CYCLOHEXANE STEREOCHEMISTRY

Reactive intermediate: CARBOCATION

Anand S. Burange, Wilson College, Mumbai

asgburange@gmail.com

The reaction intermediates formed during reaction and reactive intermediates are totally different from each other. Let us consider the example of oxidation of primary alcohols using permanganate or dichromate yields carboxylic acid where aldehyde is an reaction intermediates formed during the reaction and which is isolable and can be detected if we cease/quench the reaction earlier before the over oxidation. Whereas reactive intermediates are highly unstable, have very short life span, mostly non-isolable but they are the part of the reaction mechanism. Till date well known reactive intermediates mainly include carbocation, carbanion, carbene, nitrene, aryne, and radical. Stability of these reactive intermediates governs the rate of the reaction.

Carbocation: Carbon is tetravalent (covalency= 04); the species where carbon is trivalent bearing one vacant p orbital is called as carbocation. In carbocation, all the three bonds are in plane while the vacant p-orbital is orthogonal (perpendicular) to it. The vacant p-orbital is susceptible or prone to accept electron pair thus it favours the attack of the nucleophile.

Carbocation in which positively charge carbon (with orthogonal vacant p orbital) is attached to one, two, and three carbon atoms named as primary, secondary and tertiary respectively. Any charge species is unstable and always present in system with its counter ion. If by any means, the charge gets neutralised or delocalized then system moves towards the stability. It was observed that tertiary carbocations are more stable than secondary than primary then methyl.

Stability of carbocation

3^o  >  2 ^o  >  1 ^o  > methyl

The reason for the trend observed above for the stability of carbocation is the electron donating tendency of alkyl group. Carbon of alkyl group and positive charge bearing carbon in carbocation are bonded by a covalent bond. But how one alkyl group can donate electron to another carbon? The reason for this is hyperconjugation. Any alkyl group (C-H bond) stabilized the carbocation by hyperconjugation effect. Interaction of lone pair or bond pair with vacant or antibonding molecular orbital can be termed as hyperconjucation. This effect is recently also observed as one of the reason for the staggered conformations in alkane, well explained in Solomon and Fryhle.

Carbocations were also named as carbonium ions but now it is strictly used for CH⁵⁺ species since according IUPAC, ium suffix is used where covalency of atom is increased by one for example NH₄⁺ (ammonium), H₃O⁺ (hydronium) ion. Long time after George Olah’s discovery of CH⁵⁺ species, finally IUPAC in 1987 accepted to use carbocation strictly for trivalent positively charged carbon. Carbocations can be generated by the reaction of alkanes with the superacids which involves the evolution of hydrogen gas. Carbocations were also prepared from alkyl fluorides and alcohols. All seven pentyl fluorides give tert-pentyl cation, all propyl fluorides give isopropyl carbocation whereas all butyl fluorides give tert-butyl cation under set reaction conditions and it gives the evidence for the stability of carbocations.

In carbocations when the positive charge is localized or delocalized with adjacent pi bonds for example allylic or benzylic, they are known to be classical carbocations. In case of non-classical carbocations positive charge is stabilized by sigma bonds or pi bonds other than allylic or benzylic. Cyclopropyl methyl carbocation is an example of non classical carbocation where cyclopropyl bonds and vacant p-orbital are parallel to each other and their interaction imparts the extra stability.

It is quite difficult to generate carbocation at bridgehead atom i.e. [2.2.1] system since system can not be planar. But larger bridgehead carbocations do exist.

More stable carbocations exist as solid salts. Ph₃C⁺BF₄^- is a commercially available carbocation where three phenyl rings are responsible for the stability.

There are many chemical reaction which involve carbocation as a reactive intermediate. But one must take care that during the mechanism, formed carbocation in most cases rearranges to stable cation by either alkyl or H shift.

For more details about carbocation, I suggest to read the review article by George Olah “100 years of carbocation.”

Thank you.