Cyclohexane

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Cyclohexane

General
Molecular formula	C₆H₁₂
SMILES	C1CCCCC1
Molar mass	84.16 g/mol
Appearance and smell	Colourless liquid, greasy smell or petroleum
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 (279.55 K)
Boiling point	80.74 °C (353.74 K)
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) Severe eye irritant, may cause corneal clouding
Flash Point	-20 C
NFPA 704	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 Benzene
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.

1 Chemical conformation
2 Reactions with cyclohexane
3 Cyclohexane derivatives
4 Cyclohexane in research
5 History
6 See also
7 External links
8 References

[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. The conformation of a flat 2D planar hexagon has considerable angle strain due to the fact that its bonds are not 109.5 degrees; the torsional strain would also be considerable due to all eclipsed bonds. 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°. 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. Though another form, known as boat conformation exists, it is somewhat unstable and usually reverts back to chair formation.

A cyclohexane molecule in chair conformation, with hydrogen atoms in axial position in red, equatorial in blue.

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.

[edit] Reactions with cyclohexane

Pure cyclohexane in itself is rather unreactive, being a non-polar, hydrophobic hydrocarbon. It can react with very strong acids such as the superacid system HF + SbF₅ which will cause forced protonation and "hydrocarbon cracking". Substituted cyclohexane molecules are reactive to many different reactions, however, many which are important to organic chemistry.

[edit] Cyclohexane derivatives

The specific arrangement of functional groups in cyclohexane derivatives, and indeed in most cycloalkane molecules, is extremely important in chemical reactions, especially reactions involving nucleophiles. Substituents on the ring must be in the axial formation to react with other molecules. For example, the reaction of bromocyclohexane and a common nucleophile, a hydroxide anion (OH⁻), would result in cyclohexene:

C₆H₁₁Br + OH⁻ → C₆H₁₀ + H₂O + Br⁻

This reaction, commonly known as an elimination reaction or dehalogenation (specifically E2), requires that the bromine substituent be in the axial formation, opposing another axial H atom to react. Assuming that the bromocyclohexane was in the appropriate formation to react, the E2 reaction would commence as such:

The electron pair bond between the C-Br moves to the Br, forming Br⁻ and setting it free from cyclohexane
The nucleophile (-OH) gives an electron pair to the adjacent axial H, setting H free and bonding to it to create H₂O
The electron pair bond between the adjacent axial H moves to the bond between the two C-C making it C=C

Note: All three steps happen simultaneously, characteristic of all E2 reactions.

The reaction above will generate mostly E2 reactions and as a result the product will be mostly (~70%) cyclohexene. However, the percentage varies with conditions, and generally, two different reactions (E2 and Sn2) compete. In the above reaction, an Sn2 reaction would substitute the bromine for a hydroxyl (OH^-) group instead, but once again, the Br must be in axial to react. Once the S_N2 substitution is complete, the newly substituted OH group would flip back to the more stable equatorial position quickly (~1 millisecond).

[edit] Cyclohexane in research

The usefulness of cyclohexane in research is not to be underestimated. Although much is already known about this cyclic hydrocarbon, research is still being done on cyclohexane and benzene mixtures and solid phase cyclohexane to determine hydrogen yields of the mix when irradiated at −195 °C.

[edit] History

Unlike compounds like benzene, cyclohexane cannot easily be obtained from natural resources such as coal. Towards the end of the nineteenth century early chemical investigators had to depend on organic synthesis. It took them 30 years to flesh out the details ^[1]. In 1867 Marcellin Berthelot reduced benzene with hydroiodic acid at elevated temperatures. He incorrectly identified the reaction product as n-hexane not only because of the convenient match in boiling point (69°C) but also because he did not believe benzene was a cyclic molecule (like his contemporary August Kekule) but rather some sort of association of acetylene. In 1870 one of his sceptics Adolf von Baeyer repeated the reaction and pronounced the same reaction product hexahydrobenzene and in 1890 Vladimir Markovnikov believed he was able to distill the same compound from Caucasus petroleum calling his concoction hexanaphtene

In 1894 Baeyer synthesized cyclohexane starting with a Dieckmann condensation of pimelic acid followed by multiple reductions:

and in the same year E. Haworth and W.H. Perkin Jr. (1860 - 1929) did the same in a Wurtz reaction of 1,6-dibromohexane.

Surprisingly their cyclohexanes boiled higher by 10°C than either hexahydrobenzene or hexanaphtene but this riddle was solved in 1895 by Markovnikov, N.M. Kishner and Nikolay Zelinsky when they re-diagnosed hexahydrobenzene and hexanaphtene as methylcyclopentane, the result of an unexpected rearrangement reaction.