Page 646 - Advanced Organic Chemistry Part B - Reactions & Synthesis

has been suggested that the reactions may involve reduction of the halide by clusters 621
of magnesium atoms. 2
SECTION 7.1
Preparation and
R Br + Mg R Br + Mg(I) Properties of
–
·
R Br· – R· + Br – Organomagnesium
and Organolithium
R· + Mg(I) + Br – R Mg Br Reagents
Solutions of several Grignard reagents such as methylmagnesium bromide, ethylmag-
nesium bromide, and phenylmagnesium bromide are available commercially. Some
Grignard reagents are formed more rapidly in tetrahydrofuran than in ether. This
3
is true of vinylmagnesium bromide, for example. Other ether solvents such as
dimethoxyethane can be used. For industrial purposes, where less volatile solvents are

needed for reasons of safety, bis-2-butoxyethyl ether (butyl diglyme), bp 256 C, can
be used. The solubility of Grignard reagents in ethers is the result of Lewis acid-base
complex formation between the magnesium ion and the ether oxygens.
Under normal laboratory conditions magnesium metal is coated with an unreactive
layer of Mg OH , and the reactions do not start until the organic halide diffuses
2
4
through it. The reaction appears to begin at discrete sites, and accelerates as the surface
coating breaks up, exposing more active surface. The ether solvents are probably
involved and may assist dissociation of the metal ions from the surface. Various
techniques for initiating the reactions, such as addition of small amounts of I or
2
BrCH CH Br, appear to involve the generation of Mg 2+ salts, which serve to facili-
2 2
tate the reaction. Sonication or mechanical pretreatment can also be used to activate
5
magnesium. Organic halides that are unreactive toward magnesium shavings can often
be induced to react by using an extremely reactive form of magnesium that is obtained
6
by reducing magnesium salts with sodium or potassium metal. Even alkyl fluorides,
which are normally unreactive, form Grignard reagents under these conditions.
One of the fundamental questions about the mechanism is whether the radical is
7
really “free” in the sense of diffusing from the metal surface. For alkyl halides, there is
8
considerable evidence that the radicals behave similarly to alkyl free radicals. One test
for the involvement of radical intermediates is to determine whether cyclization occurs
in the 6-hexenyl system, where radical cyclization is rapid (see Part A, Section 12.2.2).

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