Ethyl formate, $\mathrm{HCOOC}{}_2\mathrm{H}{}_5$, is treated with excess $\mathrm{CH}{}_3\mathrm{MgBr}$ in ether, then $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The product is

The first equivalent adds to the ester and ethoxide leaves, giving acetaldehyde. The aldehyde adds a second $\mathrm{CH}{}_3{}^{-}$ to give $\mathrm{CH}{}_3\mathrm{CH}(\mathrm{OMgBr})\mathrm{CH}{}_3$. Hydrolysis gives the secondary alcohol propan-2-ol, with two identical groups from the Grignard reagent.