142 Synthetic Routes to AromaticC-Nitro Compounds
synthesis of picryl chloride (80 %) and picryl fluoride (40 %) from 2,4-dinitrochlorobenzene
and 2,4-dinitrofluorobenzene respectively.^101
Nitronium salts in sulfuric, triflic or fluorosulfuric acids are extremely reactive and well
suited for the polynitration of deactivated substrates. Olah and Lin^47 ,^107 studied the nitration
ofm-dinitrobenzene to 1,3,5-trinitrobenzene with a solution of nitronium tetrafluoroborate in
fluorosulfuric acid at 150◦C. An optimum yield of 66 % was obtained after a reaction time
of 3 hours. However, the crude reaction mixture was found to contain 17 % unreactedm-
dinitrobenzene. After a reaction time of 3.8 hours the yield of 1,3,5-trinitrobenzene dropped
to 50 % but the product was free fromm-dinitrobenzene and was essentially pure.
4.3.4.6 Nitrogen oxides
Dinitrogen pentoxide has emerged as an immensely important nitrating agent. Its versatility
is unique – in aprotic solvents it is nonacidic and mild, whereas in strong acids like sulfuric
or nitric acids it is an extremely powerful nitrating agent. Dinitrogen pentoxide has the ability
to effect the nitration of reactive, acid sensitive or easily oxidized aromatic substrates, and at
the same time, moving to a different solvent allows the polynitration of deactivated substrates.
Pollution created by mixed acid nitrations and the disposal problems posed by spent acid may
mean that dinitrogen pentoxide surpasses mixed acid as the most important industrial nitrating
agent. Nitrations with dinitrogen pentoxide are discussed in Chapter 9.
A solution of dinitrogen tetroxide in sulfuric acid is also a powerful nitrating agent. In
this medium dinitrogen tetroxide is ionized to nitronium and nitrosonium ions.^7 ,^108 Titov^109
reported using a solution of dinitrogen tetroxide in oleum for the nitration of nitrotoluene to
dinitrotoluene and then to trinitrotoluene, the two separate steps proceeding in 98 % and 85 %
yields respectively.
Dinitrogen tetroxide forms a stable complex with boron trifluoride but this is a weak nitrating
agent.^110 Aromatic nitrations with other Lewis acids have been reported, including: AlCl 3 ,
FeCl 3 , TiCl 4 , BCl 3 ,PF 3 ,BF 3 , AsF 5 and SbF 5.^1 ,^7 ,^109 ,^111
Nitrogen dioxide in the presence of ozone has been used for aromatic nitrations.^112 Such
conditions are useful for the nitration of reactive and acid sensitive substrates. Lewis acids
have been used in ozone-mediated nitrations with nitrogen dioxide.^113
Aromatic substrates containing Lewis basic substituents can undergo ortho-lithiation.
Quenching these anions with dinitrogen tetroxide at low temperature is an example of nu-
cleophilic aromatic nitration.^114 Similar examples have been reported with anions generated
from Grignard reactions with arylhalides.^115
4.3.4.7 Metal nitrates in the presence of Lewis acids, Brønsted acids and acid anhydrides
Solutions of sodium or potassium nitrates in concentrated sulfuric acid can be used as a
substitute for anhydrous mixed acid.^116 2,4,6-Trinitroaniline (picramide) has been synthesized
via the addition of a solution of potassium nitrate in concentrated sulfuric acid to a solution of
eithero-orp-acetanilide in oleum.^36
Topchiev^7 first reported on the use of metal nitrate–Lewis acid mixtures for aromatic nitra-
tion. Many of these nitrations are heterogeneous due to the poor solubility of metal nitrates in
organic solvents.
Solutions of alkali metal or ammonium nitrates in trifluoroacetic anhydride are useful
nitrating agents for a range of activated to moderately deactivated substrates and particularly