A Guidebook to Mechanism in Organic Chemistry

^ CARBANIONS AND THEIR REACTIONS

SOME organic compounds are known which function as acids, in the
classical sense, in that a proton is liberated from a Gar-H bond, the
resultant conjugate base (I) being known as a carbanion:

—C-J-H ^ —CQ + H®

/ y

m

This tendency is, not surprisingly, but little marked with aliphatic

hydrocarbons for the C—H bond is a fairly strong one and there is

normally no structural feature that either promotes acidity in the

hydrogen atom or that leads to significant stabilisation of the

carbanion with respect to the original undissociated molecule (cf.

p. 40); thus methane has been estimated to have a pKn of » 58,

compared with 4-76 for acetic acid. Triphenylmethane (II), however,

whose related carbanion (III) can be stabilised by delocalisation

PhAC^ y ~ Ph 2 C=</^P etc.

(II) (HO

is a much stronger acid (pK 0 a 25) by relative, if not by absolute,

standards. The presence of one or more strongly electron-withdraw­

ing groups has an even more marked effect; thus tricyanomethane

(IV)

(NC) 3 C-H H®+[(NC) 2 cic^N ~ (NC) 2 C=C=-N etc.]

(IV) J

approaches the mineral acids i»strength. The cumulative effect of

introducing successive cyano groups is of interest here for acetonitrile

(V), containing only one such group, still has a pl£ as high as 25:

CH 3 —C=N == H® + [CH^LC^ CH 4 =C=N]

(V)

Ph 3 C—H ^ H® +