A Guidebook to Mechanism in Organic Chemistry

Carbanions and Their Reactions

cannot be answered with any degree of confidence. A pyramidal
structure would seem more likely but direct experimental confirmation
thereof is somewhat inadequate. As soon as one or more of the groups
attached to the carbanion carbon atom are capable of stabilising the
ion by delocalisation, then limitations are imposed on its configuration
because of the near coplanarity necessary if significant delocalisation
via the overlapping of parallel p orbitals is to take place. This will
apply to the three bonds to the carbanion carbon in the triphenyl-
methyl anion (XI)

(XI)

and also in cases where the carbanion carbon is adjacent to )>C=0

NO 2, etc. A good example of this is seen in the compound (XII):

(XIII)

Here the hydrogen, despite being flanked by two carbonyl groups,

shows- little sign of acidity (cf. Me-CO-CHa* CO-Me) because the

carbanion (XIII) that would be obtained by its removal is unable to

stabilise itself by delocalisation owing to the rigid ring structure

preventing the p orbitals .on the two carbon atoms involved from

becoming parallel; significant overlapping thus could not take place

and the carbanion does not form.

In simpler examples, it is well known that asymmetric centres

carrying a hydrogen atom adjacent to carbonyl ^groups (e.g. XIV)

are very readily racemised in the presence of base. This can, never­

theless, not be taken as entirely unambiguous confirmation of the

planar nature of any carbanion intermediate involved (e.g. XV),