A Guidebook to Mechanism in Organic Chemistry

Benzoin Reaction

i.e. the so-called 'acid decomposition' of /?-keto esters. An exactly

analogous fission of /3-diketones also takes place:

o o o

II II Wr% II

R—C—CHS—C—R+ GOEt R—C-MJHS—C—R

OEt

U

_. o o

R—C+^0 CH 2 —C—R

I

OEt

(f) Benzoin condensation: Another carbanion addition is that ob­

served with aromatic aldehydes in alcoholic solution in the presence

ofeCN:

<*<> OE OH

*HI I I

Ph—C—H ^ Ph—C—H ^ Ph—CE

(^3) CN CN ^ CE^N
(XL)
o
c—PH
H
i HO HO OE
Ph—C C—Ph *= Ph—C—C—Ph
"P^1
ECN H CcN H CN H
(XLIV) (XLIII) (XLII)
Cyanide ion is a highly specific catalyst for this reaction, its effective­
ness depending presumably on the ease with which it adds to benzal-
dehyde in the first place and with which it is finally expelled from
(XLIII) to yield benzoin (XLIV). BuJ perhaps most of all, it depends
on its electron-withdrawing power which promotes the ready release,
as proton, of the hydrogen atom attached to carbon in (XL) to yield
the carbanion (XLI). The observed kinetics of the reaction
Rate oc [Ph • CHO]z[eCN]