A Guidebook to Mechanism in Organic Chemistry

Carbanions and Their Reactions

H

/ \

O O

II I

c c

/ \ X \

Me CH Me

(XXTV)

H

/ \

EtO

O

I

c

/ \

CH Me

(XXII)

Apart from the further stabilisation thereby effected, this hydrogen
bonding leads to a decrease in the polar character oftlfae enol and a
compact, 'folded up' conformation for the molecule (by contrast to
the extended conformation of the keto form) with the rather surprising
result that, where the keto and enol forms can actually be separated,
the latter usually has the lower boiling point of the two despite its
hydroxyl group. The part played by such hydrogen bonding in
effecting stabilisation is seen by comparing the proportion of the enol
form of acetylacetone (XXIV) in water and in a non-hydroxylic
solvent such as hexane. In the latter there is 92 per cent enol whereas in
the former where the keto form can also form hydrogen bonds—with
the solvent water molecules—the proportion of the enol is down to
15 per cent. #
In Jhe above examples, the composition of the equilibrium mixture
is governed by the relative thermodynamic stabilities of the two forms
under the particular circumstances being considered and this will,
of course, ultimately always be the case. An interesting situation
arises with aliphatic nitro-compounds such as phenylnitromethane
(XXV), however:

— H^9

Ph • CHj—N

®\

O®

Pseudo-acid form

(XXV)

O®

Ph CH=N

t

PhCH^-N

(XXVII)

o®

b

o©J

H® /

PhCH=N

O—H

O©

Aci-form

OCXVI)

Here the normal nitro- or pseudo-acid form (XXV) is thermo-
dynamically the more stable of the two and at true equilibrium this