Cyclohexane

Cyclohexane is a hydrogen. Due to its unique chemical and conformational properties, cyclohexane is also used in labs in analysis and as a standard.

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 substituent, then the substituent will most likely be found attached in an equatorial position, as this is the slightly more stable conformation.

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.  

Reactions with cyclohexane

Pure cyclohexane in itself is rather unreactive, being a non-polar, HF + SbF₅ which will cause forced protonation and "hydrocarbon cracking". Substituted cyclohexanes, however, may be reactive under a variety of conditions, many of which are important to organic chemistry. Cyclohexane is highly flammable.

Cyclohexane derivatives

The specific arrangement of hydroxide anion (OH⁻), would result in cyclohexene:

C₆H₁₁Br + OH⁻ → C₆H₁₀ + H₂O + Br⁻

This reaction, commonly known as an 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:

1. The electron pair bond between the C-Br moves to the Br, forming Br⁻ and setting it free from cyclohexane
2. The nucleophile (-OH) gives an electron pair to the adjacent axial H, setting H free and bonding to it to create H₂O
3. 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).

Uses

Commercially most of cyclohexane produced is converted into adipic acid and caprolactam. Practically, if the cyclohexanol content of KA oil is higher than cyclohexanone, it is more likely(economical) to be converted into adipic acid, and the reverse case, caprolactam production is more likely. Such ratio in KA oil can be controlled by selecting suitable oxidation catalyts. Some of cyclohexane is used as an organic solvent.

Cyclohexane in research

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.

History

Unlike compounds like 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 rearrangement reaction.

See also

• The Flixborough disaster, a major industrial accident caused by an explosion of cyclohexane.
• Hexane

References

1. ^ The curiously intertwined histories of benzene and cyclohexane E.W. Warnhoff J. Chem. Ed., 1996 494