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NITRATION AND NITRATING AGENTS 15

Using more precise methods of cryometric measurements Ingold [37] and co-

workers had already found in 1946 that the value of van’t Hoff i-factor for HNO 3

in sulphuric acid is 4.4. Ingold explained this by eqn. (18).

It should be pointed out that a similar equation for a solution of nitrous acid

in sulphuric acid was suggested by Hantzsch as early as in 1909, and confirmed

in his further publications of 1930-37:

HONO + 2H 2 SO 4 <-> NO+ 2HSO 4 - + H 3 O+ (17)

(with the nitrosonium cation, NO+).

The value of i can be explained by an equation assuming the formation of the

nitronium ion, NO 2 +:

HNO 3 + 2H 2 SO 4 -> NO 2 + + 2HSO 4 - + H 3 O+ (18)

Similarly for solutions of N 2 O 5 or N 2 O 4 in sulphuric acid, Ingold [38-40] and

his co-workers found i = 6, which would correspond to the equations:

N 2 O 5 + 3H 2 SO 4 -> 2NO 2 + + H 3 O+ + 3HSO 4 - (19)

N 2 O 4 + 3H 2 SO 4 -> NO 2 + + H 3 O+ + 3HSO 4 - (20)

These equations have been confirmed by an examination of the nitrating proper-

ties of such mixtures.

Titov [35] also expressed the opinion that nitric acid esters in sulphuric acid

yield nitronium ions, for example in the reaction with ethyl nitrate:

C 2 H 5 ONO 2 + 2H 2 SO 4 -> NO 2 + + C 2 H 5 OH 2 + + 2HSO 4 - (21)

The basic argument of Titov concerning the existence of the nitronium ion
in such solutions seems to be right, especially in view of the cryometric investigations

of L. P. Kuhn [41], who found that for a solution of ethyl nitrate in sulphuric acid,

i = 6. This would correspond to the equation:

C 2 H 5 ONO 2 + 3H 2 SO 4 -> NO 2 + + 2HSO 4 - + H 3 O+ + C 2 H 5 O.SO 3 H (22)

The similarity between the ultra-violet absorption spectrum of nitric acid and

that of ethyl nitrate in sulphuric acid confirms this conception.

Gillespie and Graham [42] have carried out a cryometric examination of solutions

of nitric acid in oleum. The results obtained are in agreement with the following

equation, postulating formation of the nitronium ion:

HNO 3 + 2H 2 S 2 O 7 -> NO 2 + + HS 2 O 7 - + 2H 2 SO 4 (23)

According to Titov’s paper of 1941 nitric anhydride dissociates into the nitronium

and nitrate ions:

N 2 O 5 -> NO 2 + + NO 3 - (24)

Cryometric investigations carried out by Gillespie, Hughes and Ingold [42a] con-

firm this equation. The authors examined the system nN 2 O 5 + H 2 O, where n> 1.

According to their studies, anhydrous nitric acid, present in this system, under-
goes a self-dissociation in the following way: