A Guidebook to Mechanism in Organic Chemistry

Carbanions and Tautomerism

(iii) Structure and position of equilibrium
Most information is available on this subject for keto/enol tautomers
and their near relatives and we shall confine ourselves to them.
Normally speaking, the occurrence of a significant amount of the
enol form in the keto/enol equilibrium mixture requires the presence
of a group or groups capable of stabilising the enol by delocalisation
of the w electrons of its carbon-carbon double bond:

cQ=CjO=C^OH «- 6—C=C—C=OH

III III

Thus the proportion of enol in acetone is almost negligible while with

a /3-diketone it is present as the major species:

Percentage enol

MeCOCH, V00025

MeCOCHjCOjEt (XXII) 7-5

MeCOCHPhCOjEt (XXIII) 30

MeCOCH,COMe (XXIV) 80

The relative effectiveness to this end of a second keto group as

opposed to the ^>C=0 of an ester group is seen in comparing acetyl-

acetone (XXIV) with ethyl acetoacetate (XXII), and the further

delocalisation effected, by the introduction of a phenyl group on the

methylene carbon atom by comparing ethyl acetoacetate (XXII) with

its a-phenyl derivative (XXIII).

Another factor is also contributing to the stabilisation of the enol

compared wjtb the keto form in such species, however, namely

internal hydrogen bonding: