Solution :
This reaction is an example of “Perkin reaction”. The compound X should be $${\left( {C{H_3}CO} \right)_2}O.$$
In this step the carbanion is obtained by removal of an $$\alpha - H$$ atom from a molecule of an acid anhydride, the anion of the corresponding acid acting as a necessary base.
Releted MCQ Question on Organic Chemistry >> Carboxylic Acid
Releted Question 1
When acetaldehyde is heated with Fehling’s solution it gives a precipitate of