Acetaldehyde is treated with (i) $\mathrm{CH}{}_3\mathrm{MgBr}$ in dry ether; (ii) $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The product is

The methyl carbanion of the Grignard reagent adds to the carbonyl carbon of $\mathrm{CH}{}_3\mathrm{CHO}$ giving $\mathrm{CH}{}_3-\mathrm{CH}(\mathrm{OMgBr})-\mathrm{CH}{}_3$. Hydrolysis gives the secondary alcohol $\mathrm{CH}{}_3-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}{}_3$, propan-2-ol.