One-electron redox potential of thiobenzoic acid. Kinetic characteristics of benzoylthiyl radical β-fragmentation

Article abstract of DOI:10.1021/ja974136h

By means of pulse radiolysis, one-electron oxidation of benzoylthiolate (PhCOS^- was achieved by the azide radical (N₃*) in aqueous solution. The spectrum of the resulting benzoylthiyl radical (PhCOS*) shows a broad absorption in the wavelength region from 350 to 500 nm. With N3*/N3^- as reference couple, the reduction potential E°(PhCOS*/PhCOS^-) was measured to be 1.21 V vs NHE. Using the pK(a) of 2.48 for thiobenzoic acid (PhCOSH), we derive the standard reduction potential E°(PhCOS*, H⁺/PhCOSH) to be 1.36 V vs NHE. This reduction potential implies an S-H bond energy of PhCOSH of 87 kcal/mol, which is very close to the bond energy of an alkylthiol S-H bond (87.4 kcal/mol). At 22 °C, the PhCOS* radicals decay with a rate constant of 8.5 x 10³ s^-1 to form colorless species. This process is presumed to be the β-fragmentation of the PhCOS* radical, PhCOS* → Ph* + COS. The kinetics of the β-fragmentation of the PhCOS* radical was found to follow the Arrhenius equation, log(k²/s^-1) = (12.3 ± 0.1) - (11.4 ± 0.2)/θ, where θ = 2.3RT kcal/mol. Strong evidence for this process was provided byproduct identification by GCMS, where the main products of the γ- irradiation-induced N₃* radical oxidation of PhCOS^- in 0.1 M NaN₃ solution were found to be phenyl azide (PhN₃) and aniline (PhNH₂). These products were formed via addition of the Ph* radical to N3^- to form the PhN₃(-*) radical anion, which subsequently disproportionated. In the Discussion, we summarize the reduction potentials and the bond dissociation energies of related thiols and their oxygen couterparts. The β-fragmentation of PhCOS* radicals resembles that of the oxygen counterpart, benzoyloxyl radical (PhCOO*), and their thermochemical properties are also compared.

Full text of DOI:10.1021/ja974136h

J. Am. Chem. Soc. 1998, 120, 2811-2816
2811
One-Electron Redox Potential of Thiobenzoic Acid. Kinetic
Characteristics of Benzoylthiyl Radical â-Fragmentation
Rong Zhao,* Johan Lind, Ga´bor Mere´nyi, and Trygve E. Eriksen
Contribution from the Department of Chemistry, Nuclear Chemistry, Royal Institute of Technology,
S-100 44 Stockholm, Sweden
ReceiVed December 5, 1997
Abstract: By means of pulse radiolysis, one-electron oxidation of benzoylthiolate (PhCOS-) was achieved by
the azide radical (N3 ) in aqueous solution. The spectrum of the resulting benzoylthiyl radical (PhCOS•) shows
•
•
-
a broad absorption in the wavelength region from 350 to 500 nm. With N3 /N3 as reference couple, the
reduction potential E°(PhCOS•/PhCOS-) was measured to be 1.21 V vs NHE. Using the pKa of 2.48 for
thiobenzoic acid (PhCOSH), we derive the standard reduction potential E°(PhCOS•, H+/PhCOSH) to be 1.36
V vs NHE. This reduction potential implies an S-H bond energy of PhCOSH of 87 kcal/mol, which is very
close to the bond energy of an alkylthiol S-H bond (87.4 kcal/mol). At 22 °C, the PhCOS• radicals decay
with a rate constant of 8.5 × 103 s-1 to form colorless species. This process is presumed to be the
â-fragmentation of the PhCOS• radical, PhCOS• f Ph• + COS. The kinetics of the â-fragmentation of the
PhCOS• radical was found to follow the Arrhenius equation, log(k2/s-1) ) (12.3 ( 0.1) - (11.4 ( 0.2)/θ,
where θ ) 2.3RT kcal/mol. Strong evidence for this process was provided byproduct identification by GC-
MS, where the main products of the γ-irradiation-induced N3 radical oxidation of PhCOS- in 0.1 M NaN3
•
solution were found to be phenyl azide (PhN3) and aniline (PhNH2). These products were formed via addition
of the Ph• radical to N3 to form the PhN3 radical anion, which subsequently disproportionated. In the
Discussion, we summarize the reduction potentials and the bond dissociation energies of related thiols and
their oxygen couterparts. The â-fragmentation of PhCOS• radicals resembles that of the oxygen counterpart,
benzoyloxyl radical (PhCOO•), and their thermochemical properties are also compared.
-
-•
Introduction
thiol acids, despite their uses in chemistry and polymer
synthesis,10,11 are not well studied.
Thiols and sulfur-centered reactive intermediates have im-
portant roles in both chemistry and biology.1 The free radical
chemistry of thiobenzoic acid (PhCOSH), as an example of acyl
thiol (RCOSH), is particularly interesting because it represents
the chemistries of both thiols and the sulfur counterpart of
carboxylic acids. Radical reactions of alkyl thiols (RSH, where
R denotes alkyl), especially amino acid thiols have been studied
extensively because of their important role in biochemical
systems.1-5 Aryl thiols (ArSH), their reaction kinetics and redox
properties, were recently studied and were compared to their
oxygen counterparts, phenols.6,7 However, very little is known
about the redox properties and reactivities of acyl thiols
(RCOSH) and acylthiyl radicals (RCOS•). In organic free
radical chemistry, carboxylic acids (RCOOH) are an important
class of compounds, which have been extensively studied since
they play practical roles in many areas.8,9 In contrast, the acyl
One characteristic reaction of benzoyloxyl radicals (PhCOO•)
is their â-fragmentation to form phenyl radical (Ph•) and carbon
dioxide (eq 1). PhCOO• can be formed by thermal or
PhCOO• f Ph• + CO2
(1)
photochemical decomposition of the parent diaroyl perox-
ides.12,13 Equation 1 has a first-order rate constant of ca. 2 ×
106 s-1 in CCl4.13 For the benzoylthiyl radical (PhCOS•), a
similar reaction would be expected (eq 2, where COS is carbonyl
sulfide).
PhCOS• f Ph•+ COS
(2)
Related to this topic, there have been some early electro-
chemical investigations comparing the â-fragmentation of
acyloxyl radicals (RCOO•) with that of acylthiyl radicals
(1) Von Sonntag, C. The Chemical Basis of Radiation Biology; Taylor
& Francis: Bristol, PA, 1987; pp 396-400.
(2) Armstrong, D. A. In Sulfur-centered ReactiVe Intermediates in
Chemistry and Biology; Chatgilialoglu, C., Asmus, K.-D., Eds.; Plenum:
New York, 1990; p 341.
(8) Leffler, J. E. An introduction to free radicals; John Wiley & Sons:
New York, 1993; p 126.
(3) Zhao, R.; Lind, J.; Mere´nyi, G.; Eriksen, T. E. J. Am. Chem. Soc.
1994, 116, 12010-12015.
(9) Fossey, J.; Lefort, D.; Sorba, J. Free radicals in organic chemistry;
Masson: Paris, 1995; p 109.
(4) Zhao, R.; Lind, J.; Mere´nyi, G.; Eriksen, T. E. J. Chem. Soc., Perkin
Trans. 2 1997, 569-574.
(10) Bauer, W.; Khu¨ren, K. Methoden der Organischen Chemie, 5th ed.;
Georg Thieme Publishers: Stuttgart, Germany, 1985; pp 832-890.
(11) Ishii, Y.; Nakayama, J. Organic Functional Group Transformations;
Pergamon Press: New York, 1995; Vol. 5, Chapter 5.12.
(12) Chateauneuf, J.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc. 1988,
110, 2886-2893.
(5) Surdhar, P. S.; Armstrong, D. A. J. Phys. Chem. 1987, 91, 6532-
6537.
(6) Armstrong, D. A.; Sun, Q.; Tripathi, G. N. R.; Schuler, R. H.;
McKinnon, D. J. Phys. Chem. 1993, 97, 5611-5617.
(7) Armstrong, D. A.; Sun, Q.; Schuler, R. H. J. Phys. Chem. 1996, 100,
9892-9899.
(13) Chateauneuf, J.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc. 1988,
110, 2877-2885.
S0002-7863(97)04136-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/12/1998

2812 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Zhao et al.
(RCOS•).14,15 In the electrochemical oxidation of carboxylic
acids, also known as the Kolbe reaction, the decarboxylation
of RCOO• to form alkyl radicals R• is widely used to initiate
polymerization.16 However, during electrooxidation of RCOS-,
the resulting RCOS• radicals dimerize into diacyl disulfides
without decomposition.14,16 This suggests that RCOS• radicals
have a higher stability in comparison to RCOO• radicals.
In the present investigation, thiobenzoic acid (PhCOSH) was
used to examine the characteristics of the benzoylthiyl radical
(PhCOS•). The oxidation of benzoythiolate (PhCOS-) was
•
achieved by the azide radical (N3 ) or the dibromide radical
anion (Br2-•), produced by pulse or γ-radiolysis of aqueous
solutions. The spectroscopic, kinetic, and redox characteristics
of PhCOS• radicals were examined. â-Fragmentation of Ph-
COS• radicals was found to occur as evidenced by both kinetic
analysis upon pulse radiolysis and product identification by gas
chromatography/mass spectrometry (GC-MS). The thermo-
chemical properties of related thiol species and their â-frag-
mentation reactions are summarized and compared with those
of their oxygen counterparts.
Figure 1. The transient absorption spectrum of benzoylthiyl radicals
observed 2 µs after electron pulse irradiation of a solution containing
5 mM PhCOS- and 0.1 M NaN3.
solid-phase microextraction (SPME) technique22 was also used to extract
the organic products. The extraction fiber had 65 µm carbowax/
divinylbenzene coating, selective for polar analytes (from SUPELCO).
The extracted samples were subsequently separated and identified using
a GC-MS (Finnigan SSQ 7000) connected to a Varian 3400 GC. A
30-m DB-5MS fused silica column (J & W Scientific, i.d. 0.25 mm,
film thickness 0.25 µm) was used in the GC. The injection temperature
was set at 250 °C, and the column temperature was programmed to
increase from 28 to 200 °C, at a rate of 10 deg/min.
Experimental Section
Thiobenzoic acid (PhCOSH) of commercial grade (from Sigma) was
purified by vacuum distillation before use. Diammonium 2,2′-azinobis-
(3-ethylbenzothiazoline-6-sulfonate) (ABTS), methyl viologen hydrate
(MV2+), aniline (PhNH2), sodium azide (NaN3), sodium bromide
(NaBr), potassium thiocyanate (KSCN), dichloromethane (CH2Cl2), and
sodium hydroxide (NaOH), obtained from Sigma-Aldrich, were of the
highest purity commercially available and used as received.
All experiments were carried out at room temperature (22 ( 2 °C),
except for the Arrhenius plot measurements.
Results
1. The Spectrum of PhCOS• Radical. One-electron
oxidation of PhCOS- by N3• or Br2-• yields the PhCOS• radical
(eq 3). The transient absorption observed 2 µs after electron
Pulse radiolysis was performed using doses of 10-26 Gy/pulse,
corresponding to 6 × 10-6 to 1.6 × 10-5 M radicals. The 3-MeV
linear accelerator used has a pulse length of 6 ns. The computerized
optical detection system has been described elsewhere.17 Dosimetry
was performed with a N2O-saturated 10-2 M KSCN solution taking
Gꢀ ) 4.42 × 10-3 Gy-1 cm-1 at 500 nm.18 The temperature of the
solutions was varied using a jacketed irradiation cell connected to a
thermostatically controlled bath. A thermoelement probe was inserted
into the outlet of the cell to measure the temperature of the irradiated
solutions.
PhCOS- + N3 (or Br2-•) f PhCOS• + N3 (or 2Br-) (3)
•
-
pulse irradiation of a solution containing 5 mM PhCOS- and
0.1 M NaN3 is shown in Figure 1.
The spectrum of PhCOS• exhibited a broad absorption peak
in the region from 300 to 600 nm, with a maximum between
400 and 460 nm. The extinction coefficient at the maximum
was measured to be 7900 M-1 cm-1 when the dithiocyanate
γ-Radiolysis was performed in a 60Co γ-source (AECL Gammacell
220), with a dose rate of 0.13 Gy/s as determined by the Fricke
dosimeter.19
radical anion (SCN)2 was used as reference.20 The build-up
-•
All experiments were performed in N2O-saturated aqueous solutions
where the primary radiation chemical yield of OH• radicals, GOH, was
rate of the PhCOS• absorption at 460 nm was titrated versus
the PhCOS- concentration. The rate constant of eq 3, obtained
as the slope of a linear fit of the observed rates vs [PhCOS-],
was found to be ca. 3 × 109 M-1 s-1 and 6 × 109 M-1 s-1 for
set to 5.6 × 10-7 mol J-1 20
.
In N2O-saturated solutions containing
high molar concentrations of NaN3 or NaBr, OH• radicals are
•
-•
quantitatively converted into N3 radicals or Br2 radical anions.
Solutions were prepared using Millipore-deionized water. Since
PhCOSH is an acid with a pKa value of 2.48,21 the solutions of thiolate
ion, PhCOS-, were obtained by always adding an equimolar amount
of NaOH to the acid solutions.
•
Br2-• and N3 , respectively. When measuring the rate constant
of eq 3 for N3 , a constant ratio of N3-/PhCOS- of 10 was
•
maintained, since the E°(N3 /N3-) is rather close to E°(PhCOS•/
•
PhCOS-) (see below).
Products formed on γ-irradiation were extracted by dichloromethane
and subsequently concentrated using a rotating vacuum evaporator. A
Alkylthiyl radicals are known to easily form radical anion
complexes with thiolate (equilibrium in eq 4). The RSSR-•
(14) Hirabayashi, Y.; Mazume, T. Bull. Chem. Soc. Jpn. 1966, 39, 1971-
1974.
(15) Robert, J.; Anouti, M.; Paris, J. J. Chem. Soc., Perkin Trans. 2 1997,
473.
RS• + RS- h RSSR-•
(4)
(16) Lionel Funt, B. In Organic electrochemistry; Lund, H., Baizer, M.
M., Eds.; Marcel Dekker, Inc.: New York, 1991; pp 1337-1362.
(17) Eriksen, T. E.; Lind, J.; Reitgerger, T. Chem. Scr. 1976, 10, 5.
(18) Fielden, E. M. The Study of Fast Processes and Transient Species
by Electron Pulse Radiolysis; Reidel: Dordrecht, Holland, 1982; pp 49-
62.
(19) McLaughlin, W. L.; Boyd, A. W.; Chadwick, K. H.; McDonald, J.
C.; Miller, A. Dosimetry for Radiation Processing; Taylor & Francis:
Bristol, PA, 1989; p 144.
species usually have absorption in the region of 390-450 nm,
which is in the region of the absorption peak shown in Figure
1. It is thus important to investigate whether the absorption is
attributable to PhCOS• or PhCOSSOCPh-•. The following
reasons led us to conclude that the equilibrium shown in eq 4
for the PhCOS• radical to form PhCOSSOCPh-• radical anion
is not important. First, the shape of the spectrum and the decay
rate did not change in the microsecond time region, when
(20) Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation
Chemistry; John Wiley & Sons: New York, 1990; p 106.
(21) Hipkin, J.; Satchell, D. P. N. Tetrahedron 1965, 21, 835.
(22) Zhang, Z.; Pawliszyn, J. Anal. Chem. 1993, 65, 1843-1852.

One-Electron Redox Potential of Thiobenzoic Acid
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2813
Figure 2. The inverse of optical density of PhCOS• radicals at 460
nm as a function of the ratio between N3 and PhCOS-. The linear
-
curve fit of the data is in accord with eq 6, and K3 ) 125 can be derived
by dividing the intercept with the slope of the line.
different concentrations of the thiolate were used. Second, the
400-460 nm transient was found not to react with MV2+ to
form MV+•, while RSSR-• radicals are known to reduce MV2+ 3
.
When ABTS was used to capture the oxidative radicals in
the above system, full yield of ABTS+•, as monitored at 645
nm, was observed. The PhCOS• decayed as the ABTS+• built
up (eq 5). The build-up rate of ABTS+• at 645 nm was titrated
PhCOs• + ABTS f PhCOS- + ABTS+•
(5)
Figure 3. (A) The experimental decay rate of PhCOS• (kobs) versus
its maximal optical density measured at 460 nm at (4) 10, (x) 18, (*)
23, (O) 32, ()) 41, (b) 49, (9) 60 °C. The intercepts of the linear fits
of the data represent the first-order rate constants (k2). (B) Arrhenius
plot of log(k2/s-1) versus 1/θ, where θ ) 2.3RT kcal/mol. The solutions
are N2O saturated and contain 0.1 M NaN3 and 5 × 10-3 M PhCOS-.
versus the ABST concentration in solutions containing 0.1 M
NaN3 and 5 × 10-3 M PhCOS-. The rate constant of eq 5,
obtained as the slope of a linear fit of the observed rates vs
[ABTS], was found to be 2 × 109 M-1 s-1
.
2. Reduction Potential of PhCOS-/PhCOS• Couple. The
relatively long lifetime of the PhCOS• radical (ca. 100 µs, see
below) as compared to PhCOO• radical (ca. 1 µs) made the
measurement of the reduction potential of the PhCOS-/PhCOS•
couple possible. Using I2-•/2I- as reference couple, the
approximate value of E°(PhCOS•/PhCOS-) was found to be
than its homologue, benzoic acid (pKa ca. 4.2). Thus, E°-
(PhCOS•, H+/PhCOSH) is calculated to be 1.36 V vs NHE.
3. â-Fragmentation of PhCOS• Radical. In the pulse
radiolysis experiments, the 460-nm transient decayed by mixed
first- and second-order kinetics within the dose range applied.
Kinetic traces were obtained by irradiating N2O-saturated
solutions containing 0.1 M NaN3 and 5 × 10-3 M PhCOS-.
The initial part of the decay was used to calculate a first-order
rate constant. In Figure 3A, these values are plotted against its
maximal optical density, which should be proportional to the
doses applied. By extrapolating the straight line to zero
absorption (zero dose), the intercept kint should be the first-
order component. Since phenyl radicals (Ph•) are known to
absorb only weakly in the region below 300 nm,24 along with
the evidences from product analysis (see below), kint should be
k2, the â-fragmentation of the PhCOS• radical to form Ph• and
COS. The second-order decay of the transient is due to radical
recombination reactions, which always exist as competing
reactions. The decay rate was also found to be independent of
pH in the pH range of 2-14. The rate constant k2 was measured
over a range of temperatures (10-60 °C, see Figure 3A).
The Arrhenius plot for eq 2 shown in Figure 3B yielded the
following temperature dependence
•
1.2-1.3 V. Since N3 , having a reduction potential of 1.33 V,23
rapidly oxidizes PhCOS-, it was chosen as the reference for
•
more accurate measurements. When the N3 radical oxidizes
PhCOS-, the following equilibrium is established.
•
-
PhCOS- + N3 h PhCOS• + N3 (3)/(-3)
In irradiated N2O-saturated solutions containing different con-
centrations of PhCOS- and N3-, with the latter in excess, the
following relationship is obtained.
OD°460/OD460 ) 1 + [N3 ]/K3[PhCOS-]
(6)
-
Here, OD460 is the optical density at 460 nm of PhCOS• at
equilibrium, K3 is the equilibrium constant of eq 3, and OD°460
is the optical density when the equibrium in eq 3 is completely
shifted to the right. In the experiments, the concentrations of
N3-and PhCOS- were adjusted so that the equilibrium in eq 3
was reached before significant radical decay occurred. A plot
of 1/OD460 versus [N3-]/[PhCOS-] gives a straight line (Figure
2), and K3 was found to be 125 by dividing the intercept by the
slope of the line. The rate constant of eq -3 was deduced to
be 4.8 × 107 M-1 s-1 by dividing k3 by K3. From the
equilibrium constant K3, we calculated E°(PhCOS•/PhCOS-)
log(k2/s-1) ) (12.3 ( 0.1) - (11.4 ( 0.2)/θ
(7)
where θ is 2.3RT kcal/mol and the errors correspond to 2σ.
Equation 7 gave an activation energy of 11.4 kcal/mol (47.7
•
to be 1.21 V vs NHE when E°(N3 /N3-) ) 1.33 V was used as
kJ/mol) for eq 2. At room temperature, k2 is ca. 8.5 × 103 s-1
.
reference. The pKa of thiobenzoic acid is given as 2.48,21 lower
(23) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637.
(24) Scaiano, J. C.; Stewart, L. C. J. Am. Chem. Soc. 1983, 105, 3609-
3614.

2814 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Zhao et al.
2-mercaptoethanol, k9 was reported to be 2 × 103 s-1 25
.
If we
assume similar rapid equilibrations for PhCOS• radicals in
reaction 8, eq 10 is valid.
OD°460/OD460 ) 1 + K8[O2]
(10)
A plot of OD°460/OD460 versus oxygen concentrations in Figure
4B gave an estimation of K8 to be 2.3 × 103 M-1. The first-
order decay rate of PhCOS• radicals, as given by the intercept
of the linear fit of the data in Figure 4A, follows eq 11.27
kint(1 + K8[O2]) ) k2 + k9K8[O2]
(11)
In Figure 4C, a plot of kint(1 + K8[O2]) versus the oxygen
concentration gave an estimation of k9 to be 1.7 × 104 s-1
.
5. Product Identification by GC-MS. Upon γ-radiolysis
of N2O-saturated aqueous solutions of PhCOSH, products with
low water solubility were formed. With dichloromethane as
the extraction solvent or with the SPME technique, the same
products were detected. In gas chromatograms of samples after
steady-state γ-radiolysis of N2O-saturated solutions containing
0.1 M NaN3 and 5 × 10-3 M PhCOS-, two main product peaks
were observed. The mass spectra of the two peaks were found
to resemble those of phenyl azide (PhN3) and aniline (PhNH2),
respectively, when their mass spectra were compared with those
found in the Finnigan NS library. In Figure 5, the mass spectra
of the radiolytic products PhN3 and PhNH2 are presented along
with the standard in the Finnigan NS library. These main
products are presumably formed through eqs 12 and 13.
Figure 4. (A) The decay rate of PhCOS• (kobs) versus its maximal
optical density measured at 460 nm when the solutions containing 0.1
M NaN3 and 5 × 10-3 M PhCOS- were saturated with (b) N2O, (0)
N2O/O2 (90:10) and (O) N2O/O2 (50:50). (B) Plot of OD°460/OD460
,
the maximal optical density of PhCOS• at 460 nm in N2O (OD°460
)
Ph• + N3- f PhN3
(12)
(13)
-•
divided by that in N2O/O2 saturated solutions, versus the oxygen
concentration. (C) Plot of kint(1 + K8[O2]) versus the oxygen concentra-
tion, where kint is the first-order decay rate of PhCOS• radicals given
by the intercept of the linear fit of the data in A and K8 is the equilibrium
constant for eq 8.
2PhN3-• disproportionate8 PhN3 + PhNH2 + N2
H2O, 2H+
Formation of aniline was clearly proved by GC-MS upon
comparison with the GC-MS of an authentic sample, while
authentic samples of phenyl azide are not available. The
disulfide (PhCOSSCOPh) which would be formed from dimer-
ization of the PhCOS• radicals was not found.
4. Reaction between the PhCOS• Radical and O2. Normal
alkylthiyl radicals are known to react with O2 to form peroxyl
radicals.1,25,26 The formation and the decay of PhCOS• radicals
in the presence of O2 in solutions were studied. When the
solutions containing 0.1 M NaN3 and 5 × 10-3 M PhCOS-
were saturated with three different concentrations of oxygen,
as shown in Figure 4B, with same the doses applied, the
maximal optical densities of PhCOS• radicals at 460 nm were
found to decrease with increasing oxygen concentrations. It
appears that benzoylthiyl peroxyl radicals are formed in eq 8.
Discussion
Structure of PhCOS• Radical. Normal alkylthiyl radicals
usually have weak absorption in the region from 300 to 330
nm. The PhS‚ radicals, due to their resonance stabilization by
the phenyl ring, have absorption in the 400-510 nm region
with ꢀ460nm ) 2590 M-1 cm-1 28 The spectrum of the PhCOS•
.
PhCOS• + O2 h PhCOSO2
(8)
•
radical measured in aqueous solution is rather similar to that of
PhS• radical but covers a broader wavelength interval and has
a much higher intensity. The spectrum of PhCOS• radical is
attributed to the resonance stabilization of sulfur radical by the
carbonyl group. Since for its oxygen counterpart PhCOO•
radical, the absorption in 500 to 700 or 800 nm was assigned
also to a transition that is localized in the COO• moiety of the
radical.12 A carbonyloxyl radical (e.g., ROCOO•) has a similar
absorption band in the region.29
In Figure 4A, the experimental decay rate of PhCOS• (kobs) is
ploted versus its maximal optical density measured at 460 nm.
The intercept was found to increase with increasing oxygen
•
concentrations. The PhCOSO2 radicals thus undergo a first-
order reaction presumably to form PhCOS(O)O• radicals.
PhCOSO2 f PhCOS(O)O•
(9)
•
This is seen to be an exact parallel to the case with alkylthiyl
(27) Assuming the fast equilibration in eq 8, C ) [PhCOS•] +
•
•
[PhCOSO2 ], K8 ) [PhCOSO2 ]/[PhCOS•][O2]. Thus, kintC ) k2[PhCOS•]
radicals. The rate constants of k8 and k-8 for alkylthiyls were
•
+ k9[PhCOSO2 ] ) (k2 + k9K8[O2])[PhCOS•]. Therefore, kint(1 + K8[O2])
reported to be 2 × 109 M-1 s-1, and 6 × 105 s-1 25,26
.
For
) k2 + k9K8[O2].
(25) Zhang, X.; Zhang, N.; Schuchmann, H.; von Sonntag, C. J. Phys.
Chem. 1994, 98, 6541-6547.
(26) Tamba, M.; Simon, G.; Quintiliani, M. Int. J. Radiat. Biol. 1986,
50, 595-600.
(28) Tripathi, G. N. R.; Sun, Q.; Armstrong, D. A.; Chipman, D. M.;
Schuler, R. H. J. Phys. Chem. 1992, 96, 5344.
(29) Chateauneuf, J.; Lusztyk, J.; Maillard, B.; Ingold, K. U. J. Am. Chem.
Soc. 1988, 110, 6727-6731.

One-Electron Redox Potential of Thiobenzoic Acid
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2815
Figure 5. The mass spectra of the radiolytic products, (A) PhNH2 and (C) PhN3, along with the standard in the Finnigan NS library, (B) PhNH2
and (D) PhN3.
Another noteworthy finding is that PhCOS• and PhS• radicals
Table 1. Summary of pKa, Reduction Potential E° in V, and BDE
in kcal/mol for Thiols and Their Oxygen Counterparts
behave similarly in that, unlike normal alkylthiyl radicals, they
do not favor the formation of disulfide radical anions (eq 4).28
The latter are known to form sulfur-centered three-electron
bonds (2σ/1σ*) in RS‚‚‚SR-.30 The fact that the PhSSPh-• and
the PhCOSSOCPh-• species are not favored in eq 4 also
suggests the PhS• and PhCOS• radicals to be resonance
stabilized. Upon the formation of these disulfide radical anions,
the three electron S‚‚‚S bonds are too weak to compensate for
the destruction of the resonance between the sulfur centered
radicals with the phenyl ring or the carbonyl group.
Reduction Potential and Bond Strength. In a recent
publication, Robert et al.15 reported the voltammetrically
measured one-electron oxidation potential of PhCOS- to be
0.72 V vs the reference electrode Ag/AgCl, KCl saturated in
DMA/N(Et)4ClO4, 0.1 M, when N,N-dimethylacetamide (DMA)
was used as solvent. This value corresponds to E°(PhCOS•/
PhCOS-) ) 0.94 V vs NHE in DMA. By comparison, our
present value of E°(PhCOS•/PhCOS-) ) 1.21 V vs NHE in
aqueous solution is higher by ca. 0.27 V. The higher reduction
potential of the PhCOS• radical in water can be largely attributed
to the higher solvation energy of the PhCOS- anion in water
as compared to that in DMA. Assuming similar solvation
energies for the PhCOS• radical in water and DMA, we estimate
the solvation of the PhCOS- anion to be stronger by ca. 6.2
kcal/mol in water than in DMA.
pKaa
E°(A•/A-)
E°(A•, H+/A-)
BDE
CH3S-H
RS-H
10
9.6
∼0.8j
∼1.3j
1.37d
1.08e
∼1.3j
1.36f
87.4h
0.8d
87.4h
80e
PhS-H
6.6
0.69e
∼1.1j
1.21f
CH3COS-H
PhCOS-H
CH3O-H
PhO-H
CH3COO-H
PhCOO-H
3.35b
2.48c
∼87j
87f
104.4i
86g
15.1
10.0
4.75
4.19
∼1.16j
0.79g
∼1.9j
∼2.0j
∼2.05j
1.38g
∼2.2j
∼2.2j
106i
∼106j
a Reference 31. b Reference 32. c Reference 21. d Reference 33.
e Reference 7. f This work. g Reference 34. h Reference 35. i Reference
36. j Eestimated.
them with the corresponding O-H bonds. A representative
selection is found in Table 1, where the pKa values, the reduction
potentials, and the corresponding standard reduction potentials
are compiled.
In the table, the bond dissociation energy (BDE) of CH3-
COS-H was assumed to be the same as that of PhCOS-H.
Similarly, the BDEs of CH3COO-H and PhCOO-H are also
assumed to be the same. The bond dissociation energy of the
corresponding S-H and O-H bonds in the gas phase is related
to the standard reduction potential through the equation, BDE
(kcal/mol) ) 23.1E°(A•,H+/AH) + 56.7.37,38 Using this relation,
At this point it is worthwhile and interesting to examine the
S-H bond strengths in various related compounds and compare
(31) Serjeant, E. P.; Dempsey, B. Ionization constants for organic acids
in aqueous solution; Pergamon Press: New York, 1979.
(32) Danehy, J. P.; Parameswaran, K. N. J. Chem. Eng. Data 1968, 13,
386.
(30) Go¨bl, M.; Bonifacic, M.; Asmus, K.-D. J. Am. Chem. Soc. 1984,
106, 5984.

2816 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Zhao et al.
the reduction potentials of the species were estimated, whenever
literature values were not available. Inspecting the table, we
can see that it agrees with the general trend that S-H bonds
are weaker than the corresponding O-H bonds. The trends in
thiyl radical reduction potentials and S-H bond energies upon
going from alkyl to aryl and to acyl thiols resemble those of
the corresponding oxygen counterparts. It is especially notice-
able that the S-H bond of thiobenzoic acid is almost identical
to that of the S-H bond of an alkyl thiol, while the S-H bond
of an aryl thiol is much weaker duo to the resonance stabilization
of PhS• radical. The fact that PhCOS• is also a resonance
stabilized radical indicates the similar extent of stabilization of
the PhCOSH molecule itself.39 In comparison to that of the
RSH molecule, the dipole moment of the S-H group in
PhCOSH is larger due to the electron-withdrawing carbonyl
group which stabilizes the partial negative charge on the sulfur.
The same trend is seen for the O-H bond energies in the
corresponding oxygen counterparts, i.e., the O-H bond of
benzoic acid is rather close to that of the O-H bond of an
alcohol, while the O-H bond of a phenol is much weaker.
Finally, the K8 of 2.3 × 103 M-1 for PhCOS• radicals
thiocarboxylic acids give different types of final products.
While RCOO• radicals decarboxylate, RCOS• radicals dimerize
to form the corresponding disulfide. Our product studies show
that, in aqueous solutions, the â-fragmentation of PhCOS•
radicals is the dominant route. This apparent contradiction can
be resolved knowing that the rate of â-fragmentation of PhCOS•
radicals is only 8.5 × 103 s-1 and considering the difference in
local radical concentractions between γ-radiolysis and elec-
trolysis. In the steady state γ-radiolysis of benzoyl thiolate
solutions, the radical production rate is rather low (ca 7.8 ×
10-8 M radicals per second), in contrast to the electrolytic
processes, in which high radical concentrations are usually
localized at the electrode.
The phenyl radical which is formed from the â-fragmentation
of PhCOS• radical is known to add to double bonds. The rate
constant of eq 12 was measured to be ca. 107 M-1 s-1, when
Ph• radicals were produced by the one-electron reduction of
-
iodobenzene (PhI) in the presence of N3 (Lind et al.,
-•
unpublished results). The PhN3 radical anion has a weak
absorption at 480 nm (ꢀ ) ca. 1000 M-1 cm-1) and is not
oxidized by O2 or Fe(III)(CN)63-. Only bimolecular decay was
observed, presumably to form PhN3 and PhNH2 (eq 13).
From the â-fragmentation of PhCOS• radicals in eq 2,
carbonyl sulfide, COS, is expected to be one of the main
products. Attempts to measure COS failed most probably
because it is hydrolyzed to form CO2 and H2S (at 25 °C, the
pseudo-first-order rate constant is 0.0011 s-1).40 The hydrolysis
is also known to be catalyzed by Bro¨nsted bases.40 The products
PhN3 and PhNH2 formed in the irradiated solution may catalyze
the hydrolysis of COS.
•
equilibrating with PhCOSO2 radicals is close to that, 3.2 ×
103 M-1, for the 2-mercaptoethanol thiyl radicals equilibrating
with its peroxyl radicals.25 This suggests the S-O bond
strengths in RS-O2• and PhCOS-O2• radicals are rather close
each other.
â-Fragmentation of PhCOS• Radical. Strong evidence for
the PhCOS• radicals to undergo â-fragmentation was provided
by both kinetic analysis upon pulse radiolysis and product
identification by gas chromatography-mass spectrometry (GC-
MS). The first-order rate constant of 8.5 × 103 s-1 for the
â-fragmentation of the PhCOS• radical is seen to be smaller
than for its oxygen counterpart, the PhCOO• radical. Equation
7 for the PhCOS• radical â-fragmentation in aqueous solution
can be compared with the one obtained by Chateauneuf et al.,12
log(k1/s-1) ) (12.6 ( 0.1) - (8.6 ( 0.3)/θ, for the PhCOO•
radical decarboxylation in pure CCl4. As can be seen, the
preexponential factors are very similar, suggesting simple bond
cleavage reactions. Unlike the symmetrical PhCOO• radical,
the PhCOS• radical should be more localized on the sulfur site.
The asymmetry of the PhCOS• radical could result in higher
bending energy for the O-C-S angle to form a linear state in
the â-fragmentation process.
Summary
One-electron oxidation of PhCOS- ions form PhCOS•
radicals. The reduction potential E°(PhCOS•/PhCOS-) and E°-
(PhCOS•,H+/PhCOSH) were measured to be 1.21 and 1.36 V
vs NHE, respectively. The PhCOS• radicals subsequently
undergo â-fragmentation to form Ph• radical and COS with a
rate constant of 8.5 × 103 s-1 at room temperature. The
â-fragmentation of the PhCOS• radical follows the Arrhenius
equation, log(k2/s-1) ) (12.3 ( 0.1) - (11.4 ( 0.2)/θ, where
θ ) 2.3RT kcal/mol. In 0.1 M NaN3 solutions, the Ph• radicals
-
-•
add to N3 to form PhN3 radical anion, which subsequently
disproportionate to form PhN3 and PhNH2 as final products.
The experimental data for the kinetic and thermochemical
properties of PhCOSH and PhCOS• radicals obtained in this
work are compared with those of their oxygen counterpart
PhCOOH and PhCOO• radicals.
As mentioned in the Introduction, electrochemical studies on
product formation upon oxidation of carboxylic acids and
(33) Armstrong, D. A. In S-Centered Radicals; Alfassi, Z., Ed.; John
Wiley & Sons: New York, 1997.
(34) Lind, J.; Shen, X.; Eriksen, T. E.; Merenyi, G. J. Am. Chem. Soc.
1990, 112, 479.
Acknowledgment. We thank Dr. Anna-Karin Borg-Karlsson
and her group at the organic chemistry department, KTH, for
the help during course of GC-MS experiments. Dr. Mats Jonsson
is acknowledged for valuable discussions during the writing of
this paper. We thank the Swedish Natural Science Research
Council for its financial support.
(35) Nicovich, J. M.; Kreutter, K. D.; van Dijk, C. A.; Wine, P. H. J.
Phys. Chem. 1992, 96, 2518-2528.
(36) McMillen, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982, 33,
493-532.
(37) Eberson, L. Acta Chem. Scand. 1963, 17, 2004.
(38) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287-
294.
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(39) Clark, K. B.; Wayner, D. M. J. Am. Chem. Soc. 1991, 113, 9363-
9365.
(40) Sharma, M. M. Trans. Faraday Soc. 1964, 61, 681-688.