urbaf2

NITRATION AND NITRATING AGENTS 43

Only recently Miskidzhian and Trifonov and their co-workers [101] examined
the physico-chemical properties of solutions of nitric acid in acetic acid, such as
the freezing point, viscosity, density, refraction, surface tension, electrical conduc-
tivity and heat of mixing. These authors came to the conclusion that nitric and
acetic acids combine, yielding an addition compound HNO 3 .CH 3 COOH. The
existence of Pictet’s compound has not been verified.

Confirmation of these conclusions was provided in 1954 by Titov [35] who

has studied the corrosion of steel in solutions of nitric acid in acetic acid. He found

that the solution having the composition HNO 3 .CH 3 COOH was the least corrosive.

Shatenshtein [102] drew attention to the fact that nitric acid in anhydrous

acetic acid was much less dissociated than when in water, and that this could be

explained by the protolytic properties of the solution components and by the low

dielectric constant of acetic acid.

Recently Mint and Kecki [103] examined the Raman spectra of solutions of

nitric acid in anhydrous and hydrated acetic acid. They have shown that at a con-

centration of 2 moles HNO 3 per litre of CH 3 COOH, i.e. for the solution containing

12.6% HNO 3 in acetic acid, the 1304 cm

-1

line, probably corresponding to the

nitronium ion, NO 2 +,can be seen. The intensity of the line increases with concen-

tration of nitric acid. Thus we can say that the acetic acid facilitates the formation

of the nitronium ion.

The objective of Usanovich’s work, mentioned above [50] was to elucidate

the structure of mixtures of nitric acid with acetic or chloroacetic acid by means

of conductometric measurements.

When studying the kinetics of the nitration reaction of phenol with nitric acid

in the presence of acetic acid Briner and Bolle [104] found that the rate of nitration

with this solution was higher than that with nitric acid of the same concentration.

A comparative study has also been made of the Raman spectra of the systems:

HNO 3 -H 2 SO 4 -H 2 O

HNO 3 -HClO 4 -H 2 O

HNO 3 -CH 3 COOH-H 2 O

by Mint, Kecki and Osiecki [105]. This led them to a conclusion that an increase

of the acidity of the medium not only caused the dissociation of nitric acid to

diminish, but also the Raman quantum efficiency of the 1300 cm-1 line to rise. This

line corresponds to vibrations of the undissociated HNO 3 molecule. This in turn

would indicate that the level of polarity of the bonds between the nitrogen and

oxygen atoms of the NO 2 group was decreased as a result of the influence the

medium exerts on the HONO 2 molecule. This would lead to an increase in the

polarity of the bond between the NO 2 and OH groups and hence a weakening of

this bond.

The authors express the view that the undissociated HNO 3 molecule is the

nitrating agent in the solution HNO 3 - CH 3 COOH-H 2 O. The reactivity of the mole-

cule is increased as a result of the polarized and weakened HO-NO 2 bond.