Res. Chem. Intermed., Vol. 27, No. 4,5, pp. 509–518 (2001)Ó VSP 2001.

Effect of solute structure on the nature of .OH radicalreaction with organic sulfur compounds:A pulse radiolysis study

HARI MOHANRadiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay,Mumbai 400 085, India

Abstract—The nature of .OH radical reaction with organic sul� des is observed to depend stronglyon the nature of the functional group, chain length between sulfur and the functional group and pHof the solution. The sulfur-centered dimer radical cations are observed in the neutral solutions ofdialkyl sul� des whereas high acid concentration is required to generate the dimer radical cations of2,20-thiodiethanoic acid. The concentrationof acid required to form the sulfur-centereddimer radicalcations depends on the electron-withdrawingpower of the functional group. The presence of the arylgroup changes the site of attack of .OH radical from sulfur to the benzene ring, and benzene-centeredmonomer radical cations are observed.

INTRODUCTION

The reaction of .OH radicals with dialkyl sul� des (R2S) is known to take place withthe formation of the sulfur-centered dimer radical cations, (R2S).C2 , via a complexsequence of reactions involving OH-adduct, ®-thio radicals and monomer radicalcation [1, 2]. The sulfur-centered monomer radical cations are highly unstable andare converted to the dimer radical cations. In functionalized dialkyl sul� des, thenature of the .OH radical reaction depends on the nature of the functional groupand pH of the solution [3, 4]. In aryl substituted organic sul� des, the monomericradical cations are expected to be reasonably stable due to the delocalization of thepositive charge in the aromatic ring [5–7]. With the objective to understand theeffect of substituents on the nature of the transient species formed on reaction of.OH radicals with substituted dialkyl sul� des, pulse radiolysis studies have beencarried out on a number of organic sul� des under different experimental conditionsand the results are discussed with reference to the electron-withdrawing power of

510 H. Mohan

the substituent group [8]. Studies on the reactions of .OH radicals and speci� cone-electron oxidants are also important in understanding the physico-chemicalprocesses taking place in biological systems, as radicals and radical ions derivedfrom sulfur-containing compounds play an important role in the chemistry ofbiological systems [9].

EXPERIMENTAL

2,20-thiodiethanol (TDE), 2,20-thiodiethanoic acid (TDGA), dimethyl 3,30-thio-dipropionate (DTDP), 3,30-thiodipropanol (DTP), thioanisole (TA), 4-(methylthio)benzyl alcohol (MTA) and 2-(phenylthio)ethanol, obtained from Aldrich Chemi-cals, were of high purity and were used without further puri� cation. Solutions wereprepared in ‘nanopure’ water and freshly prepared solutions were used for eachexperiment. pH was adjusted with NaOH/HClO4.

Pulse radiolysis experiments have been carried out with high energy electronpulses (50 ns, 7 MeV) obtained from a linear electron accelerator, details of whichare given elsewhere [10]. The dose delivered per pulse, determined by using anaerated aqueous solution of 1 £ 10¡2 mol dm¡3 KSCN was close to 15 Gy (1 GyD 1 J kg¡1 ). The reaction of .OH radicals in neutral aqueous solutions is carriedout in N2O-saturated conditions to convert e¡

aq to .OH radicals (N2O C e¡aq !

.OH C OH¡ C N2 ). In the acidic solutions, the reaction of .OH radicals is carriedout in aerated conditions to scavenge e¡

aq and .H atoms (e¡aq C HC ! H. C H2O;

.H C O2 ! HO.2/.

The transient absorption as a function of time is recorded on a storage oscilloscope(100 MHz) interfaced to a computer for kinetic analysis. The transient species aremonitored using a 450 W pulsed xenon arc lamp, monochromator (Kratos GM-252)and Hamamatsu R-955 photomultiplier as the detector. The rate constant values aretaken from those kinetic analyses for which a very good correlation was observedbetween the experimental and the calculated results. The rate constant values are theaverage of three experiments and the variation is within 15%. The pseudo-� rst-orderrate constant (kobs ) is determined on monitoring the formation kinetics at ¸max andfrom the plot of ln.A/ vs time. The bimolecular rate constant values are calculatedfrom the linear plots of kobs vs solute concentration.

RESULTS AND DISCUSSION

The reaction of .OH radical with dialkyl sul� des (R2S) leads to the formationof sulfur-centered dimer radical cations, (R2S).C2 . The direct evidence for theformation of the OH-adduct, ®-thio radicals and monomer radical cations isobtained in functionalized dialkyl sul� des. The ®-thio radicals and the OH-adductabsorb in the region of 280–300 and 340–360 nm, respectively. The sulfur-centered

.OH radical reactions with organic S compounds 511

Figure 1. Transient absorption spectra obtained on pulse radiolysis of: (a) N2O-saturated aqueoussolution of TDE (4 £ 10¡3 mol dm¡3, pH D 6) and (b) aerated acidic (pH D 1) aqueous solution ofTDE (1:4 £ 10¡2 mol dm¡3). Inset shows variation of transient absorbance at 505 nm as a functionof pH.

monomer radical cation absorbs in the 300 nm region and has high tendency tostabilize on coordination with another heteroatom such as O, S, N, P, halogen, etc.

Figure 1a shows the transient absorption spectrum obtained on pulse radiolysisof N2O-saturated neutral aqueous solution of TDE, which exhibits an absorptionband at 290 nm (" D 1:6 £ 103 dm3 mol¡1 cm¡1, k D 1:4 £ 1010 dm3 mol¡1 s¡1 )and decayed by second order kinetics with 2k D 1:7 £ 109 dm3 mol¡1 s¡1. Theabsorbance remained independent of solute concentration in the range 4£10¡4 –4£10¡2 mol dm¡3 which therefore suggests the formation of a monomeric species. Inthe acidic conditions, pulse radiolysis studies showed the formation of a transientband at 505 nm (Fig. 1b), whose absorbance increased with the solute concentration.It should be due to a dimeric species. The transient band at 290 nm is observedto have high reactivity with oxygen whereas the decay of the absorption bandat 505 nm is not affected. These results together with the available data in theliterature [1] suggest that the absorption bands at 290 and 505 nm, observed in theneutral and the acidic conditions respectively, are due to the ®-thio radicals andthe dimer radical cations (Scheme 1). The variation of absorbance as a function ofpH (inset of Fig. 1) shows that the dimer radical cations are observed only whenpH < 4 with an in� exion point at pH 2.2. The kinetic parameters are given inTable 1. The absorption band at 505 nm cannot be due to (1) an intra-molecular

512 H. Mohan

Table 1.Kinetic parameters for the transient species formed on reaction of .OH radicals with substitutedorganic sul� des

Solute [HClO4] ¸max " £ 103 Transient kf £ 109 2k £ 109

(mol dm¡3) (nm) (dm3 mol¡1 cm¡1/ species (dm3 mol¡1 s¡1) (dm3 mol¡1 s¡1)

TDE 0 290 1.6 ®-thio radical 14 1.7TDE 0.01 505 6.2 DRC — 2 £ 104a

TDGA 0 285 1.6 ®-thio radical 4.1 1.6TDGA 9 520 DRC 1.9 1.9DTDP 0 345 2.6 OH-adduct 18 17 £ 104a

DTP 0 420 MRC 14 4:1 £ 104a

MTA 0 320, — OH-adduct, 4.5360, — RC545 5.5 OH-adduct 3:8 £ 104a

RC

DRC, dimer radical cation; MRC intra-molecularmonomeric radical cation; RC, radical cation withpositive charge on benzene ring.

a � rst order decay (s¡1).

Scheme 1.

radical cation formed on p-orbital overlap of oxidized sulfur and oxygen with a 4-membered ring con� guration as the absorbance at 505 nm showed dependence onthe solute concentration and (2) 4-membered ring con� guration which is expectedto be unstable whereas this absorption band decayed with k D 2 £ 104 s¡1.

Pulse radiolysis studies on TDGA also showed the formation of the transientbands at 285 and 520 nm in the neutral and the acidic aqueous solutions, respec-tively. The only difference was in the concentration of HClO4 required for theformation of the dimer radical cations. The dimer radical cations of TDGA are ob-served only when the concentration of HClO4 was more than 6 mol dm¡3, in com-parison to 10¡2 mol dm¡3 required for the formation of the dimer radical cations ofTDE. The high concentration of HClO4 required for the formation of the dimer rad-ical cations of TDGA is due to the high electron withdrawing power of the -COOHgroup (¾ ¤ D C2:94) as compared to a relatively lower value for the -CH2OH group(¾ ¤ D C0:56), showing that the nature of the functional group affects the reactionof .OH radical with the substituted sul� des [11].

Pulse radiolysis of N2O-saturated neutral aqueous solution of dimethyl 3,30-thiodipropionate (DTDP) showed the formation of a transient absorption band at

.OH radical reactions with organic S compounds 513

Figure 2. Transient absorption spectra obtained on pulse radiolysis of N2O-saturated neutral aqueoussolution of (a) DTDP (2:3 £ 10¡4 mol dm¡3) and (b) TDP (1:5 £ 10¡4 mol dm¡3).

Scheme 2.

345 nm (Fig. 2a), decaying by � rst order kinetics with k D 4:1 £ 104 s¡1 . Theintensity of this absorption band remained independent of the solute concentrationin the range 1 £ 10¡2 –1 £ 10¡4 mol dm¡3 which suggests the formation of amonomeric species. The absorption band is assigned to a sulfur-centered OH-adduct(Scheme 2).

Pulse radiolysis on N2O-saturated neutral aqueous solution of 3,30-thiodipropanol(TDP) shows the formation of a transient absorption band at 420 nm (Fig. 2b).Based on electron transfer reactions and the data available in the literature, thetransient absorption band is assigned to an intra-molecular radical cation formed onp-orbital overlap of oxidized sulfur with oxygen (Scheme 3). The intra-molecularradical cation is observed due to the formation of a 5-membered ring con� guration,which is expected to have reasonably high stability.

Pulse radiolysis of N2O-saturated neutral aqueous solution of MTA showed theformation of transient absorption bands at 320, 360 and 545 nm (Fig. 3a). Theabsorption band at 545 nm decayed by � rst order kinetics with k D 3:8 £ 104 s¡1

whereas the absorption bands at 320 and 360 nm showed a mixed order decay. In

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Scheme 3.

Figure 3. Transient absorption spectra obtained on pulse radiolysis of neutral aqueous solution ofMTA (1:5 £ 10¡3 mol dm¡3, pH D 7) in (a) N2O-saturated condition; (b) in presence of tert. butylalcohol (0.3 mol dm¡3) and (c) in aerated condition.

the presence of t-butyl alcohol (0.3 mol dm¡3 ), an ef� cient .OH radical and weakH. atom scavenger, very little absorption was observed only in the 350–380 nmregion (Fig. 3b) indicating that the contribution of the H. atom reaction with thesolute is negligible. The bimolecular rate constant for the reaction of .OH radicalswith the solute, measured at 320 and 545 nm, gave similar results and the value was4:5 £ 109 dm3 mol¡1 s¡1. In aerated conditions, the nature of the transient spectrumremained the same (Fig. 3c) and the decay rate of the 545 nm absorption band alsoremained the same, however, the decay of the transient absorption band at 320 nmwas affected. The absorbance of the transient absorption bands at 320, 360 and 545nm remained independent of the solute concentration. These studies suggest theformation of the monomeric species.

The transient absorption band of Cl.¡2 (345 nm), formed on pulse radiolysisof an aerated acidic (pH D 1) aqueous solution of Cl¡.4 £ 10¡2 mol dm¡3 )showed accelerated decay on addition of low concentrations of MTA (0.8–3/ £

.OH radical reactions with organic S compounds 515

Figure 4. Transient absorption spectra obtained on pulse radiolysis of aerated acidic (pH D 1)aqueous solution of Cl¡.4 £ 10¡3 mol dm¡3) containingMTA (3£ 10¡4 mol dm¡3), 5.5 ¹s after thepulse.

10¡4 mol dm¡3, indicating electron transfer from MTA to Cl.¡2 . The time-resolvedstudies, on complete decay of Cl.¡2 (5.5 ¹s after the pulse), showed the formation oftransient absorption bands at 320 and 545 nm (Fig. 4). The entire spectrum decayedby � rst order kinetics with k D 2:5 £ 104 s¡1. The second order rate constant forthe reaction of Cl.¡2 with MTA, determined from the decay of Cl.¡2 at 345 nm andformation of the absorption band at 545 nm, showed similar results and the valuewas 3:1 £ 109 dm3 mol¡1 s¡1. The transient absorption spectrum (Fig. 4) shouldbe due to the solute radical cation. Under the present experimental conditions, theentire yield of .OH radicals would have reacted with Cl¡, and Cl.¡2 would then reactwith MTA to form the solute radical cation. The molar absorptivity at 545 nm wascalculated to be 5:5 £ 103 dm3 mol¡1 cm¡1. The ratio of the transient absorptionbands at 320/545 nm (Fig. 4) is 1.5 whereas that formed on the reaction of .OHradicals (Fig. 3) is 2.8. These studies indicate the presence of another transientabsorption band in the region of 320 nm. The mixed kinetic decay of the absorptionband at 320 nm also supports these observations.

The addition of .OH radicals to the benzene ring results in the formation of OH-adduct having an absorption band in the 310–330 nm region. The addition of.OH radicals at sulfur results in the formation of the OH-adduct at sulfur with¸max in the region of 340–360 nm. Based on the absorbance value and "545 D5:5 £ 103 dm3 mol¡1 cm¡1, about 53% of .OH radicals react with MTA to form

516 H. Mohan

Scheme 4.

the solute radical cation and remaining fraction by addition reaction at sulfur andbenzene ring (Scheme 4). Since the absorbance at 545 nm remained independentof the solute concentration, it could not be due to a sulfur-centered dimer radicalcation and the monomer radical cations are very short-lived species. Therefore thetransient absorption bands at 320 and 545 nm are due to the solute radical cationwith positive charge on the benzene ring.

Pulse radiolysis on other aryl substituted alkyl sul� des has shown similar results.Pulse radiolysis of N2O-saturated neutral aqueous solution of thioanisole has shownthe formation of the benzene-centered monomer radical cation (73%) and the sulfur-centered OH-adduct (27%). In the presence of highly acidic solution (HClO4 >

5 mol dm¡3 ), the sulfur-centered OH-adduct is converted to the benzene-centeredsolute radical cation. A very small fraction of the sulfur-centered OH-adduct isconverted to the sulfur-centered dimer radical cation. On the other hand, the reactionof .OH radicals with 4,40-thiodiphenol shows the formation of the phenoxyl radicalson fast deprotonation of the solute radical cation. The presence of -CH2OH in2-(phenylthio)ethanol does not show the formation of the phenoxyl radicals andthe benzene-centered monomer radical cation is inferred to be the transient species.

CONCLUSIONS

These studies clearly show that the presence of the aryl group changes the site of.OH attack from sulfur to the benzene ring and monomeric radical cations withpositive charge on the benzene ring are observed. The nature of the functionalgroup, chain length between sulfur and the functional group and pH have beenshown to play important roles in determining the nature of the transient speciesformed on the reaction of .OH radicals with organic sul� des.

.OH radical reactions with organic S compounds 517

Acknowledgements

Sincere thanks are due to Dr. T. Mukherjee, Head, Radiation Chemistry andChemical Dynamics Division, BARC for his interest and support of this work.

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