Generalized carbonyl addition reaction.
Reduction of acetophenone with sodium borohydride (NaBH4) is a carbonyl addition reaction. The reaction mechanism begins with nucleophilic addition of hydride to the carbonyl carbon atom (via hydride transfer from the B-H bond), resulting in an oxyanionic tetrahedral intermediate. The oxyanion is then protonated by ethanol (a protic solvent) to give the product, a secondary alcohol.