A Guidebook to Mechanism in Organic Chemistry

Carbanions and Tautomerism

EtO

V

EtO^H

P-Ph C 6 H 4 ^C^-N=F<

H

Ph p-Ph-C.H^ Ph

-r-C^ & C=N—C^H

(XX) (XXI) OEt

^OEt

whereas with a stepwise mechanism

EtO* H Ph

\. /

P-PhC,H 4 »-C—N=C

/ \

Ph H

(XX)

EtOD

(0

(^3) OEt
P-PhC,H 4 Ph p-Ph C,H 4. Ph
Ph H Ph
\
©OEt
CO EtOD
P-Ph C 6 H 4
\
H
Ph
C=N-
Ph D
(XXI)
deuteration can take place without tautomerisation by the reversal of
(i) and so should proceed the faster. In practice, both are found to
take place at the same rate so, knowing the reaction is reversible, it
can be said with confidence that it proceeds via the concerted mech­
anism.
Which mechanism is at work in a particular case is also influenced
by the medium involved, polar solvents, not surprisingly, favouring