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474 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

However, after the useful properties of TNT as a material for filling shells had

been established (early twentieth century), the importance of picric acid decreased

somewhat. Nevertheless it was still widely used mainly because of its very simple

method of preparation. In the course of time, however, as themethods of manufacture

of TNT continuously improved and new sources of toluene (for example petrochemi-

cal synthetic toluene) became available, the importance of the latter went on in-

creasing. The acidic character of picric acid and its corrosive action on metals result-

ing in the formation of salts sensitive to impact and friction led to a decline in the im-

portance of picric acid, and there is a trend to eliminate it from armaments altogether.

Because of its relatively high sensitiveness to impact, picric acid alone was unsuitable

For filling certain special purpose ammunition, for example For armour piercing

shells (p. 322).

MONONITRO DERIVATIVES OF PHENOL

Three isomeric mononitrophenols are known: ortho (m. p. 44-45°C; b.p.

214-216°C with decomposition); meta (m. p. 96°C) and para (m. p. 114°C):

The o- and p- isomers are always formed when phenol is nitrated with dilute
(e.g. 30%) nitric acid, the proportion of the o- isomer increasing with nitration
temperature.
Separation of the two isomers may be accomplished by steam distillation: the
volatile o- nitrophenol passes over, while the p- isomer remains in the residue.
Since apart from the nitrophenols some dark coloured, often resinous by-prod-
ucts are formed, o- and p- nitrophenols are Frequently prepared on an industrial
scale From the corresponding chloronitrobenzene derivatives.
Pure p- nitrophenol may also be prepared by nitrosation of phenol, followed
by oxidation of p- nitrosophenol with dilute nitric acid. The m- isomer is usually
prepared by diazotization of m- nitroaniline.

The m- and p- isomers combine with sulphuric acid to Form molecular compounds,

melting at 83°C and 90°C respectively:

while o- nitrophenol does not yield such a compound (Kendall and Carpenter [16]).