Phenylmagnesium bromide

Chembox new
Name = Phenylmagnesium bromide
ImageFile1=PhMrBrL2.png ImageSize1 = 150px
ImageName1 = Phenylmagnesium bromide, OR₂ = ether or THF
ImageFile2 = Phenylmagnesium-bromide-3D-balls.png ImageSize2 = 150px
ImageName2 = PhMgBr THF adduct
IUPACName = bromido(phenyl)magnesium
OtherNames = PMB
Section1 = Chembox Identifiers
SMILES = Br [Mg] c1ccccc1
CASNo = 100-58-3
RTECS =
Section2 = Chembox Properties
Formula = C₆H₅MgBr
MolarMass = 181.31 g/mol
Appearance = Colorless crystals
Density = 1.14
Solubility = Reacts violently
pKb = strong base
Section7 = Chembox Hazards
ExternalMSDS = [http://physchem.ox.ac.uk/MSDS/PH/phenylmagnesium_bromide_in_ether.html External MSDS]
MainHazards = flammable
FlashPt = ?°C
RPhrases = R12 R14 R15 R20 R22 R35 R41
SPhrases = S16 S26 S30 S33 S36
S37 S39 S43 S45
Section8 = Chembox Related
OtherCpds = Phenyllithium
Magnesium bromide
Methylmagnesium chloride

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 phenyl "Ph⁻" synthon.

Preparation

Phenylmagnesium bromide is commercially available as solutions of diethyl ether or THF. It may be prepared in the laboratory by treating bromobenzene with magnesium metal, usually in the form of ribbon. A small amount of iodine may be used to activate the magnesium to initiate the reaction.cite web | publisher = MiraCosta College | author = Donald L. Robertson | date = 2007-01-03 | title = Grignard Synthesis: Synthesis of Benzoic Acid and of Triphenylmethanol | accessdate = 2008-01-25 | url = http://www.miracosta.edu/home/dlr/211exp2.htm]

Polar solvents, such as ether or THF, are required to solvate (complex) the magnesium(II) center. The solvent must be aprotic solvents since alcohols and water react with phenylmagnesium bromide to give benzene. Carbonyl-containing solvents such as acetone and ethyl acetate are incompatible with the reagent.

tructure

Although phenylmagnesium bromide is routinely represented as C₆H₅MgBr, the molecule is more complex. 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. [cite journal | author = Stucky, G. D.; Rundle, R. E. | title = The Structure of Phenylmagnesium Bromide Diethyletherate and the Nature of Grignard Reagents | journal = J. Am. Chem. Soc. | year = 1963 | volume = 85 | pages = 1002 | doi = 10.1021/ja00890a039]

Chemistry

Phenylmagnesium bromide is a strong nucleophile as well as a strong base. It can abstract even mildly acidic protons, thus the substrate must be protected where necessary. It often adds to carbonyls, such as ketones, aldehydes. [OrgSynth | author = W. E. Bachmann and H. P. Hetzner | title = Triphenylcarbinol | collvol = 3 | collvolpages = 839 | prep = cv3p0839] With carbon dioxide, it reacts to give, after an acidic workup, benzoic acid.

References