The Chemistry of Powder and Explosives - Sciencemadness Dot Org

EFFECT OF GROUPS ON FURTHER SUBSTITUTION 127
on the nucleus. We arc accustomed to speak of the orienting or
directing effect of the groups already present and of their influence
in promoting or inhibiting further substitution. The two
.simple rules which summarize these effects have important implications
and wide applications in the chemistry of aromatic
substances.
Effect of Groups on Further Substitution
1. ORIENTING EFFECT. The Modified Fide of Crum Brown and
Gibson. If the atom attached to the aromatic nucleus is attached
to .some othei 1 atom by an unsaturatcd linkage (i.e., by any bond
which we commonly write as double or triple), then the next entering
group takes the meta position; otherwise it takes the ortho
and para positions.
The rule relieves us of the necessity for remembering which
groups orient meta and which ortho-para; we may write them
down on demand, thus: the —NO2, —NO, —CO—, —COOH,
—CHO, —SO2—OH, —CN groups orient meta; and the —NH.,
—NHR, — NRa> —OH, —OR, —CH3, —CH^-CH..,, —Cl, —Br
groups orient ortho-para. It is necessary, however, to take note
of three or four exceptions, only one of which is important in the
chemistry of explosives, namely, that the azo group, —N=N—,
orients ortho-para; the triehloromethyl group, —CCI3, meta;
that such conjugate systems as occur in cinnamic acid,
—CH=CH—CO—, orient ortho-para; and further that a large
excess of strong sulfuric acid reverses to a greater or less extent
the normal orienting effects of the mcthoxy and ethoxy groups,
of the amino group wholly and of the monosubstituted and disubstitutcd
amino groups in part.
In all discussions of the application of the rule we make reference
to the principal products of the reaction; substitution occurs
for the most part in accordance with the rule, or with the exceptions,
and small amounts of other materials arc usually formed
as by-products. In the mononitration of toluene, for example,
about 96 per cent of the product is a mixture of 0- and p-nitrotoluene,
and about 4 per cent is the m-compound. Under the
influence of ortho-para orienting groups, substitution occurs in
the two positions without much preference for cither one, but it
appears to be the case that, when nitro groups arc introduced,
low temperatures favor the formation of p-compounds. The effect