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chlorous acid [4, 5]. The acid also chlorinates active aromatic compounds such as
anisole and polycyclic aromatics [6, 7]. In addition to the chloro derivatives,
quinones have also been isolated. All oxidation and chlorination reactions so far
have been carried out in aqueous solution.
The mechanism and the stoichiometry of the chlorination have not been ®rmly
established, although some reports suggested that chlorine dioxide, resulting from
the decomposition of chlorous acid, is the chlorinating species [8]. However, the
chlorinating properties and the mode of decomposition of chlorous acid in non-
aqueous solutions have not yet been examined.
Results and Discussion
A standard reaction was set up in which a solution of the aromatic substrate and
trichloroacetic acid in dichloromethane was treated gradually with solid sodium
chlorite. Trichloroacetic acid was preferred to acetic acid due to the low reactivity
of the latter with sodium chlorite. The reaction proceeds according to NaOClO
Cl3CCO2H!HOClOCl3CCO2Na.
Mesitylene was the substrate of the ®rst choice because of its enhanced
reactivity, its resistance to oxidation, and its ability to afford only one monochloro
derivative. Thus, treatment of mesitylene with chlorous acid for one hour at room
temperature yielded 2-chloromesitylene nearly quantitatively. Other reactive
aromatic compounds such as anisole and acetanilide gave a 1:2 mixture of 2-
and 4-chloro derivatives. Dichlorination of some polyalkylbenzenes and anisole
was also achieved using the required stoichiometric amount of chlorous acid. The
results of aromatic chlorination are summerized in Table 1. The identity of the
products was proven by their melting or boiling points and 1H NMR spectroscopy.
The overall stoichiometry of the chlorinating reaction may be expressed by
ArH3HOClO!ArCl2ClO22H2O. Less reactive aromatic compounds, e.g.
toluene, required longer reaction times. Treatment of toluene with chlorous acid for
12 hours gave benzyl chloride (65%) and a mixture of 2- and 4-chlorotoluenes
(35%). When the molar ratio of trichloroacetic acid was doubled and the reaction
time reduced to 6 hours, benzyl chloride was isolated only in 20% yield together
with 80% of o- and p-chloro derivatives.
The tendency of chlorous acid to chlorinate the aromatic ring and the methyl group of toluene
indicates that the reaction may proceed via electrophilic substitution and free radical mechanisms. A
possible mechanism for the ring chlorination and the methyl radical reaction may involve
disproportionation of chlorous acid to chlorine dioxide and hypochlorous acid according to
3HOClO!HOCl2ClO2H2O. Hypochlorous acid is a well-known chlorinating agent for aromatic
compounds [9]. The radical chlorination mechanism of the methyl group in toluene to give benzyl
choloride may proceed via homolytic bond ®ssion of hypochlorous acid by the paramagnetic
ClO2
chlorine dioxide to give a chlorine radical according to HOCl ! HOÁ ClÁ. Homolytic C±O bond
®ssion of hypochlorous acid may also be achieved photochemically [10].
The fact that the free radical chlorination of the methyl group is predominant and restricted to
toluene when one equivalent of trichloroacetic acid is used (see Table 1) may be attributed to the low
reactivity of hypochlorous acid with the aromatic ring [11]. Consequently, the reaction of
hypochlorous acid with the weakly-activated toluene ring is rather slow compared with the more
reactive polyalkylbenzenes, e.g. xylene and mesitylene. This allows chlorine dioxide to react with