Carbanions and Their Reactions
mechanism would, in suitable cases, involve the participation of a
carbanion, while the second would not. Many common examples,
including /3-keto-esters, 0 -diketones, aliphatic nitro-compounds, i.e.
the common keto/enol systems and their relatives, are believed to
involve the stepwise mechanism
Bf>H
I
R—C—C—R'
I H
O(XVII)"R—C=C—R ~tR—C^-C—R'(XVIII)H®
R C C—R
I I
OH(XIX)and, as might be expected, the more stable the carbanion intermediate
(XVIII), i.e. the more acidic the substrate from which it may be
derived, the more is the stepwise mechanism favoured with respect to
the concerted one.«The above example enables emphasis to be laid on
the distinction between tautomerism and mesomerism which so often
apparently leads to confusion. Thus (XVII) and (XIX) are tautomers,
the so-called keto and enol forms respectively, and are quite distinct
chemical entities. Although often readily interconvertible both can,
in suitable cases (e.g. ethyl acetoacetate), actually be isolated and
characterised. By contrast, the intermediate involved in their inter-
conversion, the carbanion (XVIII), is a single species, a mesomeric
hybrid of the two hypothetical structures written, neither of which
has any real existence. It is, of course, a commonplace to find a pair
of tautomers underlain, as it were, by a carbanion stabilised by
delocalisation in this way.
By contrast, the tautomerisation of a number of compounds of the
form R 2 CH—N=CR' 2 , the azomethines, has been shown to proceed
via the concerted mechanism. Thus tautomerisation of (XX)-+
(XXI) has been carried out in EtOD with EtO® as catalyst, and been
found to result in deuterium exchange as well as tautomerisation. A
concerted mechanism must lead to deuteration and tautomerisation
proceeding at exactly the same rate for the latter cannot take place
without the former