Cyclohexane
Properties
| State | Liquid |
| Color | Colorless |
| Solubility | Immiscible with water (55 mg/L at 25°C); miscible with organic solvents |
| Melting Point | 6.5°C |
| Boiling Point | 80.7°C |
About Cyclohexane
Cyclohexane is the molecule that conformational analysis was built on, and almost everything organic chemists know about three-dimensional shape preferences in saturated rings traces back to Hassel's and Barton's work on it (Nobel Prize, 1969). The chair conformation puts every C–C–C angle at the unstrained tetrahedral 111° and every C–H bond perfectly staggered, so there's no angle strain and essentially no torsional strain — which is why cyclohexane is far more stable than the planar hexagon a textbook drawing implies. The chair-flip interconverts axial and equatorial positions through a half-chair (about 45 kJ/mol barrier) and a twist-boat intermediate (about 23 kJ/mol above the chair), with the two chair forms in dynamic equilibrium at room temperature on the NMR timescale — at -90°C you can actually freeze the equilibrium out and see separate axial and equatorial signals. Substituent A-values (the energy preference for equatorial over axial) come directly from cyclohexane analysis: methyl 7.5 kJ/mol, isopropyl 9.2 kJ/mol, t-butyl about 21 kJ/mol — large enough that t-butyl essentially locks the ring conformation. Industrially, cyclohexane is overwhelmingly a feedstock for nylon: catalytic hydrogenation of benzene over Ni or Pt gives cyclohexane, which is then air-oxidized over a cobalt naphthenate catalyst to KA oil (cyclohexanone + cyclohexanol mixture), then HNO₃-oxidized to adipic acid HOOC–(CH₂)₄–COOH, the diacid for nylon-6,6. Roughly 90% of global cyclohexane goes into this chain, with the remainder going to cyclohexanone for caprolactam and nylon-6. As a solvent, cyclohexane is the conventional non-polar reference (¹H NMR singlet at 1.43 ppm) and is used as an internal standard in NMR work.
Where you'll encounter it
If you've ever drawn a chair on an organic chemistry exam and labeled axial-up and equatorial-out, the molecule was cyclohexane. In a research lab the colorless liquid often shows up as a non-polar recrystallization solvent (bp 80.7°C, easy to rotovap) or as a non-coordinating reference solvent for ¹H NMR at 1.43 ppm. The Bhopal disaster in 1984 traced back to a methyl isocyanate plant making intermediates for carbamate pesticides; nylon-6,6 plants making cyclohexanone via cyclohexane oxidation have had their own incidents (Flixborough, 1974), where 28 tonnes of leaking cyclohexane vapor exploded.
Common Uses
- Catalytic feedstock for KA oil → adipic acid → nylon-6,6 (largest-volume use, ~90%)
- Air-oxidation feedstock for cyclohexanone → caprolactam → nylon-6 polymer manufacture
- Non-polar recrystallization solvent for moderately polar organic compounds with bp 80.7°C
- Internal NMR reference and locking solvent (¹H singlet at 1.43 ppm) for non-polar samples
- Conformational-analysis teaching molecule for axial/equatorial preferences and ring flip kinetics
- Azeotrope partner with water (bp 69.5°C, 92% cyclohexane) for Dean–Stark dehydration setups
- Calibration standard for differential scanning calorimetry (sharp 6.5°C melting transition)
- Solvent for paint, ink, and adhesive formulations where pure aliphatic non-polar character is needed
Safety Information
GHS: H225 (highly flammable, flash point -20°C), H304 (aspiration hazard — fatal if swallowed and aspirated, the lung-toxicity mode for low-viscosity hydrocarbons), H315 (skin irritation), H336 (drowsiness/dizziness from CNS depression), H410 (very toxic to aquatic life). Occupational exposure limits are comparatively permissive for a solvent — inhalation is not the controlling risk. The fire hazard is what dominates: flash point -20°C means it's flammable at any normal lab temperature, and vapor density 2.9 (heavier than air) means leaks pool at floor level and travel to ignition sources. Use only in well-ventilated areas with no sparks or open flames; bond and ground containers when transferring. The Flixborough disaster (1974) was a cyclohexane vapor cloud explosion — 28 tonnes of escaped vapor from a temporary bypass pipe killed 28 people.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.