Question
What type of esters can undergo Claisen condensation reactions?
Answer
For a standard base-driven self-Claisen condensation, the ester must have at least two α-hydrogens on the reacting α-carbon. This allows enolate formation and the product deprotonation that drives the usual reaction.
Why the α-hydrogen count matters
The α-carbon is the carbon adjacent to the ester carbonyl on its acyl side. Base first removes an α-hydrogen to form an enolate. That enolate attacks another ester molecule; elimination of alkoxide produces a β-keto ester. Removing the remaining hydrogen between its two carbonyl groups favors product formation. Acid workup then gives the neutral product.
An ester with only one α-hydrogen can form an enolate, but lacks the remaining hydrogen needed for this usual driving step after condensation.
The mixed-Claisen exception
An ester with no α-hydrogens cannot be the enolate donor, but it can be the electrophilic acceptor in a mixed Claisen condensation. Ethyl benzoate is an example. Thus, “esters without α-hydrogens never participate” is too broad.
The usual conditions use an alkoxide matched to the ester alkoxy group, such as ethoxide with an ethyl ester, to avoid exchanging that group.
Evidence boundary
The at-least-two-α-hydrogen rule applies to the standard base-driven self-Claisen pathway. Mixed Claisen reactions distinguish donor and acceptor roles; a non-enolizable ester may be an acceptor. Specialized methods are outside this conceptual answer.
Sources
These references support the concepts and methods used in the explanation above.