Claisen condensation is a fun reaction used to form a beta keto ester when reacting esters in a strong base.
This video shows you the step by step reaction mechanism including why the reaction is reversible only till the final step.
(Watch on YouTube: Claisen Condensation. Click cc on the bottom right for video transcript)
This is video 8 in the Enolates and Reactions at the Alpha Carbon Series. Click for complete series