Propanoyl chloride contains an electrophilic carbonyl center to give  nucleophilic acyl substitution reaction by removing Cl group with a strong nucleophile like methylmagnesium bromide.
Two equivalent of methyl magnesium bromide react with cabonyl center followed by protonation of negatively charged oxygen atom gives 2-methyl-2-butanol.
In the first step, nucleophilic acyl substituion occurs. In the second step, nucleophilic addition occurs. In the third step, protonation occurs.
