Organomagnesium compounds

Victor Grignard 1871-1935

Preparation of an organomagnesium compound

It is added in an absolutely dry flask - Freshly crushed magnesium turnings (no surface layer!) - A solution of an alkyl bromide ($R-Br$ ) in dry ether previously dried on sodium It is very moderately heated. The reaction starts only in absolutely anhydrous medium! If it does not start, add a few crystals of iodine that produce magnesium iodide which is very greedy for water.

Part of a video due to University of Toronto Scarborough

The reaction product is then separated from the ether. This is the alkylmagnesium bromide, an organomagnesium compound: $R-Br+Mg$ $\rightarrow$ $R-MgBr$

A nucleophile

In a simplified way, one can say that the magnesium alkyl bromide tends to dissociate in the presence of an electrophilic center $\delta +$ to form the powerful nucleophilic carbanion $R-$ $R-MgBr\rightarrow R^- + MgBr^+$

Nucleophilic attack on the carbonyl

The $C$ atom of the carbonyl is an electrophilic center, so we have the attack:

A new $C-C$ bond has been formed! Then adding a dilute acid, a hydroxyl group is obtained:

Examples

Find the final substances obtained (by following the above steps) from the reactants: - bromoethane and pentan-3-one    →

- bromopropane and ethanal    →

- bromobenzene and methanal   →