Question

What is the stereochemical result from an SN2 Reaction (Retention, Changes, Stereochemical purity?)

Answer 1

SN2   is a substitution nucleophilic bimolecular reaction. In nucleophilic substitution reaction nucleophile can attack the stereocenter in two ways. In front side attack, it attacks from the same side as the leaving group. In backside attack, it attacks from the opposite side of the leaving group. These two modes of attack give retention and inversion of stereo chemical configuration, respectively. Retention and inversion will yield two different stereoisomers.

In SN2 reaction the nucleophile has repulsion force from the outgoing nucleophile so cannot attack from the front side, hence back side attack is favoured in SN2 with inversion of configuration also called as Walden inversion.

In the above reaction chloride ion is leaving group and bromide ion is incoming nucleophile, the reaction proceeds with five membered transition state giving a inverted product due repulsion between the nucleophile and leaving group.

The changes in the substitution pattern of the alkyl halide results in dramatic changes in the rate of the reaction.* “Smaller” alkyl halides like methyl bromide are fast, while more highly substituted tertiary alkyl bromide doesn’t proceed at all or proceed very slowly.

For example when methlybromide is substituted with cyanide ion the rate of reaction is found to be 1000 (app) but when butylbromide is substituted with cyanide ion the rate of reaction is found to be 20 (app).

As SN2 reaction has to proceed with the inversion of configuration.The SN2 reaction purely gives 100% inverted product.So stereochemicaly pure product is obtained.