A Guidebook to Mechanism in Organic Chemistry

Displacement Reactions of Carbanions

but it is doubtful in most cases, whether a sufficiently negative carbon
is generated in R°~ to expel eOEt and addition, as with an ordinary
carbonyl group (p. 171), followed by elimination, is probably more
likely.
Of other organo-metallic compounds, there is some evidence that
sodium and potassium derivatives, e.g. of di- and triphenylmethane,
etc., do provide actual carbanions as the active species in the reactions
in which they take part, though even here the carbanions may be
present not as the simplest possible species but as dimers, trimers and
higher aggregates. Lithium derivatives, which are synthetically more
useful, correspond more closely to Grignard reagents in possessing
a greater degree of covalent character not normally leading to the
formation of free carbanions. They are not entirely analogous to
Grignard reagents, however, as a number of cases are known where
the two lead to different products from the same substrate.
(c) Wurtz reaction: A number of claims have been advanced that
the Wurtz reaction proceeds via a radical mechanism:

R • CHa • CH 8 Br+Na« RCHjCHj- + Br^0 + Na®

2RCH,CH,. -* R(CH,) 4 R

One of the pieces of supporting evidence quoted#to corroborate Jhis is

that disproportionate is observed to take place as well as the

expected dimerisation, an alternative reaction well known in radical

chemistry (p. 235):

2R-CHt-CH,« ->• RCH,CH,+RCH=CH,

While such a mechanism is probably valid in the vapour phase it is

somewhat less likely in solution and reaction of alkyl chlorides with

sodium in hydrocarbon solvents has been shown to lead to the forma­

tion of sodium alkyls, which can then be made to react with a second

alkyl halide to give quite high yields of the mixed hydrocarbon:

2Na R'CI

R-CHJ'CHJCI—>-Na® + Cle + RCH,CH,eNa® »RCH, CH.R

Further, the disproportionation, which has often been taken as

confirmation ofe radical mechanism, can equally be explained, on a

carbanion basis, by elimination. The carbanion acts as a base, ab­

stracting a proton from the j3-carbon atom of the halide, while chlor­

ine is lost from the a-carbon atom under the influence of Na®:

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