NITRATION AND NITRATING AGENTS 37ion and on removing, i. e. binding, the proton. An increase in water concentration
in a nitrating mixture gives rise to an increase in the HSO 4- ion concentration and
thus enhances the process of removing the proton from the nitrated compound.
Yet, at the same time, it reduces the nitronium ion concentration. Changes in the
nitration rate with the water content of a nitrating mixture would be the resultant
of the sum total of the two effects.
However, investigations on the nitration of deuterobenzene raised doubts about
the influence of the velocity of splitting off the proton on the nitration reaction rate.
By using deuteronitric acid Ingold, Raisin and Wilson [38] established that
no substitution of hydrogen atoms by deuterium atoms occurred (in this respect
the action of deuteronitric acid differs from that of deuterosulphuric acid, leading
to the formation of deuterated benzenes up to the fully substituted hexadeutero-
benzene).
From these results and later experiments (Ingold and co-workers [36,37, 39])
on the influence of solvents on the nitration rate Ingold came to the conclusion
that splitting off the proton in the nitration of aromatics does not effect the reaction
kinetics. This conclusion differed from that of Bennett and his co-workers [87].
Recently Melander [88] attempted to clarify the influence of the proton loss
on the nitration reaction rate by investigating this process using a number of aro-
matic compounds: benzene, toluene, bromobenzene and naphthalene, in which
one hydrogen atom had been replaced by tritium.
It is well known that there is no general rule applicable to the difference in the
rates of the reactions involving compounds having hydrogen or its heavier isotopes
(^2) H (deuterium) and (^3) H (tritium) in their molecules. In some cases the heavier
isotopes react more slowly, in others more quickly.
Melander points out that unless hydrogen is firmly attached by a covalent bond
and can change its position (as in tautomerism), heavy isotopes react more slowly.
The influence of heavy water on the rate of neutralization of a pseudo-acid such
as nitroethane, as observed by Wynne-Jones [89], may be cited as an example.
According to him the rate of the reaction involving deuterium loss was about
ten times lower than when the proton was lost.
Melander in his experiments on sulphonation of the monotritero aromatics,
triterobenzene and 4-triterobromobenzene, found the replacement of tritium by
the sulpho group proceeded much more slowly than the replacement of protium
(Hydrogen H).
However, on nitrating the following aromatic compounds, having one atom
of hydrogen H replaced by tritium: triterobenzene, 2-triterotoluene, 4-tritero-
toluene, 2-triterobromobenzene and α− triteronaphthalene, Melander found that the
rate of nitration of these compounds did not differ from the rate of nitration of
hydrocarbons which did not contain heavier hydrogen isotopes.
Melander nitrated naphthalene to α− dinitronaphthalene and other compounds
to dinitro derivatives. When nitrating ordinary aromatic compounds (not contain-
ing heavier hydrogen isotopes) with a nitrating mixture containing water plus a