Question

Can anyone explain the complete mechanism of a nitrile with a grignard reagent to form a ketone?

Answer 1

Mechanism for the reacton of R1MgBr with R-CN to form a ketone is shown below in the figure. The first step of the reaction involves the nucleophilic C in the organometallic reagent adding to the electrophilic C in the polar nitrile group. Electrons from the C=N move to the electronegative N creating an intermediate imine salt complex. On addition of aqueous acid, the intermediate salt protonates giving the imine. Imines undergo nucleophilic addition, but require activation by protonation (i.e. acid catalysis). Now the nucleophilic O of a water molecule attacks the electrophilic C with the pi bond breaking to neutralise the change on the N. Deprotonate the O from the water molecule to neutralise the positive charge. Before the N system leaves, it needs to be made into a better leaving group by protonation. Use the electrons on the O in order to push out the N leaving group, a neutral molecule of ammonia. Deprotonation reveals the carbonyl group ofthe ketone product. The detailed meachnism for this reaction is given as below.