Ethanenitrile is treated with $\mathrm{CH}{}_3\mathrm{MgBr}$ in dry ether and the product is hydrolysed with dilute acid. The final organic product is

The Grignard reagent adds once to the $\mathrm{C}\equiv \mathrm{N}$ bond to give an imine salt, which is hydrolysed to the ketone. Hence $\mathrm{CH}{}_3\mathrm{CN}+\mathrm{CH}{}_3\mathrm{MgBr}$ gives $\mathrm{CH}{}_3\mathrm{COCH}{}_3$. The ketone is not further attacked because the imine salt is stable in the reaction mixture.