Difference between Sonolysis and Photolysis of Bromotrichloromethane in the Presence and Absence of 1-Alkene

Full text of DOI:10.1021/jo980125a

J . Org. Chem. 1998, 63, 6719-6720
6719
Sch em e 1
Differ en ce betw een Son olysis a n d
P h otolysis of Br om otr ich lor om eth a n e in
th e P r esen ce a n d Absen ce of 1-Alk en e
Takahide Kimura,*,† Mitsue Fujita,
Hajime Sohmiya, and Takashi Ando
Department of Chemistry, Shiga University of Medical
Science, Seta, Otsu, Shiga 520-2192, J apan
Received J anuary 23, 1998
In tr od u ction
Reactions characteristic of ultrasonic irradiation have
been named sonochemistry and have become topical in
recent years. It has already been established that
sonochemical reactions originate by cavitation, i.e., the
formation and collapse of micro bubbles, which generate
the so-called hot spot.1 In the hot spot the local temper-
ature rises up to thousands of degrees and the local
pressure rises up to hundreds of atmospheres, which
produces excited species and promotes reactions while
the bulk liquid is kept at ambient temperature. Through
this activation process, sonication induces a specific
chemical reactivity, and radical reactions are generally
regarded as preferential processes.2 The detailed mech-
anism of the sonochemical excitation, however, is still
unclear.
Ta ble 1. Com p a r ison betw een Son och em ica l a n d
P h otoch em ica l Rea ction s of Br om otr ich lor om eth a n e
w ith or w ith ou t 1-Alk en e
conditionsa
yields (mmol)
Br2
DPPH
BrCCl3
run irradn
alkene
(mmol) Cl3CCCl3 adduct adduct
1
2
3
4
5
6
7
8
)))
)))
)))
hν
)))
)))
hν
)))
0.030
0.2
0.4
0.025
0.029
0.000
0.085
0.023
0.082
0.053
1-octene
1-octene
1-octene
1-octene/
1-hexene
0.092
0.049
0.061
0.033/
0.031
0.204
0.002
2.093
0.208/
0.206
0.2
a
1-Alkenes (4 mmol) in BrCCl3 (total 10 mL). Ar, 30 min, 10
To elucidate the excitation mechanism in sonochem-
istry, we performed a comparison between the sonolysis
and photolysis of bromotrichloromethane (BrCCl3) in the
presence and absence of 1-alkenes. It is interesting to
study the difference in sonochemical and photochemical
processes of the same reaction, which must reflect the
difference in the activation mechanisms. Irradiation of
either ultrasound3 or UV light4 causes fragmentation of
BrCCl3 to a bromine atom and a trichloromethyl radical.
The radicals produced may recombine to form the start-
ing BrCCl3 or dimerize to bromine (Br2) and hexachlo-
roethane (Cl3CCCl3). In the presence of 1-alkene, the
radicals attack 1-alkene to give an adduct (RCHBrCH2-
CCl3) by the following chain reaction process, together
with a bromine adduct (RCHBrCH2Br) (Scheme 1). This
reaction does not proceed without ultrasound or light, in
the absence of an initiator such as a peroxide.5
°C.
reaction solution was about 20 W as measured by calorimetry.6
Bulk temperature during sonication was maintained at 10 °C
by circulating water. The photoreaction was performed by using
a mercury lamp (Toshiba H400-P) in a Pyrex glass vessel,
jacketed with circulating water. The light was not filtered.
Reaction temperature during the irradiation of light was main-
tained at 10 °C.
BrCCl3 was purchased from Tokyo Kasei (Japan) and 1-octene,
1-hexene, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) from Wako
Pure Chemicals (J apan). They were used without further
purification.
1-Alkene (4 mmol) in BrCCl3 (total volume of a reaction
mixture, 10 mL) was irradiated for 30 min under an Ar
atmosphere. Then the reaction mixture was analyzed by gas
chromatography.
Resu lts a n d Discu ssion
Exp er im en ta l Section
The results are summarized in Table 1. In the absence
of 1-alkene, the dimer Cl3CCCl3 was the only reaction
product other than Br2 (Table 1, run 1). Even when a
nonvolatile radical scavenger, DPPH, was added to the
reaction mixture at a concentration of either 20 or 40
mM, the yield of the Cl3CCCl3 dimer did not decrease
(Table 1, runs 2 and 3). We suggest that the fragmenta-
tion of BrCCl3 and the dimerization of trichloromethyl
radical occurs in the gas phase of the cavity and that the
radicals cannot be trapped efficiently with a scavenger
of low vapor pressure.
In the presence of 1-octene, the expected adducts
(C6H13CHBrCH2CCl3 and C6H13CHBrCH2Br) were ob-
tained by the irradiation of either ultrasound or light
(Table 1, runs 5 and 7). When DPPH was added under
sonication, the yields of the adduct (C6H13CHBrCH2CCl3)
The 20 kHz ultrasonic irradiation was performed using a Heat
System immersion horn (model XL 2020) equipped with a
titanium probe. The acoustic intensity introduced into the
† Phone & Fax: +81-77-548-2102. kimura@sums.shiga-med.ac.jp.
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S0022-3263(98)00125-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/20/1998

6720 J . Org. Chem., Vol. 63, No. 19, 1998
Notes
decreased sharply (Table 1, run 6). We believe this shows
that the radical chain reactions, proceeding in the bulk
liquid, can be efficiently quenched with DPPH. A com-
petitive reaction of 1-octene and 1-hexene proves that
these alkenes have similar reactivity and give equal
amounts of the respective adducts under ultrasonic
irradiation (Table 1, run 8). The calculated vapor pres-
sures of 1-octene, 1-hexene, and bromotrichloromethane
are 7.3, 95.3, and 19.0 mmHg at 10 °C, respectively.7 If
the addition of trichloromethyl radical to alkenes takes
place mainly in the gas phase, the difference in the vapor
pressure of 1-octene and 1-hexene should give the ad-
ducts in different yields. Thus, these results indicate that
the radical chain propagation reaction occurs in the bulk
liquid phase. Presumably, some of the radicals formed
in the gas phase, in the cavitation process, dimerize to
give Cl3CCCl3 in the gas phase, while the surviving
radicals migrate into the surrounding liquid phase to
initiate the radical chain propagation.
F igu r e 1. Schematic illustration of the difference between
sonolysis and photolysis of BrCCl3.
does produce CCl3 radical. Thus, in the photochemical
reaction we can conclude that the radical is produced in
a dispersed state and its local concentration is low. The
dimer is produced only in the chain propagation step.
The present results clearly show that UV light gener-
ates radicals in a homogeneous and dispersed state, while
cavitation generates them in a heterogeneous and local-
ized state as illustrated in Figure 1. It can thus be
concluded that photolysis and sonolysis are different in
a physical sense, even though the reaction itself can be
written in the same chemical equations.
In contrast to the sonochemical reaction, the photo-
chemical reaction in the absence of 1-alkene did not give
the dimer Cl3CCCl3 (Table 1, run 4). But when 1-octene
was added to the reaction solution, the dimer Cl3CCCl3
was detected in addition to the adducts. The fact that
the presence of alkene afforded products including Cl3-
CCCl3, implies that even in the absence of alkene, BrCCl3
(7) Reid, R. C.; Prausnitz, J . M.; Poling, B. E. The Properties of Gases
and Liquids, 4th ed.; McGraw-Hill: 1987. Kudchadker, A. P.; Kud-
chadker, S. A.; Shukla, R. P.; Patnaik, P. R. J . Phys. Chem. Ref. Data
1979, 8, 514-515.
J O980125A