140 Synthetic Routes to AromaticC-Nitro Compounds
fluorosulfuric,^85 ,^86 trifluoromethanesulfonic^86 (triflic) and methanesulfonic acids^87 have all
been used in conjunction with nitric acid for aromatic nitration. Perchloric acid must be con-
sidered dangerous in the presence of organic compounds and finds little use. Trifluoroacetic
acid can protonate nitric acid and form nitronium ions but has also found little use. Nitrations
with phosphoric and polyphosphoric acids are frequently heterogeneous in nature and can give
abnormal results. Nitric acid in the presence of solid supported acid catalysts^88 like Nafion-H,
a perfluorinated resin containing sulfonic acid groups, has found some favour. Such resins are
readily recycled and spent acid disposal is not an issue.
Nitric acid in the presence of ‘super acids’ like fluorosulfuric acid and triflic acid is particu-
larly suitable for the polynitration of deactivated aromatic substrates. In such media nitric acid
is completely ionized to nitronium ions, anhydrous nitric acid–triflic acid mixtures forming
nitronium triflate and hydroxonium triflate.^86 In the presence of excess super acid these pow-
erful nitrating agents may contain the protonitronium cation (NO 2 H 2 +) as one of the active
nitrating agents.^89 Reaction rates with such reagents can be extremely high, and so methylene
chloride or other halogenated hydrocarbons can be used as co-solvents, which also aids sub-
strate solubility.^86 Temperatures as low as− 110 ◦C are used for the mono-nitration of reactive
substrates. Anhydrous nitric acid–triflic acid is more powerful than mixed acid and can be
used in cases where substrate sulfonation or oxidation may be a problem. Fluorosulfuric acid
can lead to side-reactions resulting from oxidation and sulfonation. Olah and co-workers^85
reported the use of this reagent at elevated temperature for the conversion of benzene to 1,3,5-
trinitrobenzene. Nitric acid–superacid mixtures have been used for some difficult nitrolysis
reactions (Section 5.6.1.1 and Section 6.5).^90
Numerous Lewis acids promote the formation of nitronium ions when in the presence
of nitric acid.^7 Nitric acid–boron trifluoride,^91 and the nitric acid–hydrogen fluoride–boron
trifluoride reagents described by Olah^92 are practical nitrating agents; the latter provides a
convenient preparation of nitronium tetrafluoroborate. Olah^1 reports that nitric acid–magic
acid (FSO 3 H-SbF 5 ) is extremely effective for the polynitration of aromatic substrates.
4.3.4.3 Nitric acid–mercuric nitrate
Nitric acid in the presence of catalytic amounts of mercury (II) nitrate reacts with some
substrates, under certain conditions, to give substituted polynitrophenols. The first example,
reported by Boeters and Wolffenstein^93 and known as ‘oxynitration’, involved treating benzene
with 50–55 % nitric acid in the presence of mercury (II) nitrate. The product is a mixture of
unreacted benzene, nitrobenzene,m-dinitrobenzene, 2,4-dinitrophenol and picric acid, from
which the latter can be isolated by steam distillation of this crude mixture followed by recrys-
tallization of the residue from hot water.
The mechanism of ‘oxynitration’ is generally accepted to involve the formation of phenyl
mercuric nitrate which reacts with nitrogen oxides in the nitric acid to form a nitroso compound
and then a diazonium salt; the latter forms a phenol under the aqueous conditions which is then
further nitrated.^94 The use of more concentrated nitric acid favours a process of mercuration–
nitration and suppresses the formation of phenols.^95 ,^96
Other examples of ‘oxynitration’ include the formation of: (1) 2,4,6-trinitro-m-cresol
from toluene,^96 ,^97 (2) 2,4-dinitronaphth-1-ol from naphthalene,^96 ,^97 (3) 3-chloro-2,4,6-
trinitrophenol from chlorobenzene^96 ,^97 and (4) 3-hydroxy-2,4,6-trinitrobenzoic acid from ben-
zoic acid.^93