Cyclohexane

From Wikipedia, the free encyclopedia

Cyclohexane
Image:Cyclohexane-3D-space-filling.png
General
Molecular formula	C₆H₁₂
SMILES	C1CCCCC1
Molar mass	84.16 g/mol
Appearance	Colourless liquid
CAS number	[110-82-7]
Properties
Density and phase	0.779 g/ml, liquid
Solubility in water	Immiscible
Solubility in ethanol	Miscible
Melting point	6.55 °C
Boiling point	80.74 °C
Viscosity	1.02 cP at 17 °C
Index of refraction, n_D	1.423
Thermodynamic data
Standard enthalpy of formation Δ_fH^o_liquid	-156 kJ/mol
Standard enthalpy of combustion Δ_cH^o_liquid	-3920 kJ/mol
Standard molar entropy S^o_liquid	? J.K⁻¹.mol⁻¹
Hazards
MSDS	External MSDS
EU classification	Flammable (F) Harmful (Xn) Dangerous for the environment (N)
NFPA 704	Image:NFPA 704.svg 3 1 0
R-phrases	R11, R38, R65, R67, R50/53
S-phrases	S2, S9, S16, S25, S33, S60, S61, S62
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Regulatory data	Flash point, RTECS number, etc.
Related compounds
Related cycloalkanes	Cyclopentane Cycloheptane
Related compounds	Cyclohexene
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Cyclohexane is a cycloalkane with the molecular formula C₆H₁₂. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which are intermediates used in the production of nylon. Due to its unique chemical and conformational properties, cyclohexane is also used in labs in analysis and as a standard.

[edit] Chemical conformation

Main article: cyclohexane conformation

Contrary to popular belief, the 6 vertexed ring does not conform to the shape of a perfect hexagon. Although the conformation of a flat 2D planed hexagon would reduce angle strain to 0, the torsional strain would be considerable. Therefore to reduce torsional strain, cyclohexane adopts a three-dimensional structure known as the chair conformation. The new conformation puts the carbons at an angle of 109.5 degrees. Half of the substituents are in the plane of the ring (equatorial) while the other half are perpendicular to the plane (axial). This conformation of substituents allows for the most stable structure of cyclohexane and its substituents. If cyclohexane is saturated with hydrogens with only one larger substituent attached the ring (called mono-substituted), then the larger substituent will most likely be found in the equatorial form, once again for the sake of stability.

Cyclohexane has the lowest angle and torsional strain of all the cycloalkanes, as a result cyclohexane has been deemed a 0 in total ring strain, a combination of angle and torsional strain. This also makes cyclohexane the most stable of the cycloalkanes and therefore will produce the least amount of heat when burned compared to the other cycloalkanes.

However 0 ring strain only occurs when cyclohexane is in the chair formation. Cyclohexane can take other confomational forms, however these forms are less stable and as a result, cyclohexane will be found in the chair formation 99.99% of the time (i.e. 99.99% of all molecules in a solution sample will be in the chair formation). Other conformations, starting with the most stable to the least are: the twisted boat, boat, half-chair and plane (flat) formation. Rarely are the half-chair and, even less, the flat formation, found in solution.

For di-substituted cyclohexane rings (i.e. 2 methyl groups on opposing sides of the ring), the 2 substituents can adops trans/cis formation. The trans/cis formation depends on the conformations of the substituents. A cis conformation has one methyl group in axial formation and one methyl group in equitorial formation. In cis, the methyl groups can switch from and to axial and equitorial arrangement, however, due to the equal Newman projections both have, both conformations are equally favored. In trans, the methyl groups can both take on axial of both equitorial conformations. Taking a look at both conformations' Newman projections, the favored conformation is trans-equitorial because trans-axial has a "Gauche" orientation, a formation which is less stable than the trans-equitorial formation.