A Guidebook to Mechanism in Organic Chemistry

Electrophilic and Nucleophilic Substitution in Aromatic Systems

In the absence of a catalyst and in the presence of light, chlorine will

add on to benzene; this proceeds by a radical mechanism and will be

discussed subsequently (p. 244).

SULPHONATION

The intimate details of sulphonation are less well known than those

of nitration and there has been a good deal of debate about whether

the effective electrophilic agent is the bisulphonium ion, ®S0 3 H, or

free SOa. The weight of evidence for sulphonation under normal

conditions, however, resides with the latter produced in "the following

way:

2H,S0 4 ^ SOs+H 3 0® + HS0 4 e.

The sulphur atom of the trioxide is highly electron-deficient

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and it is this atom^herefore, that becomes bonded to a ring carbon

atfll* Two features^f sulphonation that distinguish it from nitration

are that it is reversible and that it is slowed down when the hydrogen

atoms of an aromatic nucleus are replaced by the heavier, radioactive

isotope tritium,^8 H. The latter observation indicates, of course, that

the removal of proton from the a complex of benzene and sulphur

trioxide (IX) must, by contrast to nitration, be the rate-determining

step of the reaction, the formation of (IX) being fast and non rate-

determining:

(IX)

Practical use is made of the reversibility of the reaction in the
replacement of S0 3 H by H on treating sulphonic acids with steam.
Partly because of low miscibility, the reaction of hot concentrated
sulphuric acid with benzene is slow and fuming sulphuric acid in the