Acetonitrile, $\mathrm{CH}{}_3-\mathrm{CN}$, is treated with (i) $\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{MgBr}$ in dry ether; (ii) $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The product is

The Grignard carbon adds once to the nitrile carbon, giving the magnesium salt of an imine, $\mathrm{CH}{}_3-\mathrm{C}(=\mathrm{NMgBr})-\mathrm{C}{}_2\mathrm{H}{}_5$. This anionic intermediate does not add a second Grignard molecule. Aqueous acid hydrolyses the C=N to give the ketone $\mathrm{CH}{}_3-\mathrm{CO}-\mathrm{C}{}_2\mathrm{H}{}_5$, butan-2-one.