Phenylmagnesium bromide


Phenylmagnesium bromide, with the simplified formula, is an organomagnesium compound. It forms colorless crystals although its solutions generally appear dark. It is commercially available as a solution in diethyl ether or tetrahydrofuran. Phenylmagnesium bromide is a Grignard reagent. It is used as a synthetic equivalent for the phenyl "Ph−" synthon.

Preparation

Phenylmagnesium bromide is commercially available as solutions of diethyl ether or THF. Laboratory preparation involves treating bromobenzene with magnesium metal, usually in the form of turnings. A small amount of iodine may be used to activate the magnesium to initiate the reaction.
Coordinating solvents such as ether or THF, are required to solvate the magnesium center. The solvent must be aprotic since alcohols and water contain an acidic proton and thus react with phenylmagnesium bromide to give benzene. Carbonyl-containing solvents, such as acetone and ethyl acetate, are also incompatible with the reagent.

Structure

Although phenylmagnesium bromide is routinely represented as, the compound's structure is more complicated. Like most Grignard reagents, phenylmagnesium bromide is invariably observed as an adduct with various Lewis bases. Examples include the diethyl ether and tetrahydrofuran complexes and, respectively. Thus, the Mg in those adducts is tetrahedral and obeys the octet rule. The Mg–O distances are 201 and 206 pm, and the Mg–C and Mg–Br distances are 220 pm and 244 pm, respectively in the diethyl ether complex. In their reactions of these complexes, these ether ligands dissociate.

Reactions

Phenylmagnesium bromide is a strong nucleophile as well as a strong base. It can abstract even mildly acidic protons. It often adds to ketones to give tertiary alcohols. With carbon dioxide, it reacts to give benzoic acid after an acidic workup. If it is treated equivalents with one third equivalent of phosphorus trichloride, triphenylphosphine results.