The order of reactivity of Phenyl magnesium bromide (PhMgBr) with the following compounds

Options

A. `II gt III gt I`
B. `I gt III gt II `
C. `II gt I gt III`
D. All react with the same rate

Explanation

In phenyl magnesium bromide `( overset( delta-)( Ph)" "overset( delta+)(Mg)" "overset(delta-)(Br))`, Ph is attached with that C-atom of carbonyl group which has low electron density (higher electropositive charter) . Ketons are less reactive.

towards nucleophile in nucleophilic addition reaction because in ketons alkyl group ( due to `+I` effect) decrease the electropositive charge of carbon of carbonyl group. Hence, attraction of nucleophile decreases. Moreover in the tetrahedral intermediate aldehyde have less steric repulsion than ketones.

Thus, on the basis of above reason the order of reactivity of acetone ( I), acetaldehyde ( II) and benzaldehyde (III) with PhMgBr is `:`
`II gt I gt III`