2426
M. Bonifaeic and K.-D. Asmus
Free Radical Oxidation of Organic Disulfides M. BonifaEiC and K.-D. Asmus" Hahn-Meitner-lnstitut fur KernforschungBerlin GmbH, Bereich Strahlenchemie,
D 1000 Berlin 39, West Germany (Received May 17, 1976) Publication costs assisted by Hahn-Meifner-lnstitutfur KernforschungBerlin GmbH
The oxidation of aliphatic organic disulfides by 1,3,5-trimethoxybenzene and thioether radical cations, SOc- and Br2.- radical anions, and by the unstable metal ions Ag2+,Ag(OH)+,and T12+leads to the formation of RSSR.+ radical cations as primary reaction product with 100%efficiency. Oxidation by Tl(OH)+, the hydroxyl radical OH., and the bicarbonate radical HC03. leads to the formation of RSSR.+ in 80,55,and ca. 10%yield, respectively. The rate constants for the oxidation processes are in the lo7-3 x IO9 M-l s-l range. The various oxidation mechanisms and the role of RSSR.+ radical cations as a mediator for electrons are discussed.
Introduction The oxidation of organic disulfides by hydroxyl radicals in aqueous solutions has been the subject of several recent publications.'-6 It has been shown that ion pair formation and dissociative OH addition occur with about equal probability in neutral s o l ~ t i o n s . ~At f low and high pH RSH and RSO. radicals are also observed. RSSR
+
OH.
f \
RSSR' RSOH
+ +
OHRS.
(la) Ob)
The radical cations RSSR.+ have been identified by pulse radiolysis conductivity studies and by their optical absorption ~ p e c t r a . The ~ , ~ nonionic species have been characterized through steady state analysis of stable reaction products1i2,6 and EPR trapping experiment^.^^^ Oxidation reactions in general can be initiated not only by hydroxyl radicals but also by a variety of other species such as Br2.-, SOC-, T P , Ag2+, HC03., etc. These oxidizing species can be generated most conveniently by radiation chemical methods, particularly pulse radiolysis, but also by photochemical or conventional chemical methods. Oxidation of organic compounds by the above radicals often leads to the formation of the same products. In some instances, however, results suggest that the oxidation mechanism may also proceed via different pathways depending on the nature of the oxidant. For example, O'Neill et a1.,8 who studied the oxidation of methoxybenzenes, could demonstrate that T12+ and OH. lead to quite different yields of methoxy radical cations. Even the various forms of the same oxidation state of a metal ion can exhibit different oxidizing properties. For example, Tl(I1) has been found to exist in the equilibriagJO Tl(OH),
Tl(OH)+
+
OH-
T12+ -I- 20H- (2)
(pK = 7.7 and 4.6, respectively); T12+and Tl(OH)+ are good oxidants.8-10 Tl(OH)2 can even act as a reductant.1° The present pulse radiolysis study has been undertaken to obtain information on the primary reaction products of the reaction of organic disulfides and various oxidative radicals and ions. Particular emphasis has been put on the determination of the yield of radical cations RSSR.+. The results are compared with the OH. radical induced oxidation mechanism. The Journal of Physical Chemistry, Vo/.80,No. 21, 1976
Experimental Section Commercially available sulfur organic compounds were fractionally distilled and the purity (>99%) was checked gas chromatographically.6 All other chemicals were of analytical grade. Solutions were generally prepared from Millipore filtered deionized water. Deoxygenation was achieved by bubbling the solutions with argon or N20. The latter was used if hydrated electrons were to be converted to OH. radicals (N20 + eaqN2 OH- t OH.). The experimental technique has already been described.ll The pulse experiments were done with high energy electrons from a 1.6-MeV Van de Graaff generator (10 mA) a t pulse lengths of 0.5-2 fis and an absorbed dose of ca. 700 rads/l-fis pulse duration. Dosimetry was based on optical measurements of C(NO2)3ions (c35onrn = 1.5 X lo4 M-l cm-l) formed during the reduction of tetranitromethane (TNM) by eaq- and (CH&COH in pulsed, deoxygenated solutions of M TNM and 2 X 10-1 M 2-propanol (pH 4-5). This process occurs with a yield of G(C(NO&-) = 5.6.12 (The G value represents the number of species formed or destroyed per 100 eV absorbed energy in a radiation induced process.) All experiments were carried out at room temperature.
-
+
Results and Discussion Oxidation by Ag(l1). The oxidation of C ~ H ~ S S C ~byH S Ag2+ ions has been observed in pulse irradiated, N2O satuM Agf M) and 2 X rated solutions of the disulfide at pH 4. Under these conditions OH. radicals which are produced with G = 5.5 (species per 100-eV absorbed energy) will almost quantitatively react with the silver ions to form Ag2+lS (k = 6.9 X 109 M-I s-l 14).(A higher Ag+ concentration could not be used because of the possibility of direct reaction of Ag+ with eaq-.) The formation of Ag2+ was indicated by an immediate increase in optical absorption at 300 nm (Figure la). (A maximum absorption at 270 nm has been found.15) In the absence of other reactants Ag2+ has a considerable lifetime (tllz > 1 ms) and decays by a second-order process. In the presence of M C2H5SSC2H5 it disappears exponentially with t 112 = 20 ps. At 420 nm a corresponding increase in optical absorption occurs (Figure lb). The absorption spectrum taken a t the time of complete decay of the 300-nm absorption and full development of the 420-nm absorption is shown in Figure
Free Radical Oxidallon of Orwlc Disulfides
2427
are on the order of 2 X lO9'M-1 3-1. They are therefore markedly higher than for the oxidation of RSSR by Ag2+. At pH 5.5-6.0 the reaction TI(OH)+ RSSR * TI' + OHRSSR,+ (7)
+
-
1
A.nm 3 5 o m L 5 o m F M a 1. Opllcal absorption-time c w e s at 300 (a)and 430 nm (b) of pulse Inadlaled, NzOsaluated solutions of 2 X M Ag+ and IO-' M C2HSSSCzHS (pH 4). Time scale: 20 per large division: dose:ca. 700 rads. (c) Spechum taken 180 ps after the pulse.
+
was observed.The rate constantsfor these reactions are of the same order as ke. An interesting result is, however, obtained with respect to the yield of disulfide radical cations. It dropped to G(RSSR.+) = 4.7 and 4.6 for the dimethyl and diethyl disulfide, respectively. Since the TI(OH)+ absorption disappears completely in the presence of disulfide, i.e., TI(OH)+ quantitatively seems to react with RSSR,this would indicate that ca. 20%of the TI(OH)+ lead to oxidation products other than radical cations. The reaction TI(OH),
+
RSSR
+
RSSR.+
+ TI* + 20H-
(8)
investigated a t pH >7.5 is difficult to observe. It is comparatively slow, Le., rather high RSSR concentrations have to be le. I t is identical with that of the disulfide radical cation applied to obtain a pseudo-first-order disappearance of the formed in the electron transfer process TI(OH)2 absorption. To avoid direct reaction of OH. with RSSR the TI+concentration also would have to be increased Ag*+ + G H , e H , Ag' + GHSGHd (3) which, however, aLS0 leads to an unwanted TI+ e& reaction. Therefore only an upper limit of 1X 108 M-1s-1 can he estiThe rate constant for reaction 3 derived from Figures l a and mated for ks. The yield of RSSR.+ radical cations from this l b and similar curves a t several disulfide concentrations is k3 = (3.5 0.5) X 108 M-I s-l. The yield of CZH&~CZHY+ reaction could not be determined within reasonable limits of error. The observable RSSR.+ absorption from reaction 8 is radical cations calculated from the maximum absorption a t very small. This,however, is not only due to a possible reaction 420 nm and the known extinction coefficient oft = 1.8 X 103 other than electron transfer, but also results from incomplete M-I cm-I amounts to G = 5.5. This yield is identical with reaction of TI(OH)2 with RSSR as well as from partial neuG (OH.) = G(Ag2+),i.e., oxidation of the disulfide by Agz+ in tralization of RSSR.+ with OH- within this pH range. contrast to direct oxidation by OH- radicals quantitatively Oxidation by HC03. and COS-. The reactions of the oxileads to the radical cation as a primary product. dized form of carbonate with disulfides have been studied in No change in C2H5SSC2Hs+yield is observed over the pH pulse irradiated, N20 saturated solutions of 0.1-1.0 M HC03range 3.M.O. Since Ag(1I) exists in the equilibrium or CO? and low concentrations of disulfides. Since the pK Ag(OH)+ =Ag*+ + OH(4) of HCOr (5COS- + Haq+)is 9.617 experiments were carried out at pH -8 and -11. (The radicals HC03- and COS- are with a pK = 5.315 this means that the reaction formed in the reaction of OH- and HC03- or COS^-.^^.'^) The rate constants for the reactions C,H@C~H, Ag(OH)' HCOf Ag' + OH- + GH,SSGH,.+ (5) or + RSSR + products (9) also leads quantitatively to disulfide radical cations. The rate a,constant k g = (7.0 f 0.5) X 108 M-1 s--1 has been deterwere derived from the decay of the radical absorptions a t 600 mined. nm. In the absence of RSSR both HCOr and COS.- are very Basicly the same results are obtained for the oxidation of long lived (tin > 10 ms). With added RSSR they disappear CHnSSCH3 by Ag2+ and &(OH)+. The corresponding radical completely according to an exponential rate law and with t cation yields and rate constants are summarized in Table I being inversely proportional to the disulfide concentration. together with the data of the other oxdation systems studThe rate constants are found to be in the range of 107-108 M-1 ied. s-l (see Table I) and are similar to those determined in phoOxidation by T W ) .The reaction of TI(I1) with disulfides tolytic experiments for the corresponding reactions of HCOs has been o b & through the decay of the Tl(II)absorptions and COS- with some biochemically interesting disulfides.'9 (-260 nm)9J0.16and the formation of the RSSR.+ absorptions. The yield of radical cations, RSSR.+. at low pH, where Solutions were generally N20 saturated and contained 2 X neutralization by OH- ions does not yet occur, amounts to G M TI+and various lower concentrations of disulfide. The 0.6. Almost 90% of HCOs therefore do not lead to RSSR.+ pH was adjusted by HClO, or NaOH. Under these conditions in their reaction with disulfides. No radical cations could be all OH. radicals reacted with TI+within the duration of the observed for the reaction of COS- with RSSR.However, even pulse to form TI2+,TI(OH)+,or TI(OH)2depending on the pH if RSSR-+ were formed their lifetime a t the pH necessary for of the solution (see eq 2). Subsequently, reaction of these observation of COT- reactions would he too short for detection species with the disulfide could occur. The absorption-time due to neutralization by OH- ions. curves obtained were principly similar to those given in Figure Oxidation by SO,*-.The reaction of SO,.- radical anions 1. with disulfides was studied in pulsed, argon saturated soh. In acid solution (pH 3.5) the reaction tions of lo-' M SzOa2- and low RSSR concentrations under TI*+ RSSR TI+ RSR" (6) slightly acid conditions (pH ~ 4 . 5 ) .In such solutions OHradicals which are present with G = 2.7 will directly react with quantitatively leads to disulfide radical cations, i.e., occurs RSSR according to reaction 1. Since OH. forms RSSR.+ with with C = 5.5. The rate constants for reaction 6 (see Table I)
+
+
*
-
+
-
+
-
+
2428
M. Bonifaeic and K.-D. Asmus
TABLE I: Oxidation of CH3SSCH3 and C ~ H S S S C ~ H byS Various Oxidizing Speciesf
CH3SSCH3 Oxidant Ag2+ Ag(OH)+ T12+ Tl(OH)+ Tl(OH)2 HC03. CO3.-
k , M-I s-1
G (RSSR.+)
5.2 X lo8 5.6 X lo8 2.3 x 109 1.5 x 109 5108
5.5 5.4 5.5 4.7
%(RSSR*+) 100 100 100
85 420
I1
1.0 x 108 8.0 x 107 3.8 X IOs 2.2 x 109
soc-
C~HSSSC~HS
=10
-0.6
k , M-1 s-1
G(RSSR.+)
3.5 x 108 7.0 X IO8
%(RSSR.+)
5.5
100 100 100
5.5
1.4 x 109 1.4 x 109
5.5 4.6
84 5 30 =lo
11.5
5 108
4.5 x 107 6.6 X lo7 2.6 X lo8 1.8 x 109 2.1 x 109
~0.6
4.2c 100 4.3' 100 Br2.5.5 100 5.4 100 TMB-+ 2.2 x 109 5.6 100 5.5 100 b 100 (CHdzS*+ a a b 100 (CzHdzS*+ a b 100 a b 100 ((CH3)&)2S*+ 5.0 x 109 2.5d 100d OH-e 1.7 X 1010 3.1 56 1.4 X 1O'O 3.0 55 a Rate constant depends on thioether concentration (see text and ref 24). Yield depends on thioether concentration (see text and ref 24). Argon saturated solutions;total yield of RSSR.+ results from the reactions Sore- + RSSR and OH. + RSSR (see text). The thioether radical is formed with only G = 2.5 (see text). e Taken from ref 24. f k , rate constant for the oxidation process; G(RSSR-+), yield of RSSR.+ from NzO saturated solutions unless otherwise noted; %(RSSR-+),percentage yield of oxidant leading to RSSR.+ radical cations. The accuracy of the results is estimated to *lo%. 55% efficiency6 G(RSSR-+)= 2.7 X 0.55 = 1.5. The hydrated electrons which are also present with G = 2.7 react with the peroxydisulfate in a fast reaction (k = 1.1 X 1 O l o M-l s-1)20
S,O,*-
+
eaq-
-
SO4.-
+
(10)
The subsequent reaction SO4.-
+
RSSR
-+
RSSR.+
+
Sod2-
2Br-
+
+
OH*
--
Brz,-
+ OH-
(12)
Its reaction with disulfides was studied in pulsed, N20 saturated solutions of 2 X loT2M Br- and (2-5) X M RSSR at pH 4-5. The rate constants for the reactions Br2*-
(11)
is very difficult to study since the SO4.- unfortunately has a very similar optical absorption (A, = 450 nm, e = 1 X lo3 M-l cm-1)20,21and lifetime as RSSR.+. Only at about 300 nm SO4.- has a comparatively higher absorption than RSSR.+ and a faster decay is indicated in the presence of disulfide. From this estimates of k l l = 5 X lo8 and 3 X los M-l s-l are derived for CH3SSCH3 and C ~ H ~ S S C Zrespectively. H~, It should be mentioned, however, that the overall absorptions at this wavelength are very small and the observed signals could be artifacts. In order to positively prove reaction 11,the following experiments have been done: SOc- is known to react with a l ~ o h o l ,e.g., ~ ~ the , ~ ~SOc- absorption is seen to decay increasingly faster upon addition of CH30H @(SOYCH30H) = 2 X IO7M-ls-l). On the other hand, RSSR-+ does not react with CH30H. If therefore reaction 11 occurred, Le., the observable absorption was due to RSSR.+, its decay should not be affected upon addition of methanol. This was indeed found by pulsing a solution of 10-1 M S20a2-, 2 X M CH3SSCH3,and 3.2 X M CH30H (pH 3.8).The half-life for the decay of the absorption at 440 nm remained the same with and without the methanol present ( t l / 2 = 75 p s ) , whereas in the absence of the disulfide t 112 of the absorption (now due to SO4.-) is reduced to 10 11s by the alcohol. The total yield of RSSR-+ radical cations formed in the solutions mentioned at the beginning of this section amounts to G = 4.2. With G = 1.5 resulting from direct OH. reaction (see above) this leaves another G = 2.7 formed through reaction 11.It therefore seems that SO
