A Guidebook to Mechanism in Organic Chemistry

Carbanions and Their Reactions

R CHrCH^H

R—CH^i

d

CH,

R • CHj * CH3

RCH=CH 2

CI Na® CI® Na®

Whether the process is actually initiated by Na®, RCH 2 -CH 2 ® or

both is not certain, however.

Further support for such an ionic mechanism for the Wurtz reac­

tion is provided by the behaviour of some optically active chlorides,

the first formed carbanion attacking a second moledsb of chloride

by the SN 2 mechanism and leading to inversion of configuration at

the carbon atom attacked.

(d) Reimer-Tiemann reaction: An example of a species which can lay

claim to be a carbanion only because of the n delocalisation that

results in the transfer of negative change from oxygen to carbon is

the phenoxide ion (XXIX):

etc.

(XXIX)

This reacts with chloroform in the presence of strong bases to yield

the o-aldehyde, salicaldehyde (XXX), in the Reimer-Tiemann reaction,

the quasi-carbanion (XXIX) attacking the electron-deficient carbene

CCI 2 obtained by partial hydrolysis of chloroform in the alkaline

solution (cf. p. 206):

CCJ 2

H
P
ecu
(XXIX) (XXXI)

CHCI

(XXXII) (XXXffl)

CHO

CHO

(XXX)