NITRATION AND NITRATING AGENTS 13(H 2 SO 4 ) 5 .HNO 3 or N 2 O 5. 10 SO 3. 11 H 2 O with a melting point of +23°C. In 1949
Dade [31] demonstrated in the same way the existence of the compound 2N 2 O 5.
.11SO 3 .9H 2 O (m. p. +18.8°C) and Heertjes and Revallier [31a] the existence of
the compound HNO 3 .2SO 3 , which according to Spasokukotskii [59] is nitronium
pyrosulphate NO 2+
.HS 2 O 7- .
At last in papers published between 1925 and 1928, Hantzsch referred to the
analogy between above-mentioned compounds and those of nitric and perchloric
acids [21-23]. He reported that he had succeeded in isolating crystalline nitracidium
and hydronitracidium perchlorates :
(H 2 NO 3 )+(C1O 4 )- and (H 3 NO 3 )2+(C1O 4 )2-According to Hantzsch nitracidium and hydronitracidium nitrates are present
in concentrated nitric acid:(H 2 NO 3 )+(NO 3 )- and (H 3 NO 3 )^2 +(NO 3 ) 2 -The Hantzsch theory was supported by Lauer and Oda [32], who examined
the nitration of anthraquinone and nitrobenzene with mixtures of nitric and sul-
phuric acid, the sulphuric acid containing from 20% water to 5% sulphur trioxide.
They found that the activation energy of the nitration reaction was 21.65 kcal/mole
when the water content of the sulphuric acid was in the range 4.4-13%, while it
was only 13.30 kcal/mole when anhydrous sulphuric acid was present. The reaction
rate was higher in the presence of these concentrations of water than in the non-
aqueous medium. Hence Lauer and Oda came to the following conclusions:- In a mixture with 89% sulphuric acid, nitric acid exists in two forms
-true nitric acid and pseudo-nitric acid, according to Hantzsch’s theory. Only
the pseudo-acid form acts as a nitrating agent. On dilution with water the con-
centration of pseudo-acid falls. - In mixtures with 89-98% sulphuric acid, nitric acid exists principally
as pseudo-nitric acid with a certain quantity of sulphate of nitric acid also present. - When 98-100% sulphuric acid is used, the nitric sulphate content
increases. - In a non-aqueous mixture containing free SO 3 it is only the nitric sulphate
which acts as a nitrating agent.
A quite different hypothesis was suggested by Euler [33] in 1903. He assumed
that a solution of nitric acid in sulphuric acid contains the nitronium or nitryl ion,
NO 2
+. Subsequently in 1922 he confirmed this hypothesis.
This view was supported later by Walden [17]. Considering the high electrical
conductivity of nitric acid he believed that HNO 3 should be regarded as an am-
photeric electrolyte, an idea expressed by the equilibrium equation:
HNO 3 <-> H+ + NO 3 - <-> OH- + NO 2 + (9)On the basis of conductometric studies and taking into account the catalytic
action of nitrogen dioxide in the nitration Klemenc and Schöller [34] suggested