38 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVESquantity of tritium oxide, he also investigated whether the hydrogen-tritium ex-
change was taking place and found that in these conditions the exchange either
did not occur at all or was very insignificant. These observations confirmed those
of Ingold on benzene nitration with deuteronitric acid.
According to Melander, the experimental facts show that the nitration reac-
tion is a two-stage mechanism. In the first stage the intermediate compoundwould be formed and it is only in the second stage that the proton was split off.
Similar tests on nitration of monodeuterobenzene were carried out by W. M.
Lauer and Noland [90]. They found that the deuterium content in the nitration
products (nitro- and dinitrobenzene) was proportional to its content in the initial
compound, thus confirming Melander’s observations on tritium.
Bonner, Bowyer and G. Williams [91] stated that the rate of nitration of nitro-
benzene with nitric acid in 97.4 and 86.7% sulphuric acid was much the same as
the rate of nitration of pentadeuteronitrobenzene with the same mixture. Thus,
the results obtained by W. M. Lauer and Noland and by Banner and his co-workers
supported Melander’s earlier observation, that the rupture of the C-H bond
was not a determining factor for the rate of nitration of aromatic compounds.
All these investigations confirm that the nitration process is a two-stage sub-
stitution reaction, where the first slope consists in attaching the nitronium ion,
according to eqns. (22) and (23), p. 60, or according to Melander (as above).
Bunton, Halevi and Llewellyn [92] studied the oxygen atom exchange between
nitric acid and water, using heavy oxygen18
O as the labelled element. They found
that the higher the exchange rate, the higher the concentration of nitric acid. At
low HNO 3 concentrations the reaction proceeded in the presence of nitrous acid.
At high HNO 3 concentrations the presence of nitrous acid was not indispensable
for the exchange.
When comparing the rate of this reaction with the rate of nitration of aromatic
compounds with nitric acid of various concentrations, Bunton and Halevi [93]
came to the conclusion that the oxygen exchange between nitric acid and water
proceeded faster than the nitration reaction. It was only for high concentrations
of HNO 3 and for readily nitrated compounds that the values of the rates of both
the reactions were of the same order.
Bunton and Halevi have examined the following reactions (O* denotes heavyoxygen) :
HNO 3 + HNO 3 <-> H 2 NO 3 + + NO 3 - (a)
H 2 NO 3 + <-> NO 2 + + H 2 O (b)
nitration : NO 2 + + RH <-> RNO 2 + H+ (c)
oxygen exchange: NO 2 + + H 2 O <-> H 2 NO 3 + (d)
H 2 NO 3 + <-> H+ + HNO 3 (e)