Acetophenone when reacted with a base, $C_2H_5ONa$, yields a stable compound which has the structure:

Acetophenone ($C_6H_5COCH_3$) possesses $\alpha$-hydrogen atoms. In the presence of a base like sodium ethoxide ($C_2H_5ONa$), it undergoes a self-condensation reaction known as Aldol Condensation.

1. Enolate Formation: The base abstracts an $\alpha$-proton from one molecule of acetophenone to form an enolate ion.
2. Nucleophilic Attack: This enolate ion attacks the carbonyl carbon of another acetophenone molecule to form a $\beta$-hydroxy ketone (aldol adduct).
3. Dehydration: Due to the presence of the aromatic ring and the conjugation possibilities, the intermediate readily loses a water molecule (dehydration) to form a stable $\alpha,\beta$-unsaturated ketone called Dypnone (1,3-diphenylbut-2-en-1-one).

Reaction: $2C_6H_5COCH_3 \xrightarrow{C_2H_5ONa} C_6H_5-C(CH_3)=CH-CO-C_6H_5 + H_2O$