62 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVESaccording to Titov were all the molecules and ions in equilibrium, viz.: NO 2 +,
NO-ONO 2 and NO+ :(26)These agents have both nitrating and nitrosating properties. Titov believed the
nitration promoting energy was that generated by the transfer of electrons from
the aromatic compound to nitrogen atoms of a nitrating or nitrosating agent
according to the general formula NOX:(27)The difference in the electron affinities, ∆ E, approximately equal to k δ, neces-
sary for the formation of the intermediate complex, depends to a great extent on
the degree of steric accessibility and coordinative unsaturation of the nitrating
and nitrosating agents.
Since according to the experimental data, the coordination number of nitro-
gen in its oxygen compounds does not exceed 3, the nitrogen atom in the nitric
acid molecule is coordinatively saturated and has only slight electrophilic reacti-
vity. This, according to Titov, makes the formation of addition products of nitric
acid with aromatic compounds difficult.
Contrary to this nitrogen compounds with the coordination number 1 or 2
(thus, NO 2 +, NO+ ions and ONONO 2 ), may exhibit their electrophilic reactivities
and combine with the corresponding nucleophilic atoms of aromatic compounds.
To make it clear that splitting off a proton has no bearing on the rate of nitration
a mechanism should be presented where the nitration reaction proceeds in, two
steps, as pointed out by Melander [38] (p. 37).
Dewar’s diagram eqn. (23) (see p. 60) already gives this idea, the binding of
the proton by the acceptor B being the second step reaction. Titov [39] suggested
recently a scheme which would clearly show the step-wise mechanism:(28)Here aromatic bonds are designated with dotted curved lines. Plain curved lines
are π− bonds, three dots signify weaker π− bonds (as in 2,3-butadiene), the dotted
arrow (->) represents shifting of π− electrons into the cation field, and the curved
arrow ( ) the displacement of electrons. These denote the most important forms
of the conjugations.
According to Ingold nitration is the simplest form of electrophilic substitution.