A Guidebook to Mechanism in Organic Chemistry

Carbanions and Their Reactions

the stepwise mode. Although not relevant to carbanions, it should
perhaps be emphasised that keto-enol interconversions can be
catalysed by acids, e.g. HA, as well as by bases. This is another
example of general acid catalysis (cf. p. 161):

I I

AH+ >C=C—OH + Ae

The role played above by the acid anion, A®, can equally well be

played by a solvent molecule in many cases.

(ii) Rate of tautomerisation

It should perhaps be emphasised that in the tautomerisations about

which we have been speaking a C—H bond must undergo dissocia­

tion; so that although a number of the conversions are fairly rapid

they are not like ionic reactions where no such bond-breaking is in­

volved. The actual rates of a number of these conversions can, as

implied in the last section, be followed by measuring the rate at

which the compounds involved will incorporate deuterium from

D 20 ?EtOD, etc. When base is added to a pure tautomer it is, in the

more familiar examples, usually the rate of formation of the anionic

intermediate that is being observed and, on subsequent acidification,

the rate of re-formation of one or both tautomers from the ion:

O

' II

Me • C—CH. • CO.Et

^OH Me G^CH CO.Et

I

.McC=CH CO.Et

O

II

Me C—CH,-CO.Et

H®

v OH

Me C=CH COaEt

The correlation of relative rates*of tautomerisation with structure

is not quite so simple as might have been expected and while it is

broadly true that structural features leading to greater acidity, i.e.

readier loss of proton, lead to more rapid reactions, this is by no

means universally the case.