Cross aldol condensation reaction:
A few steps take place in this reaction
Step 1: Acid-base reaction occurs between base and alpha hydrogen of acetophenone, and carbanion is formed.
Step 2: In this step, formed carbanion behaves as a nucleophile and attacks carbonyl compound (benzaldehyde) carbon to form beta-hydroxy carbonyl compound.
Step 3: α,β-unsaturated carbonyl compound is formed as a product after heating.
