A Guidebook to Mechanism in Organic Chemistry

Displacement Reactions of Carbanions

form is present to the almost total exclusion of the aci-form
(XXVI).
Nevertheless, on acidification of the sodium salt of the system, i.e.
(XXVII), it is very largely (XXVI) that is obtained. This results from
the more stable pseudo-acid (XXV) being formed more slowly from
the mesomeric ion than is the aci-form (XXVI), for the transition
state between (XXVII) and (XXV) is at a considerably higher energy
level than that between (XXVII) and (XXVI), reflecting the greater
difficulty of breaking a C—H bond rather than an O—H bond. The
reaction is IIWs said to be kinetically rather than thermodynamically
controlled, leading to (XXVI) rather than (XXV).

Although the immediate result is the preferential formation of the

aci-form (XXVI), slow, spontaneous re-ionisation of (XXVI) leads

inexorably to the formation of (XXV), so that the position of

ultimate equilibrium is thermodynamically controlled.

REACTIONS OF CARBANIONS

(i) Addition reactions

Carbanions take part in a wide variety of addition reactions, many of

which involve additions to carbonyl systems? Thus Grignafd and

acetylide ion additions have already been considered (p. 170), as

have aldol (p. 172), Perkin (p. 176), Claisen ester (p. 176), and benzoin

(p. 179) reactions and carbanion addition to a/3-unsaturated carbonyl

systems in the Michael reaction (p. 155).

(ii) Displacement reactions

Carbanions are also involved in a number of displacement reactions.

(a) Alkylation: Suitable carbanions, such as those derived from

malonic ester, j3-keto-esters, /3-dicarbonyl compounds, etc., will effect

ready displacement reactions with alkylating agents such as alkyl

halides and other reactive halogen-containing compounds, this being

a useful preparative method for the formation of carbon-carbon

bonds. In most cases the carrjanion is generated in non-aqueous

solution by bases such as EtO^9 and the reaction then follows a

normal SN 2 course (see p. 222).

The SJV2 character has been confirmed kinetically in some examples

and inversion of configuration has been shown to take place at the