Claisen condensation of esters is very similar to aldol condensation (which is why we have included this methanism in this section, even though esters are carboxylic acid derivatives). In Claisen condensation, the enolate form of one ester molecule carries out nucleophilic attack on the carbonyl carbon of another ester molecule. How Claisen condensation differs from aldol condensation illustrates a general difference in the reactivity of esters vs. aldehydes and ketones. In Claisen condensation, the enolate form of one ester molecule approaches another, similarly to aldol condensation, but, in this case, the tetrahedral intermediate resolves itself along an acyl substitution pathway. Both the aldol and Claisen condensations begin with an α-substitution, but in aldol condensation the overall pathway corresponds to nucleophilic addition, while Claisen condensation resolves itself in the manner of an acyl substitution reaction with sp2-hybridization returning with the departure of the leaving group.