Radiation chemical oxidation of benzaldehyde, acetophenone, and benzophenone

Article abstract of DOI:10.1021/jp9718717

Radiation chemical reactions of ^·OH, O^·-, and SO₄^·- with benzaldehyde, acetophenone, and benzophenone have been studied using both pulse and steady-state radiolysis techniques. The observed rates for the ^·OH addition (k = (2.6-8.8)×10⁹ M^-1 s^-1) are higher than those found for the SO₄^·- reaction (k = (0.7-4.0)×10⁹ M^-1 s^-1). The rate for the reaction of O^·- with benzaldehyde is higher than that found for ^·OH, while a reverse trend is observed in the case of the two ketones. Optical absorption spectra of the intermediate transients formed in the reactions of ^·OH and SO₄^·- with all three compounds are similar with a peak around 370-380 nm. The absorption spectra from the O^·- reaction have shown a major peak at 310 nm and are somewhat different from those obtained in the reaction of ^·OH. The yields of the phenolic products formed in the reaction of ^·OH with benzaldehyde and acetophenone in the presence of 0.1 mM ferricyanide corresponded to only 30% and 50% ^·OH yields, respectively. Benzoic acid is a major product formed with benzaldehyde in the reaction of ^·OH as well as SO₄^·- with G values of 2.1 and 1.3 per 100 eV, respectively. The formation of the exocyclic OH adduct is a major pathway in the reactions of ^·OH (by addition) and of SO₄^·- from hydrolysis of the initially formed radical cation (k = 2.4×10⁴ s^-1) with benzaldehyde. The exocyclic OH adduct undergoes disproportionation to give benzoic acid. The formation of the exocyclic OH adduct of acetophenone is possibly hindered owing to the bulky COCH₃ group.

Full text of DOI:10.1021/jp9718717

8402
J. Phys. Chem. A 1997, 101, 8402-8408
Radiation Chemical Oxidation of Benzaldehyde, Acetophenone, and Benzophenone
S. B. Sharma, M. Mudaliar, and B. S. M. Rao*
Department of Chemistry, UniVersity of Pune, Pune 411 007, India
H. Mohan and J. P. Mittal†
Chemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400 085, India
ReceiVed: June 9, 1997; In Final Form: August 18, 1997X
•
•-
Radiation chemical reactions of OH, O•-, and SO4 with benzaldehyde, acetophenone, and benzophenone
•
have been studied using both pulse and steady-state radiolysis techniques. The observed rates for the OH
addition (k ) (2.6-8.8) × 109 M-1 s-1) are higher than those found for the SO4 reaction (k ) (0.7-4.0)
•-
× 109 M-1 s-1). The rate for the reaction of O•- with benzaldehyde is higher than that found for •OH, while
a reverse trend is observed in the case of the two ketones. Optical absorption spectra of the intermediate
transients formed in the reactions of •OH and SO4•- with all three compounds are similar with a peak around
370-380 nm. The absorption spectra from the O•- reaction have shown a major peak at 310 nm and are
somewhat different from those obtained in the reaction of •OH. The yields of the phenolic products formed
•
in the reaction of OH with benzaldehyde and acetophenone in the presence of 0.1 mM ferricyanide
•
corresponded to only 30% and 50% OH yields, respectively. Benzoic acid is a major product formed with
benzaldehyde in the reaction of •OH as well as SO4•- with G values of 2.1 and 1.3 per 100 eV, respectively.
•
The formation of the exocyclic OH adduct is a major pathway in the reactions of OH (by addition) and of
SO4 from hydrolysis of the initially formed radical cation (k ) 2.4 × 104 s-1) with benzaldehyde. The
•-
exocyclic OH adduct undergoes disproportionation to give benzoic acid. The formation of the exocyclic OH
adduct of acetophenone is possibly hindered owing to the bulky -COCH3 group.
Introduction
yields. Schuler and co-workers5-7 have pioneered the research
in this area during the past 2 decades. We have undertaken, in
The radiation chemical reactions of water radicals (eaq-, H•,
and •OH) and the secondary radicals (e.g., SO4•-) derived from
them with arenes in aqueous solution have been studied1-4 using
the pulse radiolysis technique since its advent more than 3
decades ago. These studies have been mainly concerned with
the evaluation of kinetic parameters and measurement of
transient absorption spectra with a view to obtain information
on the rates and reaction mechanism. Owing to the precise
knowledge of the yields of the primary radiolytic products of
the radiolysis of water (reaction 1)
the recent past, studies8-10 on radiation chemical reactions of
water radicals with various substituted benzenes, and of
particular mention among them there has been a detailed
•
•-
investigation of the reactions of OH and SO4 with chloro-
toluenes9 and cresols10 by both pulse and steady-state radiolysis
techniques. These studies have enabled us to quantify the
•
relative rates of attack of OH at different positions of o-, m-,
and p-chlorotoluenes.
The oxidation of benzaldehyde by potassium permanganate
has been known for a very long time in the literature.11-14
A
free radical reaction mechanism involving the OH radical in
basic solutions has been suggested11 for this oxidation. Since
radiation chemical methods are a clean source for •OH genera-
tion, it is interesting to examine in detail the mechanism for
radiation chemical oxidation of benzaldehyde. Although several
studies15 have been carried out on the photolysis of benzalde-
hyde, the mechanism for its radiation chemical oxidation has
not been elucidated unlike its reaction with reducing water
radicals.16-20 We have employed in this work both pulse and
steady-state radiolysis techniques to study the reactions of
(Ge
) 2.8, GOH ) 2.8, GH ) 0.55 per 100 eV energy
-
aq
absorbed) and owing to the limited solubility of arenes (e10-3
M) in water, the radiation chemical technique has been the
method of choice for the generation of specific radicals, and it
has thus proved to be an invaluable tool for studying the free
radical chemistry of substituted benzenes. An important aspect
of these studies has been the radiation chemical hydroxylation
of substituted benzenes by the OH radical. In many cases, the
detailed reaction mechanism, however, could not be reliably
established, in the absence of product distribution data, entirely
by time-resolved spectral studies because of competing parallel
processes. Thus, to establish the reaction mechanism, the pulse
radiolysis data need to be complemented by quantitative analysis
of radiation products formed.
•-
•OH, O•-, and SO4 with benzaldehyde, acetophenone, and
benzophenone.
Experimental Section
(A) Preparation of Solutions and Irradiations. Benzal-
dehyde, acetophenone, and benzophenone used in this study
were obtained from Qualigens and SRL. Their purity (g98%)
was checked by HPLC, and they were used as received except
benzaldehyde, which is known to undergo air oxidation to give
benzoic acid. Purification of benzaldehyde was carried out by
addition of ether and 10% sodium bicarbonate. The organic
layer was dried over sodium sulfate. Ether was evaporated using
Modern chromatographic methods, particularly high-pressure
liquid chromatography, now provide the radiation chemist with
the capability of examining the product distribution in low
† Also Honorary Professor, Jawaharlal Nehru Centre for Advanced
Scientific Research, Jakkur Campus, Bangalore 560 064, India.
X Abstract published in AdVance ACS Abstracts, October 1, 1997.
S1089-5639(97)01871-9 CCC: $14.00 © 1997 American Chemical Society

Radiation Chemical Oxidation
J. Phys. Chem. A, Vol. 101, No. 45, 1997 8403
TABLE 1: Second-Order Rate Constants (k/109 M-1 s-1
)
a rotary evaporator, and the residue was distilled. The purified
benzaldehyde was stored in an airtight bottle with a rubber
septum to prevent it from oxidation, and the solutions were
prepared just before use by withdrawing appropriate aliquots
by a syringe. This procedure enabled us to inhibit its oxidation
for at least 3-4 weeks. The purity of benzaldehyde was
periodically checked by HPLC analysis prior to irradiation. Fresh
solutions, prepared in water purified by the Millipore Milli-Q
system, were used.
•
and λMax/nm Obtained in the Reactions of OH, O•-, and
•-
SO4 with Benzaldehyde, Acetophenone, and Benzophenone
and with Some Other Monosubstiuted Benzenesa
•OH
•-
O•-
SO4
substrate
k
λmax
k
k
benzaldehyde
2.6 ( 0.3
(4.4)24
370
5.5 ( 0.6
0.9 ( 0.12
1.1 ( 0.15
0.71 ( 0.055
acetophenone
benzophenone
3.7 (5.4)25
370
380
1.8 ( 0.1
(0.31)28
4.0 ( 0.5
The amount of benzoic acid present in N2O and N2O/O2-
7.7 ( 1.1
(9.0)18
5.4
3.0
4.4
saturated solutions used for the •OH reaction and in N2-saturated
2-
•-
solutions in the presence of S2O8 for studying the SO4
anisole
toluene
benzonitrile
nitrobenzene
330
313
348
410
nd
4.9
3.1
0.12
e0.001
2.1
0.07
nd
reaction was less than 1%. Only in N2O-saturated basic
solutions (pH ≈ 13) was the extent of oxidation observed to be
∼25%. Hence, the studies in basic solutions were limited to
pulse radiolysis. The HPLC chromatograms obtained for
unirradiated solutions of benzaldehyde under different reaction
conditions used in this study are shown along with those
obtained following γ-radiolysis (cf. section C, Product Analysis).
The concentrations of benzaldehyde and acetophenone for
spectral measurements were usually kept at 1 mM. Owing to
its limited solubility, the concentration of benzophenone was
0.1 mM. All the experiments were carried out at room
temperature.
3.9
a ndsnot determined. Values of the rate constants for anisole, toluene,
benzonitrile and nitrobenzene are taken from refs 1 (•OH and O•-), 2,
and 28 (SO4•-).
of •OH with benzaldehyde and benzophenone were determined
from the slopes of kobs versus [solute]. However, kobs values
were not determined as a function of [acetophenone] and the
value was obtained from only one measurement. The k values
obtained in this work along with those reported1 earlier for some
monosubstituted benzenes containing electron-donating or elec-
tron-withdrawing groups are shown in Table 1.
Our value of k ) (2.6 ( 0.3) × 109 M-1 s-1 for benzaldehyde
is less than that reported earlier (4.4 × 109 M-1 s-1) by
Shevchuk et al.24 This discrepancy may be due to the difference
in the methods employed, since the latter determined their value
by the indirect method of competition kinetics. Our value (k
) (7.7 ( 1.1) × 109 M-1 s-1) for benzophenone is in reasonable
agreement with that reported (9.0 × 109 M-1 s-1) by Brede et
al.18 The k value of 3.7 × 109 M-1 s-1 in the case of
acetophenone is between the values obtained for benzaldehyde
and benzophenone. However, it is lower than the value (k )
5.4 × 109 M-1 s-1) reported by Wilson et al.25
The reactions of •OH and O•- were studied in N2O-saturated
solutions at pH 7 and 13, respectively, while the SO4•- radicals
-
were produced by the reaction of eaq and H• with peroxydis-
ulfate in N2-saturated solutions containing 15 mM K2S2O8 and
0.2 M tert-butyl alcohol.
High-energy electron pulses (7 MeV, 50 ns) from a linear
accelerator were used for the experiments, and the details of
the facility have been described elsewhere.21,22 Dosimetry was
carried out with aerated aqueous solutions of 10 mM KSCN
solution by optical measurements, taking Gꢀ500 ) 21 522 M-1
cm-1 per 100 eV for the transient23 (SCN)2•-. A 60Co source
was employed for steady-state radiolysis experiments.
(B) Separation and Estimation of the Radiation Products.
The HPLC system from Perkin-Elmer (Series 10 liquid chro-
matograph) coupled to an LC-235 photodiode array detector
and reverse-phase Nucleosil C-18 (250 mm × 4.6 mm, Machery
Nagel) or a Lichrospher 100 RP-18 column (125 mm × 4.6
mm, Merck) was used for the identification and estimation of
the yields of the products formed from radiolysis of benzalde-
hyde and acetophenone. The radiation products were monitored
at 220 (benzaldehyde) and 210 nm (acetophenone). The solvent
was 30% acetonitrile in water, and a flow rate of 0.8 mL min-1
was used in the case of benzaldehyde while 30% methanol with
a flow rate of 1.2 mL min-1 was used for acetophenone. The
formation of benzoic acid was further confirmed by monitoring
the chromatogram at 205 nm using water as the solvent and a
The observed k values with all three systems indicate that
the rates are nearly diffusion controlled. The increase in the
values on going from benzaldehyde to benzophenone is in
accord with the increase in electron density due to the replace-
ment of -H by -CH3 or -C6H5 in the functional group. The
rate constant for the •OH reaction with benzaldehyde is similar
to that reported1 for toluene, while those for acetophenone and
benzophenone are comparable to nitrobenzene and anisole,
respectively (Table 1). It is, therefore, evident that there is no
•
significant effect of substituent on the reactivity of OH with
monosubstituted benzenes unlike in the case of substituted
purines, the respective F+ values26,27 being -0.5 and -2.5.
(ii) Reaction of O•-. Similar to the •OH reaction, the second-
order rate constants for the reaction of O•- were determined by
monitoring the formation of the transients at the λmax ) 310
nm. As mentioned in the Experimental Section, not more than
25% of benzaldehyde was oxidized to benzoic acid in unirra-
diated basic solutions (pH ≈ 13) on the time scale (e2 h) of
our experiments. Thus, the amount of benzoic acid present in
unirradiated solutions for the highest [benzaldehyde] used in
our study (1 mM) was not more than 0.25 mM. The reported1
upper limit of the rate constant for the reaction of O•- with
benzoic acid is 4.0 × 107 M-1 s-1. Based on the reactivities
flow rate of 1 mL min-1
. The standards were obtained
commercially, and the products were characterized by their
retention times and UV spectra. The concentration of the
products was determined from the calibration curves of area
versus concentration of the authentic samples. The G values
were determined from the yield versus dose plots.
Results and Discussion
(A) Evaluation of Kinetic Parameters. (i) Reaction of •OH.
The rates of the reaction of •OH with benzaldehyde, acetophe-
none, and benzophenone were determined from the buildup of
the maximum absorption of the transients at 370-380 nm. The
rates of formation were found to be first order and were observed
to increase linearly with the solute concentration in the range
0.2-1 mM. The second-order rate constants for the reaction
of O•- with benzaldehyde (kobs ) k[solute] ) 7.2 × 104 s-1
)
and benzoic acid (kobs ) 1.0 × 104 s-1), it is seen that nearly
90% of O•- reacted with benzaldehyde. Thus, the rate constants
measured in our work indeed represent those for the O•- reaction
with benzaldehyde.

8404 J. Phys. Chem. A, Vol. 101, No. 45, 1997
Sharma et al.
respective values being (9.0 ( 1.2) × 108 and (1.10 ( 0.15) ×
109 M-1 s-1. The rate of the O•- reaction with benzaldehyde
•
was found to be higher than that for the OH, while a reverse
trend is seen in the case of acetophenone and benzophenone.
The higher reactivity of O•- with benzaldehyde compared to
that with the two ketones could be due to the different
mechanism by which it reacts. The major pathway for its
reaction with benzaldehyde is by H abstraction from the
aldehyde group, while addition to the benzene ring predominates
in the case of acetophenone and benzophenone. The decrease
•
in rates of the O•- reaction relative to the OH in the case of
acetophenone and benzophenone is due to the nucleophilic
nature of O•-.
•-
(iii) Reaction of SO4
. The second-order rate constants
obtained for the reaction of SO4•- with benzaldehyde, acetophe-
none, and benzophenone are (7.10 ( 0.55) × 108, (1.8 ( 0.1)
× 109, and (4.0 ( 0.5) × 109 M-1 s-1, respectively. Our rate
Figure 1. Different traces obtained in this work for decay at (A) 370
•-
constant value for the SO4 reaction with acetophenone is 6
•
•-
nm and (B) 350 nm obtained in the reactions of OH and SO4 with
benzaldehyde, respectively. Buildup (λmax ) 310 nm) at (C) 2.5 µs
and (D) 100 µs was measured in the reaction of O•- with benzaldehyde.
Dose per pulse ) 1.5 krad; [benzaldehyde] ) 1 mM.
times higher than that reported by Neta et al.,28 and the reason
for this difference is not clear. Electron-withdrawing groups
cause deactivation of the benzene ring for the SO4•- attack, and
hence, substituted benzenes containing electron-withdrawing
substituents have apparently low reactivity with SO4•-. The
increase in the k value on going from benzaldehyde to
benzophenone is due to the decrease in the deactivation of the
The rate of buildup at 310 nm with benzaldehyde has shown
an initial fast growth followed by a fall-off of the rate, indicating
a change in reaction mechanism (cf. section D). The traces
recorded on a 2.5 and 100 µs scale are shown in Figure 1. The
rate of the former was found to be linearly dependent on
[benzaldehyde] and the second-order rate constant evaluated
from this is (5.5 ( 0.6) × 109 M-1 s-1. The rate of the latter
•-
benzene ring for the SO4 attack. The large difference in the
magnitude of observed k values for the •OH and SO4•- reactions
•-
is in line with the relatively stronger electrophilic nature of SO4
(F+ ) -2.5) found28 in its reaction with monosubstituted
from the trace recorded using 1 mM solutions is 4.3 × 104 s-1
.
benzenes.
Such a behavior was, however, not observed in the case of
acetophenone and benzophenone. The rate constants evaluated
from the linear kobs versus [solute] plots for the O•- reaction
with acetophenone and benzophenone are nearly equal, the
(B) Transient Absorption Spectra. (i) Reaction of •OH and
O•- Radicals. Optical absorption spectra of the intermediate
transients formed in the reaction of •OH with benzaldehyde were
Figure 2. Time-resolved absorption spectra obtained in the reactions of •OH (A-C) and SO4•- (D-F) with 1 mM benzaldehyde (A and D), 1 mM
acetophenone (B and E), and 0.1 mM benzophenone (C and F).

Radiation Chemical Oxidation
J. Phys. Chem. A, Vol. 101, No. 45, 1997 8405
Figure 4. HPLC chromatograms obtained for unirradiated (A-C) and
60Co γ-irradiated (D-F) benzaldehyde solutions (1 mM): (A, D, and
E) N2O-saturated solutions containing K3Fe(CN)6 (0.1 mM); (B) at pH
≈ 13; (C and F) N2-saturated solutions containing potassium persulfate
(15 mM) and tert-butyl alcohol (0.2 M); (A-D and F ) 220 nm,
eluents30% acetonitrile in water; (E) 205 nm, eluents100% water;
(peak 1) ferricyanide; (peak 2) benzoic acid; (peak 3) benzaldehyde;
(peak 4) potasssium persulfate; (peak 5) p-hydroxybenzaldehyde; (peak
6) m-hydroxybenzaldehyde. Dose ) 20 krad.
Figure 3. Time-resolved absorption spectra obtained in the reactions
of O•- with (A) 1 mM benzaldehyde, (B) 1 mM acetophenone, and
(C) 0.1 mM benzophenone.
•
monitored in the range 280-600 nm. In Figure 2 is shown the
transient absorption spectrum obtained in N2O-saturated neutral
solutions of benzaldehyde (1 mM) after completion of the
reaction (5 µs after the pulse). The spectrum exhibited a peak
at 370 nm with a shoulder around 310 nm. The spectra obtained
for the transients formed in acetophenone and benzophenone
are also similar with characteristic peaks at 370 nm, though the
shoulder at 310 nm in the benzaldehyde transient spectrum
transformed into a peak in benzophenone. Such a transforma-
tion is not, however, apparent in the case of acetophenone
(Figure 2). The absorption spectra obtained for acetophenone
and benzophenone are similar to those reported in the litera-
ture.26,29 The λmax in all the three systems is red-shifted by
nearly 50 nm compared to the λmax of substituted benzenes (e.g.,
anisole and toluene)30 with electron-donating groups. Such a
shift,26 which was also noticed in the OH adducts of benzonitrile
and nitrobenzene, is due to the extended conjugation in these
systems. The time-resolved spectrum recorded at 45 µs after
the pulse has shown the usual bimolecular decay of the
transients.
Figure 2, is similar to that obtained in its reaction with OH,
i.e., with a peak at 370 nm and a shoulder at 310 nm. In
addition, a very weak absorption in the region between 450 and
480 nm was observed in the SO4•- reaction. The spectra with
acetophenone and benzophenone (Figure 2) are more or less
similar, though the intensities of the absorption at 370 nm were
different. The order for the intensities found is acetophenone
> benzaldehyde > benzophenone. An important difference in
the •OH and SO4•- reactions is the variation in the rates of the
decay of the absorption at 370 nm. The decay is much faster
in the latter than in the former, as can be seen from the traces
shown in Figure 1 for the benzaldehyde system. Note that the
•
time scales of the decay are 1000 and 200 µs for the OH and
SO4•- reactions, respectively. The rate of decay from the trace
obtained in the SO4•- reaction was found to be first order, and
the k value obtained from the trace is 2 × 104 s-1. The decay
rate for the •OH reaction is at least 5 times lower and is ascribed
to the usual bimolecular decay of the transients.
(C) Product Analysis. The major stable products formed
in the OH reaction with substituted benzenes are the corre-
•
Pulse radiolysis experiments were also carried out in N2O-
saturated solutions of these compounds at pH 13 in order to
ascertain the differences in the mechanism of the •OH and O•-
reactions. The spectra after the completion of the reaction for
the three systems are shown in Figure 3, which are somewhat
different from those obtained in its reaction with OH radicals.
As can be seen from Figure 3, the absorption spectra obtained
in the O•- reaction exhibited an intense peak at 310 nm and a
weaker one at 370 nm in the case of benzaldehyde, the ratio of
the intensities being 3:1. The intensities of the two peaks are,
however, nearly equal in the case of acetophenone and ben-
zophenone.
sponding phenols. Most commonly employed oxidants are
3-
2-
Fe(CN)6 (E° ) 360 mV),31 IrCl6 (E° ) 870 mV),31 and
methylviologen (E° ) -448 mV).31 All three oxidants were
used in our experiments for the oxidation of the OH adducts of
benzaldehyde.
The HPLC chromatograms for the unirradiated solutions
under different conditions are shown in parts A-C of Figure
4. The chromatogram of the unirradiated solution obtained in
•
the reaction of OH with benzaldehyde in the presence of 0.1
mM ferricyanide has shown two major peaks corresponding to
the ferricyanide and benzaldehyde (peaks 1 and 3 in Figure 4A)
and also a very small peak due to benzoic acid (peak 2) whose
concentration was estimated as e1 × 10-5 M accounting for
an impurity level of e1%. Furthermore, the peak of benzal-
(ii) Reaction of SO4•-. The transient absorption spectrum
•-
measured in the SO4 reaction with benzaldehyde, shown in

8406 J. Phys. Chem. A, Vol. 101, No. 45, 1997
Sharma et al.
TABLE 2: Yields per 100 eV of Products Formed in 60Co
γ-Radiolysis Following the OH and SO4 Reactions with
Benzaldehyde and Acetophenone under Different Solution
Conditionsa
The HPLC analysis of the products formed in the γ-radiolysis
of N2O-saturated solutions of acetophenone in the presence of
ferricyanide has shown the formation of three major products
that were identified as the hydroxy isomers of acetophenone.
The G values of o-, m-, and p-hydroxyacetophenones were found
to be 0.6, 1.6, and 0.6 per 100 eV, respectively. In the presence
of IrCl62-, the yields of m- and p-hydroxyacetophenones
remained the same while that of o-hydroxyacetophenone
increased by about 3 times. In N2O/O2 (4:1)-saturated solutions,
negligible amounts of m- and p-isomers were formed while
the formation of o-hydroxyacetophenone was not observed
(Table 2).
•
•-
•OH
N2O
0.1 mM
N2O Fe(CN)6
N2O/O2
(4:1) (v/v) SO4
3-
•-
(A) Benzaldehyde
benzoic acid
1.8
2.1 (1.0)
0.8 (1.2)
0.4 (0.5)
2.1
0.2
0.1
1.3
nf
nf
m-hydroxybenzaldehyde 0.2
p-hydroxybenzaldehyde
o-hydroxyacetophenone
0.1
(B) Acetophenone
(D) Reaction Mechanism. The different OH adducts that
nf
0.6 (2.0)
1.6 (1.5)
0.6 (0.6)
nf
0.4
0.3
nd
nd
nd
•
m-hydroxyacetophenone trace
p-hydroxyacetophenone 0.1
can be formed in the reaction of OH with benzaldehyde are
•
shown in Scheme 1. These can arise from the attack of OH
a ndsnot determined. nfsnot formed. The values in parentheses are
on the ring (reaction 2) and on the -CHO group (reaction 4).
The attack of •OH on the ortho position of benzaldehyde must
be negligible as is evident from the lack of formation of
salicylaldehyde under steady-state conditions. The two hy-
droxycyclohexadienyl radicals (structures 2 and 3) except the
ipso OH adduct (structure 4) can be oxidized to the correspond-
ing phenols in the presence of an oxidant (reaction 3). Since
the yields of the phenolic products correspond to 30% •OH yield,
the ipso adduct formation is limited to 10% •OH yield. Buxton
et al.33 have shown that the rate of oxidation of the OH adducts
of substituted benzenes by ferricyanide is dependent on the
substituent with the second-order rate constants varying from
2.3 × 109 M-1 s-1 for anisole to about 5 × 106 M-1 s-1 for
chlorobenzene (F+ ) -3.0). They have further shown that the
behavior with the stronger oxidant IrCl62- has not revealed any
electronic effects of the substituent with the rates close to
diffusion controlled, except in the case of the OH adducts of
2-
obtained in the presence of IrCl6
.
dehyde in unirradiated solution shows a small hump whose area
is negligible. This may be due to some impurity level of e10-7
M. No increase in the area of this peak was seen with dose,
and only at doses g30 krad was a slight increase in its peak
area noticed.
The products formed in the 60Co γ-radiolysis of N2O-saturated
solutions of benzaldehyde in the absence of any oxidant were
identified as benzoic acid and m- and p-hydroxybenzaldehydes,
and the estimated G values are tabulated in Table 2. o-
Hydroxybenzaldehyde was found to be eluted within 1 min after
benzaldehyde, and its formation was not observed as can be
seen from the chromatograms recorded in Figure 4. Even if
the impurity trace noticed in the benzaldehyde peak is ascribed
to o-hydroxybenzaldehyde, its yield as a radiolytic product must
be negligible. The G value of benzoic acid in deoxygenated
solutions was found to be 1.8 per 100 eV, whereas the yields
of phenolic products are marginal. Although the yields of m-
and p-hydroxybenzaldehydes were enhanced in the presence of
ferricyanide, the G value of benzoic acid remained more or less
unaffected. Their yields were estimated to be 0.8, 0.4, and 2.1
per 100 eV, respectively.
benzonitrile33 where k ) 4.5 × 108 M-1 s-1
.
Since the yields of the phenolic products formed from
benzaldehyde are very low, even in the presence of the oxidants,
pulse radiolysis experiments to evaluate the rates of oxidation
of the OH adducts by ferricyanide were not attempted.
However, it is assumed that the rates of oxidation of the hydroxy
adducts of benzaldehyde by IrCl62- must be comparable to those
found for benzonitrile. It is thus expected that their quantitative
conversion to the corresponding phenols under γ-radiolysis
occurs. The yields of the hydroxybenzaldehydes measured in
the presence of IrCl62- must then represent the extent of initial
•OH attack. Since the combined G value of m- and p-
hydroxybenzaldehydes in N2O-saturated solutions in the pres-
ence of IrCl62- is 1.7, only 30% •OH seems to add to the ring.
The HPLC chromatogram of the products formed in the
γ-radiolysis of N2O-saturated solutions of benzaldehyde in the
presence of ferricyanide for a typical dose of 20 krad is depicted
in Figure 4D, and the peaks marked 2, 5, and 6 correspond to
benzoic acid and p- and m-hydroxybenzaldehydes, respectively.
The formation of benzoic acid as a radiation product, as
mentioned earlier, was further confirmed using 100% water as
the solvent and by monitoring it at 205 nm. The chromatogram
obtained from radiolysis using this solvent has shown a single
peak due to benzoic acid (Figure 4E). The phenolic products,
however, could not be separated using this solvent. When
ferricyanide was replaced by a less powerful oxidant, methyl-
viologen, m- and p-hydroxybenzaldehydes were not at all
formed, whereas their yields were slightly enhanced (G(total)
The reason for low phenolic yields in the case of benzalde-
hyde is the possibility of another reaction channel leading to
the formation of the exocyclic OH adduct (structure 5) from
•
the addition of OH to the carbonyl group of benzaldehyde
(reaction 4). The total yields of hydroxybenzaldehydes and
benzoic acid account for about 90% •OH yield. The interesting
finding is that the G values of benzoic acid are more or less the
same with and without any oxidant (2.1 and 1.8 per 100 eV,
respectively). Radiation chemical oxidation of benzaldehyde
to benzoic acid is interpreted in terms of the disproportionation
of the exocyclic OH adduct (structure 5) to give benzoic acid
(reaction 5), which can account for the complete consumption
2-
) 1.7) when IrCl6 was used. Trace amount of o-hydroxy-
benzaldehyde was formed in the presence of IrCl62-. The yields
of phenols with benzaldehyde are among the lowest found for
substituted benzenes containing electron-withdrawing substit-
uents. For instance, the total phenolic yields in benzonitrile in
the presence of Fe(CN)63- are reported32 to be 85% •OH yield.
•
of OH. Dimerization of this adduct seems to be a minor
The chromatogram recorded in N2O/O2 (4:1)-saturated solu-
tions of benzaldehyde following γ-radiolysis (not given in
figure) has shown the formation of the same products with
nearly identical yields as in N2O-saturated solutions. In its
reaction with SO4•-, the formation of only benzoic acid with a
G value of 1.3 was observed (Figure 4F).
process.
The addition of O2 to the OH adducts of benzene and its
•
derivatives has been reported32 to be reversible, and HO2
elimination from the peroxyl radical results in the formation of
the corresponding phenol. Considering one of the OH adducts

Radiation Chemical Oxidation
J. Phys. Chem. A, Vol. 101, No. 45, 1997 8407
SCHEME 1
of benzaldehyde as an example, this process is shown in
reactions 18 and 19:
by addition-elimination and/or direct electron-transfer from the
-CHO group is hydrolyzed in neutral solutions to give the
corresponding exocyclic OH adduct (reaction 10). As in the
•
case of the OH reaction, this OH adduct disproportionates to
give benzoic acid (reaction 5) that will account for the
•-
•-
consumption of 80% SO4 yield. The SO4 attack (reaction
11) on the ring seems to be a marginal process, and it occurs
only to an extent of 20%.
The yields in oxygenated solutions of benzene derivatives are
usually lower9e,32 than in deoxygenated solutions in the presence
of an oxidant other than O2, especially with derivatives
containing electron-withdrawing groups. This is due to the low
values of K ()kf/kr) e 1 and the rate of HO2• elimination. This
possibly explains the significant reduction in the yields of the
hydroxybenzaldehydes in the presence of O2.
Thus, the •OH and SO4•- addition to the carbonyl group seems
to predominate over addition to the benzene ring. The proposed
mechanism is also in accord with the transient absorption
spectrum measured in the SO4•- reaction. The initial spectrum
with λmax at 350 nm (Figure 2) is assigned to the radical cation
(structure 6) formed in reaction 9, and the first-order decay (2.4
× 104 s-1) is attributed to the formation of the exocyclic OH
adduct (reaction 10). The exocyclic adduct is not expected to
absorb above 300 nm. The rate of hydrolysis of radical cation
6 (k ) 2.4 × 104 s-1) must be relatively slower than the
corresponding value for the σ adduct radical.
The proposed mechanism for the formation of benzoic acid
from the disproportionation of the exocyclic OH adduct
(structure 5) is also in accord with the observed yields of benzoic
•-
acid (G ) 1.3) in the SO4 reaction under steady-state
conditions (Table 2). The radical cation (structure 6) formed

8408 J. Phys. Chem. A, Vol. 101, No. 45, 1997
Sharma et al.
The reaction of O•- seems to proceed mainly through H
abstraction from the aldehyde group (reaction 14), leading to
the radical 7. This is evident from the peak observed at 310
nm in the transient absorption spectrum. The decrease in the
rate of absorption at 310 nm is ascribed to the attack of radical
7 by OH-. This radical in the presence of high [OH-] leads
to the radical anion 8. The relatively less intense peak at 370
nm is assigned to the addition of O•- to the benzene ring
(reaction 17).
(3) Steenken, S. In Free Radicals in Synthesis and Biology; Minisci,
E., Ed.; Nato ASI Series C-260; Kluwer Academic: Dordrecht, The
Netherlands, 1989; p 213.
(4) (a) Sonntag, C. v.; Schuchmann, H.-P. Angew. Chem., Int. Ed. Engl.
1991, 30, 1229. (b) Sonntag, C. v.; Schuchmann, H.-P. In Peroxyl Radicals;
Alfassi, Z. B., Ed.; Wiley: Chicester, 1997; p 173. (c) Schuchmann H.-P.;
Sonntag, C. v. In Peroxyl Radicals; Alfassi, Z. B., Ed.; Wiley: Chicester,
1997; p 439.
(5) (a) Bhatia, K.; Schuler, R. H. J. Phys. Chem. 1973, 77, 1356. (b)
Ye, M.; Schuler, R. H. J. Phys. Chem. 1989, 93, 1898. (c) Chen, X.; Schuler,
R. H. J. Phys. Chem. 1993, 97, 421.
(6) (a) Klein, G. W.; Schuler, R. H. Radiat. Phys. Chem. 1978, 11,
167. (b) Kanodia, S.; Madhavan, V.; Schuler, R. H. Radiat. Phys. Chem.
1988, 32, 661. (c) Schuler, R. H. Radiat. Phys. Chem. 1992, 39, 105.
(7) Boyland, E.; Sims, P. J. Chem. Soc. 1953, 2966.
(8) (a) Mohan, H.; Mudaliar, M.; Aravindakumar, C. T.; Rao, B. S.
M.; Mittal, J. P J. Chem. Soc., Perkin Trans. 2 1991, 1387. (b) Mohan, H.;
Mudaliar, M.; Rao, B. S. M.; Mittal, J. P. Radiat. Phys. Chem. 1992, 40,
513. (c) Mohan, M. Ph.D. Thesis, University of Poona, India, 1993.
(9) (a) Merga, G.; Aravindakumar, C. T.; Mohan, H.; Rao, B. S. M.;
Mittal, J. P. J. Chem. Soc., Faraday Trans. 1994, 90, 597. (b) Merga, G.;
Rao, B. S. M.; Mohan, H.; Mittal, J. P. J. Phys. Chem. 1994, 98, 9158. (c)
Merga, G. Ph.D. Thesis, University of Poona, India, 1995 (d) Merga, G.;
Schuchmann, H.-P.; Rao, B. S. M.; Sonntag, C. v. J. Chem. Soc., Perkin
Trans. 2 1996, 551. (e) Merga, G.; Schuchmann, H.-P.; Rao, B. S. M.;
Sonntag, C. v. J. Chem. Soc., Perkin Trans. 2 1996, 1097.
(10) Choure, S.; Bamatraf, M. M.; Rao, B. S. M.; Das, R.; Mohan, H.;
Mittal, J. P. Manuscript in preparation.
(11) Wiberg, K. B.; Stewart, R. J. Am. Chem. Soc. 1955, 77, 1786.
(12) Rocek, J. In The Chemistry of Carbonyl Group; Patai, S., Ed.; John
Wiley and Sons: London, 1966; p 461.
(13) Waters, W. A. In Mechanisms of Oxidation of Organic Compounds;
Methuen and Company: London, 1964; p 82.
(14) Stewart, R. In Oxidation in Organic Chemistry, Part A; Wiberg,
K. B., Ed.; Academic Press: New York, 1965; p 54.
(15) Khudyakov, I. V.; McGarry, P. F.; Turro, N. J. J. Phys. Chem.
1993, 97, 13234 and references therein.
The proportion of the yields of ortho:meta:para isomers of
phenolic products formed in radiolysis of acetophenone in the
presence of 0.1 mM ferricyanide is 0.5:1.3:1 per position (Table
2). When 0.2 mM IrCl62- was used as the oxidant, the G value
of the ortho isomer was enhanced from 0.6 to 2.0, giving a
proportion of 1.6:1.3:1 per position for ortho, meta, and para
isomers, respectively, with the total phenolic yield (G ) 4.1)
corresponding to 75% OH yield. IrCl62-, being a stronger
•
oxidant than Fe(CN)63-, must be able to oxidize all the OH
adducts formed from its attack. The other two possible
channels, i.e., addition to the ipso carbon and the functional
group -COCH3, together should account for the remaining 25%
•
•OH yield. Assuming that the extent of OH attack at these
two positions is equally likely, the proportion of the yields of
ortho:meta:para:ipso:exo (1.6:1.3:1:1.1:1.1) indicates that there
•
is not much preference for OH addition to any of these
positions. It seems that the formation of the exocyclic OH
adduct of acetophenone compared to benzaldehyde is hindered
owing to the bulky -COCH3 group.
(16) Hayon, E.; Ibata, T.; Lichtin, N. N.; Simic, M. J. Phys. Chem. 1972,
76, 2072.
Conclusions
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277.
(20) Solar, S.; Getoff, N.; Holeman, J.; Sehested, K. J. Phys. Chem.
1995, 99, 9425.
•
The OH addition to the functional group of benzaldehyde
seems to predominate over addition to the benzene ring, leading
to the formation of the exocyclic OH adduct, while addition to
the ring is predominant in the case of acetophenone. Benzoic
•
•-
acid is a major product formed in both the OH and SO4
reactions, and its formation has been explained by dispropor-
tionation of the exocyclic OH adduct. The major reaction
pathway for the reaction of O•- with benzaldehyde is by H
abstraction from the aldehyde group, while addition to the ring
predominates in the case of both the ketones. This study
demonstrates the usefulness of radiation chemical methods in
the elucidation of the oxidation reaction mechanism of aromatic
aldehydes.
(21) Guha, S. N.; Moorthy, P. N.; Kishore, K.; Naik, D. B.; Rao, K. N
Proc. Indian Acad. Sci., Chem. Sci. 1987, 99, 261.
(22) Panajkar, M. S.; Moorthy, P. N.; Shirke, N. D. BARC Rep. 1988,
1410.
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USSR5 1969, 1655.
(25) Wilson, R. L.; Greenstock, C. L.; Adams, G. E.; Wageman, R.;
Dorfman, L. M. Int. J. Radiat. Phys. Chem. 1971, 3, 211.
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(27) Vieira, A. J. S. C.; Steenken, S. J. Phys. Chem. 1987, 91, 4138.
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Soc. 1977, 99, 163.
(29) Land, E. J. Proc. R. Soc. A 1968, 305, 457.
(30) Dorfman, L. M.; Taub, I. A.; Harter, D. A. J. Chem. Phys. 1964,
41, 2954.
Acknowledgment. The authors thank Professor M. S. Wadia
and Dr. M. G. Kulkarni for their useful suggestions. S.B.S. is
also thankful to the Nuclear Science Centre and the University
Grants Commission, New Delhi for providing financial as-
sistance. This work is partly supported by Board of Research
in Nuclear Sciences.
(31) (a) In CRC Handbook of Chemistry and Physics; Weast, R. C.,
Ed.; CRC Press Inc.: Boca Raton, FL, 1978; p D-193. (b) Wardman, P. J.
Phys. Chem. Ref. Data 1989, 18, 1637.
References and Notes
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