68 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES
The rule was checked by Tronov and Ber [15] when nitrating aromatic compounds
with nitric acid in nitrobenzene. They found there was no simple correlation be-
tween the influence of a substituent on the reaction rate and its orienting activity.
The rule is true only for the groups with strong orienting activity (e.g. NO 2 , CN,
CH 2 CN, C 6 H 5 CO).
Ingold [65] gave the following order, representing activating properties of sub-
stituents in the nitration of an aromatic ring:CH 3 >H>F,I>Cl,Br>COOC 2 H 5 >SO 3 H>NO 2The introduction of several groups intensifies the action of the substituents.
Thus Tronov and Ber have found that o-, m-, and p- xylene are nitrated 1.6-1.9,
4.5-4.9 and 5.7-10.5 times faster respectively than toluene. According to Ingold,p- dichlorobenzene is nitrated more slowly than chlorobenzene.
Striking data for the deactivating properties of nitro groups have been provi-
ded by Westheimer and Kharasch [65a]. They are related to the known fact that
the nitration of nitrotoluene proceeds much faster than that of dinitrotoluene.
Tronov, Kamay and Kovalenko [66] have measured the rate of nitration of
aromatic hydrocarbons and their halogenides with a mixture of nitric and acetic
acids. The compounds examined were arranged according to increasing rate
of nitration the relative rate is given in brackets, taking 1 for benzene:C 6 H 5 Cl (0.15) < C 6 H 5 Br (0.25) < C 6 H 5 CH 2 Cl (0.4) < m- CH 3 C 6 H 4 Cl(0.5) < C 6 H 6 (1) << p- CH 3 C 6 H 4 Cl(1.15) < o- CH 3 H 4 Cl(1.2) < m- xylene (7) < mesitylene (2.5)If there are two substituents on the ring directing the NO 2 group into different
positions more isomers may of course, be formed. Holleman [55] analysed minutely
the problem as to which isomers were obtained in such cases. He established that
their position, or positions, depend on the effect of separate groups on the re-
action rate.
He arranged ortho- and para- orienting groups, according to their decreasing
substitution rates, as follows :OH>NH 2 >I>Br>CI>CH 3and did likewise for meta-orienting groups:
COOH>SO 3 H>NO 2When nitrating aromatic compounds containing more than one substituent,
the position of a new entrant group may be predicted from the number of isomers
obtained by nitrating the compound with each of the substituents taken separately.
Wibaut [67] studied this problem in detail when nitrating chlorotoluenes. He found
that all possible isomers could be formed from o- chlorotoluene (formula I). The
figures in different positions give the percentage of the corresponding isomers.
By nitrating m- chlorotoluene, three isomers were obtained (II) and by nitrating
p- chlorotoluene, two isomers (III):
il....m / -.-.-__ I- 1. -_. _I.“_,, ..“. ._. ,. -.. ,..~ ~...-. .-._--_ .-_ I