Kinetic modeling of the photooxidation of dimethyldisulfide in the liquid phase

Article abstract of DOI:10.1002/(SICI)1097-4601(1997)29:11<825::AID-KIN3>3.0.CO;2-O

A reaction mechanism for the photooxidation of dimethyldisulfide (DMDS) in aqueous acetonitrile has been established by kinetic modeling the UV absorbance vs. time curves under continuous irradiation. The model, built according to the known solution reactivity of oxysulfur radicals, consists of 22 steps involving 6 radical and 10 nonradical species. The first steps of the mechanism are the homolytic cleavage of the DMDS S-S bond with formation of methanethiyl radicals (CH₃S^·) followed by addition of these radicals to molecular oxygen. There are photoequilibria between thiyl (CH₃S^·), sulfinyl (CH₃S^·), and sulfonyl (CH₃SO₂^·) radicals and the corresponding molecular species (methyl methanethiosulfinate CH₃S(O)SCH₃ or MMTSI, methyl methanethiosulfonate CH₃S(O)₂SCH₃ or MMTS and methanesulfinic acid CH₃S(O)OH or MSIA) which appear as long lived intermediates. Reactions of sulfonyl radicals with oxygen lead to methanesulfonic acid (CH₃S(O)₂OH) or MSA. Cleavage of sulfonyl radicals gives SO₂ and CH₃^·, the parent compounds of sulfuric (H₂SO₄) and methanoic (HCOOH) acids. The predictive power of the model was tested at higher initial concentration of DMDS in anhydrous and aqueous acetonitrile. In these conditions, the proposed mechanism gives a semiquantitative description of the course of the reaction and reproduces the kinetic behavior of the long lived intermediates.

Full text of DOI:10.1002/(SICI)1097-4601(1997)29:11<825::AID-KIN3>3.0.CO;2-O

Kinetic Modeling of the
Photooxidation of
Dimethyldisulfide in the
Liquid Phase
EVELYNE ROBERT-BANCHEREAU,1 SYLVIE LACOMBE,1 JEAN OLLIVIER,2
JEAN CLAUDE MICHEAU,3 DOMINIQUE LAVABRE3
1Laboratoire de Physico-Chimie Mole´culaire, UMR 5624, University of PAU, F-64000 Pau, France
2Groupement de Recherches de Lacq, Elf, Elf Atochem, BP 34, F-64170 Artix, France
3Laboratoire des Interactions Mole´culaires, Re´activite´ Chimique et Photochimique, UMR 5623, University Paul Sabatier,
118 route de Narbonne, F-31062, Toulouse, France
Received 18 November 1996; accepted 17 March 1997
ABSTRACT: A reaction mechanism for the photooxidation of dimethyldisulfide (DMDS) in
aqueous acetonitrile has been established by kinetic modeling the UV absorbance vs. time
curves under continuous irradiation. The model, built according to the known solution reac-
tivity of oxysulfur radicals [1], consists of 22 steps involving 6 radical and 10 nonradical species.
The first steps of the mechanism are the homolytic cleavage of the DMDS S—S bond with
и
formation of methanethiyl radicals (CH3S ) followed by addition of these radicals to molecular
и
и
oxygen. There are photoequilibria between thiyl (CH3S ), sulfinyl (CH3S ), and sulfonyl
и
(CH3SO2 ) radicals and the corresponding molecular species (methyl methanethiosulfinate
CH3S(O)SCH3 or MMTSI, methyl methanethiosulfonate CH3S(O)2SCH3 or MMTS and meth-
anesulfinic acid CH3S(O)OH or MSIA) which appear as long lived intermediates. Reactions of
sulfonyl radicals with oxygen lead to methanesulfonic acid (CH3S(O)2OH) or MSA. Cleavage
и
of sulfonyl radicals gives SO2 and CH3 , the parent compounds of sulfuric (H2SO4) and meth-
anoic (HCOOH) acids. The predictive power of the model was tested at higher initial concen-
tration of DMDS in anhydrous and aqueous acetonitrile. In these conditions, the proposed
mechanism gives a semiquantitative description of the course of the reaction and reproduces
the kinetic behavior of the long lived intermediates. ᭧ 1997 John Wiley & Sons, Inc. Int J Chem
Kinet: 29: 825–834, 1997.
INTRODUCTION
sphere either from natural sources or from human ac-
tivities [2]. The atmospheric photooxidation of DMDS
[3–10] and DMS [4,5,7,8,11–14] has been exten-
sively studied over the last decade. Both methanesul-
fonic (MSA) and sulfuric acids are produced, which
contribute to acid rain, a current environmental con-
cern. Depending on the reaction conditions, other
products such as sulfur dioxide (SO2), methanal
(HCHO), methanol (CH3OH), or methanoic acid
(HCO2H) have also been detected. Although the yield
of each product cannot always be accurately deter-
Reduced sulfur compounds such as hydrogen sulfide
(H2S), methanethiol (CH3SH), dimethylsulfide
(CH3SCH3 or DMS), and dimethyldisulfide
(CH3SSCH3 or DMDS) are released into the atmo-
Correspondence to: S. Lacombe
Contract grant Sponsor: Elf Atochem
᭧ 1997 John Wiley & Sons, Inc.
CCC 0538-8066/97/110825-10

826
BANCHEREAU ET AL.
mined under gas-phase conditions, the crucial role of
quartz cuvette via an optic fiber from a nonfiltered high
pressure mercury lamp (Oriel 200W). Only the
254 nm line is absorbed significantly by the reaction
medium allowing us to make the monochromatic
и
methanethiyl radicals (CH3S ) and their further reac-
tion with oxygen has been demonstrated [6–
10,13,14].
In addition, recent studies on alkyldisulfides have
shown that short-wavelength (Ͻ320 nm) irradiation
in the presence of oxygen in alcohols or aqueous ace-
tonitrile leads to the corresponding sulfonic and car-
boxylic acids along with sulfuric acid [15,16]. In the
case of DMDS, intermediate compounds such as
methyl methanethiosulfonate (CH3S(O)2SCH3 or
MMTS) and methanesulfinic acid (CH3S(O)OH or
MSIA), as well as SO2, have been detected. The rates
of formation and disappearance of these species have
been monitored in different solvents. Further experi-
ments have indicated the involvement of thiyl radicals
and a rough tentative mechanism has been proposed.
In order to gain further understanding of the pho-
tooxidation of DMDS in dilute acetonitrile solution,
we recorded by UV spectroscopy the absorbance vs.
time curves at four different wavelengths under con-
tinuous irradiation. Two independent experiments
were performed and the data obtained were used to
test a kinetic model of the reaction.
assumption; the volumic photon flux I0 could be
6
varied using
a
diaphragm from
2
10Ϫ to
10Ϫ5 mol LϪ1sϪ1. Oxygen was bubbled in the reactor
ؒ
during the reaction and the solution was continuously
stirred with a magnetic bar.
Concentrated DMDS solutions (.2 mol LϪ1) were
ؒ
irradiated using a 250 mL immersion well photoreac-
tor equipped with a 450W Hanovia medium pressure
mercury lamp.
MMTSI was synthesized by oxidation of DMDS
with m-chloroperbenzo¨ıc acid in CHCl3 at Ϫ20ЊC, ac-
cording to Small et al. [17].
UV/visible spectra were recorded by a diode-array
spectrophotometer (HP 8451) connected to a HP 9000/
710 series workstation.
Simulation and Fitting
To simulate the model, the differential equations gov-
erning the kinetics of all the proposed chemical species
were integrated numerically (semiimplicit Runge–
Kutta method [18]). Some parameters were fixed to a
known value: the initial concentrations of the species
and the molar extinction coefficients of the known sta-
ble compounds (Table I). Other parameters such as
rate constants, quantum yields, and molar extinction
coefficients of postulated nonisolated intermediates
(polysulfides, oxidation products from alkyl radicals,
and sulfur dioxide-acid complex) were given an initial
value, either from literature data or chosen arbitrarily
and then fitted numerically using an iterative mini-
mization algorithm of the Powell type [19]. The
change in absorbance was deduced from the calculated
EXPERIMENTAL
Materials and Methods
Solutions: Dimethyldisulfide (DMDS) (obtained from
Elf Atochem) and acetonitrile (Standard HPLC quality
99.9%) were used without further purification. The
water content of acetonitrile was determined by Karl–
Fischer titration.
The photooxidation experiments were performed
as follows: 2 ml of dilute DMDS solution (2–4
10Ϫ3 mol LϪ1) were irradiated in a 1 cm ϫ 1 cm
ؒ
Table I Molar Extinction Coefficients (L·molϪ1 ·cmϪ1) of Known Stable Compounds
Epsilon
Maximum
(␭nm)
Wavelength (nm)
Dimethyl-disulfide
234
252
254
400
270
318
290
126
400
(254)
72
(236)
2300
(254)
380
Methyl methanethio-sulfonate
Methyl methanethio-sulfinate
Sulfur dioxide
71
1266
43
47
2300
99
21
1371
292
0
6
369
342
0
(280)
60
Methanoic acid
35
5
(210)

KINETIC MODELING OF THE PHOTOOXIDATION OF DIMETHYLDISULFIDE IN THE LIQUID PHASE
827
Table II Rate Constant and Quantum Yields from the Literature in the Gas Phase and in the Liquid Phase and
after Inverse Treatment. Literature References are into Brackets. Units are: sϪ1 for k4, k15, k16, and k22; L·molϪ1 ·sϪ1 for
k1, k3, k5, k6, k9, k12, k18, and k21; and L2 ·molϪ2 ·sϪ1 for k11 and k20
Rate Constants in Gas Phase
Rate Constants in Liquid Phase
Constants Issued from Optimization
⌽
k1
0.13
0
3 1010 [4]
1.4 1013
Ϫ
3
⌽
2
2 10
3.5 106
k3
k4
k5
k6
k7
⌽
3.5 104 [25]
102 [4]
4.8 1010 [4]
107 to 109 [21,25]
Ϫ2
9.7 10
1.9 1010
6.5 106
3.5 1012
0.29
6 107 to 1.1 1010 [26]
2.5 1010 [4]
8
k9
3.5 107
0.38
3.2 1010
⌽
10
k11
k12
k13
k14
k15
k16
⌽
Ϫ3
1.5 10
1.1 101 to 5.1 102 [4], [29]
2 108 to 1010 [4], [28], [31]
108 [34]
8.3 104
7.3 108
5.9 103
Ϫ5
8.2 10
5.4 10
Ϫ4
17
k18
k19
k20
k21
k22
9.3 104
8.5 103
8.3 1014
4.0 1010
1.6 105
4.5 109 [31]
4.5 109 [31]
1.6 105 [39]
concentrations in each species and from their molar
extinction coefficients according to Beer–Lambert’s
law.
only shown for the purpose of demonstration as they
tend to iron out small imperfections in the model.
RESULTS AND DISCUSSION
Absorbing Species
Absorption spectra of the known stable species were
recorded in acetonitrile.
Kinetic Study of the Photooxidation of
DMDS in Aqueous Acetonitrile
Unknown molar extinction coefficients derived
from the minimization procedure are listed on Table
III (see below). They were attributed to nonisolated
species such as polysulfides, the products of oxidation
of carbon compounds, and a SO2/H2SO4 complex.
However, these values of extinction coefficients are
UV/visible spectroscopy was chosen in order to mon-
itor this reaction. This technique allows continuous
measurements of all absorbing species including sub-
strates, products, and long lived intermediates without
any perturbation. Two separate experiments with
somewhat different values of [DMDS]0, [H2O]0, and
I0 (incident volumic photon flux), gave results quali-
tatively comparable. The change in the UV/visible ab-
sorption spectrum recorded in experiment 1 is illus-
trated in Figure 1.
Table III Optimized Molar Extinction Coefficients of
1
Nonisolated Intermediates (L·molϪ1 ·cmϪ
)
From the onset of irradiation, a fall of the absorp-
tion band of DMDS was observed concomitant with
the appearance of a peak at 280 nm attributed mainly
to SO2. This peak decreased slowly up to the end of
the reaction. Four wavelengths for which the changes
in absorption vs. time were independent (234, 254,
270, and 290 nm) were selected to represent the over-
Wavelength (nm)
234 254 270 290
S2 (polysulfides)
383 314 532 397
Methanal and other interme-
diates from alkyl radical
oxidation
186
69
94 101
Sulfur dioxide-acid complex
518 119 292 342

828
BANCHEREAU ET AL.
anesulfonic acid (MSA) and sulfuric acid by ion ex-
change chromatography at the end of experiment 2.
MSA and sulfuric acids corresponded to 80% and
20%, respectively, of the initial sulfur. These values
were of the same order of magnitude as those found
in previous experiments (75% and 15%) for a 100 fold
higher concentration of DMDS in a 250 mL immer-
sion well reactor [15].
Putative Mechanism
From these results and literature data on the reactivity
of molecular and radical intermediates [1], several
possible mechanisms were analyzed and tested quali-
tatively. However, the best results were obtained with
the mechanism of Scheme I using the parameters listed
in Tables I, II, and III (see below for Tables II and
III). Some processes were deliberately simplified in
order to facilitate the kinetic modeling. In particular,
all the unstable molecular intermediates which were
not detected experimentally were not introduced ex-
plicitly into the model. Thermal steps were taken as
pseudo-elementary (characterized by a rate constant
ki ). For photochemical steps (characterized by a quan-
tum yield ␾j ), competitive absorption of the different
species was taken into account by introducing the pho-
tokinetic factor:
Figure 1 Evolution of the UV-Visible spectrum of a
Ϫ
3
2.5 10 M solution of DMDS in aqueous acetonitrile
Ϫ
2
([H2O] ϭ 4.5 10 M) during irradiation. (Time interval be-
Ϫ
6
tween spectra ϭ 2400 s, volumic photon flux: I0 ϭ 3 10
Ϫ
Ϫ1
moles of photons. L 1s at 254 nm).
all change of the UV spectrum of the reaction mixture.
It should be noted, however, that none of these wave-
lengths are pure, since the observed absorbance is that
of the sum of all the absorbances of the species in
solution. For an optical path length of 1 cm:
Abs ϭ ͚ ⑀i[Xi]
Ј
F ϭ (1 Ϫ 10ϪAbs )/AbsЈ
[Xi] is the concentration of the absorbing species i of
molar extinction coefficient ⑀i at the observation wave-
length ␭. The change in absorbance at these four wave-
lengths was plotted as a function of time (Fig. 2).
We also determined the concentrations of meth-
where AbsЈ is the total absorbance of the reaction mix-
ture at the irradiation wavelength ␭Ј ϭ 254 nm. More-
over, for the overall processes 15, 16, and 17, a sim-
plified rate equation was employed.
Numerical Simulation and Fitting of
Mechanism
A set of differential equations corresponding to the
kinetic rates of the all species involved in the mecha-
nism was established (Scheme II).
Numerical integration of this system reconstitutes
the change in concentrations of all species and in turn
the change in absorbance at all observation wave-
lengths. The experimental observations at the four
monitoring wavelengths for the two experiments were
subjected to a least-squares fitting procedure using an
iterative algorithm designed to minimize the residual
error RE.
Figure 2 Absorbance vs. time at four wavelengths 234,
254, 270, and 290 nm during experiment of Figure 1.
234 nm represents a global change for all the absorbing spe-
cies, 254 nm corresponds to the peak absorption of DMDS,
while 270 and 290 nm encompass the peak absorption of
SO2.
(RE) ϭ (͚ ͚ ͚n (Absexp Ϫ Abscalc)2)/Nwn
N
W
where N is the number of experiments, w the number
of wavelengths, and n the number of points in each

KINETIC MODELING OF THE PHOTOOXIDATION OF DIMETHYLDISULFIDE IN THE LIQUID PHASE
829
⌽
(0) CH3SSCH3 90
9: 2 CH3S
V0 ϭ ⌽0I0F[CH3SSCH3 ] ⑀Ј
DMDS
и
k1
2
и
и
(1) 2 CH3S 9
9
: CH3SSCH3
V1 ϭ k1[CH3S ]
⌽
(2) CH3SSCH3 92
9
: 2 CH3 ϩ [S2 ]
V2 ϭ ⌽2I0F[CH3SSCH3 ]⑀Ј
и
DMDS
k3
и
и
и
(3) CH3S ϩ O2 9
9
: CH3SOO
V3 ϭ k3[CH3S ][O2 ]
k4
и
и
и
(4) CH3SOO 9
9
: CH3S ϩ O2
V4 ϭ k4[CH3SOO ]
k5
и
и
и
и
и
(5) CH3S ϩ CH3SOO 9
9
: 2 CH3SO
V5 ϭ k5[CH3SOO ][CH3S ]
k6
2
и
и
и
и
(6) 2 CH3SO 9
9
: CH3SO2 ϩ CH3S
V6 ϭ k6[CH3SO ]
k7
и
9
и
и
(7) CH3SO2 ϩ CH3S 9
(8) CH3S(O)2SCH3 98
9: CH3S(O)2SCH3
V7 ϭ k7[CH3SO2 ][CH3S ]
⌽
и
и
: CH3SO2 ϩ CH3S
V8 ϭ ⌽8I0F[CH3S(O)2SCH3 ]⑀Ј
MMTS
k9
9
и
и
и
и
(9) CH3SO ϩ CH3S 9
: CH3S(O)SCH3
V9 ϭ k9[CH3SO ][CH3S ]
⌽
10
и
и
(10) CH3S(O)SCH3 99: CH3SO ϩ CH3S
V10 ϭ ⌽10I0F[CH3S(O)SCH3 ]⑀Ј
MMTSI
k11
и
и
и
и
(11) CH3SO2 ϩ CH3SO ϩ H2O 9
9
: 2 CH3SO2H
V11 ϭ k11[CH3SO2 ][CH3SO ][H2O]
k12
(12) 2 CH3SO2H 9
9
: CH3SO2 ϩ CH3SO ϩ H2O
V12 ϭ k12[CH3SO2H]2
и
и
k13
и 9
(13) CH3SO2
и
и
9
: CH3 ϩ SO2
V13 ϭ k13[CH3SO2 ]
k14
9
и
и
и
(14) CH3 ϩ SO2 9
: CH3SO2
V14 ϭ k14[CH3 ][SO2 ]
k15
34 O2
99
: HCHO ϩ 1 H3O
V15 ϭ k15[CH3 ]
и ϩ
(16) HCHO ϩ 1 O2 9
и
(15) CH3
2
k16
9
: HCOOH
V16 ϭ k16[HCHO]
2
⌽
17
(17) SO2 ϩ H2O ϩ 1 O2 9
9
: H2SO4
V17 ϭ ⌽17I0F[SO2 ]⑀Ј
SO2
2
k18
(18) SO2 ϩ H2SO4 9
9
: H2SO4 ,SO2
V18 ϭ k18[SO2 ][H2SO4 ]
k19
(19) H2SO4 ,SO2 9
9
: H2SO4 ϩ SO2
V19 ϭ k19[H2SO4 ,SO2 ]
k20
2
и
и
и
и
и
(20) 2 CH3SO2 ϩ O2 9
9
: 2 CH3SO3
V20 ϭ k20[CH3SO2 ] [O2 ]
k21
9
2
и 9
(21) 2 CH3SO2
и
и
: CH3SO3 ϩ CH3SO
V21 ϭ k21[CH3SO2 ]
k22
V22 ϭ k22[CH3SO3 ]
и 9
(22) CH3SO3
9: CH3SO3H
RH
Scheme I Mechanism and rate equations for the photooxidation of dimethyldisulfide in the liquid phase. ⑀Ј stands for
x
254
⑀
of species X
experimental curves. Absexp and Abscalc are, respec-
tively, the experimental and calculated absorbance.
The unknown parameters were fitted numerically in
the minimization procedure, while the others were
fixed at their known values. Figure 3 shows the results
of the optimization for each wavelength for both ex-
periments.
There is a good correlation between the calculated
and experimental points. It can be seen from Table II
that the calculated rate constants are within one order
of magnitude of literature values in the liquid phase
(when available). The calculated absorption coeffi-
cient of unisolated putative species are given in Table
III.
at the end of the reaction in experiment 2 (80% and
20%, respectively) were in agreement with those de-
rived from the model (67% and 24%, respectively).
However, in order to evaluate the predictive power of
the model, we wanted to find out whether the model
could predict the results of new experiments carried
out under very different conditions (concentrated
DMDS solutions, immersion well photoreactor, sol-
vent . . . ).
Predictive Power of the Model
We simulated another photooxidation experiment us-
ing a concentrated solution of DMDS in aqueous (H2O
added) acetonitrile in a 250 mL immersion well pho-
The yields of sulfonic and sulfuric acid determined

830
BANCHEREAU ET AL.
d[CH3SSCH3 ]/dt ϭ Ϫ v0 ϩ v1 Ϫ v2
и
d[CH3 S ]/dt ϭ 2v0 Ϫ 2v1 Ϫ v3 ϩ v4 Ϫ v5 ϩ v6 Ϫ v7 ϩ v8 Ϫ v9 ϩ v10
и
d[CH3 ]/dt ϭ v13 Ϫ v14 Ϫ v15
d[S2]/dt ϭ v2
d[O2]/dt ϭ Ϫ v3 ϩ v4 Ϫ 0.75v15 Ϫ 0.5v16 Ϫ 0.5v17 Ϫ v20
и
d[CH3SOO ]/dt ϭ v3 Ϫ v4 Ϫ v5
и
d[CH3SO ]/dt ϭ 2v5 Ϫ 2v6 Ϫ v9 ϩ v10 Ϫ v11 ϩ v12 ϩ v21
и
d[CH3SO2 ]/dt ϭ v6 Ϫ v7 ϩ v8 Ϫ v11 ϩ v12 Ϫ v13 ϩ v14 Ϫ 2v20 Ϫ 2v21
d[CH3S(O)2SCH3]/dt ϭ v7 Ϫ v8
d[CH3S(O)SCH3]/dt ϭ v9 Ϫ v10
d[H2O]/dt ϭ Ϫ v11 ϩ v12 ϩ 0.5v15 Ϫ v17
d[CH3SO2H]/dt ϭ 2v11 Ϫ 2v12
d[SO2]/dt ϭ v13 Ϫ v14 Ϫ v17 Ϫ v18 ϩ v19
d[HCOH]/dt ϭ v15 Ϫ v16
d[HCOOH]/dt ϭ v16
d[H2SO4]/dt ϭ v17 Ϫ v18 ϩ v19
d[H2SO4/SO2]/dt ϭ v18 Ϫ v19
и
d[CH3SO3 ]/dt ϭ 2v20 ϩ v21 Ϫ v22
d[CH3SO3H]/dt ϭ v22
Scheme II Kinetic equations for the mechanism shown in Scheme I (fractional coefficients are
explained in the text)
toreactor. The results of this simulation (change in
concentration of reactant and products) were com-
pared with those observed experimentally. Figure 4
shows a semiquantitative good agreement between ex-
perimental and model-predicted curves.
The model also reproduces well the main differ-
ences between the reactions carried out in aqueous and
anhydrous acetonitrile (simulation data in anhydrous
acetonitrile not shown). In anhydrous acetonitrile,
DMDS is more slowly consumed and no induction
period is observed for the production of MSA. A
greater peak concentration of MMTS and a lower peak
concentration of MSIA than in aqueous acetonitrile are
also predicted in agreement with experiments.
Support for Proposed Mechanism on
Scheme I
It is based on the known solution reactivity of oxy-
sulfur radicals [1] and on the photochemical reactivity
of long-lived intermediates previously detected
[15,16].
Figure 3 Simultaneous curve fitting for the two photoox-
idation experiments (numbered 1 and 2) for the four chosen
Ϫ
3
Ϫ
1
wavelengths (exp. 1: [DMDS]0 ϭ 2.5 10 mol · L
;
;
Photolysis of DMDS
[H2O]0 ϭ 4.5 10 mol · L 1; I0 ϭ 3 10 mol · L 1s
Ϫ
2
Ϫ
Ϫ
6
Ϫ
Ϫ
1
exp. 2: [DMDS]0 ϭ 3.13 10 mol · L 1; [H2O]0 ϭ 1.57
Ϫ
3
Ϫ
Homolytic cleavage of DMDS under photolysis gives
rise to thiyl radicals [6,20] (step 1) which may dimer-
ize to the starting DMDS [1,4]. A cleavage of the
10 mol · L 1; I0 ϭ 9.7 10 6 mol · L 1s 1). Solid line: cal-
Ϫ
2
Ϫ
Ϫ
Ϫ
Ϫ
culated curve and squared dots: experimental points.

KINETIC MODELING OF THE PHOTOOXIDATION OF DIMETHYLDISULFIDE IN THE LIQUID PHASE
831
tioned in the model) between a thiylperoxyl and a thiyl
radical leads to the formation of two sulfinyl radicals
и
CH3SO (step 5).
и
Formation of CH3SO2
It is known [1,26] that the main reaction of sulfinyl
radicals is a mediamutation via a sulfenylsulfinate in-
termediate (CH3S(O)OSCH3 ) giving rise to sulfonyl
and thiyl radicals (step 6).
Participation of MMTS
Recombination of a sulfonyl and a thiyl radical (step
7) [1] leads to the photochemically reactive methyl
methanethiosulfonate MMTS (steps 7 and 8).
Participation of MMTSI
By analogy with the formation of thiosulfonate, sul-
finyl radicals may well recombine with the thiyl rad-
icals, which by step 9 give rise to MMTSI. MMTSI is
photochemically unstable (step 10). (Homologous
thiosulfinate was only detected experimentally for di-
t-butyldisulfide).
Participation of MSIA
MSIA derives from the hydrolysis of the sulfinylsul-
fone (CH3S(O)2S(O)CH3 ) [27], formed on reaction of
a sulfonyl radical with a sulfinyl radical (steps 11 and
12). This possibility is supported by the formation of
larger amounts of MSIA in aqueous acetonitrile and
the consistent detection of the ester of sulfinic acid in
the alcohols.
Ϫ
Figure 4 Photooxidation of 0.2 mol · L 1 DMDS solution
in aqueous acetonitrile (containing 1 mole of water per
Ϫ
1
Ϫ4
mole of sulfur: [H2O] ϭ 0.4 mol · L
and I0 ϭ 5 10
mol · L 1s 1) in a 250 mL immersion well photoreactor.
(A): experimental concentration curves (determined by po-
tentiometric titration, gas or ion-exchange chromatography)
[16] and (B) simulated concentration curves derived from
the model. (᭺): MSA; (ٗ): MMTS; (᭛): DMDS; (*):
MSIA; (Ͼ): H2SO4; and (ϩ): HCOOH.
Ϫ
Ϫ
и
Thermal Decomposition of CH3SO2
Cleavage of the C—S bond in the sulfonyl radicals
leads to the formation of methyl radicals and SO2
[4,28–30] (step 13). The value of the rate constant
k13 seems to be realistic. Assuming a preexponential
factor of 1013 for the dissociation reaction, its activa-
C—S bond leading to polysulfides (denoted here as
ϽϽS2ϾϾ) is considered (step 2) but it is much less fa-
vored [3,6].
tion energy was estimated at 11 Kcal molϪ1, of a
ؒ
comparable order of magnitude to that calculated in
previous studies (18 to 20 Kcal molϪ1) [31,32]. The
ؒ
reverse reaction has also been investigated [28]
(step 14).
и
Formation of CH3SO
The addition of oxygen to thiyl radicals leads to the
и
formation of thiylperoxyl radicals CH3SOO (step 3)
Formation of Methanoic Acid
either in liquid [21–25] or in gas phase [3–8,13] with
significantly different rate constants (Table II). The
reverse reaction was also considered [4] (step 4). The
formation of a highly unstable adduct [4,25] (not men-
Methyl radicals undergo photochemical oxidation giv-
ing rise to oxygenated species such as methanal and
methanoic acid [33,34]. In addition, alcohols will un-

832
BANCHEREAU ET AL.
dergo photooxidation in the presence of oxygen [35].
mediates such as sulfenylsulfinate, sulfinylsulfone,
sulfinylsulfonate, and peroxides are neglected. This
mechanism provides an overall kinetic network for a
semiquantitative description of the photooxidation of
DMDS in solution.
The precise determination of the kinetic rate con-
stants is not the main purpose of such a study. Our
approach is summarized in Figure 5 which syntheti-
cally collects the same chemical steps as in Scheme I
but evidences interactions between chemical species
and couplings between processes.
и radicals into methanal
The overall oxidation of CH3
can be described by step 15 which summarizes steps
(23) to (25).
и
и
2CH3 ϩ 2O2 !: 2CH3OO
(23)
и
2CH3OO !: CH2O ϩ CH3OH ϩ O2 (24)
1
CH3OH ϩ O2 !: CH2O ϩ H2O
(25)
(15)
2
3
4
1
2
и
CH3 ϩ O2 !: CH2O ϩ H2O
“CH2O” denotes all the putative oxidation products.
The kinetic equations represented in Scheme II were
thus established on the basis of this assumption. For
step 15, this gives a coefficient of 0.75 for the oxygen
consumption and 0.5 for the formation of water. The
formation of methanoic acid by step 16 is also an over-
all process which requires a coefficient of 0.5 for the
consumption of oxygen.
The first hexagonal part shows multiequilibria and
photoequilibria between three sulfur-containing radi-
и
и
cals: thiyl CH3S , sulfinyl CH3SO , and sulfonyl
и with four molecular parent species: DMDS,
CH3SO2
MMTSI, MSIA, and MMTS. Irrespective of the start-
ing compound (DMDS, MMTS, or MMTSI), one ob-
tains the same distribution of radicals along with the
same long lived intermediates and products as it was
experimentally observed.
Formation of Sulfuric Acid
и
The second part (starting from CH3SO2 ) summa-
We checked experimentally that the photochemical
oxidation of SO2 in the presence of water and oxygen
gives rise to sulfuric acid (step 17). In the calculation
of the consumption of oxygen, the coefficient of 0.5
in front of the term v17 takes account of the stoichi-
ometry of the oxidation of SO2. Steps 18 and 19 were
introduced to take into account the slight increase in
absorption of SO2 at short wavelengths observed in
the presence of H2SO4.
rizes the escape reactions from the sulfonyl radicals,
which in the presence of oxygen give rise mainly to
sulfonic (CH3SO3H), sulfuric, and methanoic acids
(steps 15, 16, 17, and 20).
At the onset of irradiation, the reaction starts with
the accumulation of stable molecular species (long
lived intermediates) such as MSIA and MMTS which
act as radical “reservoirs.”
The reaction then shifts towards the escape path
(step 13) leading to the formation of acids thereby ex-
hausting the supply of radicals and of their molecular
parents.
Production of MSA
The sulfonyloxyl radicals may be formed either via
the dimerization of sulfonyl radicals [1,31] (step 21)
(accounting for the formation of MSA even in the ab-
sence of oxygen) or via the addition of oxygen to these
radicals (step 20). This latter reaction has been de-
scribed for the sulfoxidation of paraffins [36] and the
autooxidation of sulfinic acids [37,38], giving rise to
CONCLUSION
A complete mechanism for the photooxidation of
DMDS in acetonitrile solution is proposed. The model
was established by kinetic modeling and inverse treat-
ment of two independent experiments each carried out
and monitored continuously by UV/visible spectros-
copy at four different wavelengths. The model pro-
vided a semiquantitative simulation of several other
experiments carried out in a 250 mL immersion well
photoreactor. It is based on known reactions of various
sulfur-(thiyl, sulfinyl and sulfonyl) and carbon-con-
и
sulfonylperoxyl radicals CH3SO2OO . These radicals
may react with further sulfonyl radicals to give two
и
sulfonyloxyl radicals CH3SO3 . MSA is derived from
the trapping of a hydrogen atom from the solvent by
these sulfonyloxyl radicals (step 22) [39].
For the sake of simplicity, the reaction of recom-
bination of radicals derived from the solvent was not
included in the mechanism.
и
taining radicals (CH3 ).
The dynamic behavior predicted by this kinetic
model can account for: (i) the transient formation of
MSIA and MMTS and the formation of acids mainly
at the end of the reaction; (ii) the formation of meth-
Validity and Operation of the Mechanism
The mechanism proposed here involves chemically re-
alistic steps although some possible unstable inter-

KINETIC MODELING OF THE PHOTOOXIDATION OF DIMETHYLDISULFIDE IN THE LIQUID PHASE
833
Figure 5 Mechanism of the photooxidation of DMDS in aqueous acetonitrile. Numbering of reactions is the same as in
Scheme I.
anoic, sulfuric, and sulfonic acids from either DMDS,
MMTS, or MMTSI; all these molecular species derive
from the recombination of the same sulfur- or oxysul-
fur-radicals; and (iii) the influence of the solvent
(aqueous vs. anhydrous acetonitrile): we found a dif-
ferent kinetic behavior of intermediates and a higher
yield of acids in aqueous rather than in anhydrous ace-
tonitrile.
CH SOи ϩ O2, self-reactions of CH S(O)OOи and
3
3
CH S(O )OOи
. . . , and of the reactivity and ab-
3
2
sorbances of nonisolated molecular species such as
sulfinylsulfone, sulfenylsulfinate, and sulfonic perox-
ide. These intermediates, which have been little stud-
ied so far, may account for some of the unattributed
UV absorption.
Improvements of the model could be made by
taking into more account some kinetic pathways that
have been postulated in the gas phase such as:
E. R. B. and S. L. would like to thank Elf Atochem for
financial support.

834
BANCHEREAU ET AL.
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