2-Methyloxirane (propylene oxide) is treated with $\mathrm{CH}{}_3\mathrm{MgBr}$ in dry ether, followed by aqueous $\mathrm{NH}{}_4\mathrm{Cl}$. The product is

The Grignard reagent is a strong nucleophile and opens the epoxide by SN2 at the less hindered $\mathrm{CH}{}_2$ carbon. This gives $\mathrm{CH}{}_3\mathrm{CH}{}_2-\mathrm{CH}(\mathrm{OMgBr})-\mathrm{CH}{}_3$, which on protonation is butan-2-ol. Attack at the $\mathrm{CH}$ carbon would give 2-methylpropan-1-ol.