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NITRATION THEORIES 63

Recently Ingold and his co-workers [40] summarized the results of their

investigations [41-48] into the three forms of nitration: C-, O- and N- nitration.

The nitronium ion is formed in two stages, the first of which (a) - protonation

-is always fast:

HNO 3 + HNO 3 <-> H 2 NO 3 + + NO 3 - (fast) (a)

H 2 NO 3 + <-> NO 2 + + H 2 O (slow) (b)

(slow) (c)

(fast) (d)

If O- or N- substitution takes place, the reactions (a) and (b) proceed in the same

way. In O- nitration, the subsequent reactions take place according to the fol-

lowing scheme :

(slow) (c’)

(fast) (d’)

Ingold gives the following examples of N-nitration reactions:

(slow) (c”)

(fast) (d”)

In all these diagrams splitting off the proton is the last stage of the reaction.

ORIENTATION OF NITRO GROUPS

The orientation of a nitro group entering an aromatic ring depends first of

all on the substituent already present in the ring and to a lesser degree on the

composition of the nitrating acids, the nitrating conditions, etc. It is well known

that the nitro group itself directs substituents to the meta position.

The problem of the orientation of nitro groups is connected with that of ni-

tration kinetics. Nitration kinetics have already been discussed in connection with

the composition of nitrating mixtures, especially with regard to the presence
and the role of nitronium and other ions in the nitration process. Now nitration