Photochemistry of chlorobenzene in ice

Article abstract of DOI:10.1016/S0040-4039(00)01336-8

An unusual photobehaviour of chlorobenzene in ice is reported. Its photolysis at >254 nm provided very different photoproducts from those observed in liquid water where phenol derivatives are almost exclusively isolated. Thus, biphenyl and terphenyl as well as their chlorinated isomers were formed in ice, possibly via a free-radical mechanism, thanks to aggregation of the starting molecules even in very dilute solid solutions. In addition, an unusual multi-step triphenylene photoproduction is reported. Environmental aspects are also discussed. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01336-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 7785–7789
†
Photochemistry of chlorobenzene in ice
a,
b
b
b
Petr Kl a´ n, * Alena Ansorgov a´ , David Del Favero and Ivan Holoubek
a
Department of Organic Chemistry, Faculty of Science, Masaryk University, Kotlarska 2,
6
11 37 Brno, Czech Republic
b
Department of Environmental Chemistry and Ecotoxicology, Faculty of Science, Masaryk University,
Veslarska 230b, 637 00 Brno, Czech Republic
Received 5 August 2000; accepted 7 August 2000
Abstract
An unusual photobehaviour of chlorobenzene in ice is reported. Its photolysis at >254 nm provided
very different photoproducts from those observed in liquid water where phenol derivatives are almost
exclusively isolated. Thus, biphenyl and terphenyl as well as their chlorinated isomers were formed in ice,
possibly via a free-radical mechanism, thanks to aggregation of the starting molecules even in very dilute
solid solutions. In addition, an unusual multi-step triphenylene photoproduction is reported. Environmen-
tal aspects are also discussed. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: chlorobenzene; photochemistry; ice; photodegradation; effective reaction cavity; environmental; aggrega-
tion.
Photochemically induced homolysis of the carbonꢀhalogen bond in haloarenes has been
1
known since the 1960s. Among the aryl halides, chlorobenzene is the best-studied chloroaro-
matic compound. Photoreductive dechlorination and aryl free-radical coupling reactions were
2
3
4
observed in all H-donating non-polar solvents, in the gas phase, and in the solid state. On the
5
other hand, photosubstitution is the major pathway in methanol or water/acetonitrile mix-
6
−1
7
tures. Thanks to its low solubility (ꢀ510 mg l at 25°C ), comparatively few reports dealt with
8
studies in water, usually with the emphasis on its environmental fate and photodegradation.
Park and his coworkers have analyzed all the photoproducts from photolysis of chlorobenzene
8
b
in water. In addition to phenol and chloride ion, small amounts of all three phenylphenol
isomers were detected (several orders of magnitude lower yield than that of phenol).
Here we present an original investigation of the photobehaviour of chlorobenzene in the solid
matrix of ice. Such a medium presents surroundings which might prevent drastic conforma-
9
tional, translational, and rotational changes along the reaction coordinate. This research is a
10
part of our programme on photochemistry in solid media.
*
Corresponding author. E-mail: klan@sci.muni.cz
Dedicated to Dr. Alfred Bader for his ‘Chemist Helping Chemists’ activities.
†
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01336-8

7786
1
1
Photolysis of chlorobenzene in ice (Scheme 1) was found to be entirely different from that
8
observed in liquid water where phenol derivatives—products of the radical nucleophilic
substitution reaction with the solvent—were almost exclusively isolated. Table 1 lists 22
photoproducts detected after the irradiation of chlorobenzene samples at various concentrations.
Scheme 1.
Table 1
Ice photochemistry of chlorobenzene at −20°C
11
−1 a,b
−1 a,c
−1 a,d
Photoproduct
Retention time
c=157 mg l
c=27 mg l
c=4 mg l
Benzene
Phenol
3.919
–
–
6.054
6.349
1.04
ND
ND
1.64
0.14
1.59
2.22
0.08
0.12
0.12
0.76
0.12
0.50
ND
ND
0.26
0.01
0.11
0.18
0.01
0.02
0.02
0.10
0.01
0.10
ND
ND
0.14
ND
B0.01
–
B0.01
B0.01
B0.01
ND
o-, m-, p-Phenylphenol
Biphenyl
o-Chlorobiphenyl
m-Chlorobiphenyl
p-Chlorobiphenyl
o-Terphenyl
m-Terphenyl
p-Terphenyl
Chloroterphenyl
f
6.464
7.060
17.097
18.525
19.456
17.320–23.379
23.525
e
Triphenylene
ND
a
Irradiation of chlorobenzene (with the initial concentration c) at \254 nm for 5 hours. The reproducibility was
−
1
9
10%. Product concentrations are in mg l . The meaning of ‘ND’ is ‘under the detection limit’. The reaction
b
c
d
e
f
conversions: ꢀ10%, ꢀ30%, and ꢀ90%, respectively. 13 different isomers found. The total amount of the
m- and p-isomers.
The results suggest that the coupled photoproducts were formed in ice thanks to intermolec-
ular reactions of the starting molecules, which, remarkably, showed no reactivity towards the
water molecules of the solvent cavity walls (neither phenol or phenylphenol were detected). Since
no diffusion of the chlorobenzene molecules in ice is anticipated, such reactions were caused by
the aggregation of molecules in the frozen solvent before and/or during the process of ice
−
1
solidification. In experiments with a lower chlorobenzene concentration (ꢀ4 mg l ), many of
the photoproducts were still detectable, which means that aggregation still played an important
role. The 10–15% loss in the mass balance observed during the experiments suggests the
formation of higher-mass compounds that were also detected by the GC analysis.
On the other hand, irradiation of an approx. 0.1% (w/w) mixture of chlorobenzene in ice for
2
4 hours produced a much more complicated photoproduct mixture, consisting of more than 50
different compounds, including dichlorobenzene, dichlorobiphenyl, and dichloroterphenyl iso-
1
0b
mers. Such observations parallel the study of the photochemistry of neat chlorobenzene in
4
that the formation of various polychlorinated biphenyls was reported.

7787
We assume that bond homolysis and reactions of the free radicals so formed take part in most
of the transformations in ice. The cleavage of the aryl carbonꢀhalogen bond affords aryl free
radicals that can either arylate a suitable aromatic reaction partner or abstract hydrogen from
a H-donor:
’
’
PhCl*“Ph +Cl
’
’
Ph +HꢀAr“PhꢀH+Ar
’
’
Ph +HꢀAr“[ArꢀPhꢀH] “ArꢀPh
The quantum efficiencies for the reactions in ice (Table 2) were estimated by photolyzing
separately chlorobenzene samples in ice and in methanol, in which the quantum yield of the
5
a
anisole production at −20°C is known. The quantum efficiencies were correlated with the
optical densities of the liquid solutions. The values in the solid solutions are expected to be
10a
smaller due to light scattering, reflection, and a different absorption by the solid material.
Table 2
a
Quantum efficiencies of the photoproduct formation at −20°C
−
1 b
−1 b
Photoproduct
c=157 mg l
c=27 mg l
−
−
−
−
−
−
−
−
3
3
3
3
3
3
3
3
−3
Biphenyl
2.01×10
0.14×10
1.60×10
2.23×10
0.07×10
0.10×10
0.10×10
0.10×10
5.59×10
−3
o-Chlorobiphenyl
m-Chlorobiphenyl
p-Chlorobiphenyl
o-Terphenyl
m-Terphenyl
p-Terphenyl
0.18×10
−3
1.94×10
−3
3.17×10
−3
0.14×10
−3
0.29×10
−3
0.29×10
−3
Triphenylene
0.14×10
a
The reaction conversion was kept below 15% to avoid the photoproduct interference. Chlorobenzene in methanol
5a
was used as an actinometer (b=0.022 for anisole formation at −20°C).
b
Initial chlorobenzene concentration c.
The quantum efficiencies of the chlorobiphenyl and terphenyl photoproduction were found to
be comparable at both chlorobenzene concentrations. However, a regular increase in biphenyl
efficiency with decreasing concentration suggests that dimerisation becomes the most probable
reaction due to the smaller number of the starting molecules in the aggregates.
The concentration independent regioselectivity of chlorobiphenyl and terphenyl formation (m-
and p-isomers are strongly preferred) indicates the presence of steric rather than electronic
effects in the aggregation. The production of triphenylene in relatively large amounts is quite
intriguing. We propose a mechanism, in which photochemically formed 2¦-chloro-[1,1%;2%,1¦]-
terphenyl cyclises to give triphenylene by either a radical or ionic process as was observed
1
2
in some other systems (Scheme 2).

7788
Scheme 2.
In conclusion, chlorobenzene does not undergo any photosubstitution reactions with water
9
molecules in ice: the walls of the ice cavity are, in terms of the effective reaction cavity, entirely
hard. Production of chlorinated biphenyls and terphenyls is, moreover, an environmentally
interesting topic. Such reactions can be a secondary source of pollution in the polar regions as
well as in tropospheric ice cloud particles.
Acknowledgements
We thank Alfred and Isabel Bader for funding through the Foundation for a Civil Society.
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−
3
−5
1. Aqueous chlorobenzene solutions (1.4×10 to 4×10 M) were prepared by centrifugation of the saturated
−
3
solution (ꢀ3×10 M) for 5 min at 4°C, followed by dilution. Oxygen was removed from the samples either by
bubbling the solutions with argon for 5 min or by sonication. The samples were sealed in quartz tubes (>254 nm
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0
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