Definition

Alkyl halides (or tosylates) will react with carboxylate ion to form esters in SN2 reaction.

Structural reaction scheme for Ester formation [RCO2(–)], diagram 1 of 2
Figure 1. Ester formation [RCO2(–)]

Explanation

This is a one-step transformation. The nucleophile (acetate) attacks alkyl halide (methyl bromide), with a simultaneous expulsion of the leaving group (bromide). This is a concerted process because bond making takes place at the same time as a bond breaking.

In the transition state, the negative charge on the nucleophile has spread partly onto the leaving group. As the reaction comes to completion, the leaving group evolves to a fully charged anion.

Structural reaction scheme for Ester formation [RCO2(–)], diagram 2 of 2
Figure 2. Ester formation [RCO2(–)]