Phenylmagnesium bromide

Phenylmagnesium bromide, with the simplified formula C₆H₅MgBr, is a a magnesium-containing organometallic compound. It is so commonly used that it is commercially available as a solution in diethyl ether or tetrahydrofuran (THF). Phenylmagnesium bromide is a Grignard reagent. It is often used as a synthetic equivalent for the "Ph⁻" phenyl synthon.

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Preparation and structure

Phenylmagnesium bromide can be prepared in the laboratory by treating bromobenzene with magnesium metal, usually in the form of ribbon. A polar solvent is required to solvate the magnesium portion of the molecule, hence the use of ether or THF. Protic solvents such as alcohols and water react with phenylmagnesium bromide to give benzene, and halogenated solvents are also incompatible with the synthesis. A small amount of iodine may be used to activate the magnesium by removing the oxide surface.

Although in organic synthesis it is convenient to represent this species as C₆H₅MgBr, the molecule is more complex as shown in the figure. The compound invariably forms an adduct with two OR₂ ligands from the ether or THF solvent. Thus the Mg is tetrahedral and obeys the octet rule. The Mg-O distances are 2.01 and 2.06 Å whereas the Mg-C and Mg-Br distances are 2.2 and 2.44 Å, respectively.^[1]

Chemistry

Phenylmagnesium bromide is a strong nucleophile as well as a strong base. It can abstract many labile protons, thus the substrate must be protected where necessary. It often adds to carbonyls, which are electron-deficient, such as in the laboratory synthesis of benzoic acid and triphenylmethanol.

References

1. ^ Stucky, G. D.; Rundle, R. E. "The Structure of Phenylmagnesium Bromide Diethyletherate and the Nature of Grignard Reagents" Journal of the American Chemical Society 1963, volume 85, pp1002. DOI 10.1021/ja00890a039

Further reading

• Oxford University, Phenylmagnesium bromide in ether, Aug 19 2005. Accessed 28 Oct 2006.