A Guidebook to Mechanism in Organic Chemistry

Addition of Carbanions

bromide has, however, been isolated in crystalline form as the com­
pound C 8 H 8 MgBr-2Et 2 0, in which C 6 H 6 , Br and the two molecules
of ether are arranged tetrahedrally about the magnesium atom. The
known reactions of Grignard reagents indicate the possible participa­
tion of all of the species:

2Re+2MgHal

2R Mg Hal =F=^2R- + 2-MgHal

• ^ RjMg+MgHal,
  Thus free radical reactions with them are known, but in most of
  their useful synthetic applications they tend to behave as though
  a- «+
  polarised in the sense R-Mg-Hal, i.e. as sources of negative carbon
  if not necessarily of carbanions as such.
  In reactions with carbonyl groups it appears that two molecules of
  Grignard reagent are involved in the actual addition. One molecule
  acts as a Lewis acid with the oxygen atom of the carbonyl group, thus
  enhancing the positive nature of the carbonyl carbon atom, and so
  promotes attack on it by the R group of a second molecule of
  Grignard reagent via a cyclic transition state:
  Br Br ^ Br
  I I I
  Mg Mg Mg

R 2 C Mg-Br R,C® Mg—Br R 2 C ° Mg—Br

\ \ / \ /

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If such a course is followed it might be expected that Grignard

reagents of suitable structure, i.e. those having hydrogen atoms on a

/3-carbon, might undergo conversion to olehnes as a side-reaction,

transfer of hydride ion to the positive carbon atom of the carbonyl

group taking place:

V V \/

«

/C

><- -

H

i& -? V

R 2 C Mg—Br R 2 C® Mg—Br R 2 C Mg—Br

\ \ / \ /

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