NITRATION AGENTS AND METHODS MORE RARELY USED 121studied by Shills, Rybakov and Pal [168], Shilov and Stepanova [169] and
Halfpenny and P. L. Robinson [170].
The experiments so far performed have sometimes been contradictory, but they
have led to the conclusion that pernitrous acid does exist.
Nitration with pernitrous acid. The first investigations into the nitration
of benzene with per-nitrous acid were performed by Trifonov [166]. In 1922
he found o- nitrophenol to be the reaction product and suggested the appli-
cation of the reaction for detecting benzene and other aromatic hydrocarbons.
His results were confirmed by the experiments of Halfpenny and P. L. Ro-
binson [170].
It has been shown that aromatic hydrocarbons can be nitrated by pernitrous
acid even at very low acid concentrations (e.g. 2%) at room temperature. The
hydroxylation of the hydrocarbon takes place simultaneously in many cases. A charac-
teristic feature of the reaction is that the nitro group mostly takes the meta posi-
tion with respect to the substituent already present. If a nitro and a hydroxyl group
are introduced simultaneously the hydroxyl group nearly always takes the ortho
or para position with respect to the substituent already present. Diphenyl deriv-
atives are also formed.
Toluene also yields nitro derivatives of o- and p- cresol besides nitrotoluenes
and benzaldehyde :Nitrobenzene gives m- dinitrobenzene along with small quantities of the ortho-
and para- isomers, a mixture of o-, m- and p- nitrophenols and a small quantity
of 3,3’-dinitrodiphenyl.
Chlorobenzene is converted mainly into m- chloronitrobenzene and o- chloro-
phenol and 2-nitro-4-chlorophenol are also formed.
The yield of nitro compounds is, however, not high and does not generallyexceed 10%. The method is therefore not of practical importance as yet, although
it is interesting from the theoretical point of view. A high dilution of nitrous acid
during nitration provides evidence that the nitronium ion cannot be the nitrating
agent in the reaction. Halfpenny and Robinson assume the nitration does not takeplace through nitrosation and oxidation as in the case of nitrophenols (p. 117),
and they advance the hypothesis that the nitrating action of pernitrous acid con-
sists in the formation of a free aromatic radical, which reacts with nitrogen dioxide,
split off from the pernitrous acid:
HOONO -> HO* + *NO 2 (74)