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
3o
> 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.
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