Hexane
Hexane is an organic compound classified as a straight-chain alkane hydrocarbon with the molecular formula C₆H₁₄.[1] It exists as a colorless, volatile liquid at standard temperature and pressure, characterized by a boiling point of approximately 69 °C and a density of 0.66 g/cm³, rendering it less dense than water and insoluble therein.[1] As a non-polar solvent, hexane is widely employed in industrial applications such as the extraction of edible oils from seeds, in chemical reactions, and as a component in gasoline formulations, though pure n-hexane—the unbranched isomer—is primarily utilized in laboratory settings.[2] Its vapors are heavier than air, contributing to its utility in solvent-based processes, but also posing significant risks due to high flammability, with a flash point of -9 °F and explosive potential in confined spaces.[1][3] Notably, prolonged inhalation of n-hexane vapors can induce peripheral neuropathy, manifesting as numbness and motor weakness in extremities, a neurotoxic effect substantiated through occupational exposure studies.[4][5]History
Discovery and Early Characterization
Hexane isomers were initially isolated from petroleum distillates in the mid-19th century amid growing interest in refining crude oil for illuminants and solvents, using rudimentary fractional distillation to separate light naphtha fractions boiling between approximately 60°C and 70°C.[6] These efforts, spurred by the 1859 discovery of oil in Pennsylvania, yielded mixtures of C6 hydrocarbons, though pure isomers remained challenging to obtain without chemical treatments like chlorination or selective absorption to distinguish saturated chains from unsaturated or aromatic contaminants.[7] Carl Schorlemmer, a German-born chemist working in Manchester, advanced the characterization of normal hexane (n-hexane) in the 1870s through both isolation from natural paraffin sources and laboratory preparation. By distilling mannitol (a sugar alcohol) with excess hydriodic acid, Schorlemmer obtained a pure sample of n-C6H14, confirming its identity via elemental combustion analysis that yielded the empirical formula consistent with saturated alkanes (CnH2n+2).[8] This method provided a reference compound free from petroleum impurities, enabling precise measurement of its boiling point at 69°C and highlighting its high volatility compared to heavier fractions.[9] Early empirical tests further delineated hexane as a saturated alkane, revealing its chemical inertness: unlike aromatic hydrocarbons such as benzene (isolated earlier by Faraday in 1825), n-hexane exhibited no rapid decolorization of bromine solutions in the absence of light or catalysts, nor did it undergo addition reactions typical of unsaturates.[8] These observations, grounded in Schorlemmer's systematic studies of paraffin derivatives up to octane, established hexane's straight-chain structure and non-reactive nature, distinguishing it from cyclic or unsaturated petroleum components through differences in density, refractive index, and thermal stability during distillation. By the 1880s, such characterizations informed the nomenclature "hexane," first recorded around 1875-1880, reflecting its six-carbon chain.Commercial Development and Key Milestones
Hexane's commercial production scaled with the development of thermal and catalytic cracking processes in petroleum refining during the 1920s, which increased yields of light aliphatic hydrocarbons like the C6 fractions from which hexane is distilled.[11][12] These advances, including the Burton thermal cracking method adopted by major refiners by the mid-1920s, provided a reliable supply of naphtha streams containing hexane isomers for industrial use as solvents and fuels.[13] Demand for hexane accelerated during World War II due to expanded refining capacity to meet aviation and military fuel needs, indirectly boosting production of by-product solvents like hexane for adhesives and extraction processes.[14] Post-war, its role in solvent extraction of vegetable oils—particularly soybeans—gained prominence in the 1940s, as hexane-based systems replaced mechanical pressing for higher efficiency and yield, driving adoption in the food processing industry.[15][16] In the 1960s, occupational studies in Japan and Italy identified n-hexane's neurotoxicity, linking chronic exposure in shoe manufacturing to peripheral polyneuropathy among workers using hexane-based glues, prompting initial regulatory scrutiny and shifts toward lower-toxicity isohexane mixtures.[17][18] This recognition, based on epidemiological data from solvent-exposed cohorts, led to isomer-specific exposure limits by the 1970s, influencing formulation standards in solvent applications.[19] As of 2025, the global n-hexane market is valued at approximately USD 2.5 billion, with projections to reach USD 3.45 billion by 2032, driven primarily by sustained demand in edible oil extraction and adhesives amid refining optimizations.[20] Growth reflects causal ties to petrochemical supply chains, though tempered by toxicity-aware substitutions in some sectors.[21]Chemical Structure and Isomers
Molecular Formulas and Configurations
Hexane and its isomers share the molecular formula C₆H₁₄, adhering to the general alkane formula CₙH₂ₙ₊₂ for n=6, where all bonds are single sigma bonds between carbon and hydrogen atoms or between carbons.[1] In these structures, each carbon atom adopts sp³ hybridization, blending one s and three p orbitals to form four equivalent sp³ hybrid orbitals arranged in tetrahedral geometry with bond angles near 109.5°, enabling maximal overlap for sigma bond formation and inherent structural stability./05%3A_Bonding_in_Polyatomic_Molecules/5.02%3A_Valence_Bond_Theory_-_Hybridization_of_Atomic_Orbitals/5.2D%3A_sp3_Hybridization) This hybridization applies uniformly across the straight-chain n-hexane, represented as CH₃(CH₂)₄CH₃ with six carbons in a continuous chain, and its four branched constitutional isomers: 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane.[22] n-Hexane features an unbranched carbon skeleton, maximizing linear extent and surface interactions, whereas branched isomers exhibit compact configurations due to methyl substitutions on the chain, reducing overall molecular surface area.[23] For instance, 2-methylpentane incorporates a methyl group at the second carbon of a pentane chain, forming CH₃CH(CH₃)CH₂CH₂CH₃; 3-methylpentane has the branch at the third carbon, CH₃CH₂CH(CH₃)CH₂CH₃; 2,2-dimethylbutane features two methyl groups on the second carbon of butane, (CH₃)₃CCH₂CH₃; and 2,3-dimethylbutane has methyl groups on adjacent second and third carbons, CH₃CH(CH₃)CH(CH₃)CH₃.[24] These variations in connectivity preserve the C₆H₁₄ stoichiometry while altering spatial arrangements, with sigma bonds ensuring covalent saturation and no pi bonds or unsaturation.[25] Commercial mixtures labeled "hexanes" predominantly feature n-hexane alongside these branched forms, with n-hexane comprising 20% to 85% of the blend depending on refining processes.[1] The straight-chain dominance in such mixtures arises from distillation fractions in petroleum processing, where linear alkanes elute before more volatile branched counterparts, though exact compositions vary by supplier and application.[1] Branched isomers, by virtue of their reduced chain length and increased branching, exhibit diminished intermolecular surface contact compared to n-hexane, influencing packing efficiency despite identical hybridization and bonding fundamentals.[23]Comparative Properties of Isomers
The constitutional isomers of hexane differ in their physical properties due to varying degrees of branching, which alters molecular shape and reduces the surface area for London dispersion forces, leading to weaker intermolecular attractions. This results in lower boiling points and higher volatility for more branched isomers compared to the linear n-hexane. For example, n-hexane boils at 68.7 °C, while 2,2-dimethylbutane, the most branched isomer, boils at 49.7 °C.[26][27] The intermediate isomers follow this trend: 3-methylpentane at 63.3 °C, 2-methylpentane at 60.3 °C, and 2,3-dimethylbutane at 58.0 °C.[28]| Isomer | Boiling Point (°C) |
|---|---|
| n-Hexane | 68.7 |
| 3-Methylpentane | 63.3 |
| 2-Methylpentane | 60.3 |
| 2,3-Dimethylbutane | 58.0 |
| 2,2-Dimethylbutane | 49.7 |
Physical Properties
n-Hexane appears as a clear, colorless liquid with a mild petroleum-like odor and is less dense than water, with vapors heavier than air.[1][31] It exhibits low solubility in water, approximately 0.0013 g/100 mL at 20 °C, rendering it practically insoluble, though it mixes readily with organic solvents such as alcohols, chloroform, and ethers.[32][1] Key thermal properties include a melting point of −95.3 °C and a boiling point of 68.7 °C at standard pressure.[33] At 25 °C, its density measures 0.6606 g/mL, with a refractive index of 1.3727 under the same conditions.[33] Viscosity is approximately 0.31 mPa·s at 20 °C, contributing to its flow characteristics as a nonpolar solvent.[34]| Property | Value | Conditions |
|---|---|---|
| Flash point | −22 °C | Closed cup |
| Vapor pressure | 13 mm Hg | 20 °C |
| Surface tension | 17.91 dyn/cm | 25 °C |