In Acetonitrile $\left({\mathrm{CH}}_3\mathrm{CN}\right)$, there is an electronegativity difference in between the carbon and nitrogen; hence, the carbon behaves as an electrophile site on which the methyl anion from the Grignard reagent attacks, and forms the ketone as shown below:(here $\mathrm{R}={\mathrm{CH}}_3, {\mathrm{R}}^{\text{'}}={\mathrm{CH}}_3$)