Nitration 139is the higher concentration of protonated carboxylic acid in the 100 % sulfuric acid. Studies
have shown that nitration rates are at a maximum when conducted in 90 % sulfuric acid for a
range of deactivated substrates.^77 The use of more concentrated sulfuric acid or oleum often
retards the nitration of deactivated substrates by leading to an increase in the proportion of
protonated substrate. The use of oleum in mixed acid also reduces substrate solubility, and
hence, makes nitration more heterogeneous in nature, further reducing reaction rate. However,
oleum is frequently needed in reactions where polynitration is required in order to mop up
water formed during the reaction and keep the concentration of nitronium ions high. Other
deactivated substrates containing carbonyl functionality i.e. carboxylic esters, aldehydes and
ketones, are also affected by high sulfuric acid concentrations, which usually results in an
increase in the proportion ofm-isomer formed.
4.3.4 Other nitrating agents
Olah^1 has given an excellent review of the reagents used for acid-catalyzed electrophilic nitra-
tion, which supplements an earlier review.^78 These reagents are composed of either Brønsted
acids or Lewis acids in the presence of a nitrating agent of general formula NO 2 X which acts as
a source of nitronium ions or other electrophilic nitrating species. Topchiev^7 gives an extensive
discussion of the practical use of such reagents in nitrating aromatic substrates. This is a brief
overview of an otherwise huge area of nitration chemistry.
4.3.4.1 Nitric acid
Nitric acid is a weak nitrating agent when used alone, although it is a strong enough acid
to protonate itself and generate the nitracidium ion (H 2 ONO 2 +). The latter is probably the
active nitrating agent when concentrated or fuming nitric acids are used in nitrations and,
consequently, only fairly activated substrates undergo nitration under these conditions. Draper
and Ridd^79 nitrated a variety of electron-rich aromatic compounds with nitric acid of different
strengths. Nitration of phenolic substrates with dilute nitric acid often involves a nitrosation–
oxidation pathway and this is discussed further in Section 4.4. Other activated substrates like
phenol ethers, anilines etc. are readily nitrated with dilute nitric acid.
Fuming nitric acid is generally a good solvent for most organic compounds but the addition
of sulfuric acid often lowers substrate solubility. The use of fuming nitric acid either neat or
as a solution in carbon tetrachloride, chloroform, nitromethane, acetic acid etc. is restricted to
reactive substrates. Fuming nitric acid in acetic acid is a mild, not very active nitrating agent, but
is useful because the oxidizing properties of nitric acid are largely suppressed in this medium.
Fuming nitric acid is a reasonable nitrating agent but its activity rapidly diminishes as water
is formed during a nitration, and so it is commonly used in excess. Fuming nitric acid at
elevated temperatures is reported to convert toluene to 2,4-dinitrotoluene.^43 The same reagent
converts mesitylene to a mixture of 2,4-di- and 2,4,6-trinitromesitylenes.^80 Concentrated nitric
acid heated to 100◦C is reported to give a mixture of isomeric 2,4- and 2,6-dinitroxylenes from
m-xylene.^80
4.3.4.2 Nitric acid in the presence of Lewis acids and Brønsted acids
Strong Brønsted acids promote the formation of nitronium ions when mixed with ni-
tric acid. Perchloric,^81 hydrofluoric,^82 phosphoric,^83 polyphosphoric,^83 trifluoroacetic,^84