Esters are one member of the group of compounds that are referred to as carboxylic acid derivatives. As shown by the examples of the various classes of carboxylic acid derivatives below, these have the general formula RCOX where the group X has an electronegative atom bonded to the carbonyl carbon.
 |  |
| acid chloride | anhydride |
 |  |
| ester | amide |
 |
| propanoic acid |
Acyl chlorides are the most reactive of these compounds and, as shown in the scheme at the right, acyl chlorides are converted rapidly, on reaction with an appropriate reagent to an anhydride, an ester, an amide or a carboxylic acid. The overall reaction is substitution, with HCl being a co-product of the reaction.
In addition any of the carboxylic acid derivative can be converted to a carboxylic acid derivative below it in the diagram when treated with the reagent shown over the arrow.
Thus esters may be prepared from reaction of an anhydride with an alcohol.
Acyl chlorides may be prepared from the reaction of a carboxylic acid with SOCl
2. This reaction gives the more reactive derivative because the reaction with SOCl
2 is a two-step process. In the first step the carboxylic acid is converted to a carboxylic acid derivative that is more reactive than the acyl halide. In the second step this activated intermediate is converted to the acyl halide.
