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44 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

In 1950 Ingold, Hughes and Reed [39], when studying the kinetics of nitration

with nitric acid in the presence of acetic acid, found the reaction to be of zero

order in the case of aromatic compounds which are readily nitrated (benzene and

its homologues), and its rate Va could be expressed by the approximate equation:

Va = k 0 at [HNO 3 ] = const.

While in case of compounds more difficult to nitrate (di- and tri-chlorobenzenesj.

the reaction is of the first order and its rate V'a may be expressed by the equation :

V'a = k 1 [ArH] at [HNO 3 ] = const.

According to the authors, the nitronium ion is the main nitrating agent in the

presence of acetic acid.

Nitric acid and acetic anhydride. In their further work in 1907 Pictet and

Khotinsky [106] found that acetic anhydride formed acetyl nitrate with N 2 O 5 :

(48)

Cohen and Wibaut [107] found when studying the kinetics of interaction of

nitric acid and acetic anhydride, that after mixing the two substances, free nitric

acid was present in the mixture during the first 75-100 min, then the HNO 3 con-

centration decreased in consequence of the reaction:

HNO 3 + (CH 3 CO) 2 O <-> CH 3 CONO 3 + CH 3 COOH (49)

The reaction did not stop at this stage but proceeded further since the nitrating

action of acetyl nitrate on acetic anhydride yielded acetic nitroanhydride and

consequently the concentration of acetyl nitrate decreased.

According to Cohen and Wibaut, it was nitric acid which was the nitrating

agent at the beginning and later acetyl nitrate took its place.

Konovalov [108] observed an important fact as long ago as 1895. He reported

that aromatic hydrocarbons with side chains were nitrated with this mixture chief-

ly in the side chain. Shorygin and Sokolova [109] confirmed this observation in 1930.

The kinetics of nitration of aromatic hydrocarbons with nitric acid in the
presence of acetic anhydride was first studied by Tronov, Kamay and Kovalenko

[110]. Their studies will be discussed later.

Menke [2] inferred from the results of his studies on nitration of aromatic

hydrocarbons with mixtures of nitric acid and acetic anhydride that the latter

acted not only as a dehydrating substance but also as a catalyst.

Recently Paul [111] examined kinetics of the nitration of benzene using solutions

of nitric acid in acetic anhydride and acetic acid at 25°C. In a number of his experi-
ments he also added sulphuric acid in a small quantity (ca. 0.01 M).

The results favour nitronium ion attack on benzene as the rate-determining step:

HNO 3 + H+ <-> H 2 NO 3 + (a)