Effect of Alkyl Groups.

In organic chemistry, we consider alkyl groups to be electron donating. They are able to donate electrons via a means called hyperconjugation. When alkyl groups are linked to a carbon with a positive charge or a carbon with an unpaired electron, the alkyl groups with its electron donating properties stabilises the carbocation (or radical).

On the other hand, alkyl groups when attached to the benzene ring, donate electron density towards the ring and hence making the benzene ring more reactive toward electrophiles.

Hence, is the electron donating property of alkyl groups stabilising or destabilising?

Well, the answer is lies in the effect that the electron donating property causes. In the case of carbocations (and radicals), the alkyl groups donates their electron density towards the electron deficient carbon.

Hence, by donating the electron density, the alkyl group is no longer neutral in charge but rather carry some partial positive charge; inevitably sharing the positive charge (or the extent of electron deficiency) with the carbon that has the positive charge.

However, in the case of the methylbenzene, the alkyl groups donate electron density to the electron ring. This makes the pi electron ring richer in electron density and hence more susceptible to electrophilic attacks.

It is timely to remind ourselves that in the electrophilic substitution mechanism, the slow step is the reaction between the benzene ring and the electrophilc.

Hence, seeing the effect caused by the alkyl groups will lead us to making the correct choice to whether they stabilises or destabilises.

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Article written by Kwok YL 2010.}

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