828 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 828±829$
Halogenation of Aromatic Compounds by using
Sodium Perborate as an Oxidant$
Anjali P. Deshmukh, Kamlesh J. Padiya, Vidyadhar K. Jadhav and
Manikrao M. Salunkhe*
Department of Chemistry, The Institute of Science, 15 Madam Cama Road, Mumbai-400 032, India
A facile method for the halogenation of benzene and its derivatives by aqueous haloacids (HCl and HBr) using sodium
perborate as an oxidising agent in the presence of tertabutylammonium bromide as a phase transfer catalyst is described.
Halogenation is an important synthetic strategy for prep-
aration of halogenated aromatic compounds which are
of great importance as intermediates in pharmaceutical,
pesticide, agrochemical industries, etc. The classical direct
halogenation (chlorination and bromination) of aromatic
compounds suers from waste of halogen if halogen is
employed. In large scale operations this is an environmental
as well as an economical problem. A possible solution is to
recycle the byproduct, i.e. HCl or HBr. These problems can
be overcome by use of haloacid in presence of an oxidising
agent, the method is found to be very eective and avoids
the waste of halogen and recycling of HCl or HBr and
hence can be said as ecofriendly.
Halogenation of an aromatic ring by aqueous HCl or
Scheme 2
HBr using hydrogen peroxide as an oxidising agent has
already been attempted.1 However, the strong oxidising
nature of H2O2 makes the method less selective, also H2O2
is hazardous and not easily handled, therefore this method
is not convenient for large scale synthesis.
Sodium perborate is a very cheap, safe and easy to handle
and therefore the ability of sodium perborate to release
oxidative species in organic medium has made it a useful
reagent in organic synthesis.2,3 By considering all these
properties of sodium perborate it has been successfully
employed as a convenient substituent for H2O2 in various
oxidation reactions. Also sodium perborate is widely used
for selective functional group transformation,4,6 however, as
yet it has not been used for halogenation reactions.
At present continuous research is directed to modi®ed
and improved procedures for higher eciency and cleaner
selectivity considering economic and environmental pro-
blems. In light of this, the properties of sodium perborate
and the importance of halogenation reactions, we report
here, an exceptionally simple yet ecient method for the
halogenation of aromatic compounds by aqueous halo-
acids using sodium perborate as an oxidising agent in the
presence of tetrabutylammonium bromide (TBAB) as a
phase transfer catalyst (Scheme 1).
Sodium perborate oxidises haloacid to hypohalous acid
which is a slower halogenating agent compared to halogen,
as HO is a poorer leaving group from HOX than X is
from X2. The reaction is speeded up in the presence of
TBAB, as HOX is then converted into the more reactive
X2. TBAB acts as a phase transfer catalyst and as a
Lewis acid in the halogenation of aromatic compounds.
The probable mechanism is shown in Scheme 2. Halogen
molecules are known to be activated by quaternary
ammonium salts for electrophilic attack on the aromatic
ring. This may well be achieved by the formation of adducts
of general structure R4NXn (n 3 or 5) in which the
halogen molecules are partially polarized. This type of
adduct is known to function as a mild halogenating agent.7
Probably due to this, only activated aromatic rings undergo
electrophilic substitution under the reaction condition
used.
In conclusion, the present procedure is simple and con-
venient. In addition the reagent used, sodium perborate, is
inexpensive, non-toxic, easy to handle and does not involve
any euent or byproduct problem.
Experimental
The results of the synthesis of various halogenated
compounds are given in Table 1.
Activated benzene rings undergo halogenation smoothly
to yield a mixture of both ortho and para substituted
products, whereas deactivated rings do not undergo halo-
genation even after a prolonged reaction time.
GC analysis were performed on a Chromosorb W AW DMCS,
using a 10 ft 1/8 in 20% SE±30 column.
Table 1 Halogenation of aromatic compounds using sodium
perborate as oxidant
Chlorination
Bromination
Reaction
time/h
Yield
(%)
Yield
(%)
R
o-
p-
o-
p-
Me
H
OMe
OH
COMe
CO2H
NO2
6
6
6
40
Ð
Ð
22
Ð
Ð
Ð
57 74
90a
37
Ð
Ð
20
Ð
Ð
Ð
60 78
88a
Ð
100 66
78 80
Ð
100 70
80 77
Scheme 1
2
24
24
24
Ð
Ð
Ð
0
0
0
Ð
Ð
Ð
0
0
0
*To receive any correspondence.
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
aReaction proceeds up to the monohalogenated state, no
dihaloganated product was observed by GC analysis.
Deshmukh, Anjali P.
