A Guidebook to Mechanism in Organic Chemistry

Halogenation of Ketones

more acidic, and therefore more readily abstracted by base, but it will

also help to stabilise the resultant carbanion, leading to the formation

of (XL) rather than (XLI):

O H
II t
CH,—C—CH^-Br
(XXXIX)

JOH

CH, -C—CH->-Br

(XL)

Br, o

CH,—C—CHBr, + Br^6

O

CH,—C—CH 2 Br

(XLI)

This is, of course, the reason for the exclusive production of

MeCOCXj in the base-induced halogenation of acetone. As a

final stage in the haloform reaction, this species then undergoes attack

by base, e.g. ®OH, on the carbonyl carbon because of the highly

positive character that that atom has now acquired:

O Br O

Me—C)Pb^Br -> Me— c" +^0 CBr 3 -» Me— C +HCBr,

^OH Br OH Oa

In the base-induced halogenation of the ketone, R • CH 2 • CO • CH 3 ,

it is the methyl rather than the methylene group that is attacked,

for the inductive effect of the R group will serve to decrease the acidity

of the hydrogen atoms attached to the methylene group, while those

of the methyl group are unaffected, thus leading to preferential

formation of the carbanion (XXLII) rather than (XLIII):

O

R—CH,—C—CH, ^l->--CH—C—CH,

(XLII) (XLIII)

The halogenation of ketones is also catalysed by acids, the reaction

probably proceeding through the enol form of the ketone (cf. p. 218)

whose formation is the rate-determining step of the reaction:

. • ' 227