Acetophenone when reacted with a base, C₂H₅ONa, yields a stable compound which has the structure :

Acetophenone, $C_6H_5COCH_3$, has $\alpha$-hydrogen atoms, so with a base like $C_2H_5ONa$ it undergoes the aldol condensation reaction.
Step 1: the base removes an $\alpha$-hydrogen to give the carbanion (enolate) $C_6H_5CO\overset{\ominus}{C}H_2$.
Step 2: this carbanion attacks the carbonyl carbon of a second molecule of acetophenone.
$C_6H_5COCH_3 + CH_3COC_6H_5 \xrightarrow{C_2H_5ONa} C_6H_5-C(OH)(CH_3)-CH_2-CO-C_6H_5$ (a $\beta$-hydroxy ketone)
Step 3: the $\beta$-hydroxy ketone readily loses a molecule of $H_2O$ because the product is stabilised by conjugation with the carbonyl group and the benzene ring.
$C_6H_5-C(OH)(CH_3)-CH_2-CO-C_6H_5 \xrightarrow{-H_2O} C_6H_5-C(CH_3)=CH-CO-C_6H_5$
The stable product is the $\alpha,\beta$-unsaturated ketone $C_6H_5-C(CH_3)=CH-CO-C_6H_5$ (1,3-diphenylbut-2-en-1-one, also called dypnone).