Structure-reactivity studies on .OH radical reaction with substituted dialkyl sulfides

Article abstract of DOI:10.1039/a900830f

The results of the reaction of an ^.OH radical with a number of functionalized organic sulfides reported here demonstrate that the pH, the nature of the functional group and the chain length affect the nature of the ^.OH radical reaction with sulfides. The transient absorption spectrum (λ_max = 295 nm), formed on reaction of ^.H atoms and ^.OH radicals with an aqueous solution of 2-(methylthio)ethanol and 2-(ethylthio)ethanol is assigned to the α-thio radical. It decayed by second order kinetics with 2k = 5.2 × 10⁹ dm³ mol^-1 s^-1 and was quenched by oxygen. The transient absorption spectra (λ_max = 480 and 500 nm respectively), obtained on reaction with ^.OH radicals (pH = 1 ), are assigned to a sulfur centered dimer radical cation. The variation of the transient absorbance with pH showed an inflection point at pH =2.1. The reaction of ^.OH radicals (pH = 6) with 2,2′-thiodiethanoyl chloride showed the formation of α-thio radicals (λ_max = 300 nm) and an OH-adduct (λ = 350-380 nm), whereas in acidic solutions, the transient spectrum (λ_max = 340 nm, τ = 0.8 μs) is assigned to an intra-molecular radical cation with a 4-membered ring configuration. The transient species (λ_max = 370 nm, τ = 17 μs) formed on reaction of ^.OH radicals with a 3,3′-thiodipropionyl chloride is assigned to a 5-membered intra-molecular radical cation and remained independent of pH. The OH-adduct (λ_max = 350 nm, pH = 11) of 3,3′-thiodipropionamide is observed to undergo transformation to an intra-molecular radical cation (λ_max = 370 nm). The transformation is not observed in acidic solutions and only an intra-molecular radical cation (λ_max = 370 nm) is observed immediately after the pulse. The contribution of α-thio radicals of 2,2′-thiodiethanamide decreased with pH, and in neutral solutions ^.OH radicals are observed to react mainly by OH-adduct formation whereas in acidic solutions (pH = 1), as intra-molecular radical cation (λ_max = 335 nm) with a 4-membered ring configuration is inferred to be the transient species.

Full text of DOI:10.1039/a900830f

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ؒ
Structure–reactivity studies on OH radical reaction with
substituted dialkyl sulfides
V. B. Gawandi, H. Mohan and J. P. Mittal*†
Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
Received (in Cambridge) 1st February 1999, Accepted 12th May 1999
ؒ
The results of the reaction of an OH radical with a number of functionalized organic sulfides reported here
ؒ
demonstrate that the pH, the nature of the functional group and the chain length affect the nature of the OH radical
ؒ
ؒ
reaction with sulfides. The transient absorption spectrum (λmax = 295 nm), formed on reaction of H atoms and OH
radicals with an aqueous solution of 2-(methylthio)ethanol and 2-(ethylthio)ethanol is assigned to the α-thio radical.
It decayed by second order kinetics with 2k = 5.2 × 109 dm3 molϪ1 sϪ1 and was quenched by oxygen. The transient
ؒ
absorption spectra (λmax = 480 and 500 nm respectively), obtained on reaction with OH radicals (pH = 1), are
assigned to a sulfur centered dimer radical cation. The variation of the transient absorbance with pH showed an
ؒ
inflection point at pH = 2.1. The reaction of OH radicals (pH = 6) with 2,2Ј-thiodiethanoyl chloride showed the
formation of α-thio radicals (λmax = 300 nm) and an OH-adduct (λ = 350–380 nm), whereas in acidic solutions, the
transient spectrum (λmax = 340 nm, τ = 0.8 µs) is assigned to an intra-molecular radical cation with a 4-membered
ؒ
ring configuration. The transient species (λmax = 370 nm, τ = 17 µs) formed on reaction of OH radicals with a 3,3Ј-
thiodipropionyl chloride is assigned to a 5-membered intra-molecular radical cation and remained independent
of pH. The OH-adduct (λmax = 350 nm, pH = 11) of 3,3Ј-thiodipropionamide is observed to undergo transformation
to an intra-molecular radical cation (λmax = 370 nm). The transformation is not observed in acidic solutions and only
an intra-molecular radical cation (λmax = 370 nm) is observed immediately after the pulse. The contribution of α-thio
ؒ
radicals of 2,2Ј-thiodiethanamide decreased with pH, and in neutral solutions OH radicals are observed to react
mainly by OH-adduct formation whereas in acidic solutions (pH = 1), as intra-molecular radical cation (λmax = 335
nm) with a 4-membered ring configuration is inferred to be the transient species.
strong electrophile, it would immediately remove an electron
Introduction
from the dialkyl sulfide and the intermediate OH-adduct would
ؒ
The reaction of the hydroxyl radical ( OH) in aqueous solu-
be highly unstable. In the presence of electron withdrawing sub-
tions of dialkyl sulfides (R2S) and their substituted derivatives
stituents, the electron density at sulfur would decrease and the
OH-adduct and α-thio radicals may have long enough life times
has been the subject of recent interest.1–4 These studies have
gained in importance as sulfur centered radical species are
to be observed as the intermediates. With this objective, the
considered to be possible intermediates in redox reactions
of biomolecules.5–7 These studies are also important in under-
standing the physico-chemical processes taking place in bio-
logical systems as radicals and radical ions derived from sulfur
containing compounds play an important role in the chemistry
of biological systems.8
ؒ
nature of the OH radical reaction with substituted alkyl sul-
fides has been investigated using pulse radiolysis and it is shown
that the reaction is influenced by the substituents, the pH and
the chain length between sulfur and the substituted group.
Experimental
Synthesis of thiodialkanoyl chlorides
Hydroxyl radicals and specific one-electron oxidants, on
reaction with organic sulfur compounds produce sulfur cen-
tered radical cations either directly or via a complex sequence
of reactions involving an OH-adduct, an α-thio radical and
monomer radical cations.3,9,10 The OH-adduct and α-thio radi-
cals absorb in the 345–360 and 290–310 nm regions respect-
ively. Sulfur centered monomer radical cations are highly
unstable and absorb at 300 nm. Oxidized sulfur has a high ten-
dency to stabilize itself by coordinating with the free p-electron
pair of a second sulfur or other hetero atom such as O, P, N or
a halogen, both inter- and intra-molecularly.1–3,11,12 Such inter-
actions are represented by a 2c–3e bonded species containing
two bonding σ electrons and one antibonding σ* electron. The
The dry acids (2,2Ј-thiodiethanoic acid and 3,3Ј-thio-
dipropionic acid) were converted to thiodialkanoyl chlorides by
treating with an excess of thionyl chloride (reaction (1)) and
R(COOH)2 ϩ SOCl2 → R(COCl)2 ϩ SO2 ϩ HCl (1)
R = S(CH2)2; S(CH2CH2)2
refluxing the mixture for 80 min.13 The excess of thionyl chlor-
ide was removed on distillation at 78 ЊC.
ؒ
Synthesis of thiodialkanamides
nature of the OH radical reaction with substituted alkyl sul-
fides is observed to depend on the nature of the substituents,
the pH of the solution and the chain length between sulfur and
the substituent group. Sulfur in dialkyl sulfides has a high elec-
tron density due to the presence of two lone pairs of electrons
Concentrated ammonia solution was added to thiodialkanoyl
chlorides at 0 ЊC. The reaction mixture was heated to dryness.
The thiodialkanamide formed (reaction (2)) was recrystallised
ؒ
and electron releasing alkyl groups. As the OH radical is a
R(COCl)2 ϩ 2 NH3 → R(CONH2)2 ϩ 2 HCl (2)
from ethanol. The purity of the compounds was determined
with NMR and IR analysis.
† Also affiliated as Honorary Professor with the Jawaharlal Nehru
Centre for Advanced Scientific Research, Bangalore, India.
J. Chem. Soc., Perkin Trans. 2, 1999, 1425–1432
1425

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2-(Methylthio)ethanol (MTE) and 2-(ethylthio)ethanol
(ETE) obtained from Aldrich Chemicals were of high purity
and were used without further purification. All other chemicals
were also of high purity. The solutions in deionized ‘nanopure’
water were prepared in 1 × 10Ϫ3 mol dmϪ3 phosphate buffer
using Na2HPO4 and KH2PO4. Freshly prepared solutions
were used for each experiment. The pH was adjusted with
NaOH–HClO4.
Pulse radiolysis experiments were carried out with high
energy electron pulses (7 MeV, 50 ns) obtained from a linear
electron accelerator whose details are described elsewhere.14
The dose delivered per pulse was determined by the use of an
aerated aqueous solution of KSCN (1 × 10Ϫ2 mol dmϪ3) and
it was close to 12–15 Gy. All other experimental details are
described elsewhere.4 The transient absorption as a function of
time was recorded on a storage oscilloscope (100 MHz) inter-
faced to a computer for kinetic analysis.15 The transient species
were monitored using a 450 W pulsed xenon arc lamp, mono-
chromator (Kratos, GM-252) and Hamamatsu R-955 photo-
multiplier as the detector. The rate constant values were taken
from those kinetic analyses for which a very good correlation
was observed between experimental and calculated results. The
rate constant values are the average of three experiments and
the variation was within 15%. The structures of various com-
pounds used in this study are shown in Scheme 1.
Fig. 1 Transient absorption spectrum obtained on pulse radiolysis
of MTE (4.6 × 10Ϫ3 mol dmϪ3) (a) N2O-saturated, pH = 6; (b) N2-
saturated, pH = 1, tert-butyl alcohol = 0.3 mol dmϪ3; (c) N2-saturated,
pH = 1 and (d) aerated (pH = 1). Inset shows absorption–time profiles
at 480 nm in (e) N2 and (f) aerated solutions.
species of water radiolysis do not react with the solute initially
and only the specific one-electron oxidant reacts with the solute.
ؒ
Br atoms were generated on pulse radiolysis of a N2O-
saturated aqueous solution of 1,2-dibromoethane (2 × 10Ϫ2
CH2CH2CONH2
CH2CH2CONH2
CH2CH2COCl
CH2CH2COCl
mol dmϪ3) via reactions (8) and (9).
S
S
ؒ
ؒ
ؒ
CH2BrCH2Br ϩ H/ OH → CH2Br CHBr ϩ H2/H2O (8)
3,3′-thiodipropionamide (TDP)
3,3′-thiodipropionyl chloride (TDPC)
ؒ
ؒ
CH Br CHBr → CH ᎐CHBr ϩ Br
(9)
᎐
2
2
CH2CONH2
S
CH2CONH2
CH3
S
CH2CH2OH
The rate constant for the reaction of radiolytic species with
the solute was determined by monitoring the growth of the
transient absorption at λmax as a function of solute concen-
tration (2–8) × 10Ϫ4 mol dmϪ3 and the pseudo-first order rate
(kobs) was observed to increase with solute concentration. The
bimolecular rate constant was determined from the linear plot
of kobs vs. solute concentration. The molar absorptivity of the
transient species was determined under conditions such that the
radiolytic species had completely reacted with the solute and
only one pathway was available for the reaction.
2,2′-thiodiethanamide (TDE)
2-(methylthio)ethanol (MTE)
CH2COCl
S
CH2COCl
CH2CH3
S
CH2CH2OH
2,2′-thiodiethanoyl chloride (TDEC)
2-(ethylthio)ethanol (ETE)
Scheme 1
Radiolysis of N2-saturated neutral aqueous solution leads to
the formation of three highly reactive species ( H, OH, eaqϪ) in
addition to the formation of less reactive or inert molecular
ؒ
ؒ
Results
The ground state optical absorption spectra of various substi-
tuted sulfides in aqueous solutions, used in the present studies,
showed absorption maxima at λ < 280 nm with a very small
absorption at λ > 300 nm and remained the same at pH = 1–11.
The absorption spectra of 2,2Ј-thiodiethanoyl chloride and
3,3Ј-thiodipropinoyl chloride were different from those of the
corresponding acids, indicating that these acyl chlorides are not
hydrolysed under the present experimental conditions, within
the experimental time. These results suggest that pulse radi-
olysis investigations with optical absorption as the detection
technique can be employed without any correction for the
ground state absorption in the pH 1–11 region.
products (H2, H2O2).16 The reaction of OH radicals in neutral
ؒ
H2O → H, OH, eaqϪ, H3Oϩ, H2, H2O2
(3)
ؒ
ؒ
aqueous solutions was carried out under N2O-saturated con-
ditions where eaq are quantitatively converted to OH radicals
(reaction (4)). In acidic solutions, the reaction of OH radicals
Ϫ
ؒ
ؒ
Ϫ
Ϫ
ؒ
N2O ϩ eaq → OH ϩ OH ϩ N2
(4)
ؒ
was carried out in aerated solutions to scavenge H atoms
Ϫ
ؒ
(reaction (5)) and eaq are converted to H atoms (reaction (6)).
ؒ
ؒ
2-(Methylthio)ethanol
H ϩ O2 → HO2
(5)
(6)
Fig. 1a shows the transient optical absorption spectrum
obtained on pulse radiolysis of an N2O-saturated neutral aque-
ous solution of MTE (4.6 × 10Ϫ3 mol dmϪ3), which exhibits an
absorption band with λmax = 295 nm. The kinetic parameters of
the transient absorption band are shown in Table 1. The band
was observed to decay by second order kinetics with 2k =
5.2 × 109 dm3 molϪ1 sϪ1. Fig. 1b shows the transient absorption
spectrum obtained on pulse radiolysis of an N2-saturated acidic
Ϫ
ϩ
ؒ
eaq ϩ H → H ϩ H2O
ؒ
The reaction of H atoms was carried out at pH = 1 in the
presence of 0.3 mol dmϪ3 tert-butyl alcohol to scavenge OH
ؒ
radicals.
ؒ
ؒ
(CH3)3COH ϩ OH → CH2(CH3)2COH ϩ H2O (7)
(pH = 1) aqueous solution of MTE (4.6 × 10Ϫ3 mol dmϪ3
tert-butyl alcohol = 0.3 mol dmϪ3), which also showed an
,
ؒ
The reaction with a specific one-electron oxidant (Br ) was
carried out under conditions such that the primary radiolytic
1426
J. Chem. Soc., Perkin Trans. 2, 1999, 1425–1432

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ؒ
Table 1 Kinetic and spectral parameters of the transient formed on reaction of OH radicals with different substituted sulfides
λmax
nm
/
εmax/dm3
kf/dm3
kd/dm3
No.
1
Reaction
pH
6
molϪ1 cmϪ1
molϪ1 sϪ1
molϪ1 sϪ1
1.8 × 103
5 × 109
5.2 × 109 a
CH3
S
CH2CH2OH
295
+
+
+
+
+
+
+
+
•OH
•H
1.7 × 103
6.6 × 103
1.7 × 103
6.8 × 103
—
—
CH3
S
CH2CH2OH
2
3
4
5
6
1
1
6
1
6
295
480
295
500
300
3.2 × 109
4.5 × 109
3.8 × 109
6.5 × 109
1 × 104 b
5.1 × 109 a
3.7 × 104 b
6.2 × 106 c
CH3
S
CH2CH2OH
•OH
•OH
•OH
•OH
•OH
•OH
CH2CH3
S
CH2CH2OH
CH2CH3
S
CH2CH2OH
CH2COCl
S
CH2COCl
—
—
350–380
5.1 × 103
1.0 × 1010
2.4 × 109
1.2 × 106 b
4.9 × 104 b
CH2COCl
S
CH2COCl
7
8
1
6
340
3.4 × 103
CH2CH2COCl
S
370
CH2CH2COCl
CH2CH2CONH2
CH2CH2CONH2
CH2CH2CONH2
CH2CH2CONH2
CH2CH2CONH2
CH2CH2CONH2
CH2CONH2
9
10
11
12
13
14
11
350
370
1.5 × 103
—
7.3 × 109
—
1.4 × 105 b
3.1 × 104 b
S
S
S
S
S
S
+
+
+
+
+
+
•OH
5.5
370
370
—
4.5 × 104 b
•OH
•OH
•OH
•OH
•OH
1
11
6
3.6 × 103
6.1 × 109
—
3.7 × 104 b
310
340
—
2.3 × 104 b
2.3 × 105 b
1.7 × 103
CH2CONH2
3.5 × 105 b
8.2 × 103 b
CH2CONH2
345
310–330
—
—
—
CH2CONH2
CH2CONH2
1
335
2.4 × 109
2.7 × 105 b
CH2CONH2
a Second order decay (2k). b First order decay (sϪ1). c Second order decay (2k/εl)/sϪ1
.
absorption band at 295 nm. Pulse radiolysis of an N2-saturated
acidic (pH = 1) aqueous solution of MTE (4.6 × 10Ϫ3 mol
dmϪ3) showed an absorption band with λmax = 480 nm and a
small shoulder in the 290–310 nm region (Fig. 1c). The nature
of the spectrum and decay kinetics remained the same in an
aerated solution (Fig. 1d and inset of Fig. 1) indicating that the
ؒ
spectrum is due to the reaction of OH radicals and the contri-
ؒ
butions of the H atom is negligible. The variation of absorb-
ance at 480 nm as a function of pH showed an inflection point
at pH = 2.1. The yield and the life-time of the transient absorp-
tion band were observed to increase with solute concentration.
2-(Ethylthio)ethanol
Fig. 2a shows the transient absorption spectrum obtained on
pulse radiolysis of N2O-saturated neutral aqueous solution of
ETE (3.8 × 10Ϫ3 mol dmϪ3), which exhibits an absorption band
at 295 nm. At pH = 1, the transient absorption spectrum
showed a peak at 500 nm with a small shoulder in the 290–310
nm region (Fig. 2b). The kinetic parameters are shown in Table
1. The variation of absorbance at 500 nm, in aerated solutions,
showed an inflection point at pH = 2.1 (inset of Fig. 2). The
variation of the absorbance (500 nm) and decay rate with ETE
concentration are shown in Fig. 3 and the saturation value was
attained when the solute concentration was in the range of
Fig. 2 Transient absorption spectrum obtained on pulse radiolysis of
an aqueous solution of ETE (3.8 × 10Ϫ3 mol dmϪ3) (a) N2O-saturated at
pH = 6 and (b) N2-saturated (pH = 1). Inset shows variation of absorb-
ance at 500 nm as a function of pH.
2,2Ј-Thiodiethanoyl chloride
Fig. 4a exhibits the transient absorption spectrum obtained on
pulse radiolysis of an N2O-saturated neutral aqueous solution
(6–7) × 10Ϫ3 mol dmϪ3
.
J. Chem. Soc., Perkin Trans. 2, 1999, 1425–1432
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Fig. 3 Variation of (a) absorbance and (b) first order rate constant at
500 nm as a function of ETE concentration, formed on pulse radiolysis
of aerated aqueous solution of ETE at pH = 1. Dose = 16 Gy per pulse.
Fig. 5 (a) Transient absorption spectrum obtained on pulse radiolysis
of an N2O-saturated aqueous solution of TDPC (2.6 × 10Ϫ3 mol dmϪ3
)
at pH = 6. Dose = 58 Gy per pulse. (b) Transient absorption spectrum
obtained on pulse radiolysis of aerated acidic (pH = 1) solution of 1,2-
dibromoethane (4 × 10Ϫ2 mol dmϪ3) containing TDPC (1 × 10Ϫ3 mol
dmϪ3). Dose = 18 Gy. Inset shows variation of absorbance at 370 nm as
a function of solute concentration. Dose = 45 Gy per pulse.
Fig. 4 Transient absorption spectra obtained on pulse radiolysis of an
aqueous solution of TDEC (2.6 × 10Ϫ3 mol dmϪ3) (a) N2O-saturated at
pH = 6 and (b) N2-saturated (pH = 1). Inset shows variation of absorb-
ance at 340 nm as a function of pH.
Fig. 6 Transient absorption spectra obtained on pulse radiolysis of an
N2O-saturated aqueous solution of TDP (1 × 10Ϫ3 mol dmϪ3, pH = 11)
(a) 1 µs and (b) 7 µs after the pulse and (c) at pH = 6. Inset shows
absorption–time profiles at (d) 350 nm (pH 11) and (e) 370 nm
(pH 6).
of TDEC (2.6 × 10Ϫ3 mol dmϪ3) with an absorption band at
300 nm and a small shoulder in the 350–380 nm region. The
transient absorbance at 300 nm remained independent of solute
concentration in the (1–5) × 10Ϫ3 mol dmϪ3 region and decayed
by second order kinetics with 2k/εl = 6.2 × 106 sϪ1. The decay
kinetics in the 350–380 nm region could not be studied accur-
ately due to the very small size of the signal. As the pH of the
solution was decreased, the transient absorption was observed
to increase. Fig. 4b (pH = 1) shows the transient absorption
spectrum having an absorption band at 340 nm, decaying by
first order kinetics with k = 1.2 × 106 sϪ1. The absorbance at 340
nm remained independent of solute concentration, suggesting
it to be due to a monomeric species. The variation of absorb-
ance (340 nm) as a function of pH gave an inflection point at
pH = 1.2 (inset of Fig. 4). All other kinetic parameters are
shown in Table 1.
3,3Ј-Thiodipropionamide
Fig. 6a shows the transient absorption spectrum obtained on
pulse radiolysis of an N2O-saturated basic (pH = 11) aqueous
solution of TDP (1 × 10Ϫ3 mol dmϪ3), which exhibits an
absorption band with λmax = 350 nm. The analysis of the
absorption–time profile (Fig. 6d) revealed that the transient
absorption has two different types of decay. The initial portion
decayed by first order kinetics with k = 1.4 × 105 sϪ1 and the
latter portion with k = 3.1 × 104 Ϫ1. Time-resolved studies
s
showed the formation of another transient absorption band
with λmax = 370 nm (Fig. 6b). The absorbance of this band
remained independent of solute concentration and the rate
constant was determined to be equal to 7.3 × 109 dm3 molϪ1 sϪ1
.
Fig. 6c shows the transient absorption spectrum obtained on
pulse radiolysis of N2O-saturated aqueous solution of TDP
(1 × 10Ϫ3 mol dmϪ3, pH = 5.5). The absorption band at 370 nm
decayed by first order kinetics with k = 4.5 × 104 sϪ1. Pulse radi-
olysis of N2-saturated aqueous solution of TDP (1 × 10Ϫ3 mol
dmϪ3) at pH = 1, also showed one transient absorption band at
370 nm which decayed by first order kinetics with k = 3.7 × 104
3,3Ј-Thiodipropionyl chloride
Fig. 5a shows the transient absorption spectrum obtained on
pulse radiolysis of an N2O-saturated neutral aqueous solution
of TDPC (2.6 × 10Ϫ3 mol dmϪ3), which exhibits an absorption
band with λmax = 370 nm. The kinetic parameters of this band
are shown in Table 1. The nature of the transient absorption
spectrum remained independent of pH (1–11) and of solute
concentration (inset of Fig. 5).
s
Ϫ1. The absorbance at 370 nm remained independent of solute
concentration (0.6–4) × 10Ϫ3 mol dmϪ3. The molar absorptivity
1428
J. Chem. Soc., Perkin Trans. 2, 1999, 1425–1432

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Fig. 9 Transient absorption spectrum obtained on pulse radiolysis of
Fig. 7 Transient optical absorption spectra obtained on pulse radioly-
sis of an N2O-saturated aqueous solution of 2,2Ј-thiodiethanoamide
(1.4 × 10Ϫ3 mol dm3, pH = 11) (a) 1.5 µs and (b) 6 µs after the pulse.
Transient absorption spectra under aerated conditions (c) 1.5 µs and (d)
6 µs after the pulse and (e) absorption–time profile at 350 nm.
aerated aqueous solution of 2,2Ј-thiodiethanoamide (1.4 × 10Ϫ3 mol
dm3, pH = 1) (a) 1.5 µs and (b) 12 µs after the pulse. Inset shows (c) the
absorption–time profile at 335 nm and (d) the variation of absorbance
(335 nm) as a function of pH.
(Fig. 9c). The variation of absorbance at 335 nm showed an
inflection point at pH = 1.4 (Fig. 9d).
Discussion
Reactions with MTE and ETE
The nature of the transient spectrum, obtained on reaction of
ؒ
ؒ
OH radicals (pH = 6) and H atoms with MTE, and its decay
and formation kinetics, were similar for both MTE and ETE
(Fig. 1). The band was completely quenched by oxygen and is
therefore assigned to a carbon centered α-thio radical formed
ؒ
ؒ
ؒ
on H atom abstraction by H/ OH (reaction (10), Scheme 2).
CH3
CH3
S
•H/•OH
S
+
+
H2/H O (10)
2
CH2CH2OH
•CHCH2OH
Scheme 2
Fig. 8 Transient absorption spectrum obtained on pulse radiolysis of
an N2O-saturated aqueous solution of 2,2Ј-thiodiethanoamide (1.4 ×
10Ϫ3 mol dm3, pH = 6) (a) 1.5 µs and (b) 6 µs after the pulse and transi-
ent absorption spectra under aerated conditions (c) 1.5 µs and (d) 6 µs
after the pulse and (e) absorption–time profile at 345 nm.
In analogy with these studies, the transient absorption band
formed on reaction of OH radicals with ETE at neutral pH is
also assigned to α-thio radicals. In acidic solutions, the nature
ؒ
of the spectrum and the decay kinetics observed on reaction of
ؒ
OH radicals with MTE and ETE remained the same in N2 and
was determined to be 3.6 × 103 dm3 molϪ1 cmϪ1 and other
kinetic parameters are given in Table 1.
aerated conditions indicating that the transient species is not
due to a carbon centered radical. Since the yield and the life-
time of the transient species were observed to increase with
solute concentration (Fig. 3), it is probably a dimer radical
cation (reaction (11), Scheme 3). It is also known that oxidized
2,2Ј-Thiodiethanamide
Fig. 7a shows the transient absorption spectrum obtained on
pulse radiolysis of an N2O-saturated aqueous solution of TDE
(1 × 10Ϫ3 mol dmϪ3, pH = 11), which exhibits absorption bands
at 310 and 340 nm. The time resolved studies revealed that the
absorption at 340 nm decays faster (k = 2.3 × 105 sϪ1) than that
at 310 nm (k = 2.3 × 104 sϪ1) and only one band (310 nm) with a
small shoulder in the 330–370 nm region was observed 5 µs after
the pulse (Fig. 7b). The absorption–time profile also suggests
the presence of two types of species (Fig. 7e). The transient
absorption spectra in aerated solution showed the presence of
only one band at 340 nm (Fig. 7c). The transient absorption
spectrum of TDE under N2O-saturated conditions at pH = 6,
showed only one band at 345 nm (Fig. 8a) with some absorp-
tion in the 310–330 nm region (Fig. 8b) which could be seen 5 µs
after the pulse. The spectrum in aerated conditions (Fig. 8c)
showed only one band at 345 nm. The transient absorption
spectrum at pH = 1, showed only one band at 335 nm (Fig. 9a)
which decayed by first order kinetics with k = 2.7 × 105 sϪ1
CH3
S
CH2CH2OH
•+
CH3
CH3
CH3
•OH + H+
CH2CH2OH
S
•+S
S
+
(11)
CH2CH2OH
CH2CH2OH
2
Scheme 3
sulfur has a tendency to a stabilize on coordination with oxygen
forming an intra-molecular 3-electron bonded species
S
+O
•
••
(
).1 The large difference in the electronegativity of
S and O (1.06 eV) and the relatively unstable nature of a 4
•
S
+O
••
membered ring for the intra-molecular
3-electron
bonded species, may prevent its formation. 2-(Ethylthio)-
ethanol is similar in structure to MTE and pulse radiolysis
J. Chem. Soc., Perkin Trans. 2, 1999, 1425–1432
1429

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ؒ
studies at pH = 1 also showed similar results and thus support
the reaction mechanism. In case of ETE, the transient absorp-
tion band of the dimer radical cation (500 nm) showed a red
shift, which is due to the presence of an additional electron
releasing methyl group. The life-time of the dimer radical cation
of ETE (27 µs) was shorter than that of MTE (100 µs), which
may be due to the bulkier ethyl group present in the molecule.
The reaction of OH radicals with TDPC showed the form-
ation of one transient absorption band at 370 nm and remained
independent of pH (1–11) and solute concentration (inset of
Fig. 5). The nature of the transient absorption spectrum also
remained the same in aerated solutions. These studies suggest
that the transient absorption band is due to a monomeric
cationic species. The band is assigned to the intra-molecular
radical cation forming a 5-membered ring on p-orbital overlap
of oxidized S and Cl (Scheme 6).
ؒ
The reaction of the Br radical, which is a specific one-electron
Ϫ
oxidant (Eo for Br /Br couple = 1.9 V), with MTE and ETE
ؒ
also produced transient absorption spectra similar to those
ؒ
observed on reaction of OH radicals with MTE and ETE at
CH2CH2COCl
H2C CH2
+
•OH
+
OH– (14)
S
pH = 1, thus supporting the assignment of the transient absorp-
tion band to cationic species. The OH-adduct formed in an
acidic medium may be too short lived to detect under our
experimental conditions.
O
C
S
CH2CH2COCl
••+
•
Cl
Scheme 6
Reactions with TDEC and TDPC
A 5-membered ring configuration is expected to have higher
stability as compared to the 4-membered ring configur-
ation observed in the case of TDEC. The higher stability is
reflected in the increased life-time of the intra-molecular radical
cation of TDPC (20 µs) as compared to that of TDEC (0.8 µs).
The intra-molecular radical cation of TDPC (Fig. 5a) was
formed even at pH = 11, which also supports the expectation
that the 5-membered ring is more stable, unlike the 4-membered
ring configuration of TDEC which was formed in acidic
solutions. The transient absorption spectrum obtained on reac-
As mentioned earlier, the α-thio radicals formed on abstraction
ؒ
ؒ
ؒ
of H atoms by H/ OH radicals from dialkyl sulfides absorb in
the 290–310 nm region. Therefore, the transient absorption
band at 300 nm (Fig. 4a) formed on reaction of OH radicals
ؒ
with TDEC at pH = 6 is assigned to α-thio radicals. The small
shoulder in the 350–380 nm region may be due to the OH-
adduct as the OH-adducts of dialkyl sulfides absorb in the 350–
ؒ
370 nm region. The reaction of OH with TDEC is represented
by Scheme 4.
ؒ
tion of Br atoms with TDPC (Fig. 5b) was similar to that
•CHCOCl
ؒ
+
H2O
(12a)
obtained on reaction of OH radicals, thus supporting the
S
CH2COCl
formation of the cationic species. BrϪ formed an oxidation
ؒ
CH2COCl
CH2COCl
of these acyl chlorides by Br may react with a solute radical
+
•OH
S
cation (>S ϩ) to form a 3-electron bonded species >S∴Br. But
᝽
ؒ
CH2COCl
CH2COCl
the transient absorption band obtained on reaction of Br atom
•
HO
S
(12b)
••
with TDEC and TDPC were similar to those obtained on
ؒ
reaction with OH radicals at pH = 1, suggesting that the
Scheme 4
formation of >S∴Br is not taking place.
The contribution of individual processes could not be deter-
mined. The absorbance of the transient absorption band at 340
nm at pH = 1 remained independent of solute concentration,
suggesting the formation of a monomeric species. Simple sulfur
centered monomer radical cations of dialkyl sulfides are very
unstable and are known to stabilize on coordination with
another heteroatom. Therefore, the transient absorption could
not be due to a simple sulfur centered monomer radical cation.
The small difference in the electronegativity of S and Cl (0.39
eV) may favour the formation of an intra-molecular radical
Reactions with TDP and TDE
In analogy with earlier studies and with those reported in the
literature, the transient absorption band at 350 nm, formed on
ؒ
the reaction of OH radicals with TDP (Fig. 6a) is assigned to
the OH-adduct. The transient band with λmax = 370 nm (Fig.
6b) is assigned to the intra-molecular radical cation formed
with a 5-membered ring configuration. The 5-membered ring
can be formed by bonding between S and O or N. But the
difference in the electronegativity between S and N (0.63 eV) is
less than that between S and O (1.06 eV). Therefore, it is
expected that the intra-molecular radical cation is formed
between S and N (Scheme 7). Intra-molecular radical cations
formed between S and N are known to absorb in this region.1,3,9
•
(
S
+Cl
)
cation
in competition with the formation of the
••
dimer radical cation. The larger difference in the electronega-
tivity of S and O (1.06 eV) may not favour the formation of an
intra-molecular radical cation between sulfur and oxygen. In
acidic solutions, the transient absorption band at 340 nm is
therefore assigned to the intra-molecular radical cation formed
on p-orbital overlap of oxidized sulfur and Cl forming a 4-
(a)
H2NOCH2CH2C
H2NOCH2CH2C
H2NOCH2CH2C
H2NOCH2CH2C
•OH
S
OH
Product (15a)
Product (15b)
•
S
+
••
ؒ
membered ring (Scheme 5). The reaction of Br atoms with
CH2COCl
H2C CH2
O
C
CH2
S
•OH
+
H+
+ H2O (13)
+
OH–
+
S
O
C
S
Cl
CH2COCl
••+
••+
•
•
H2N
Scheme 5
(b)
TDEC also showed the formation of a transient absorption
spectrum similar to that obtained on reaction of OH radicals
Scheme 7
ؒ
at pH = 1. The nature of the transient absorption spectrum
It is possible that either (1) the OH-adduct and the monomer
radical cation are formed simultaneously or (2) the OH-adduct
is converted on decay to the intramolecular radical cation as
shown in Scheme 7. If the second mechanism is operative, then
the decay of the OH-adduct should be affected by pH and the
OH-adduct should not be observed at lower pH. Only one band
at 370 nm was observed at pH = 5.5 (Fig. 6c), which showed one
component decay by first order with k = 4.5 × 104 sϪ1 (Fig. 6e).
remained the same in aerated and deaerated solutions suggest-
ؒ
ing that the contribution of the H atom reaction is negligible
and the transient species is not due to a carbon centered radical.
These studies support the assignment of the transient absorp-
tion spectrum (Fig. 5b) to a cationic species. The OH-adduct
and α-thio radicals if formed in acidic conditions may be too
short lived to detect under our experimental conditions.
1430
J. Chem. Soc., Perkin Trans. 2, 1999, 1425–1432

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ؒ
ؒ
Therefore, OH radicals are reacting with the solute to form the
obtained on reaction of Br atoms with TDE, thus supporting
the assignment to the solute radical cation.
OH-adduct, which on decay is converted to an intra-molecular
radical cation absorbing at 370 nm as shown in Scheme 7. The
OH-adduct would decay very fast at lower pH. At pH = 1 only
one band at 370 nm was observed. The decay of this band was
not affected in aerated solutions and the nature of the transient
absorption spectrum was similar to that observed on reaction
of Br atoms with TDP at pH = 1, thus supporting its assign-
ment to the solute radical cation.
The normalized absorbance [∆A/G(OH)]‡ of the transient
band (370 nm) observed on pulse radiolysis of N2O-saturated
aqueous solution of TDP at pH = 6 was 3.1 × 103, much less
The 4-membered ring configuration is expected to be less
stable than the 5-membered ring configuration and this is sup-
ported by the decreased life-time (4 µs) for the radical cation
of TDE in comparison to that of TDP (27 µs). The spectral
ؒ
changes observed on reaction of OH radicals with these two
ؒ
amides are not due to the reaction with the amide group as the
observed absorption spectra are different from those observed
with various simple amides.17 The OH-adduct, if formed,
may be too short lived to observe under our experimental
conditions.
ؒ
than that obtained on reaction of OH radicals with TDP at
pH = 1 [∆A/G(OH) = 5.3 × 10Ϫ3] and also on reaction of
Effect of pH
ؒ
specific one-electron oxidant Br atoms with TDP [∆A/
G(OH) = 5 × 10Ϫ3]. It may be due to the fact that at pH = 6,
some of the OH-adduct may be decaying by reaction (15a). At
pH = 6 removal of OHϪ may not be favoured as compared to
removal of H2O in acidic solutions. The remaining fraction of
OH-adduct may decay to a stable product at this pH. The trans-
formation of the OH-adduct to solute radical cation at this pH
may be too fast to observe distinctly as at pH = 11. The trans-
formation would slow down at higher pH.
In case of 2,2Ј-thiodiethanoamide (TDE), the time resolved
studies at pH = 11 revealed the formation of two transient
species absorbing at 310 nm and 340 nm. The absorption at 310
nm was not seen in aerated solutions whereas the absorption at
340 nm was still present in aerated solutions and is assigned to
the OH-adduct. The band at 310 nm is assigned to a carbon
centered radical (α-thio radicals) as these are known to have a
high reactivity with oxygen. Both the species might be formed
simultaneously (Scheme 8) as time resolved studies do not show
the formation of a band with decay of another band.
The acid-catalysed oxidation of a number of organic com-
pounds has been reported in the literature.4,18 Since the decay of
OH-adduct or α-thio radicals do not follow the formation of
the radical cation in acidic solutions, the acid-catalysed oxid-
ation of substituted sulfides cannot be taking place on reaction
of Hϩ with OH-adduct or α-thio radicals. The rate constant for
ؒ
the reaction of OH radicals remained independent of pH,
ؒ
showing that OH radicals react directly with the solute (in the
presence of Hϩ) to form the radical cation. The increase in the
ؒ
absorbance with decreasing pH is due to the fact that more OH
radicals are reacting by the acid-catalysed mechanism than by
OH-adduct or α-thio radical formation. The saturation value at
lower pH represents the complete formation of radical cations.
The variation of absorbance with pH does not represent
a thermodynamic equilibrium but shows the existence of
different processes at different pHs. The electron transfer
reaction, at neutral pH, would take place with the removal of
OHϪ, which may not be a favourable process in comparison
with the removal of H2O in acidic solutions.19 Therefore
•CHCONH2
ؒ
OH radicals may react with different mechanisms at different
S
+ H2O (16a)
pHs.
CH2CONH2
CH2CONH2
CH2CONH2
+
•OH
S
CH2CONH2
CH2CONH2
•
(16b)
Conclusions
HO
S
••
The hydroxyl radicals react with dialkyl sulfides to form a
3-electron bonded sulfur centered (>S∴ϩS<) dimer radical
cation. In the presence of a functional group containing a
heteroatom (Cl, N, O), the nature of the transient species
Scheme 8
The contribution of individual processes could not be deter-
mined as the bands are close together. Based on the absorbance
at 340 nm in aerated solutions and G(OH) = 2.7, the molar
absorptivity at 340 nm was determined to be 1.7 × 103 dm3
molϪ1 cmϪ1. At pH = 6, the contribution of α-thio radicals is
observed to be less than at pH = 11 as the transient absorption
spectrum (Fig. 8a) showed only one band at 345 nm with a
small contribution by α-thio radicals. In aerated solutions only
one band at 345 nm was observed.
In acidic solutions (pH = 1), only one band at 335 nm was
observed (Fig. 9a) and its absorption time profile was different
from those at pH = 6 and 11. This band was observed when the
pH was less than 3. Its molar absorptivity was also different
from that of the OH-adduct. The band is assigned to the intra-
molecular radical cation (Scheme 9) forming a 4-membered
ؒ
formed on reaction of OH radicals depends strongly on the
nature of the substituent group, the pH of the solution and
the chain length of the functional group. The presence of the
-CH2CH2OH group results in the formation of α-thio radicals
and dimer radical cations in neutral and acidic solutions
respectively. In the case of the -CH2CH2CH2OH group, an
intra-molecular radical cation with p-orbital overlap between
oxidized sulfur and O forming a 5-membered ring is observed
in neutral solutions.4c The reaction of OH radicals with 2,2Ј-
ؒ
thiodiethanoyl chloride leads to the formation of α-thio rad-
icals in neutral solutions. An intra-molecular radical cation
forming a 4-membered ring between oxidized sulfur and chlor-
ine is inferred to be the transient species in acidic solutions. In
the case of 3,3Ј-thiodipropionyl chloride, an intra-molecular
radical with a 5-membered ring is observed even in neutral solu-
O
C
CH2
S
ؒ
CH2CONH2
CH2CONH2
tions. The reaction of OH radicals with 2,2Ј-thiodiethano-
S
+
OH
+
H+
(17)
H2N
amide forms α-thio radicals and OH-adduct/intra-molecular
••+
•
radical cation in neutral and acidic solutions respectively. The
ؒ
Scheme 9
OH-adduct formed on reaction of OH radicals with 3,3Ј-
thiodipropionamide is observed to undergo transformation to
the intra-molecular radical cation in basic solutions and the
intra-molecular radical cation without any transformation from
OH-adduct in acidic solutions. Thus the nature and the chain
length of the substituent group play important roles in the
ring on p-orbital overlap of oxidized sulfur with nitrogen. The
nature of the transient absorption spectrum at pH = 1 remained
the same in aerated solutions and was also similar to that
ؒ
ؒ
nature of the OH radical reaction with substituted alkyl
sulfides.
‡ G(OH) denotes the number of OH radicals per 100 eV of absorbed
energy.
J. Chem. Soc., Perkin Trans. 2, 1999, 1425–1432
1431

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