Question

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

and I II III <!--td {border: 1px solid #cccccc;}br {mso-data-placement:same-cell;}--> AIPMT MAIN 2011

• A. $II>I>III$
• B. $III>II>I$
• C. $I>III>II$
• D. $I>II>III$

Answer 1

The correct option is D $I>II>III$

The reactivity of the carbonyl group towards the nucleophilic addition reactions depends upon the magnitude of the positive charge on the carbonyl carbon atom (electronic factor) and also on the crowding around the carboxyl carbon atom in the transition state (steric factor). Both these factors predict the following order.
$-C{H}_3$ is electron donating group, it will reduce the partial positive charge on carbonyl carbon.
$Ph-$ will also show +R effect but there will be a lot more crowding near the Carbonyl Carbon.

Thus, the order of reactivity of $PhMgBr$ follows the order,

I II IIIHence, option (d) is correct.