to improve processability. As plasticizers, they reduce melt in polyolefins like (HDPE) and ultrahigh-molecular-weight polyethylene (UHMWPE), enhancing flexibility and impact resistance without significantly compromising mechanical strength. In polyethylene wax form, they control melt flow during and molding, lowering friction and enabling uniform distribution of fillers or pigments for smoother surface finishes and higher production efficiency. This application is particularly valuable in manufacturing durable plastic components, where precise flow control prevents defects like warping. In niche applications, higher alkanes serve as electrical insulators due to their non-polar nature and high , forming protective layers that prevent current leakage in cables and components. For instance, multilayers on substrates like black have demonstrated effective electrical insulation by delaying degradation and maintaining conductivity isolation. Additionally, specific higher alkanes like n-octacosane (C28_{28}H58_{58}), with a of approximately 62°C, are employed as phase-change materials in thermal storage systems, absorbing and releasing heat at consistent temperatures to regulate in building envelopes or cooling. This latent heat capacity, around 240 J/g, supports efficient thermal management without volume expansion issues.

Biological Aspects

Bioactivity

Higher alkanes, particularly branched variants in the C23-C29 range, play significant roles in communication as contact pheromones. These compounds, embedded in cuticular hydrocarbons, serve as species-specific signals that facilitate mate recognition and behaviors. Similar specificity is observed in other , such as the parasitic wasp Lariophagus distinguendus, where 3-methylheptacosane elicits strong behavioral responses in males, underscoring the structural sensitivity of these signals across taxa. Long-chain n-alkanes from C21 to C27, commonly found in cuticular waxes, exhibit mild antimicrobial effects, primarily against . These hydrocarbons contribute to the of plant surfaces, inhibiting bacterial and growth through hydrophobic interactions and disruption of cell membranes. Broader studies on cuticular waxes confirm that n-alkanes in this chain length range enhance overall plant resistance to microbial colonization, though their activity is generally weaker compared to other wax components like fatty acids. Higher alkanes display low in mammals, with oral LD50 values exceeding 5 g/kg for compounds like (C9), indicating minimal risk from single exposures. However, chronic of vapors from C9-C16 alkanes poses risks, including respiratory and effects, leading to their classification as irritants under occupational health guidelines. These effects stem from their volatility and solvent-like properties, which can cause ocular and upper at concentrations above 100 ppm.

Biosynthesis

Higher alkanes are biosynthesized in and primarily as components of cuticular hydrocarbons (CHCs), which provide and serve signaling functions. In , the pathway begins with de novo in the plastids, producing (C16), which is elongated in the to very-long-chain fatty acids (VLCFAs, typically C26–C30) by elongases. These VLCFAs are then reduced to aldehydes by fatty reductases and decarboxylated to odd-numbered n-alkanes (e.g., C27, C29, C31) via aldehyde-decarboxylating enzymes, such as P450s in the CYP96A family. Branched alkanes arise from of iso- or anteiso-branched fatty acids during elongation. In , CHC biosynthesis occurs in oenocyte cells embedded in the . The process mirrors pathways: and elongation produce VLCFAs, which are converted to and then hydrocarbons. Key enzymes include reductases (e.g., FAR family) for formation and CYP4G family P450s for oxidative to alkanes. Methyl-branched CHCs, crucial for pheromones, are produced by incorporation of propionyl-CoA or methylmalonyl-CoA during chain elongation, leading to iso-, anteiso-, or mono-methyl structures. is regulated by hormonal signals like , with chain lengths typically C23–C29 in many . These pathways ensure the production of nonpolar, long-chain hydrocarbons essential for resistance and chemical communication, with variations across taxa reflecting evolutionary adaptations.

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