M. Fujita et al. / Ultrasonics Sonochemistry 27 (2015) 247–251
249
presence of KBr takes place rather than decomposition of anisole,
the OH seems to attack Br rather than an aromatic ring directly
as follows:
which was determined by UV at 417 nm (log
hexene was then added to the brown solution, the color disap-
peared and bromocyclohexanol was obtained in 41% yield based
e
= 2.32). When cyclo-
Å
ꢀ
on amount of generated Br
during the sonication.
2 2
, emphasizing the generation of Br
ÞÞÞ
Å
Å
H
2
O ! OH þ H
What is the exact role of CCl
4 2
? The production rate of I from KI
Å
OH þ Brꢀ ! OHꢀ þ Br
Å
in the presence of CCl is faster by 2 orders of magnitude than that
4
in the absence of CCl
CCl4 traps H so that H cannot recombine with OH [55] leading
thus to an increase in OH concentration:
4
[43,52–54]. Zheng et al. proposed that added
Å
Å
Å
Å
Å
Å
OH þ Br ! HBrO
Å
Å
Br þ Br ! Br
2
ÅH þ CCl
! HCl þ CCl
Å
Å
ꢀ
4
3
The second order rate constants of the reaction of OH with Cl ,
Br , and
ꢀ
ꢀ
9
10
Å
I
were reported 4.3 ꢁ 10 , 1.1 ꢁ 10
,
and
The increased concentration of OH by the presence of CCl
accelerates the oxidation of Br , and thereby bromination occurs
4
1
0
ꢀ1 ꢀ1
ꢀ
1
.1 ꢁ 10
M
s
, respectively, although depending on pH of
Å
reaction solution [49]. Then, atomic Br or combined Br
2
might
at meaningful rate. This may be the reason why CCl
4
is required.
Å
attack an aromatic ring to produce brominated compounds.
The OH radicals can also recombine into H
2
O
2
[37]. It has been
ꢀ
One more argument in this sense is the impact of the used inci-
dent ultrasonic frequency. Indeed, the rate of sono-bromination is
higher at 480 kHz than at 36.6 kHz as shown in Fig. 1.
reported that H
2
O
2
or other oxidants can oxidize Cl to liberate
molecular chlorine or HClO, which can chlorinate the active aro-
matic compounds [56,57]:
It is well known that the sonolysis of water is more effective at
ÞÞÞ
Å
Å
several hundreds kHz ultrasound than at several dozen kHz one
H O ! OH þ H
2
Å
[
40,50] meaning that OH species might be involved in the whole
mechanism. Moreover, t-butanol is known as a good hydroxyl rad-
ical scavenger [51]. Addition of 1.05 mmol of t-butanol quenched
Å
Å
OH þ OH ! H
2
O
the reaction completely (entry 6, Table 2). A production of C
and C Cl was also decreased. This result clearly indicates that
bromination takes place via radical mechanisms. Probably, OH is
2 4
Cl
H
2
O
2
þ HCl ! HClO þ H
2
O
2
6
Å
ꢀ
Å
a key radical species, although oxidation of Br by Cl or HClO pro-
duced from the sonolysis of CCl cannot be ruled out:
2
H O þ 2HCl ! Cl þ 2H O
2
2
2
4
However, it is also reported that below 30% of H
tion does not proceed at all [57]. Since the production of such a
high concentration of H by sonication cannot be expected
2 2
O , chlorina-
ꢀ
Å
Å
ꢀ
Br þ Cl ! Br þ Cl
2 2
O
Å
Å
Br þ Br ! Br
2
[38], reaction mechanisms of our present system must be different
ꢀ
from the above even if HCl (Cl ) was formed during sonolysis of
Br þ HClO ! Clꢀ þ OHꢀ þ Br
ꢀ
2
aqueous CCl4.
When KCl is used instead of KBr, chlorination occurred slightly
faster than that without KCl (entries 1–4, Table 3). Selectivities of
Br þ HClO þ HCl ! 2Clꢀ þ H
ꢀ
O þ Br
2
2
2
-/4- and monochloro-/dichloro-were not so affected. When KI is
used, iodination occurred in 11% and 14% in respectively 1 and
h (entries 7 and 8, Table 3), but chlorination did not take place
at all.
In the case of KI, color of CCl
became persistently purple indicating that I
sonication [47,48]. However I did not react well with anisole
2
As shown in entries 9 and 10 in Table 3, Br is readily reactive to
anisole under both sonication and mechanical stirring conditions.
The resulted 2-/4-selectivities of entries 5, 9 and 10 were similar
to each other. These results probably indicate that the main bromi-
2
4
layer of the reaction solution
was formed during
nating species is Br
ment. When aqueous CCl
ultrasonically irradiated (480 kHz), the color of the CCl
became progressively dark brown. In 20-min sonication,
2
. This point was clarified by a ‘‘blank’’ experi-
with KBr in the absence of anisole was
layer
2
4
2
4
under our experimental conditions (entries 11 and 12, Table 3).
ꢀ5
ꢀ4
ꢀ
Since only chlorination occurs without alkaline metal halide in
4
.86 ꢁ 10 mol of Br from 1.50 ꢁ 10 mol of Br was formed,
2
Å
H
Cl
2
O–CCl
4
(entry 1, Table 3), all chlorinating species such as Cl or
ꢀ
2
may be more preferably used to oxidize I to I
2
than to attack
an aromatic ring.
Phenol, aniline, and acetanilide were also sono-brominated, as
summarized in Table 4. Toluene and xylenes gave mainly
a-brominated products. These results are similar to those by
bromination with NaBr/H /light [21]. Nitrobenzene was not
2 2
O
brominated under the present sonication conditions. This result
is generally observed at electrophilic bromination of
electron-deficient arenes by Br .
2
The advanced oxidation process (AOP) [36–38] is characterized
Å
by production of the hydroxyl radical ( OH) as a primary oxidant,
which is used to depollute charged liquid effluents. However, the
AOP must be used not only for degradation of persistent organic
pollutants in aqueous effluents but also for the creation of more
efficient chemical technologies. In the present study, ultrasonic
AOP was applied for the bromination of aromatic compounds.
Navarro, et al. showed that the sonolysis of HCOOH initiates
Fisher–Tropsch hydrogenation of carbon monoxide [39]. In their
Fig. 1. The time profile of the production of bromoanisole and the consumption of
anisole under experimental conditions; Anisole (0.1 mmol), KBr (0.15 mmol), CCl
1 mL). H O (5 mL), time (1 h), ultrasonic powder (5 W). d, bromoanisole/480 kHz;
s, anisole recovered/480 kHz; j, bromoanisole/36.6 kHz; h, anisole recovered/
6.6 kHz.
4
(
2
3
4
case, the formation of CH was explained by the secondary CO