Carbanions and Their Reactions(iii) Halogenation of ketones
One of the earliest observations having a bearing on the subject of the
occurrence of carbanions as reaction intermediates was that the
bromination of acetone in the presence of base followed the kinetic
equation
Rate a [Me-CO-Me]and was independent of [Br 2 ]. The rate was, however, found to be
proportional to [base] and this was subsequently iqfgrpreted as in
volving the formation of a carbanion as the rate-determining step,
followed by rapid bromination:O
Br, II a
Me—C—CH, ~—> Me—C—CH 2 Br+Br 9(^3) OH
slow
HI A
Me—C-LCH 2
i
e
o
I
l_Me—C=CH 2
In support of thjs view it has subsequently been shown that under
these conditions ketones undergo chlorination, iodination, deuterium
exchange in D 2 0 and racemisation (if of suitable structure)
R O
•V * II ©OH
R—C—CR =====
/ H,O
slow
(+)
R crS
Xo-V IK
C-^CR
/
R'
\ I
C=CR
(XXXVIII)
R O
H.O . ||
===RVC—C-R
©OH /
H
(-)
at exactly the same rate that they undergo bromination, indicating
the participation of a common intermediate in all. The reconversion
to the ketone of the planar intermediate (XXXVIII), or the enol
derived from it (cf. p. 215 ), leads as readily to the D- as to the L- form
and thus results in racemisation. 0
Further base-induced halogenation of a mono-halogenated ketone
(XXXIX) will take place preferentially at the carbon atom that has
already been substituted, provided that it still carries a hydrogen
atom; for not only will the inductive effect of the halogen atom make
the hydrogen atoms attached to the halogen-substituted carbon atom