# Claisen Condensation vs. Trans-Esterification

In the Claisen condensation, an alkoxide $\ce{RO-}$ acts as a base. However, in trans-esterification, an alkoxide $\ce{R'O-}$ attacks the carbonyl carbon of an ester as a nucleophile. Is this because a different 'R' group is used in trans-esterification?

• RO- doesn't act as a nucleophile in the claisen condensation. It's like an Aldol but the alkoxy group leaves giving a beta keto ester. – RobChem Jul 7 '16 at 12:23
• my bad, I meant it acted as a base – Reeshabh Ranjan Jul 7 '16 at 14:55