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). OSHA PEL 300 ppm (TWA), NIOSH REL 300 ppm, IDLH 1300 ppm. The fire hazard is significant — 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.