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J . Org. Chem. 1997, 62, 236-237
running dark controls at 80 °C and at room temperature
for a period of several days. In all of these experiments,
o-xylene (1) was used as organic starting material being
brominated. In water, photobromination of o-xylene (1)
afforded predominantly side-chain bromination products,
whereas in the dark, exclusively ring bromination oc-
curred (Table 1).
F r ee-Ra d ica l Br om in a tion of Selected
Or ga n ic Com p ou n d s in Wa ter
Henry Shaw,* Howard D. Perlmutter, and Chen Gu
Department of Chemical Engineering, Chemistry and
Environmental Science, New J ersey Institute of Technology,
Newark, New J ersey 07102
In water, the photobromination reactions of toluene (2),
p-xylene (3), m-xylene (4), diphenylmethane (5), and
triphenylmethane (6) produced the corresponding side-
chain bromination products in high yields (see Table 2).
In all of these alkylaromatics, the corresponding ring
bromination products were produced in trace amounts.
Also, all of these photobrominations evinced an interest-
ing partitioning of organics, the unbrominated reactant
residing on top of the middle layer that containined the
unreacted aqueous HBr and the brominated products
forming a lower, heavier than water layer.
On the other hand, in water, the bromination of
mesitylene (7) and durene (8) either in the presence or
absence of light, produced solely ring bromination prod-
ucts (see Table 3). In CCl4, bromination of mesitylene
(7), a highly nucleophilic aromatic compound that pro-
duces 2-bromomesitylene, could be done either in the
dark or with light.9
The photobromination of cyclohexane (9) in water (see
Table 4) was complete in a few minutes, giving monobro-
mocyclohexane and vicinal dibromocyclohexane, with the
same three-phase partitioning effect that was observed
with the alkylaromatics. No reaction occurred in the
dark.
The bromination of cyclohexene (10) in water either
in the presence or absence of light produced trans-1,2-
dibromocyclohexane as the only product; no free-radical,
allylic bromination occurred (see Table 5).
Susan D. Arco and Titos O. Quibuyen
Institute of Chemistry, University of the Philippines,
Diliman, Quezon City 1101, Philippines
Received February 27, 1995 (Revised Manuscript Received
November 12, 1996 )
Environmental consideration prompts an urgent need
to redesign commercially important chemical processes
and products or to invent new ones that make use of
chemicals that are less toxic and more environmentally
benign.1 Water could also be used as a substitute solvent
for free-radical bromination. Like supercritical carbon
dioxide,2 it is nontoxic and environmentally benign.
Water is an excellent medium for free-radical reactions
because its OH bond is remarkably strong,3 so hydrogen
abstraction from the solvent is unlikely. It does not
possess reactive multiple bonds to which radicals might
add. Indeed, in the field of pulse radiolysis, reactions
are normally carried out in water.4-7 The potential
problems with water as the medium for radical reactions
relate to the limited solubility of most organic materials
and its high polarity, which could influence the course
of the bromination. In this work, we report on the free-
radical bromination of some simple hydrocarbons in
water. Also, for two representative compounds we ran
the bromination neat, i.e., without any solvent, to ascer-
tain the role of water in the reaction.
The following organic compounds, o-xylene (1), toluene
(2), p-xylene (3), m-xylene (4), diphenylmethane (5),
triphenylmethane (6), mesitylene (7), durene (8), cyclo-
hexane (9), and cyclohexene (10), have been used as the
organic starting materials in the free-radical bromination
reactions in water. Since Br2 shows significant absorp-
tion of light in the visible region, λmax ≈ 415 nm,8 the
reaction was initiated photolytically using an ordinary
incandescent light bulb.
To ascertain the role water plays in this free radical
system, we ran the photo bromination of toluene (2) and
cyclohexane (9) neat, i.e., without any water. As Tables
6 and 7 show, product yields were comparable to reac-
tions run in the presence of water.
It thus seems likely that (a) in the presence of water,
the reaction takes place in the organic phase and (b)
water exerts no significant effect on the bromination
itself.10 Nevertheless, water serves a number of impor-
tant functions: (1) The coproduct HBr is efficiently
scavenged, while in the neat reaction, HBr escapes un-
trapped. (2) By providing a diluting medium, free radi-
cals are uniformly distributed over the reaction volume.
Thus, recombinations are minimized near the window
where the radiation is entering the reactor. (3) The dilu-
ting medium also promotes uniform reaction tempera-
ture, which helps obtain better product distribution. (4)
An interesting and potentially more important feature
of water-based reactions is the property of water to
partition the heavier-than-water bromination reaction
product from the lighter-than-water starting materials.
All free-radical brominations gave reaction mixtures
consisting of three layers: a lower organic layer analyzed
for reaction product(s), a middle aqueous layer titrated
for the stoichiometric amount of HBr, and an upper
organic layer that was analyzed for starting material.
This partitioning phenomena, which can be generalized
to most brominations, will dramatically simplify reaction
workup and product isolation procedures, compared to
reactions run neat or in CCl4.
The absorbed light intensity (photon flux) from the
incandescent light bulb was also measured with the aid
of potassium ferric oxalate solutions in a method devel-
oped by Hatchard and Parker,8 and the average value
was calculated to be 7.31 × 1015 quanta/s of absorbed
radiation. It was observed that due to the proximity of
the light bulb (∼5 cm distance) to the flask, the reaction
temperature rose to ∼80 °C. The free-radical bromina-
tion was shown not to be brought about thermally by
(1) Illmann, D. Chem. Eng. News 1994, 72, 22.
(2) Tanko, J .; Blackert, J . Science 1994, 263, 203.
(3) Morrison, R.; Boyd, R. Organic Chemistry, 5th ed.; Allyn and
Bacon, Inc.: Boston, 1987; p 22.
(4) Lind, J .; J onsson, M.; Erikson, T. J . Phys. Chem. 1993, 97, 1610.
(5) Gabor, M.; Lind, J . J . Am. Chem. Soc. 1994, 116, 7872.
(6) Zhang, X.; Zhang, N.; Schuchmann, H. J . Phys. Chem. 1994, 98,
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(7) Domae, M.; Katsumara, Y.; J iang, P. J . Phys. Chem. 1994, 98,
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(8) Calvert, J .; Pitts, J . Photochemistry; J ohn Wiley: New York,
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(9) Smith, L. In Organic Synthesis; Blatt, A., Ed.; J ohn Wiley: New
York, 1943; Collect. Vol. II, p 94.
(10) We acknowledge with gratitude one of the reviewers for incisive
comments and suggestions regarding the function of water.
Our results show that aliphatic and alkyl-substituted
aromatic substrates that are not highly nucleophilic with
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