A Guidebook to Mechanism in Organic Chemistry

Carbanions and Tautomerism

CARBANIONS AND TAUTOMERISM

The enolisation mentioned above, is, of course, an example of the
larger phenomenon of tautomerism: the existence of two or more
readily interconvertible structures that differ only in electron distri­
bution and in the position of a mobile atom. The mobile atom con­
cerned is hydrogen in the overwhelming majority of examples, in
which case the phenomenon is known as prototropy; familiar exam­
ples are ethyl acetoacetate and aliphatic nitro-compounds:

O OH

II I

MeC—CH,COaEt Me • C=CH • COaEt

Keto Enol

O OH

/ /

R-CHa—N RCH=N

O© Os

Pseudo-acid form Aci-form

(i) Concerted v. stepwise mechanism
These interconversions could, in tfllory, take place by either a step­
wise or a concerted, mechanism depending on whether the abstraction
of hydrogen from one atom takes place prior to, or simultaneously
with, the addition of hydrogen to the other; both types are en­
countered in practice. If the conversion is base-induced, the former

despite its likelihood on other grounds, for the enol form (XVI)

which must be planar will also be in equilibrium with it:

H O R <X

R»-C—C— C * C

/ /

R' (XIV) R' (XV)

B:Hffl1l. fB:H®

R 0s, R OH

c±c— =? c=c—

/ /

R' (XV) R' (XVI)