The activating effect of
the phenylazo substituent in electrophilic substitution has been examined. The
rates and partial rate factors for chlorination of azobenzene with molecular
chlorine and protonated chlorine acetate have been determined relative to benzene.
Whereas the chlorine acetate reaction proceeds readily (relative rate 4900)
there is virtually no activation to chlorination by molecular chlorine.
Complexes between azobenzene and bromine were, however, isolated and chatacterized.
Their formation implies that during molecular halogenation reactions the
electrophile is essentially unavailable. The relative chlorination rates for
azobenzene and azoxybenzene have also been established: the phenylazo group is
more activating towards protonated chlorine acetate whereas azoxybenzene (which
does not complex with halogens) is the more reactive with molecular chlorine.
The chlorination results confirm the versatility of the phenylazo group which
is the first substituent for which kinetic data have been obtained quantifying
activation of aromatic electrophilic, radical and nucleophilic substitution.
