A Guidebook to Mechanism in Organic Chemistry

Stability of Carbanions

STABILITY OF CARBANIONS

As might be expectgd, the effect of electron-withdrawing (i.e. activat­
ing) groups is most pronounced when they can exert a mesomeric as
well as an inductive effect; for then they not only promote acidity in
the relevant hydrogen atom but stabilise the resultant carbanion as

FORMATION OF CARBANIONS

Although the tendency of many C—H bonds to such spontaneous
dissociation is but low, structural factors, particularly the presence
of electron-withdrawing groups, can lead to sufficient acidity in such
hydrogen atoms as to permit their ready removal by bases:

RCH.X+B: -* RCHX+B:H

Thus the following compounds

EtO,CCH,CO,Et CH 3 NO,

(VI) pK.»15 pK„=10-2

MeCOCH,CO,Et Me • CO • CH, • CO • Me

(VII) pK„=10 1 (VIII) pK„ = 8-8

readily yield carbanions in this way which are, with similar ones, of the
utmost importance as intermediates in a wide variety of reactions.
Other compounds which lose a proton less readily may sometimes
be converted into carbanions by treatment with very strong bases in
anhydrous media, as is the case with acetylene (pK?= 26):

• •
  ©NH,
  HC=CH v HC=Ce + NH 3
  NH, liq.

Where the actual isolation or ready detection of a carbanion as
such is not feasible, the transient formation of such a species may be
inferred from dissolving the compound, in the presence of base, in
DaO or EtOD and observing if deuterium becomes incorporated.
Thus chloroform is found to undergo such an incorporation:

-H® D,O

HCC1, ©CCI, DCCI3

H.O -D©