But why in the world would a nucleophile want to attack a carbon. The answer lies in that opposite charges attract. A nucleophile is a species that is attracted to positive charges, and oftentimes may even have a full negative charge. Since the leaving group invariably draws electrons from the carbon that it is attached to, it gives the carbon a partially positive charge (+), making it an attractive candidate for nuclophilic substitution.

The Sn2 Reaction involves displacement of a leaving group (usually a halide or a tosylate), by a nucleophile. For steric reasons, the fastest reactions occur with methyl and primary halides, but sometimes do go with secondary halides (although it is usually accompanied by elimination), and will not react at all with tertiary halides. In the following example, the hydroxide ion is acting as the nucleophile and bromine is the leaving group:

It was found, however, that inversion of configuration occurs. Therefore the following mechanism was proposed--the backside attack of the nucleophile, with the leaving group leaving in a concerted step. This backside attack causes the inversion of stereochemistry shown thus:

This mechanism was consistent with the fact that when the concentration of either the bromomethane or the hydroxide ion concentration was doubled the reaction rate was doubled as well, as defined by the rate equation (rate = kr[CH3Br][-OH].) This is because both are involved in the rate-determining step.

As we can see from the reaction diagram, the transition state (), is where the hydroxide ion has begun to bond and the halide has started to leave.

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