_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Grignard reagents
Mgl 2 , a good drying agent for traces of moisture) or merely by stirring the metal
under nitrogen is sometimes necessary. The formation of a Grignard reagent is
strongly exothermic, so that once the induction period is over it commonly
accelerates very markedly. Care is therefore necessary to avoid adding too much
halide before the reaction is underway or the boiling of the solvent can get out of
control. An exception is sometimes made for unreactive halides. Here the
'entrainment method' is useful. The inert halide is added together with 1,2-
dibromoethane to an excess of the stoichiometric quantity of magnesium. l, 2-
Dibromoethane (CARE-suspect carcinogen) reacts approximately according to
the equation
BrCH 2 CH 2 Br + 2Mg----> MgBr 2 + C 2 H 4
exposing fresh metal which can attack the desired halide concurrently. *
While diethyl ether is usually the preferred solvent, the more basic tetrahydrofuran
is useful for making arylmagnesium chlorides and vinyl Grignard reagents.
Aryl chlorides, unlike the more expensive bromides and iodides, do not react with
magnesium in diethyl ether, neither do vinyl chlorides. In contrast to organolithium
compounds, Grignard reagents do not normally cleave ethers. Organomagnesium
compounds can also be prepared in hydrocarbons, although only primary
and aryl halides give good yields. The soluble products from chlorides are
essentially MgR 2 , as MgC1 2 precipitates from solution. Bromides and iodides afford
solutions which contain unsolvated 'R"MgX 2 -n· (n > 1 ).
Among side reactions which can arise in Grignard preparations, Wurtz
coupling to form hydrocarbons R -Ris the most significant. It occurs especially with
bulky R groups and also when Ris allyl or benzyl. Iodides are worse than chlorides
or bromides. Excess magnesium and slow addition of the halide helps to reduce
this problem. Coupling can be either through direct reaction between RX and
RMgX or through radical coupling. The attack of an organic halide on
magnesium is thought to occur by a free radical mechanism. It is those alkyl
groups which form the most stable radicals which are most prone to these side
reactions.
3.4.2 Composition and structures
Careful evaporation of solvent from solutions of Grignard reagents leaves
colourless solids or viscous liquids. It is difficult to remove ali traces of ether below
100°C. Prolonged pumping to constant weight at 90-120°110- 3 mm leads to
dismutation of 'RMgX' to [R 2 Mg]" and MgX 2 (R = Me, Et; X= CI, Br). Careful
crystallization, however, has yielded materials suitable for X-ray analysis. Some
*Solutions of Grignard reagents can sometimes be prepared without the protection of an
inert atmosphere, air being displaced by ether vapour during the exothermic reaction.
Such a procedure is not recommended; if it is adopted boiling must not be allowed to stop
before the formation and use of the reagent is completed.
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