A Guidebook to Mechanism in Organic Chemistry

Halogenation

After fission of the bromine-bromine bond in forming the a complex
with benzene, the anionic complex, ®Br-FeBr 3 , so obtained then re­
moves the proton to yield bromobenzene.
Halogenation may also be carried but with the aqueous hypohalous
acid, HO-Hal, provided that a strong acid is also present. Here the
evidence is very strong that in, for example, chlorination, the chlor­
inating agent is actually CI®, produced as follows:

HO—Q HO—CI > HjO + Cl®
fast J-J slow
The further attack on benzene is then exactly*analogous to Hfffa-
tion by ®N0 2. A further similarity between the two is provided by
the fact that HOC1 alone has, like HN0 3 , very little action on ben­
zene; the presence of a further entity, i.e. strong acid, is necessary in
either case to release the highly electrophilic species, CI® or ®N0 2
by protonation of their 'carrier molecules':

HO^CI and HO^-N0 2 i

H H

Further support for the idea that a halonium ion or a positively

polarised halogen-containing complex is the effective substituting

agent is provided by a study of the reactions of interhalogen com­

pounds with aromatic substances. Thus Br-CI leads only to bromin-

ation and I-CI only to iodination, i.e. it is the./ess electronegative

halogen that is introduced, due to:

«+ «-

Br-»-CI, etc.

H Br

V-'