The above procedure differs from that recorded in the literature,
mainly in the use of a fairly large excess of sulfuric acid.
This shortens the reaction time from forty hours to about one hour,
which is especially convenient in the preparation of the acid
on a laboratory scale. Because of the use of this large
excess of sulfuric acid, the reaction is apt to be rather
violent if the directions given are not carefully followed.
The oxidation should be carried out under a hood. Small amounts of
nitrotoluene are lost by volatilization, but this loss is not serious,
as can be seen from the yield of product obtained.
Ten or 20 g. of unchanged nitrotoluene can be recovered from
the reaction mixture by steam distillation, but the value of
the by-product would not pay for the time spent in recovery.
The washing of the crude reaction product with dilute sulfuric acid
is advisable, if good material is to be obtained. If an efficient
centrifuge is available for use at this stage of the operation,
this separate washing may prove to be less essential.
When a sparingly soluble organic acid is precipitated from
fairly concentrated solution, the precipitate is liable
to carry down with it some of the salt of the organic acid.
Addition of the salt solution to the mineral acid, with stirring,
avoids this difficulty. 3. Other Methods of Preparation
The nitration of benzoic acid produces only very small yields
of the _p_-nitro product.[1] The only practical method for the
preparation consists in the oxidation of _p_-nitrotoluene, although
for this purpose various oxidizing agents are used. In addition
to nitrotoluene, _p_-nitrobenzyl alcohol, _p_-nitrocinnamic acid
and similar compounds may be oxidized, but their cost is prohibitive
in comparison with that of the cheaper nitro hydrocarbon.
_p_-Nitrotoluene may be oxidized by means of strong nitric acid,[2]
chromic acid mixture,[3] or permanganates.[4] Electrolytic
oxidation[5] has also been proposed. The procedure given above
involves the use of chromic acid mixture, but, owing to a change
in the concentration of sulfuric acid, the time of reaction is greatly
shortened and the preparation is thus considerably improved.
[1] Ber. 8, 528, 536 (1875)
[2] Ann. 127, 137 (1863); 128, 257 (1863)
[3] Ann. 139, 335 (1866).
[4] J. Am. Chem. Soc. 41, 1575 (1919).
[5] R. P. 117, 129; Frdl. 6, 112.