Ethyl acetate, $\mathrm{CH}{}_3-\mathrm{CO}-\mathrm{OC}{}_2\mathrm{H}{}_5$, is treated with excess $\mathrm{CH}{}_3\mathrm{MgBr}$ in dry ether and then with $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The product is

The first $\mathrm{CH}{}_3{}^{-}$ adds and expels $\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{O}{}^{-}$ to give acetone. Acetone is more reactive than the ester and takes up a second $\mathrm{CH}{}_3{}^{-}$, giving $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{OMgBr}$. Hydrolysis gives the tertiary alcohol 2-methylpropan-2-ol.