A Guidebook to Mechanism in Organic Chemistry

Displacement Reactions of Carbanions

Thus attack on CC1 2 yields (XXXI) which undergoes a proton shift
to form (XXXII). This undergoes hydrolysis in the system to yield
(XXXIII), the free aldehyde not being obtained until the reaction
mixture is acidified after the reaction proper is over. Some p-hydroxy-
benzaldehyde is also obtained for the negative charge on the nucleus
of (XXIX) is not of course confined to the o-position.
Some support for the above mechanism involving attack by
dichloro-carbene is provided by the fact that when the reaction is
carried out on the anion of p-cresol (XXXIV), in addition to the
expected o-alaeTyde (XXXV), it is also possible to isolate the un-
hydrolysed dichloro-compound (XXXVI) arising from attack by
CC1 2 at the p-position:
O O OH
X H H H

Me (XXXIV) Me Me (XXXV)

t

O o o

Q ^ 0 ^ 0 *

Me Me CC1 2 Me CHC1 2

(XXXIV) (XXXVID (XXXVI)

The initial product of the attack (XXXVII) has no hydrogen atom

that can be eliminated as H® from the relevant carbon atom to allow

reformation of an aromatic structure, so the introduced eCCl 2 group

acquires H® from the solvent. The dichloro-compound (XXXVT) is

somewhat resistant to further hydrolysis, probably due to its in­

solubility in aqueous alkali.

Somewhat analogous attack of C0 2 on sodium phenate (the

Kolbe-Schmidt reaction) is used to prepare sodium salicylate:

\

o

(XXIX)

o