Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

ALIPHATIC NITKO-COMPOUNDS 263
In benzoylacetylacetone the tendency to enolise is so pronounced that
this substance exists as enol only. The keto-form is unknown.
C6HS.CO.C=C—CH3
H3C—CO OH
Nor can there be any question of real tautomerism in the case of
phenol. In its chemical properties phenol resembles the aliphatic
enols in all respects. We need only recall the agreement in the acid
character, the production of colour with ferric chloride, and the reactions
with halogens, nitrous acid, and aromatic diazo-compounds (coupling),
caused by the " activity " of the double bond and proceeding in the
same way in phenols and aliphatic enols. The " enol nature " of phenol
provides valuable support for the conception of the constitution of
benzene as expressed in the Kekule-Thiele formula, since it is an expression
of the tendency of the ring to maintain the " aromatic " state
of lowest energy. In this connexion the hypothetical keto-form of
phenol (A)—not yet obtained—would be of interest in comparison with
the aliphatic ketone (B).
CO
CH CH2
II 1
CH CH
\ / CH
B
CO
CH CH2
II I
CH2 CH=CH2
The tautomerism of the aliphatic nitro-compounds is very closely
allied to that of the ketones and aldehydes. Here also there are two
forms—the neutral and the acid, or so-called oci-form (Hantzsch) :
—C=O O=N=O
—C—H —C—H '
Ketone True nitro-compound
—C—OH O=N—OH
I I
—c —c
I I
Enol aci-Nitro-compound
In respect of their properties, conditions of rearrangement, and
reactions, we simply refer to what was said about the keto-enol change.
Here, also, the bromine method enables the points of equilibrium to be
determined quantitatively. The oldest and most important example
of desmotropy in nitro-compounds was found in •phenylnitromethane,