48 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES
The work of Bennett and his co-workers [87] (discussed in detail on the p. 312)
was an exception: a 50/50 mixture of di- and tri-nitrotoluene was nitrated by shaking
with mixed acids of various compositions for a fixed time. The reaction was then
quenched with cold water and the proportion of the dinitrotoluene which has been
converted to trinitrotoluene was determined. The conversion, and the reaction
rate, approach zero as the mole ratio water: sulphuric acid approaches unity.
This is significant, because if this ratio considerably exceeds 1.0 the NO 2 + ion is
spectroscopically undetectable in sulphuric acid-nitric acid-water solutions. Ben-
nett showed that various acid mixtures that gave the same conversion contained
practically the same concentration of the NO 2 + ion, as determined by Raman
spectra. Hetherington and Masson [84] had already found that the reaction rate
became negligibly small at certain concentrations and that a line drawn through
the limiting boundary almost coincides with the boundary of the area of spectro-
scopic detection of NO 2 + ions.
However, a few authors offered evidence showing that nitration can also occur
with mixtures in which concentrations of the nitronium ion are too weak to be
spectroscopically detectable (Lowen, Murray, Williams [116]; Bunton, Halevi,
for C-nitration [93], and Urbanski and Hackel [80] for O-nitration). Also Bren-
necke and Kobe [117] drew attention to the experiments of McKinley and White
[118] who found that the acids for mononitration lie outside the nitronium ion
envelope.
All this created doubt as to whether the nitronium ion mechanism can be applied
to all cases. As will be pointed out later, it seems that the mechanism of the nitration
reaction is more complicated and may proceed under the influence of nitrating
agents other than the NO 2 + ion. According to Titov in his early work of 1941,
nitration with nitric acid of moderate concentration (60-80% HNO 3 ) occurs through
the action of NO 2 (or N 2 O 4 ) and nitric acid plays only the role of a source of NO 2.
However, Bunton and Halevi [93] have expressed the view that the nitronium
ion mechanism is still responsible for C-nitration even in highly aqueous con-
ditions. Bunton, Halevi and Llewellyn [92] in their work described above (p. 38)
examined the mechanism of oxygen exchange between nitric acid and water in
an aqueous nitric acid medium by using isotopically labelled water. From the
identity of the absolute rate of exchange in such a medium with the absolute rate
in the same medium used for an aromatic nitration they suggested that the nitro-
nium ion mechanism is responsible for nitration with nitric acid containing for
example 60 mole % of water. Banner and Frizel [99b] have also suggested the ni-
tronium ion mechanism for O-nitration of alcohols by nitric acid in aqueous sol-
ution.
It should be pointed out that phenols and anilides can be nitrated with very
dilute nitric acid. The mechanism of such reactions might differ essentially from that
of the reactions of C-nitration in other aromatic compounds, as it most likely
passes through the formation of nitroso compounds which are then oxidized to the
corresponding nitro compounds. This problem is discussed later (e.g. p. 85, 116).