Ultrasound effects on the photopinacolization of benzophenone

Article abstract of DOI:10.1021/jo000611+

Ultrasonic irradiation is able to modify the course of several photochemical reactions, especially bimolecular, proceeding via triplet states. These effects were illustrated in the study of benzophenone photopinacolization in ethanol. The rates and yields increase when sonication is applied simultaneously to UV irradiation. An explanation is based on a 2-fold effect: (i) light-absorbing transient species undergo sonolytic decomposition, making the photoconversion more efficient, and (ii) sonication induces the triplet state quenching, as shown by Stern-Volmer plots from experiments run in the presence of naphthalene, probably due to the easier collisional deactivation processes favored by the homogeneous distribution of the activated species.

Full text of DOI:10.1021/jo000611+

8444
J . Org. Chem. 2000, 65, 8444-8447
Ultr a sou n d Effects on th e P h otop in a coliza tion of Ben zop h en on e
Anton Gaplovsky,† Martin Gaplovsky,† Stefan Toma,*,† and J ean-Louis Luche*,‡
Faculty of Sciences, Comenius University, SK 842 15 Bratislava, Slovakia, and Laboratoire de Chimie
Mole´culaire et Environnement, Universite´ de Savoie-ESIGEC, F-73376 Le Bourget du Lac, France
J ean-Louis.Luche@univ-savoie.fr
Received April 19, 2000
Ultrasonic irradiation is able to modify the course of several photochemical reactions, especially
bimolecular, proceeding via triplet states. These effects were illustrated in the study of benzophenone
photopinacolization in ethanol. The rates and yields increase when sonication is applied simulta-
neously to UV irradiation. An explanation is based on a 2-fold effect: (i) light-absorbing transient
species undergo sonolytic decomposition, making the photoconversion more efficient, and (ii)
sonication induces the triplet state quenching, as shown by Stern-Volmer plots from experiments
run in the presence of naphthalene, probably due to the easier collisional deactivation processes
favored by the homogeneous distribution of the activated species.
In tr od u ction
homogeneous distribution of the reactive excited states
throughout the solution by cavitational microstreaming,
known to provide an ideal stirring.10,11 A recent study of
the photorearrangement of diphenyl ether could support
the idea that cavitation enhances the quenching of some
excited states or cleaves intermediate excimers or exci-
plexes.12 To explore further the effects of the combined
actions of light and acoustic waves, the photoreduction
of benzophenone was investigated, and the results are
described in this paper.
The specificities of sonochemistry with respect to
photochemistry have been envisaged in a number of
papers. The photo- and sonolyses of metal carbonyls,1 and
of bromotrichloromethane in the presence or absence of
1-alkenes,2 display significant differences, absent in the
decomposition of arenediazonium salts.3 Much less at-
tention has been paid to reactions performed under
simultaneous ultrasound and light irradiations. The
photolyses of water,4 trifluoromethyl phenyl ketone,5 and
dihalo benzils6 were shown to be substantially influenced
by sonication. Not only photocleavages, but also photo-
isomerization and -coupling reactions, can also be affected
by ultrasound.7 From the data reported, general trends
seem to appear: (i) a monomolecular process, e.g., the
photoisomerization of 2-phenylindane-1,3-dione to 3-ben-
zylidenephthalide,8 is practically unchanged by sonica-
tion; (ii) bimolecular reactions undergo significant changes
in rates and selectivity as observed in the photodimer-
ization of acenaphthylene,8 the addition of cyclohexanone
to cyclohexene, and the Paterno-Bu¨chi reaction of ace-
tone to ethyl vinyl ether.9 An explanation can be the
Resu lts a n d Discu ssion
The experiments were run in a quartz cuvette (for UV
spectroscopy, 1 cm × 1 cm section) sealed with a septum
and placed in a HP-8452A array spectrophotometer.
Photolyses were effected at 366 nm with an OSRAM
xenon lamp (Figure 1), the incident light (“excitation
beam”) covering the totality of the cuvette surface.
Nitrogen bubbling through a capillary placed in the top
half of the cuvette was used to ensure agitation in the
nonsonicated experiments. This agitation method proved
to be efficient enough in the small volume of the cell and
did not interfere with the analytic system, whose beam
(“analytic beam”) propagates in the bottom half of the
cell perpendicularly to the excitation beam. The ultra-
* To whom correspondence should be addressed. For S. Toma:
TOMA@fns.uniba.sk. Phone: +421 760 296 208; Fax: +421 760 296
690. For J .-L. Luche: Phone: +33 479 75 88 59; Fax: +33 479 75 88
85.
† Comenius University.
sonic microtip horn (3 mm diameter, fitted to a Branson
‡ Universite´ de Savoie-ESIGEC.
13
Sonifier 250, power setting 21.5 ( 1 W‚cm-2
)
was
(1) Suslick, K. S.; Goodale, J . W.; Schubert, P. F.; Wang, H. H. J .
Am. Chem. Soc. 1983, 105, 5781-5785. Suslick, K. S.; Schubert, P. F.
J . Am. Chem. Soc. 1983, 105, 6042-6044. See also: Suslick, K. S. In
Ultrasound, its Chemical, Physical and Biological Effects; Suslick, K.
S., Ed.; VCH: Weinheim, 1988; pp 123-163.
(2) Kimura, T.; Fujita, M.; Sohmiya, H.; Ando, T. J . Org. Chem.
1998, 63, 6719-6720.
(3) Rehorek, D.; J anzen, E. G. J . Prakt. Chem. 1984, 326, 935-940.
(4) Harada, H. Ultrason. Sonochem. 2000, 7, in press.
(5) Theron, A.; Pichat, P.; Guillard, C.; Petrier, C.; Chopin, T. Phys.
Chem. Chem. Phys. 1999, 1, 4663-4668.
(6) Sohmiya, H.; Kimura, T.; Fujita, M.; Ando, T. Ultrason. Sonochem.
2000, 7, in press.
inserted through the septum, at a reproducible position
obtained by a three-axes screw device. A solution of
benzophenone (Aldrich, recrystallized from methanol) in
96% ethanol (Merck, UV grade) placed in the cuvette was
degassed under 10-4 Torr by three freeze-thaw cycles
(10) Gondrexon, N.; Renaudin, V.; Petrier, C.; Clement, M.; Boldo,
P.; Gonthier, Y.; Bernis, A. Ultrason. Sonochem. 1998, 5, 1-6. Gonze,
E.; Gonthier, Y.; Boldo, P.; Bernis, A. Can. J . Chem. Eng. 1997, 75,
245-255.
(11) Andre´, J . C.; Viriot, M. L.; Said, A. J . Photochem. Photobiol. A.
Chem. 1988, 42, 383-396.
(12) Gaplovsky, A.; Donovalova, J .; J akubikova, B.; Toma, S. J .
Chem. Soc., Perkin Trans. 2, submitted.
(7) Gaplovsky, A.; Donovalova, J .; Toma, S.; Hrnciar, P. Chem. Listy
1986, 80, 989-993.
(8) Gaplovsky, A.; Donovalova, J .; Toma, S.; Kubinec, R. J . Photo-
chem. Photobiol. A: Chem. 1998, 115, 13-19.
(9) Gaplovsky, A.; Donovalova, J .; Toma, S.; Kubinec, R. Ultrason.
Sonochem. 1997, 4, 109-115.
(13) Measured calorimetrically: Mason, T. J .; Lorimer, J . P.; Bates,
D. M.; Zhao, Y. Ultrason. Sonochem. 1994, 1, S91-S95.
10.1021/jo000611+ CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/15/2000

Ultrasound Effects on Photopinacolization
J . Org. Chem., Vol. 65, No. 25, 2000 8445
F igu r e 2. Spectra of oxygen-free ethanolic benzophenone
before (solid line) and after (dotted line) UV irradiation.
F igu r e 1. Experimental setup.
Ta ble 1. P h otor ed u ction of Ben zop h en on e 1: Rela tive
Am ou n ts of Resid u a l Ben zop h en on e 1, Ben zp in a col 2,
a n d Ben zh yd r ol 3 a fter Rea ction for 1000 s
1 (C0)a
conditions
1
2
3
7.2
UV
UV +))))
UV
UV +))))
UV
UV +))))
4.34 ( 0.04
3.37 ( 0.03
1.95 ( 0.02
1.80 ( 0.02
0.91 ( 0.01
0.89 ( 0.01
1.39 ( 0.01
1.85 ( 0.02
0.81 ( 0.01
0.87 ( 0.01
0.39 ( 0.01
0.42 ( 0.01
0.08 ( 0.01
0.07 ( 0.01
0.07 ( 0.01
0.05 ( 0.01
0.04 ( 0.01
0.02 ( 0.01
3.63
1.76
a
All the concentrations in mmol‚L-1
.
Sch em e 1
F igu r e 3. Time dependence of LAT formation and decomposi-
tion under photolysis (upper curve) and photosonolysis (lower
with repressurization by oxygen-free nitrogen and ther-
mostatized to 23 ( 0.3 °C. All the experiments were run
for 1000 s. Comparative experiments prolonged for 2000
s provide practically identical results. The course of the
reaction was followed by recording simultaneously with
irradiation the UV spectra of the mixture every 10 s. The
rate constants were calculated from the 100 spectra thus
obtained. The composition of the mixture was determined
by HPLC with a UV array detector at 230 nm. The
results given in Table 1 represent an average of at least
two runs. The accuracy of the measurements can be
estimated to better than 1%. Table 1 gives the amounts
of the photoproducts which can actually be isolated. In
the absence of light, sonication does not initiate any
detectable reaction.
curve), C0 Ph2CO ) 0.6 mmol‚L-1
.
to benzophenone (Figure 2), cannot be isolated due to
their high sensitivity to oxygen and thermal lability.
Even at low concentrations (ca. 1% of the initial Ph2-
CO),16 they act as internal light filters (LAT ꢀ333 5 × 104
M-1‚cm-1; Ph2CO ꢀ333 167 M-1‚cm-1) and quench the
benzophenone triplet.14 The higher conversion observed
under sonication can thus mean that lower amounts of
LAT are formed, a hypothesis that was checked by
recording the absorbance of the solution as a function of
time: the LATs formed when sonication is applied reach
only 25% of the amount formed without sonication,
independently from the initial concentration (Figure 3).
Photosonolysis of a solution previously photolyzed for
an initial 400 s period [Figure 4 (top), curve 1] produces
a sharp decrease of the absorbance as soon as ultrasound
is switched on. Conversely, sonication inhibits the photo-
chemical formation of the LATs, which appear as soon
as ultrasound is switched off [after 500 s, curve 2]. With
continuous simultaneous illumination and sonication, the
Previous workers have shown that during the photoly-
sis of benzophenone in the presence of hydrogen donors,
light-absorbing transients (LAT, Scheme 1) are formed.14,15
These species, characterized by a λmax at 327 nm, hypso-
chromically shifted by several nanometers as compared
(14) Weiner, S. A. J . Am. Chem. Soc. 1971, 93, 425-429.
(15) Chilton, J .; Geiring, L.; Steel, C. J . Am. Chem. Soc. 1976, 98,
1865-1870.
(16) Churio, M. S.; Grela, M. A. J . Chem. Educ. 1997, 74, 436-438.

8446 J . Org. Chem., Vol. 65, No. 25, 2000
Gaplovsky et al.
F igu r e 5. Quenching of benzophenone triplet by naphthalene.
[EtOH] ) 17.15 mol‚L-1; slope of (366 nm + 20 kHz) line: 710
mol-1‚L; slope of (366 nm) line: 440 mol-1‚L.
setting 3) to 50 ( 2 W‚cm-2 (power setting 5). Kinetic
measurements show that the LAT decomposition is very
slow without sonication (ksono ) 0.128 ( 0.007 mol-1‚s-1
at 21.5 ( 1 W‚cm-2; ksilent ) 0.0092 ( 0.0002 mol-1‚s-1).
An explanation was envisaged by considering the known
sensitivity of the LATs to oxygen, which can form by
sonolysis of the solvent and the water contained in it.
However, this hypothesis is invalidated by the results
shown in Figure 4 (top), curve 2: if oxygen is generated
in the solution by sonolysis (endogenous oxygen), the
steep rise in absorbance as soon as sonication is stopped
should not occur. In addition, the spontaneous decay of
LATs under oxygen without sonication requires 1-2
days.16 This sonochemical decomposition of LATs remains
an experimental observation for which it is premature
to propose a reasonable mechanism.
The net result is that in the absence of the LAT
internal light filters, the photoconversion increases, to a
higher degree for the more concentrated solutions. The
higher conversion yields under sonication demonstrate
that the effect of the cavitation bubble cloud is minimum.
If light scattering by this cloud was important, a lesser
amount of excited molecules should be produced, leading
to lower yields, in opposition to the experiment. Similarly,
if light scattering decreases the intensity received by the
detector, the “apparent” absorbance should increase at
the ultrasound switch-on, and decrease at the switch-
off, again the opposite of what is observed.
It was, however, suspected that lowering the LAT
amount was not the sole effect of sonication, and further
experimentation was undertaken to determine if ultra-
sound can actually quench some activated species. The
same experiments as above were run in the presence of
naphthalene,17 and the results expressed by Stern-
Volmer plots (Figure 5).
F igu r e 4. Absorbance of the continuously photolyzed solution
vs time. Changes in the sign of the slope coincide with the
switching on or off of ultrasound. Top frame: curve 1: initial
366 nm irradiation followed after 400 s by simultaneous (366
nm + 20 kHz) irradiations; curve 2: initial (366 nm + 20 kHz)
irradiations followed after 500 s by UV only; curve 3: continu-
ous (366 nm + 20 kHz) irradiations. Middle frame: alternate
366 nm and (366 nm + 20 kHz) irradiations. Bottom frame:
same as top, curve 1, with ultrasound power setting 3 (5.7 (
0.5 W‚cm-2) and 5 (50 ( 2 W‚cm-2).
level of LAT formed remains very low (curve 3). The
absorbance increase/decrease from alternate UV/
(UV+ultrasound) irradiations can be repeated [Figure 4
(middle)], illustrating the reversibility of the reactions.
That sonication plays a direct role in the LAT concentra-
tion lowering is also demonstrated in Figure 4 (bottom):
the absorbance decreases faster when the ultrasound
intensity is increased from 5.7 ( 0.5 W‚cm-2 (power
Sonication of the mixtures with naphthalene increases
the Stern-Volmer constant KSV, with a ratio KSV(sono)
/
(17) Moore, W. M.; Ketchum, M. J . Am. Chem. Soc. 1962, 84, 1368-
1371.

Ultrasound Effects on Photopinacolization
J . Org. Chem., Vol. 65, No. 25, 2000 8447
Sch em e 2
determine. Obviously these reactions should occur in the
bulk solution, out of the cavitation bubbles, where the
thermal effects vanish rapidly as the distance from the
bubble increases. Pressure and/or shock waves effects can
propagate at longer distances, and a parallel could be
given to the pressure enhancement of the quenching
reaction, observed previously.22 Last, it is observed that,
whereas sonication increases the conversion and the yield
of benzpinacol, the formation of benzhydrol 3 does not
follow the same tendency. Benzhydrol acts as an efficient
hydrogen donor to triplet Ph2CO (Scheme 2, reaction 7),
a reaction most probably accelerated by sonication ac-
cording to the perfect stirring effect, contributing to keep
the amount of 3 at a low level in all the cases and to
accelerate the conversion of 1 and the quenching.
As a conclusion, this work has established experimen-
tally that sonication can play a direct role in photochemi-
cal mechanisms. The higher benzophenone conversion
and pinacol yields observed under sonication should find
their origin in the sonochemical decomposition of the
internal light filtering LATs leading to a higher efficiency
of the excitation. Other interpretations at this stage are
certainly premature, and further experimentation is
required. One can just point out that the homogenization
of sonicated solutions enhances all the processes, imply-
ing the encounter of two species, (i) the triplet benzophe-
none with naphthalene, (ii) the same triplet with a
hydrogen donor (solvent, benzhydrol), (iii) and the dimer-
ization of radical [1-H]• to benzpinacol.
KSV(silent) ) 1.61. In the equation KSV ) kq/(ka‚[RH]) applied
to the silent reaction, it can be assumed that the
diffusion-controlled quenching rate constant is similar to
the diffusion rate constant:15 kq(silent) ) kdiff ) 5.4 × 109
mol-1‚s-1 18
The hydrogen abstraction rate constant
.
ka(silent) (Scheme 2),19,20,21 is calculated as 7.19 × 105
mol-1‚s-1
.
In a first hypothesis, if the hydrogen abstraction
reaction 4 is assumed to be independent of cavitation,
then ka(silent) ) ka(sono) and the ratio kq(sono)/kq(silent) is
equivalent to KSV(sono)/KSV(silent) ) 1.61, giving the sonochem-
ical quenching rate constant kq(sono) ) 8.7 × 109 mol-1‚s-1
.
On the other hand, if reaction 4 is also accelerated by
sonication (in agreement with the general effects of
sonication on mass transport dependent processes), the
sonochemical quenching rate constant will reach even
larger values. The origin of this effect is difficult to
Ack n ow led gm en t. This work has been carried out
under the auspices of the European COST Chemistry
Networks, Project D10/0008.
(18) Wagner, P. J .; Kocherar, J . J . Am. Chem. Soc. 1968, 90, 2232-
2238.
(19) Pitts, J . N., J r.; Letsinger, R. S.; Taylor, R. P.; Patterson, J .
M.; Recktenwald, G.; Martin, R. B. J . Am. Chem. Soc. 1959, 81, 1068-
1077.
J O000611+
(20) Weizmann, C.; Bergmann, E.; Hinsberg, Y. J . Am. Chem. Soc.
1938, 60, 1530-1533.
(21) Beckett, A.; Porter, G. Trans. Faraday Soc. 1963, 59, 2039-
(22) Okamoto, M.; Teranishi, H. J . Am. Chem. Soc. 1986, 108, 6378-
6380.
2050.