A Guidebook to Mechanism in Organic Chemistry

Electrophilic and Nucleophilic Substitution in Aromatic Systems*^9

NITRATION

The aromatic substitution reaction that has received by far the closest

study is nitration and, as a result, it is the one that probably provides

the most detailed mechanistic picture. Preparative nitration is most

frequently carried out with a mixture of concentrated nitric and

sulphuric acids, the so-called' nitrating mixture'. The classical explan­

ation for the presence of the sulphuric acid is that it absorbs the

water formed in the nitration proper

OH. + HNO, -*• C.H 6 NO, + H,0

and so prevents the reverse reaction from proceeding. This explan­

ation is unsatisfactory in a number of respects, not least that nitro­

benzene, once formed, appears not to be attacked by water under the

conditions of the reaction! What is certain is that nitration is slow in

the absence of sulphuric acid, yet sulphuric acid by itself has virtually

no effect on benzene under the conditions normally employed. It

would thus appear that the sulphuric acid is acting on the nitric acid

rather than the benzene in the system. This is borne out by the fact

that solutions of nitric acid in concentrated sulphuric acid show a

four-fold molecular freezing-point depression, which has been inter­

preted as being dueTo formation of the four ions:

HNO,+2H 2 S0 4 ^ ®N0 2 +H 3 0©+2HS0 4 e

I,e" H.SOi ®* H.SO,

HO—NO, • HSO 40 +HO-^NO, > H,0®+HS0 4 ©+©N0 2

H

The presence of ®N0 2 , the nitronium ion, in this solution and also in
a number of salts has now been confirmed spectroscopically, and
some of the salts, e.g. eN0 2 ClOf, have actually been isolated. Nitric
acid itself is converted in concentrated sulphuric acid virtually entirely
into ®N0 2 , and there can be little doubt left that this is the effective
electrophile in nitration under these conditions. If the purpose of the
sulphuric acid is merely to function as a highly acid medium in which
®N0 2 can be released from HO—N0 2 , it would be expected that
other strong acids, e.g. HC10 4 , would also promote nitration. This is
indeed found to be the, case, HF and BF 3 also being effective. The
poor performance of nitric acid by itself in the nitration of benzene