CHAPTER XNITRO DERIVATIVES OF HIGHER BENZENE
HOMOLOGUES
HIGHER benzene homologues, which have more or longer side chains than toluene,
undergo oxidation more readily during the nitration process than toluene or ben-
zene. They should therefore be nitrated with great care; the higher homologues
in particular should be nitrated at a temperature as low as possible. As a rule the
range of permissible concentrations of nitrating acids is narrower than for nitration
of benzene or toluene. An excess of nitric acid should also be avoided. For example
Kobe and Langworthy [1] gave the following data on the nitration of p- cymene
to the mononitro derivative. An 89% yield can be obtained by nitration at 10°C,
with a mole ratio of nitric acid to p- cymene of 0.9 and a weight ratio of sulphuric
acid to p- cymene of 4.0, initially using a sulphuric acid concentration of 85 weight %
and very vigorous stirring.
NITRO DERIVATIVES OF XYLENES
m- Xylene is the most important of the three xylene isomers (ortho-, meta- and
para-) as a starting material for the preparation of nitro derivatives, as it enables
three nitro groups to be introduced into the 2-, 4, and 6- positions. Ortho- and
para- xylenes can give a stable system when only two nitro groups are introduced,
for when three groups are introduced one of them is compelled to occupy an inap-
propriate position and may easily undergo substitution reactions.
Xylene for nitration should therefore be as rich in the m- isomer as possible.
The requirements for the nitration of xylene will be discussed on pp. 406-412.
The presence of two methyl groups on the benzene ring greatly facilitates the
introduction of nitro groups. In consequence less concentrated mixtures may be
used for the preparation of trinitro-m-xylene (TMX) than for TNT.
On the other hand, the methyl groups are easily oxidized so the nitration should
be married out under conditions less favourable for oxidation, for example at a tem-
perature as low as possible. Low concentration of the nitrating mixture also facili-
tates oxidation reaction of xylene. Gorst [2] reported that the rate of oxidation
is reduced with increase in the factor Φ or concentration of H 2 SO 4 in nitrating13951