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Short Answer
The normal boiling point of pure n-hexane is 68.7 °C (155.7 °F; 341.9 K) at standard atmospheric pressure (760 mmHg). Commercial reagent-grade “hexanes” consists of an isomeric hydrocarbon mixture that distills across a boiling range of 65 °C to 70 °C. The solvent exhibits a low flash point of -22 °C (-7.6 °F) and high volatility.
Definition and Scope
Hexane (C6H14, molar mass 86.18 g/mol) is a straight-chain aliphatic hydrocarbon assigned CAS Registry Number 110-54-3. In experimental laboratories, hexane serves as an essential nonpolar solvent for thin-layer chromatography, organic partition extractions, and mobile phases in normal-phase HPLC.
Understanding the exact physical constants of hexane is essential for laboratory workflows. Because commercial stock bottles range from certified pure n-hexane to multi-isomer mixtures, research staff must distinguish single-compound constants from industrial blends when establishing rotary evaporation conditions, calculating partition coefficients, or assessing ventilation requirements.
How It Works and How It Is Used in Practice
Hexane functions as a lipophilic solvent governed solely by weak London dispersion intermolecular forces. The net dipole moment is 0.0 Debye, making hexane polarity virtually zero. Its electrical resistance is demonstrated by a hexane dielectric constant of 1.88 at 20 °C, while chromatographic retention behavior is defined by a Snyder hexane polarity index of 0.1. Consequently, it selectively dissolves neutral, lipophilic analytes such as lipids, hydrocarbons, and uncharged natural products.
Bench Extractions and Phase Partitioning
In liquid-liquid extraction protocols, hexane density (0.659 g/cm³ at 20 °C; 0.6606 g/cm³ at 25 °C) determines phase separation. Because water has a density of 0.998 g/cm³, hexane forms the upper layer in separatory funnels. Hexane miscibility is negligible in aqueous phases (water solubility is 9.5 mg/L at 25 °C), whereas it is fully miscible with ethanol, diethyl ether, and chloroform. Furthermore, hexane pKa is estimated at ~50, meaning its sp3 carbon-hydrogen bonds undergo no acid-base dissociation in aqueous or organic media.
Rotary Evaporation and Solvent Removal
Stripping solvent after synthetic steps or column chromatography relies on hexane vapor pressure (120 mmHg at 20 °C; 180 mmHg at 25 °C) and its enthalpy of vaporization (28.85 kJ/mol). On a standard rotary evaporator equipped with a 40 °C water bath, the boiling point of n-hexane drops to 40 °C at a vacuum of 335 mbar (251 mmHg). Lower vacuum settings accelerate evaporation but require condenser chilling to 0 °C to prevent vapor bypass.
Types and Related Concepts
Constitutional isomerism determines alkane boiling points, fluid properties, and toxicity. Hexane has five constitutional isomers (C6H14), each displaying different boiling points and densities due to molecular branching.
| Compound Name | CAS Number | Boiling Point (°C) | Density at 20 °C (g/cm³) | Primary Laboratory Role |
|---|---|---|---|---|
| n-Hexane (linear) | 110-54-3 | 68.7 | 0.659 | HPLC mobile phases, analytical extractions |
| 2-Methylpentane (isohexane) | 107-83-5 | 60.3 | 0.653 | Non-neurotoxic alternative chromatography solvent |
| 3-Methylpentane | 96-14-0 | 63.3 | 0.664 | Component of commercial hexanes mixtures |
| 2,3-Dimethylbutane | 79-29-8 | 58.0 | 0.662 | Hydrocarbon analytical standard, volatile solvent |
| 2,2-Dimethylbutane (neohexane) | 75-83-2 | 49.7 | 0.649 | Low-boiling extraction solvent |
Commercial “hexanes” typically contains roughly 60% n-hexane blended with branched isomers and methylcyclopentane, resulting in a distillation range of 65 °C to 70 °C. The hexane freezing point is -95.3 °C (-139.5 °F). Fluid optical and transport properties include a hexane refractive index of 1.3727 at 25 °C and a hexane viscosity of 0.326 mPa·s at 20 °C, ensuring low flow resistance during column elutions.
Where It Matters for Labs, Clinics, and Supply Decisions
Knowledge of hexane physical constants directly dictates occupational health controls, storage safety, and sustainable solvent substitution.
Occupational Health and Neurotoxicity Mechanism
The defining chronic hazard of n-hexane is peripheral sensorimotor neuropathy (“glove-and-stocking” numbness). In hepatocytes, cytochrome P450 enzymes metabolize n-hexane to 2,5-hexanedione (2,5-HD). This gamma-diketone reacts with lysine amino groups on neurofilaments, generating pyrrole adducts that crosslink structural proteins. This molecular crosslinking causes neurofilament accumulation, axonal swelling, and distal nerve degeneration.
Crucially, branched hexane isomers (such as 2-methylpentane) cannot yield 2,5-hexanedione due to their methyl branching. Laboratory managers frequently purchase technical isohexane or heptane (boiling point 98.4 °C) to eliminate chronic neurotoxicity risks.
Exposure Standards
- OSHA PEL (Table Z-1): 500 ppm (1,800 mg/m³) 8-hour TWA.
- Cal/OSHA PEL: 50 ppm (180 mg/m³) 8-hour TWA.
- NIOSH REL: 50 ppm (180 mg/m³) up to 10-hour TWA.
- ACGIH TLV: 50 ppm 8-hour TWA with a skin notation.
- NIOSH IDLH: 1,100 ppm (10% of lower explosive limit).
Flammability and Vapor Management
Hexane is an OSHA Class IB Flammable Liquid (flash point -22 °C; explosive limits 1.1% to 7.5% by volume). With a vapor density of 2.97 relative to air, hexane vapors sink and pool along floors and bench troughs, posing ignition risks across long distances. Conformance with chemical hygiene standards requires handling hexane in an active chemical fume hood equipped with spark-proof electrical fittings.
Common Misunderstandings
- Misunderstanding: Pure n-hexane and reagent hexanes are interchangeable. Linear n-hexane has a precise boiling point of 68.7 °C, while mixed hexanes distills between 65 °C and 70 °C, altering rotary evaporation rates and chromatographic selectivity.
- Misunderstanding: All C6 alkanes produce peripheral neuropathy. Only unbranched n-hexane forms the toxic metabolite 2,5-hexanedione; branched isomers such as 2-methylpentane and 3-methylpentane do not share this pathway.
- Misunderstanding: The 500 ppm OSHA PEL indicates complete workplace safety. Federal OSHA limits reflect older standards; NIOSH and ACGIH recommend 50 ppm to prevent subclinical nerve conduction slowing.
- Misunderstanding: Hexane vapors disperse upward. A vapor density of 2.97 means vapors accumulate near the floor, requiring ground-level ventilation.
Frequently Asked Questions
What is the boiling point of acetonitrile under vacuum?
Acetonitrile (CH3CN) has a normal atmospheric boiling point of 81.6 °C (354.8 K) at 760 mmHg. Under reduced pressure, its boiling point decreases predictably: at 235 mbar (176 mmHg) it boils at 40 °C; at 155 mbar (116 mmHg) it boils at 30 °C; at 97 mbar (73 mmHg) it boils at 20 °C; and at 30 mbar (22.5 mmHg) it boils at approximately 7 °C. By comparison, n-hexane boils at 40 °C under a milder vacuum of 335 mbar (251 mmHg), evaporating faster on rotary evaporators.
What are the different isomers of hexane?
Hexane exists as five constitutional isomers (formula C6H14): straight-chain n-hexane (boiling point 68.7 °C), 2-methylpentane (isohexane, BP 60.3 °C), 3-methylpentane (BP 63.3 °C), 2,3-dimethylbutane (BP 58.0 °C), and 2,2-dimethylbutane (neohexane, BP 49.7 °C). Increased branching creates a more spherical molecular profile, reducing surface contact area and lowering boiling points.
What are the health hazards of hexane?
Acute health hazards of n-hexane include eye and respiratory irritation, dizziness, central nervous system depression, and severe aspiration chemical pneumonitis. Chronic exposure causes sensorimotor peripheral neuropathy mediated by the metabolite 2,5-hexanedione, leading to bilateral numbness, weakness, and loss of reflexes.
Why is the boiling point of hexane higher than butane?
The boiling point of hexane (68.7 °C) is significantly higher than that of n-butane (-0.5 °C) because of molecular weight and surface area. Both molecules are nonpolar hydrocarbons governed by London dispersion forces. Hexane’s six-carbon backbone (86.18 g/mol) contains more electrons and a larger surface area than butane’s four-carbon chain (58.12 g/mol), generating stronger intermolecular attractions that require more thermal energy to break.
Sources
- National Institute of Standards and Technology (NIST). (2026). Hexane: Phase Change Data and Thermochemical Properties. NIST Chemistry WebBook, SRD 69. URL: https://webbook.nist.gov/cgi/cbook.cgi?ID=C110543&Units=SI&Mask=4
- National Oceanic and Atmospheric Administration (NOAA). (2026). Chemical Datasheet: N-HEXANE (UN 1208). CAMEO Chemicals. URL: https://cameochemicals.noaa.gov/chemical/851
- National Center for Biotechnology Information (NCBI). (2026). Compound Summary: n-Hexane (CID 8058). PubChem. URL: https://pubchem.ncbi.nlm.nih.gov/compound/8058
- Occupational Safety and Health Administration (OSHA). (2026). Hexane (n-Hexane): Permissible Exposure Limits (29 CFR 1910.1000 Table Z-1). Occupational Chemical Database. URL: https://www.osha.gov/chemicaldata/112
