Ethyl formate, $\mathrm{HCOOC}{}_2\mathrm{H}{}_5$, is treated with an excess of $\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{MgBr}$ in dry ether and then $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The major organic product is

The first $\mathrm{C}{}_2\mathrm{H}{}_5{}^{-}$ adds to the formate ester and expels ethoxide, giving propanal. Propanal is more reactive than the ester and adds a second $\mathrm{C}{}_2\mathrm{H}{}_5{}^{-}$. Hydrolysis of $(\mathrm{C}{}_2\mathrm{H}{}_5){}_2\mathrm{CH}-\mathrm{OMgBr}$ gives the secondary alcohol pentan-3-ol. A tertiary alcohol would need an ester of a carboxylic acid other than formic acid.