Q.From which of the following one, tertiary butyl alcohol is obtained by the action of methyl magnesium iodide ?

$CH_3MgI$ adds across the carbonyl of acetone to form an addition product that on hydrolysis gives tertiary butyl alcohol, $(CH_3)_3COH$.

Grignard reagents add to the carbonyl group $\big(>C=O\big)$, and the class of alcohol obtained depends on the carbonyl substrate: formaldehyde ($HCHO$) gives a primary alcohol, any other aldehyde gives a secondary alcohol, and a ketone gives a tertiary alcohol. Here $CH_3MgI$ attacks acetone: $CH_3-CO-CH_3 + CH_3MgI \rightarrow (CH_3)_3C-OMgI \xrightarrow{H_3O^+} (CH_3)_3C-OH$ (tert-butyl alcohol), because the incoming methyl group joins the two methyls already on the carbonyl carbon to give a carbon bonded to three methyl groups and one $-OH$. $CH_3CHO$ (option a) would give a secondary a …