502 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVESAs this scheme indicates, phenolsulphonic acid is first nitrated to 2-nitro-4-sul-
phonic and 4-nitro-2-sulphonic acids. The former yields 2,4-dinitrophenol and
2,6-dinitro-4-sulphonic acid, and the latter 2,4-dinitro-6-sulphonic acid. All these
dinitro compounds are finally converted into picric acid.
The introduction of nitro groups proceeds the more rapidly the more sulpho
groups “sulphophenol” contains. The nitration of disulphonic acid is faster
than the nitration of monosulphonic acid, but not so fast as the nitration of
trisulphonic acid.
Thus, when heating 1-hydroxy-2,4,6-trisulphonic acid with 61% nitric acid
at 110°C, picric acid is produced in a yield amounting to 90% of the theoretical (220
parts of picric acid per 100 parts of phenol), whereas from 1,2,4-phenoldisulphonic
acid under analogous conditions, the yield of picric acid is 86% of the theoretical
(208-210 parts of picric acid per 100 parts of phenol).
Moreover, it has been proved by Marqueyrol and Loriette [1] that the rate of
formation of picric acid from 2,6-dinitro-4-sulphonic acid is higher than that from
2,4-dinitro-6-sulphonic acid. Since for the formation of both p- phenolsulphonic
and 2,4-disulphonic acids high sulphonation temperatures are favourable, it is
evident that efforts should be made to attain sulphonation temperature as high
as possible. This is especially important if dilute nitric acid is to be used.
King [3] has shown that phenolsulphonic acid can be nitrated by the action
of concentrated nitric acid to yield nitrophenolsulphonic acid. On the contrary,reaction with dilute nitric acid leads to the exchange of the sulphonic group by
the nitro group:
(2)If sulphonation is carried
tration should be used or a
nitration. The latter must,
this, higher temperatures
oxidation).
out at a low temperature, nitric acid of higher concen-
higher temperature should be maintained during the
however, be kept within safety limits. Apart from
naturally favour undesirable side reactions (e.g.