Octadecane

Octadecane is a straight-chain alkane hydrocarbon with the molecular formula C₁₈H₃₈, consisting of eighteen carbon atoms linked by single bonds in a linear chain, making it the eighteenth member of the n-alkane series.^[1] It appears as a colorless, waxy solid at room temperature due to its relatively high melting point of 28.2°C (82.8°F), with a boiling point of 317°C (603°F) and a density of approximately 0.78 g/cm³ for the liquid phase.^[2] Chemically inert and non-polar, octadecane is insoluble in water but soluble in organic solvents, and it serves as a model compound in studies of hydrocarbon behavior.^[1] Naturally occurring as a plant and bacterial metabolite, octadecane is found in various sources including papaya and sunflowers, where it contributes to the composition of waxes and lipids.^[3] It has been detected in biological samples from petroleum-derived sources and environmental contexts. Industrially, octadecane is utilized as a solvent in organic synthesis, a calibration standard in analytical chemistry, and a component in paraffin waxes for candles and polishes.^[1] Its phase-change properties—absorbing and releasing heat during melting and solidification—make it valuable as a phase-change material (PCM) in thermal energy storage applications, such as building insulation, solar energy systems, battery cooling, and electronics thermal management.^[4] Additionally, it is used in high-performance lubricants,^[5] cosmetic formulations as an emollient and solvent,^[6] and microencapsulation for latent heat transfer.^[4]

Structure and nomenclature

Molecular formula and structure

Octadecane is a saturated hydrocarbon with the molecular formula C₁₈H₃₈, consisting of 18 carbon atoms arranged in a continuous chain and 38 hydrogen atoms that satisfy the tetravalency of each carbon through single bonds.^[1][7] The structural formula of octadecane is CH₃(CH₂)₁₆CH₃, representing a linear, unbranched alkane chain where the terminal methyl groups (CH₃) are connected by 16 methylene units (CH₂). This configuration forms a straight-chain molecule, with all carbon-carbon bonds being single (σ) bonds, allowing for free rotation around them under ambient conditions.^[7][1] In terms of bonding geometry, each carbon atom in octadecane adopts a tetrahedral arrangement due to sp³ hybridization, resulting in bond angles of approximately 109.5° for H-C-H, C-C-H, and C-C-C interactions. The average C-C bond length is about 1.54 Å, while C-H bonds measure roughly 1.09 Å, values typical for alkanes and derived from standard quantum chemical models and experimental X-ray diffraction data.^[8][9] Octadecane exhibits conformational flexibility along its chain, but in the solid state, it predominantly adopts an all-anti (extended) conformation, manifesting as a zig-zag arrangement of the carbon backbone to minimize steric repulsion and maximize chain packing efficiency in the crystal lattice.^[10]

Naming and isomers

Octadecane is the IUPAC name for the straight-chain alkane with 18 carbon atoms, systematically derived from the Greek numerical prefixes octa- (meaning eight) and deca- (meaning ten), combined to denote the total of 18 carbons in the unbranched chain, followed by the suffix -ane for alkanes.^[11] The term "n-octadecane" serves as a trivial or common name to explicitly distinguish the linear isomer from its branched counterparts.^[1] The systematic nomenclature of alkanes, including octadecane, emerged in the late 19th century as organic chemistry expanded beyond simple compounds isolated from natural sources like petroleum or fats. Early naming conventions were often descriptive or source-based—such as "paraffin" for waxy hydrocarbons—but proved inadequate for the increasing number of isomers discovered through synthesis and distillation techniques. By 1892, the International Congress of Chemistry in Geneva laid the groundwork for IUPAC rules, formalizing numerical prefixes for chain length to ensure unambiguous identification amid the combinatorial explosion of possible structures.^[12] This evolution reflected broader efforts to codify organic nomenclature, culminating in the 1957 IUPAC recommendations that standardized alkane naming for global use. The molecular formula C$_{18}$18​H$_{38}$38​ admits 60,523 constitutional isomers, each featuring a different connectivity of carbon atoms while maintaining the same overall composition.^[13] These structural variants range from the linear n-octadecane to highly branched forms, such as 2-methylheptadecane (a chain of 17 carbons with a methyl group at position 2) or 2,2,4,4-tetramethylpentadecane (featuring multiple methyl branches for greater compactness). Naming follows IUPAC rules by selecting the longest continuous chain as the parent, numbering it to give substituents the lowest possible locants, and listing branches alphabetically with multipliers like di- or tri- for repeats. For instance, 3-ethyl-2-methylhexadecane would denote a 16-carbon main chain with an ethyl group at carbon 3 and a methyl at carbon 2.^[14] Constitutional isomers differ from stereoisomers, the latter involving variations in spatial configuration without altering atomic connections. Alkanes like octadecane and most of its constitutional isomers are achiral, lacking tetrahedral carbons with four different substituents, and thus exhibit no stereoisomerism; the straight-chain form, for example, possesses a plane of symmetry rendering it superimposable on its mirror image./Chirality/Chirality_and_Stereoisomers) Only a subset of branched constitutional isomers with chiral centers—such as those incorporating asymmetric branching—could potentially have enantiomeric stereoisomers, but these are exceptions rather than the rule for C$_{18}$18​H$_{38}$38​.^[15]

Physical properties

Thermodynamic data

Octadecane, a long-chain alkane, displays characteristic thermodynamic properties influenced by its non-polar linear structure, which promotes weak intermolecular forces and phase behaviors typical of hydrocarbons.^[16] Key phase transition temperatures include a melting point of 28 °C (301 K) and a boiling point of 317 °C (590 K) at standard atmospheric pressure.^[16] The enthalpy associated with these transitions is significant: the heat of fusion is 61.0 kJ/mol, reflecting the energy required to disrupt the solid lattice into a liquid state, while the heat of vaporization at 298 K is approximately 91 kJ/mol.^[16] These properties underscore octadecane's hydrophobicity, with extremely low water solubility and a high octanol-water partition coefficient, making it poorly miscible with polar solvents.^[1] The critical point values indicate the conditions under which the liquid and vapor phases become indistinguishable.^[16]

Spectroscopic characteristics

Octadecane, as a linear alkane, exhibits characteristic infrared (IR) absorption bands typical of saturated hydrocarbons, with no peaks indicative of functional groups such as carbonyls or unsaturations. The IR spectrum shows strong C-H stretching vibrations for methylene (CH₂) and methyl (CH₃) groups at approximately 2920 cm⁻¹ and 2850 cm⁻¹, respectively, along with a C-H bending mode around 1465 cm⁻¹. Additionally, a prominent rocking mode for the long CH₂ chain appears near 720 cm⁻¹.^[17][18] In nuclear magnetic resonance (NMR) spectroscopy, the ¹H NMR spectrum of octadecane in CDCl₃ displays a characteristic triplet at δ 0.88-0.90 ppm for the terminal methyl (CH₃) protons (6H) and a broad multiplet at δ 1.25-1.30 ppm for the methylene (CH₂) protons (32H), reflecting the symmetric chain structure with minimal splitting due to long-range coupling. The ¹³C NMR spectrum reveals nine distinct signals due to the molecule's symmetry, with the terminal methyl carbon at approximately δ 14 ppm, methylene carbons ranging from δ 23 to δ 32 ppm, and no signals beyond δ 35 ppm, confirming the absence of branching or unsaturation.^[19][20][21] Mass spectrometry of octadecane under electron ionization (70 eV) shows a weak molecular ion peak at m/z 254 (M⁺, corresponding to C₁₈H₃₈), with base peak at m/z 57 (C₄H₉⁺) from alkyl fragmentation; a notable fragment at m/z 239 arises from loss of a methyl group (M-15). Common alkane fragmentation patterns include sequential losses of C₂H₄ units, yielding peaks at m/z 71, 85, and 99.^[22] Ultraviolet-visible (UV-Vis) spectroscopy of octadecane reveals no absorption bands above 200 nm, as expected for a saturated hydrocarbon lacking π-electrons or conjugated systems, limiting its utility for UV detection in analytical methods.^[23]

Chemical properties

Reactivity profile

Octadecane, as a saturated alkane, exhibits significant chemical inertness under ambient conditions, primarily due to the high bond dissociation energies of its carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds, which are approximately 347 kJ/mol and 410 kJ/mol, respectively.^[24] These strong sigma bonds render octadecane resistant to most common reagents, including acids, bases, and oxidizing agents at room temperature, preventing spontaneous reactions without external energy input or catalysts.^[25] Despite its inertness, octadecane participates in free radical reactions, such as halogenation, where ultraviolet light or heat initiates the process. For instance, chlorination with chlorine gas under UV irradiation substitutes one or more hydrogen atoms, yielding a mixture of chlorinated octadecanes through a chain mechanism involving radical formation, propagation, and termination steps.^[26] This reactivity is typical of alkanes and highlights the vulnerability of C-H bonds to homolytic cleavage under radical conditions.^[27] Octadecane undergoes complete combustion in the presence of sufficient oxygen, producing carbon dioxide and water as the primary products. The balanced equation for this exothermic oxidation is: $$2 \ce{C18H38} + 55 \ce{O2} \rightarrow 36 \ce{CO2} + 38 \ce{H2O}$$2\ceC18H38+55\ceO2→36\ceCO2+38\ceH2O ^[28] In petrochemical processes, octadecane can be subjected to thermal or catalytic cracking, which breaks its long carbon chain into smaller alkanes and alkenes. Thermal cracking proceeds via a free-radical mechanism initiated by C-C bond fission at elevated temperatures, while catalytic methods enhance selectivity for valuable lighter hydrocarbons.^[29] Lacking pi bonds, octadecane does not undergo electrophilic or nucleophilic addition reactions characteristic of unsaturated hydrocarbons, further underscoring its stability toward such pathways.^[25]

Stability and decomposition

Octadecane shows thermal stability with negligible mass loss below approximately 100 °C in inert atmospheres, though significant weight loss occurs between 100–207 °C according to thermogravimetric analysis (TGA).^[30] Its autoignition temperature is reported as 227°C, beyond which spontaneous combustion can occur in the presence of air.^[31] Oxidative decomposition of octadecane proceeds via slow autoxidation in air at ambient temperatures, primarily forming hydroperoxides through a free-radical chain mechanism involving hydrogen abstraction and oxygen addition. This process is significantly accelerated above 100°C, leading to faster accumulation of peroxides and subsequent breakdown products.^[32][33] Photolytic stability of octadecane is high under visible light, with minimal decomposition observed; however, exposure to ultraviolet radiation can initiate radical chain reactions, resulting in structural alterations and fragmentation.^[34] Under ambient soil conditions, octadecane exhibits variable environmental persistence due to its low water solubility and limited microbial accessibility, with biodegradation rates depending on dosing and conditions (e.g., 18-88% degradation in 28-day OECD tests). In soil, biodegradation half-lives for similar long-chain alkanes are typically in the range of weeks to months under aerobic conditions, though slower in anaerobic environments.^[35][1] In high-temperature pyrolysis, octadecane decomposes primarily into alkenes and smaller hydrocarbons via free-radical β-scission, alongside aldehydes from partial oxidation pathways and carbon monoxide/carbon dioxide at elevated temperatures exceeding 500°C.^[36][37]

Synthesis and occurrence

Natural sources

Octadecane occurs as a minor component in petroleum and natural gas deposits, where it forms part of the straight-chain alkane fraction derived from the diagenesis of ancient organic matter, such as plankton and terrestrial plants, under sedimentary conditions.^[38] In crude oil, n-octadecane typically represents 0.1-2% of the aliphatic hydrocarbons in diesel and kerosene-like fractions, varying by source rock and maturation level.^[39] It is also present in shale oils, contributing to the light hydrocarbon content alongside other n-alkanes.^[40] In biological systems, octadecane serves as a plant and bacterial metabolite, identified in species such as Camellia sinensis (tea), Vanilla madagascariensis, papaya, sunflowers, and hop oil, as well as in alcoholic beverages; it often appears as part of cuticular waxes that provide protective barriers against environmental stress.^[1][3] For example, in flower surface waxes, octadecane can be a predominant alkane, comprising significant portions of the total wax load, up to several hundred micrograms per sample in certain plants.^[41] In bee wax (Apis mellifera), it accounts for approximately 12% of the hydrocarbon fraction, aiding in structural integrity and waterproofing of the comb.^[42] Octadecane is a component of cuticular hydrocarbons in insects, where it contributes to waterproofing and chemical communication; for instance, it appears as a species-specific compound in the blowfly Phormia regina.^[43] In marine environments, trace levels occur in algal lipids from phytoplankton and are deposited in sediments, reflecting both biogenic inputs and early diagenetic processes, with concentrations often below 1% of total hydrocarbons in planktonic samples.^[44] These occurrences highlight octadecane's role as a biomarker for organic matter transformation in both terrestrial and aquatic ecosystems.^[1]

Production methods

Octadecane is industrially produced on a large scale through the fractionation of petroleum-derived paraffin waxes, isolating the C18 hydrocarbon fraction via distillation processes.^[45] One key synthetic method involves the Ziegler process, where ethylene undergoes oligomerization using triethylaluminum and titanium-based catalysts to form linear alpha-olefin chains, which are then hydrogenated to yield n-octadecane.^[46] The Fischer-Tropsch synthesis provides another route, converting syngas (a mixture of CO and H2) over iron or cobalt catalysts into a broad spectrum of hydrocarbons, with the C18 fraction, including octadecane, separated from the product mixture.^[47] Additionally, octadecane can be synthesized via the hydrodeoxygenation of stearic acid or related fatty acid derivatives, employing catalysts such as platinum supported on niobia (Pt/Nb2O5) to remove oxygen and produce the saturated alkane.^[48] Purification of crude octadecane typically involves fractional distillation at its boiling point of 317°C for bulk separation, while high-purity grades are obtained through chromatographic techniques or crystallization from solvents like methanol.^[1]

Applications

Industrial uses

Octadecane serves as a phase-change material (PCM) in thermal energy storage systems due to its melting point of 28°C and high latent heat of fusion, enabling efficient heat absorption and release.^[1] These properties make it suitable for applications in building insulation, where it is incorporated into materials like concrete or composites to regulate indoor temperatures and reduce energy consumption for heating and cooling.^[49] In electronics cooling, octadecane-based PCMs are used in thermal management systems to dissipate heat from devices such as batteries and circuits, maintaining operational stability under varying loads.^[50] In the lubricant industry, octadecane functions as an additive in greases and oils, contributing to high-temperature stability and low volatility, which enhances performance in automotive engines and industrial machinery.^[51] Its straight-chain structure provides shear resistance and reduces wear in closed systems like hydraulic fluids and motor oils.^[51] Octadecane acts as an intermediate in the production of surfactants, which are incorporated into detergents and emulsifiers for their ability to lower surface tension and stabilize mixtures. These derivatives improve cleaning efficiency in household and industrial formulations by enhancing wetting and dispersion properties.^[52] As a fuel component, octadecane is blended into diesel and kerosene to adjust the cetane number, with its own derived cetane number exceeding 100, promoting smoother ignition and combustion efficiency.^[53] This application leverages its physical properties, such as high boiling point and low reactivity, to optimize fuel blends for better engine performance.^[54] The global market for octadecane is primarily within specialty chemicals, valued at approximately USD 250 million in 2024, with projected growth to USD 400 million by 2033 driven by increasing adoption in green energy applications like solar thermal storage since 2020.^[55]

Research and standards

Octadecane serves as a key calibration standard in gas chromatography for the analysis of hydrocarbons. It is included in NIST Standard Reference Material (SRM) 1494, a solution containing 20 aliphatic hydrocarbons ranging from n-decane to n-triacontane, designed to calibrate chromatographic instruments for accurate quantification of these compounds in environmental and petroleum samples.^[56] This SRM ensures traceability and precision in measurements, with certified concentrations for each component, including n-octadecane at 0.122 mg/g (121.9 μg/g).^[56] In biomarker research, octadecane functions as a proxy for paleoclimate reconstruction in sediment cores, where distributions of n-alkanes reveal odd-over-even carbon number preferences indicative of organic matter sources, such as terrestrial versus aquatic inputs, and environmental changes like humidity or temperature shifts. For instance, ratios such as pristane/n-C17 and phytane/n-C18 in core sediments from the Yellow Sea have been used to infer redox conditions and paleoenvironmental settings during the Holocene.^[57] These preferences, often quantified via the carbon preference index (CPI), highlight octadecane's (C18, even-numbered) role in distinguishing biogenic versus petrogenic origins and reconstructing past climate variability.^[57] As a model compound in alkane studies, octadecane is widely utilized in surface science to examine adsorption behaviors on metals. Early investigations demonstrated its coadsorption with polar compounds like stearic acid on iron, copper, silver, and gold surfaces, providing insights into the mechanisms of nonpolar hydrocarbon interactions and film formation.^[58] In polymer research, octadecane mimics the long-chain segments of polyethylene, facilitating studies on chain dynamics, solubility, and catalytic degradation processes. For example, it has been employed as a substrate to model the hydrogenolysis of polyethylene under ruthenium catalysis, achieving high selectivity for C(sp³)–C bond cleavage at secondary sites.^[59] Recent studies since 2015 have incorporated octadecane in nanotechnology applications, particularly for self-assembling structures and simulations relevant to advanced materials. In surface nanotechnology, derivatives like octadecanethiol form self-assembled monolayers (SAMs) on coinage metals, with investigations into selenolate alternatives revealing enhanced stability and charge transfer properties compared to traditional thiolates.^[60] For drug delivery simulations, octadecane-based phase change microcapsules have been modeled using molecular dynamics to optimize encapsulation and controlled release, demonstrating improved thermal responsiveness in lipid-like nanoparticle systems.^[61] Analytical standards of octadecane are certified with purity exceeding 99%, ensuring reliability for spectroscopic and chromatographic applications. Their melting points, traceable to NIST values of 28.2 °C, provide a benchmark for thermal analysis and phase behavior studies.^[16]

Safety and environmental aspects

Toxicity and handling

Octadecane exhibits low acute toxicity via oral exposure, with an LD50 greater than 5,000 mg/kg in rats according to OECD Test Guideline 401.^[62] Inhalation toxicity data are limited due to its low volatility, but analogous studies on similar higher alkanes indicate an LC50 exceeding 5 mg/L with minimal respiratory irritation.^[63] Dermal acute toxicity is also low, with an LD50 exceeding 3,160 mg/kg in rabbits.^[64] No significant irritation to skin or eyes has been observed in standard OECD tests.^[62] It does not appear to cause skin sensitization based on available hydrocarbon solvent assessments.^[63] Chronic or repeated skin exposure may lead to dermatitis, as observed with prolonged contact to petroleum-derived aliphatic hydrocarbons.^[65] Its solid or waxy physical state at room temperature limits significant dermal absorption.^[1] Occupational exposure limits for octadecane are not substance-specific from OSHA, which has no dedicated PEL. ACGIH establishes a TLV of 1,200 mg/m³ as an 8-hour time-weighted average for C14–C20 aliphatic hydrocarbon vapors, treating octadecane within this category of total hydrocarbons to prevent irritation.^[63] Aerosol forms are limited to 5 mg/m³.^[63] Safe handling of octadecane requires use in well-ventilated areas to minimize vapor accumulation, given its flash point of approximately 166°C (331°F).^[1] Personal protective equipment includes nitrile gloves to prevent skin contact, as this material resists hydrocarbon penetration, along with safety goggles and protective clothing.^[62] Avoid ignition sources during storage and transfer, and do not induce vomiting if ingested due to aspiration hazard risk.^[62] Octadecane is listed on the TSCA inventory as an active substance in the United States.^[66] In the European Union, it is registered under REACH and classified as a low-concern substance with no specific restrictions beyond general handling precautions.^[67]

Ecological impact

Octadecane is readily biodegradable by microorganisms in aerobic environments, achieving 74% biodegradation in 28 days under OECD 306 marine conditions.^[68] The primary biodegradation pathway involves terminal oxidation, where microbes convert the alkane to primary alcohols, aldehydes, and subsequently carboxylic acids via cytochrome P450 monooxygenases, followed by beta-oxidation for further breakdown. Due to its high octanol-water partition coefficient (log Kow ≈ 9.2, estimated via KOWWIN model), octadecane has low bioaccumulation potential, with a bioconcentration factor (BCF) below 2,000 L/kg in fish modeled using BCFBAF.^[67] However, actual bioaccumulation is limited by its low water solubility (approximately 0.006 mg/L at 25°C), which restricts uptake in aqueous environments and reduces exposure risk to aquatic organisms.^[1] Ecotoxicity studies indicate low acute effects on aquatic life, with LC50 values exceeding 1000 mg/L for fish (e.g., >1028 mg/L in 96-hour tests) and algae (e.g., EC50 >10,000 mg/L for marine diatoms), suggesting no significant adverse impacts at typical environmental concentrations. Chronic effects are also minimal due to rapid biodegradation and partitioning away from water columns. In the environment, octadecane demonstrates slow volatilization, with a vapor pressure of approximately 4.5 × 10^{-5} Pa at 25°C.^[1] It primarily partitions to soil and sediment (Koc ≈ 10^7, estimated via KOCWIN), where it persists longer but remains subject to microbial degradation, minimizing long-term mobility in ecosystems.^[1] In oil spill scenarios, octadecane serves as a diagnostic hydrocarbon marker for tracking petroleum degradation, with ratios such as pristane/n-C18 used to assess weathering and source identification.^[69] Response protocols involve absorption using inert materials like sand or vermiculite to contain and recover spills, preventing widespread dispersion while leveraging natural biodegradation processes.^[2]