We mentioned above that adjacent π systems increase the rate of the SN2 reaction by stabilizing the transition state, and likewise increase the rate of SN1 reactions by stabilizing the carbo cation. The effect on the SN2 reaction applies to both C=C (electron-rich) and C=O (electron-deficient) π systems, but only C=C π systems increase the rate of SN1 reactions. Adjacent C=O groups in fact significantly decrease the reactivity of alkyl halides towards SN1 reactions because the electron-withdrawing effect of the carbonyl group greatly destabilizes the carbocation. Electron-withdrawing or -donating groups can also tip finely balanced cases from one mechanism to another. For example, benzylic compounds react well by either SN1 or SN2, and a change of solvent, as just discussed, might switch them from one mechanism to another. Alternatively, a benzylic compound that has a well-placed electron-donating group able to stabilize the cation will also favour the SN1 mechanism. Thus 4-methoxybenzyl chloride reacts by SN1 for this reason: here we show the methoxy group stabilizing the cation intermediate by assisting departure of the chloride.

On the other hand, an electron-withdrawing group, such as a nitro group, within the benzylic compound will decrease the rate of the SN1 reaction and allow the SN2 mechanism to take over.

We have considered the important effects of the basic carbon skeleton and of solvent on the course of SN1 and SN2 reactions and we shall now look at two final structural factors: the nucleophile and the leaving group. We shall tackle the leaving group fi rst because it plays an important role in both SN1 and SN2 reactions.

مواضيع ذات صلة

A very bad nucleophile in a good SN1 reaction: the Ritter reaction

The nucleophile in SN1 reactions

Epoxides as electrophiles

Ethers as electrophiles

Substituting alcohols with the Mitsunobu reaction

Nucleophilic substitutions on alcohols: how to get an OH group to leave

The effect of solvent

A closer look at steric effects

Adjacent C=C or C=O π systems increase the rate of SN2 reactions

An adjacent C=C π system stabilizes a carbocation: allylic and benzylic carbocations

Alkyl substituents stabilize a carbocation

Shape and stability of carbocations