The Chemistry of Powder and Explosives - Sciencemadness Dot Org

128 AROMATIC NITRO COMPOUNDS
of temperature on sulfonations appears to be exactly the opposite.
2. EASE OF SUBSTITUTION. Ortho-para orienting groups promote
substitution; meta orienting groups hinder it and make it more
difficult. The rule may be stated otherwise: that substitution
under the influence of ortho-para orienting groups occurs under
less vigorous conditions of temperature, concentration of reagents,
etc., than it does with the unsubstituted aromatic hydrocarbon
itself; under the influence of meta orienting groups more vigorous
conditions than with the unsubstituted hydrocarbon are necessary
for its successful accomplishment. The rule may also be stated
that ortho-para substitution is easier than meta. In this last form
it fails to make comparison with substitution in the simple hydrocarbon,
but does point clearly to the implication, or corollary,
that the orienting effect of an ortho-para orienting group dominates
over that of one which orients meta. To the rule in any of
these forms, we must add that, when more than one group is
already present on the nucleus, the effect of the groups is additive.
Toluene nitrates more easily than benzene; aniline and phenol
more easily still. Higher temperature and stronger acid are
needed for the introduction of a second nitro group into benzene
than for the introduction of the first, for the second is introduced
under the influence of the me fa-orienting first nitro group which
tends to make further substitution more difficult. The inhibitory
effect of two nitro groups is so great that the nitration of dinitrobenzene
to the trinitro compound is extremely difficult. It is more
difficult to nitrate benzoic acid than to nitrate nitrobenzene. The
common experience of organic chemists indicates that the order
of the groups in promoting substitution is about as follows:
—OH> —NH2> —CH3> —Cl> —H> —NO2> —SO2(OH)> —COOH
Any one of these groups makes substitution easier than the
groups which are printed to the right of it.
Xylene nitrates more easily than toluene. Two methyl groups
promote substitution more than one methyl group does, and this
appears to be true whether or not the methyl groups agree among
themselves in respect to the positions which they activate. Although
a nitro group may be said to "activate" a particular position,
inasmuch as it points to that position as the one in which
substitution will next occur, it nevertheless makes substitution
more difficult in that position, as well as in all other positions on