144 Synthetic Routes to AromaticC-Nitro Compounds
nitration of methoxybenzoic acid (anisic acid).^132 Picric acid has been prepared in yields of
95 % by treating 4-hydroxybenzoic acid with 8 M nitric acid.^133 Such examples illustrate the
vulnerability of ring functionality during polynitration.
As a general rule, the lower the temperature of a nitration, the fewer the oxidation by-
products that are formed.
4.4 Nitrosation–oxidation
Phenolic groups make aromatic nitration a very facile process, so much so that the selective
introduction of one or even two nitro groups into the aromatic nucleus can be a problem. The
ease with which phenolic substrates undergo oxidative side-reactions can also pose a problem.
Taking phenol as an example, the action of commercial 70 % nitric acid on this substrate readily
introduces three nitro groups into the aromatic ring, although the process is accompanied
by much oxidative degradation. The problem is more evident with polyhydric phenols like
resorcinol, where the introduction of three nitro groups into the aromatic ring is very facile.
Nitrosation of phenolic substrates usually uses nitrous acid preparedin situfrom a di-
lute mineral acid and an alkali metal nitrite.^134 In general, for every phenolic group present
in a substrate an equal number of nitroso groups can be introduced into the aromatic ring;
phenol, resorcinol and phloroglucinol react with nitrous acid to form 4-nitrosophenol, 2,4-
dinitrosoresorcinol and 2,4,6-trinitrosophloroglucinol respectively.
OH OHNOOHNO 2OH OHNO NO 2OH242322H 2 SO 4 , NaNO 296–100 %dil. HNO 3 ,PhCH 3 , -5 °CFigure 4.14Resorcinol (22) is readily converted to 2,4-dinitrosoresorcinol (23) in quantitative yield
by slowly adding an aqueous solution containing two mole equivalents of sodium nitrite to a
solution of resorcinol that has been acidified with two mole equivalents of sulfuric acid.26a,b
The 2,4-dinitrosresorcinol (23) from this process can be oxidized to 2,4-dinitroresorcinol (24)
with a binary phase system of toluene and dilute nitric acid at− 5 ◦C.26cTreatment of either 2,4-
dinitrosoresorcinol or 2,4-dinitroresorcinol with concentrated nitric acid provides a convenient
route to styphnic acid (2,4,6-trinitroresorcinol).^27
Both 2,4-dinitrosoresorcinol (23) and 2,4-dinitroresorcinol (24) are important in the explo-
sives industry. The lead salt of 2,4-dinitrosoresorcinol has a low ignition temperature and finds
use in priming compositions and in electrical igniters. The lead salt of 2,4-dinitroresorcinol is
a weak initiator but is found to exhibit high sensitivity to friction and stab action without being
highly sensitive to impact, and as such, this compound has found use in primers.
2,4,6-Trinitrophloroglucinol has been synthesized via the nitrosation of phloroglucinol fol-
lowed by nitric acid oxidation.^30 ,^32 The direct nitration of phloroglucinol must be conducted at
subambient temperature to avoid excessive oxidation, and even then, the yield rarely exceeds
70 %.^28 –^30
Some phenolic substrates are readily nitrated with nitric acid of 5 % concentration or
lower.^134 Such reactions are catalyzed by nitrous acid either already present in the nitric acid
or from initial oxidation of the phenolic substrate. Reaction of the substrate with nitrous acid