Cyclohexene
Cyclohexene is a cycloalkene hydrocarbon with the molecular formula C₆H₁₀, characterized by a six-membered carbon ring containing one carbon-carbon double bond.[1] This structure renders it a key intermediate in organic chemistry, distinguishing it from the fully saturated cyclohexane (C₆H₁₂) by its unsaturation, which imparts reactivity typical of alkenes.[2] As a physical entity, cyclohexene presents as a colorless liquid with a sweet odor, exhibiting a boiling point of 83 °C, a melting point of -103.5 °C, and a density of 0.81 g/cm³ at 20 °C.[1] It is insoluble in water but highly miscible with common organic solvents such as ethanol, ether, and benzene, reflecting its nonpolar nature.[2] Chemically, it undergoes typical alkene reactions, including electrophilic additions like hydrogenation to cyclohexane or oxidation to form epoxides and diols, and it can participate in Diels-Alder cycloadditions.[3] However, it poses hazards as a highly flammable substance with a flash point of -7 °C and potential to form explosive peroxides upon prolonged exposure to air.[1] Industrially, cyclohexene is produced via partial catalytic hydrogenation of benzene or dehydration of cyclohexanol.[4][2] It serves as an intermediate in the production of various chemicals, including precursors for nylon, pharmaceuticals, and polymers, as well as a solvent and gasoline stabilizer. Naturally occurring in coal tar, cyclohexene plays a role in petrochemical and fine chemical industries.[1][2]Structure and Properties
Molecular Structure
Cyclohexene has the chemical formula C₆H₁₀ and a molecular weight of 82.14 g/mol. It features a six-membered carbon ring with one endocyclic double bond, distinguishing it from the fully saturated cyclohexane. This unsaturated structure imparts specific geometric constraints that define its chemical identity.[1] The double bond is located between carbons 1 and 2, where these atoms are sp²-hybridized, leading to a planar arrangement with a bond angle of approximately 120°. This hybridization enforces trigonal planar geometry around the unsaturated carbons, while the remaining four carbons are sp³-hybridized with tetrahedral angles near 109.5°. The C=C double bond length is approximately 1.34 Å, shorter than typical C-C single bonds due to the π-component, and the adjacent C-C single bonds (C2-C3 and C1-C6) measure about 1.50 Å, reflecting partial double-bond character from hyperconjugation.[1] At room temperature, cyclohexene adopts a half-chair conformation as its most stable form, characterized by C₂ symmetry and a pseudorotation that minimizes torsional strain along the ring. In this arrangement, the double bond and adjacent carbons remain nearly coplanar, while the opposite side of the ring puckers out of plane to avoid eclipsing interactions. This contrasts with the fully relaxed chair conformation of cyclohexane, where all bonds are staggered without such puckering; the double bond in cyclohexene introduces rigidity that prevents a complete chair and enforces the distorted half-chair geometry.[1]Physical Properties
Cyclohexene is a colorless liquid at room temperature with a sweet odor.[1][5] Its density is 0.811 g/cm³ at 20 °C.[6] The melting point is -103.5 °C, and the boiling point is 83 °C at standard pressure.[1][7] Cyclohexene exhibits low solubility in water, approximately 0.021 g/100 mL at 25 °C, but is miscible with common organic solvents such as ethanol, diethyl ether, acetone, and benzene.[1][8] Additional physical characteristics include a dynamic viscosity of 0.625 mPa·s at 25 °C and a refractive index of 1.446 at 20 °C.[1] Key thermodynamic properties are a standard heat of vaporization of 33.5 kJ/mol and a critical temperature of 287 °C.[9][1] The notably low melting point can be attributed to the flexible half-chair conformation of the molecule, which facilitates disorder in the solid state.[1]| Property | Value | Conditions | Source |
|---|---|---|---|
| Density | 0.811 g/cm³ | 20 °C | Fisher SDS |
| Melting point | -103.5 °C | - | PubChem |
| Boiling point | 83 °C | 760 mmHg | PubChem |
| Water solubility | 0.021 g/100 mL | 25 °C | PubChem |
| Viscosity | 0.625 mPa·s | 25 °C | PubChem |
| Refractive index | 1.446 | 20 °C (D line) | PubChem |
| Heat of vaporization | 33.5 kJ/mol | Standard | NIST |
| Critical temperature | 287 °C | - | PubChem |