Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C₆H₁₂, consisting of a six-carbon ring that predominantly adopts a stable chair conformation to minimize angle and torsional strain.[1] This saturated hydrocarbon appears as a clear, colorless liquid with a mild, petroleum-like odor, exhibiting non-polar characteristics that render it insoluble in water but miscible with organic solvents.[1] At standard conditions, it has a boiling point of 80.7 °C, a melting point of 6.5 °C, and a density of 0.779 g/mL at 20 °C, making it a volatile and flammable substance with a flash point of -20 °C.[1][2] Cyclohexane is primarily produced on an industrial scale through the catalytic hydrogenation of benzene, utilizing catalysts such as nickel, platinum, or palladium in either liquid or vapor phase processes, often derived from petroleum feedstocks.[1] Alternative methods include separation from petroleum liquids, though hydrogenation accounts for the majority of global production, approximately 9.6 million metric tons in 2024.[3] Its chemical stability under normal conditions allows it to serve as a versatile non-polar solvent in applications like lacquers, resins, adhesives, and paint removers, as well as an analytical reagent in laboratories.[4] A key industrial role of cyclohexane lies in its oxidation to cyclohexanol and cyclohexanone, which are precursors for adipic acid and caprolactam, essential monomers in nylon-6 and nylon-6,6 production, accounting for over 90% of its consumption.[1] It also finds use as a co-solvent in pesticide formulations and in the extraction of various compounds.[5] However, cyclohexane poses significant safety hazards: it is highly flammable, can cause irritation to the skin, eyes, and respiratory tract upon exposure, and may lead to central nervous system depression or aspiration pneumonia if ingested or inhaled in large amounts.[6] Proper handling requires ventilation, protective equipment, and adherence to regulations classifying it as a hazardous substance under frameworks like the U.S. Toxic Substances Control Act.[4]Structure and conformation
Molecular structure
Cyclohexane has the molecular formula C₆H₁₂ and a molecular weight of 84.16 g/mol.[1] It is a saturated cycloalkane composed of a six-membered carbon ring in which adjacent carbon atoms are connected by single bonds, with each carbon atom exhibiting sp³ hybridization and bonded to two hydrogen atoms.[1][7] The C–C–C bond angles in the ring are approximately 109.5°, aligning closely with the ideal tetrahedral geometry and resulting in minimal angle strain relative to smaller cycloalkanes such as cyclopropane (60°) or cyclobutane (90°).[1][8][7] Due to its highly symmetric structure and absence of polar functional groups, cyclohexane is a non-polar molecule with no net dipole moment.[1] The molecule is commonly represented in Kekulé form, which explicitly shows all six carbon atoms, twelve hydrogen atoms, and the single bonds forming the ring, or in skeletal formula, where carbon atoms are implied at the vertices of a hexagon and hydrogen atoms are omitted for simplicity (equivalent to the SMILES notation C1CCCCC1).[1]Conformational analysis
Cyclohexane adopts a variety of conformations due to the flexibility of its six-membered ring, with the chair conformation representing the global energy minimum. In this form, all carbon-carbon bonds are staggered, and the hydrogen atoms are positioned either axially (perpendicular to the ring plane) or equatorially (roughly in the ring plane), minimizing torsional strain and avoiding eclipsed interactions./12%3A_Cycloalkanes_Cycloalkenes_and_Cycloalkynes/12.03%3A_Conformations_of_Cycloalkanes) Alternative conformations include the boat and twist-boat forms, which are higher in energy relative to the chair. The boat conformation features eclipsed bonds along four carbons and flagpole hydrogens that introduce steric repulsion, resulting in an energy approximately 6.9 kcal/mol above the chair. The twist-boat, a distorted version that relieves some of these interactions, lies about 5.5 kcal/mol higher than the chair but serves as a local minimum.[9][10] Ring inversion in cyclohexane interconverts equivalent chair forms through a half-chair transition state, where one bond becomes nearly planar, leading to significant torsional strain. This process has an activation barrier of approximately 10-12 kcal/mol, allowing rapid equilibration at room temperature on the order of 10^5 times per second.[10] In the chair conformation, axial and equatorial positions differ in their steric environment; axial substituents experience 1,3-diaxial interactions with syn-axial hydrogens, which contribute to higher energy for axial orientations compared to equatorial. These gauche-like interactions, each worth about 0.9 kcal/mol for hydrogen pairs, are absent in the unsubstituted ring but explain substituent preferences./12%3A_Cycloalkanes_Cycloalkenes_and_Cycloalkynes/12.03%3A_Conformations_of_Cycloalkanes) The low ring strain in cyclohexane, totaling 0.0-1.0 kcal/mol, arises from near-zero angle strain (bond angles close to the ideal 109.5°) and minimized torsional strain in the chair due to staggered arrangements, making it a strain-free model for cycloalkanes./Alkanes/Properties_of_Alkanes/Cycloalkanes/Ring_Strain_and_the_Structure_of_Cycloalkanes) The conformational equilibrium between chair and twist-boat is governed by \text{chair} \rightleftharpoons \text{twist-boat} with a free energy difference of about 5.5 kcal/mol favoring the chair, resulting in population ratios at room temperature of approximately 99.9% chair and 0.1% twist-boat.[10]Solid phases
Cyclohexane displays two primary solid phases, distinguished by their molecular ordering and thermodynamic properties. The high-temperature phase I, known as the plastic crystal phase, exists between approximately 186 K and the melting point of 6.5 °C (279.7 K). In this phase, the molecules exhibit significant orientational freedom, rotating nearly isotropically about their centers of mass while maintaining positional order in a face-centered cubic lattice with space group Fm\bar{3}m. This disorder contributes to the plastic-like mechanical behavior observed in such crystals.[1][11] The low-temperature phase II forms below 186 K and represents a fully ordered crystalline state with a monoclinic structure in the space group C2/c. Here, the cyclohexane molecules adopt fixed chair conformations, aligned in the lattice without the rotational mobility seen in phase I; as noted in conformational analyses, the chair form is the predominant low-energy structure for cyclohexane. The intermolecular interactions in this phase are primarily van der Waals forces, stabilizing the ordered arrangement.[12][13] The transition between phase II and phase I at 186.09 K is a first-order phase change, marked by a significant entropy increase of 35.93 J/mol·K due to the onset of orientational disordering in the higher-temperature phase. This entropy change reflects the gain in rotational degrees of freedom as the system moves from the rigid ordered lattice of phase II to the more dynamic plastic phase I. The melting point of 6.5 °C further delineates the upper limit of the solid state, with the liquid density at 25 °C measured at 0.7785 g/cm³, providing context for the material's behavior near the solid-liquid boundary.[2][1]Properties
Physical properties
Cyclohexane is a colorless liquid at room temperature, exhibiting a mild, characteristic odor reminiscent of petroleum or chloroform.[1] Its phase transition temperatures include a boiling point of 80.7 °C at standard atmospheric pressure and a melting point of 6.5 °C.[14] The density of liquid cyclohexane is 0.7785 g/cm³ at 20 °C.[15] Cyclohexane is practically insoluble in water, with a solubility of 0.0058 g/100 mL at 25 °C, due to its non-polar nature. It is miscible with common organic solvents such as ethanol, ether, acetone, and benzene.[16][1] The refractive index is 1.426 at 20 °C, and the dynamic viscosity is 0.98 cP at the same temperature.[1] Thermodynamic properties include a heat of vaporization of 30.0 kJ/mol at the boiling point and a molar heat capacity of 155.5 J/mol·K for the liquid phase at 25 °C.[14][17] Regarding flammability, cyclohexane has a flash point of -20 °C and an autoignition temperature of 245 °C.[1]| Property | Value | Conditions | Source |
|---|---|---|---|
| Boiling point | 80.7 °C | 760 mm Hg | NIST WebBook |
| Melting point | 6.5 °C | - | PubChem |
| Density | 0.7785 g/cm³ | 20 °C | LSU Solvents |
| Solubility in water | 0.0058 g/100 mL | 25 °C | ICSC |
| Refractive index | 1.426 | 20 °C | PubChem |
| Viscosity | 0.98 cP | 20 °C | PubChem |
| Heat of vaporization | 30.0 kJ/mol | Boiling point | NIST WebBook |
| Molar heat capacity (liquid) | 155.5 J/mol·K | 25 °C | NIST WebBook |
| Flash point | -20 °C | Closed cup | PubChem |
| Autoignition temperature | 245 °C | - | PubChem |