J. CHEM. SOC. FARADAY TRANS., 1990, 86(19), 3261-3266 3261

Mechanisms of Peroxide Decomposition : An Electron Paramagnetic Resonance Study of the Reaction of the Peroxomonosulphate Anion (HOOSO,) with Cu' A marked Contrast in Behaviour with that of Tillland Fe"

Bruce C. Gilbert* and the late Jonathan K. Stell Department of Chemistry, University of York, Heslington, York YO1 5DD, UK

EPR spectroscopy has been employed in conjunction with a continuous-flow system to show that Cu' (produced from the rapid reaction of Cu" with Ti"') reacts rapidly with the peroxomonosulphate anion (k = 2 x lo4 dm3 mol-' s-' at pH 2 and 20°C) to give the hydroxyl radical (HO') which is trapped via its addition to alkenes. In contrast, Fe" and Ti"' react with HOOSO, predominantly to give the sulphate radical-anion (Soh-). It is argued that with Cu' the reaction shows characteristics of an outer-sphere electron-transfer process, whereas in t he inner-sphere reaction of Fe" and Ti"' t h e reaction is facilitated by attack of the hydroxyl oxygen atom on t h e smaller, more highly charged metal ions.

Evidence is also presented that radicals formed by the addition of both HO' and SO;- to alkenes ('CR'R2CR3R4Y, Y = OH, OSO,) are readily oxidized by HOOSO, (k z lo5 dm3 mol-' s-') , a reaction which leads to the formation of SO,-.

The generation of oxygen-centred free radicals during the decomposition of various peroxides by some low-valence metal ions can be convincingly demonstrated by the use of EPR spectroscopy in conjunction with continuous-flow rapid-mixing techniques : these experiments can provide information on the first-formed radicals, the kinetics of their formation, and a variety of subsequent radical reactions.

For example, we have recently described applications which include a kinetic and mechanistic study' of the Fenton reaction and its analogues

M"+ + H,O, - M"" + HO' + OH -

Fe" + S,Oi- - Fe"' + SO;- + SO:-

Ti"' + HOOSO, -TiIV + HO- + SO;-

(1)

(2)

(3)

HOOSOg HO' + SO:- (4)

[reaction (l), M"' = Fe", Ti"'] and the related decomposi- tion of peroxodisulphate [e.g. reaction (2)] and per- oxomonosulphate :' for the latter peroxide, our finding that reaction with Ti"' and Fe" proceeds to give predominately SO;- and OH- [reaction (3)], with no evidence for 'OH pro- duction, is in complete contrast with the results of pulse- radiolysis studies3 which suggest that for this case the formation of 'OH (ca. 80%) is the dominant mode [reaction (4)]. We have argued that, as with the Ti"'-H,O, peroxide reaction (but possibly unlike the corresponding Fenton reaction), this is evidence for an inner-sphere electron-transfer mechanism.

It has also been shown4 that addition of Cu" ions to flow experiments employing Ti"' leads to the generation of Cut after mixing

and that this provides an excellent means for monitoring directly the subsequent reactions of this ion with both perox- ides and organic free radical^.^ For example, we have con- firmed the extremely rapid reaction of Cu' with several alkyl radicals [k = 2 x 10" and 1.9 x 10" dm3 mol-' s-' for its reaction with 'CH and 'CH ,CH,OH, respectively; see also ref. (5) ] and also shown that the one-electron transfer

between Cu' and S,Oi-

CU' + s,o;- + CU" + so;- + soy (6)

is much faster (with k 2 lo6 dm3 mol-' s-') than both the reactions between Cu' and H,O, and the reactions of S,O;- with Fe" and Ti"'. The major aim of the work described here was to continue to probe the factors which underlie the control of metal-peroxide electron-transfer reactions, via a kinetic and mechanistic study of the reaction between Cu' and the peroxomonosulphate anion HOOSO, .

Experimental EPR spectra were recorded on a Varian E-104 and a Bruker ESP-300 spectrometer, each equipped with X-band klystron and 100 kHz modulation. Hyperfine splittings were measured directly from the field scan [with the ESP-300 by determi- nation with an NMR Gaussmeter ER 035M; with the E-104 the field scan was calibrated with an aqueous solution of Fremy's salt, a(N) 1.309 mT '1; g values were determined by comparison with that for' 'CHMeOH (g = 2.00321) obtained from the reaction of 'OH (from Ti'll-H,O,) with ethanol. Relative radical concentrations were determined both by spectrum simulation using a program supplied by Dr M. F. Chiu and by direct double integration (using the ESP-300); absolute radical concentrations were determined by compari- son of doubly integrated signals with those from a standard solution of vanadyl sulphate.

Flow experiments were conducted by pumping three reagent streams through a mixer which allowed simultaneous mixing ca. 30 ms before passage through the cavity of the spectrometer (using a Watson-Marlow 502 peristaltic pump). The solutions used were typically as follows: stream (i) con- tained titanium(II1) sulphate (typically [Ti"'] = 0.005 mol dm-3), in some cases with added sequestering agent (EDTA), stream (ii) contained HOOSO; at concentrations in the range 0.00M.06 mol dm-3 and stream (iii) contained the substrate (at concentrations up to ca. 1 mol dm-3). Copper(@ sulphate was normally added to stream (iii), to give a concen- tration in this stream of between lo-' and mol dm-3. The pH was adjusted to ca. 2 by addition of sulphuric acid. pH measurements were made using a Pye-Unicam pH meter PW 9410 with the electrode inserted into the effluent stream.

Publ

ishe

d on

01

Janu

ary

1990

. Dow

nloa

ded

by L

aure

ntia

n U

nive

rsity

on

29/1

0/20

13 0

5:30

:46.

View Article Online / Journal Homepage / Table of Contents for this issue

3262

;o 0

J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86

0 x x

All solutions were deoxygenated both before and during use by purging with oxygen-free nitrogen. The source of peroxomonosulphate anion was the triple salt 2KHS0, K,SO, - KHSO,, kindly provided by Interox Chemicals Ltd. Other chemicals employed were commercial samples (used as supplied).

Reaction of Cu' with HOOSO, in the Presence of Alkenes

But-2-ene-I ,I-diol In order to determine whether the reaction of Cut with HOOSO, produces either or both HO' or SO;- [cf: reac- tions (3) and (4)], alkenes were first chosen as substrates for reaction with the Cut-HOOSO, couple in the rapid-flow mixing system. As noted p rev iou~ ly ,~ .~ it is possible unam- biguously to assign separate sets of splitting constants for the HO' and SO;- adducts 'CR,CR;X,X = OH, OSO;.But-2- ene- 1,4-diol was first chosen as substrate since only one adduct can be formed from each radical.

But-2-ene-1,Cdiol was found to react in the presence of Ti"' and HOOSO, (with the concentration of the latter typi- cally lo-, mol dmP3)t at pH 2 to give solely the sulphate adduct 'CH(CH ,OH)CH(CH,OH)OSO, [see table 1 and fig. l(a)], as anticipated on the basis of our earlier study which established the occurrence of reaction (3).$ However on addi- tion of low concentrations of Cu" to the substrate stream [to give concentrations of (1-30) x mol dm-3 overall] it was surprising to observe the EPR signal of the hydroxyl adduct 'CH(CH ,OH)CH(CH,OH)OH in addition to that of the sulphate adduct [see fig. l(h)].

The effect on the concentrations of the two adducts of increasing the concentration of copper ions is shown in fig. 2. At high [Cu"], where the HO' adduct predominates over the SO;- adduct, the ratio of the concentrations of the two adducts appears to be independent of [Cu"]. In a separate set of experiments it was established that similar results are obtained on addition of Cu" to the peroxide stream; there

t Concentrations quoted hereafter in the text are those after mixing.

$ Hyperfine splitting constants and g values for these (and other) adducts have been reported previously ' v 9 and will not be repeated here.

Table 1. Relative radical concentrations of adducts of HO' and SO:- to various alkenes in the Ti"'/HOOSO;/Cu" system'

relative concentrations (96) of alkene adducts

alkene SO:- adduct HO' adduct

HO,CCH=CHCO,H * 2 98 HO,CCH=CMeCO,H ca. 0 ca. 100 HOCH,CH=CHCH,OH 20 80 HOCH,CH=CH,' 20 80

Me,C=CH,' ca. 100 ca. 0 HOCH,CMe=CH,' 52 48

' In experiments at room temperature, pH ca. 2, and mixing time ca. 30 ms, and with [Cu"], 2 3.3 x lop4 mol dmw3 (see text), [Ti"'], 1.7 x lop3 mol drnp3, and [HOOSOJ, lo-' mol dm-j. * Fumaric acid. Addition observed in each case to the least-substituted end of the alkene's double bond.

was no significant decomposition of the peroxide on standing over a period of ca. 30 min, which rules out any reaction between Cu" and peroxide as the explanation for our obser- vations.

As significant concentrations of radicals are observed in the presence of copper ions it is believed that Cu' reacts very

A

1 I I I 1 I 1 I I

0 2 1 6 8

Fig. 2. Variation in the relative radical concentrations of HO' (0) and SO:- (0) adducts to but-2-ene-1,4-diol with [Cu"], in experi- ments with the Ti"'/HOOSOJ couple at pH 2. ([Ti"'], 1.7 x [HOOSO;] 1 x mol dm-3, [but-2-ene-l,Cdiol] 2% v/v).

[ C U " ] / ~ O - ~ rnol dm-3

Publ

ishe

d on

01

Janu

ary

1990

. Dow

nloa

ded

by L

aure

ntia

n U

nive

rsity

on

29/1

0/20

13 0

5:30

:46.

View Article Online

J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86 3263

rapidly with HOOSO; ; we note that, in contrast, addition of Cu" (in similar concentrations to those employed here) to the Ti"'/H ,02/but-2-ene- 1,4-diol system caused almost complete removal of all signals, evidently a consequence of the rapid reduction of radicals by CU' in conjunction with a relatively slow rate of decomposition of H,O, by Cu' (the mechanism of reaction is unclear, but it appears that the rate constant is ca. lo3 dm3 11101-' s - l : see e.g. ref. (10) and (11).

At this stage, two possible explanations of the formation of the alkene hydroxyl adduct in the Cul/HOOSO; system may be advanced. The first is that decomposition of HOOSO, (as with that brought about by the hydrated electron itself,) gives largely 'OH which adds to the alkene

Cu' + HOOSO; -+ Cu" + HO' + SO:- (7)

HO' + HOCH ,CH=CHCH,OH

1 (8) HOCH,CHCH(CH ,OH)OH

The second is that formation of SO;- is followed by oxida- tion of Cu" to Cu"', and subsequent one-electron oxidation of the alkene [cf ref. (12)]:

Cu' + HOOSO, -+ Cu" + HO- + SO;- (9)

CU" + so;- -+ CU"' + so;- Cu"' + HOCH,CH=CHCH,OH

1

I Cu" + [HOCH,CH=CHCH,OH]'+ (1 1)

I H20p-H+ + HOCH,CHCH(CH ,OH)OH

In order to investigate the possibility that Cu*" might be involved, we carried out experiments with SO:- (from Ti1''/S,O;-) and the alkene in the presence Of CU". Studies over a wide range of copper concentrations failed to provide any evidence of hydroxyl-adduct formation, which suggests that reactions (10) and (11) do not occur to any significant extent (under either sets of conditions). Further evidence against this mechanism is that in experiments with Cu'/HOOSO, the ratio of hydroxyl to sulphate adduct was found to be independent of [alkene] (which would not be expected if the alkene and Cu" were to be in competition for

Other experiments were performed in an attempt to verify so;-). the occurrence of reaction (7) in the Cu'/HOOSO; system.

Experiments with Other Alkenes When fumaric acid was employed as substrate for the Cu'/HOOSO, couple the hydroxyl adduct was again detected; it seems most unlikely that Cu"' would readily oxidize such an electron-deficient alkene to a radical-cation and this result therefore also implies that 'OH is indeed formed. It was also noted the even at relatively high [Cu"] ( > 3 x mol dmP3) only the hydroxyl radical could be detected (in contrast to the results for but-2-ene-l,4-diol).

For a range of alkenes (see table 1) it was found that the relative concentrations of hydroxyl and sulphate adducts detected (at high [Cu"]) depended critically upon the struc- ture of the substrate. Electron-deficient alkenes (such as fumaric acid) give solely the HO' adducts, whereas the more electron-rich alkene 2-methyl-prop-2-ene-1-01 gives the two adducts in similar proportions, and the most electron rich alkene employed (2-methylpropene) gives only the SO;- adduct. [Electron-rich alkenes of even lower ionization potential, such as 3-methyl-but-2-ene-1-01 were not used in this comparison since it is known that their reaction with

SO;- (generated using alternative techniques) leads to the detection of only the hydroxyl radical-adducts.' The mecha- nism of this reaction, which is believed to be quite different from that discussed here for other alkenes, evidently involves either electron-transfer from the alkene to SO;-, followed by hydrolysis of the radical-cation, or rapid hydrolysis of the sulphate a d d ~ c t . ~ . ' 3]

We ruled out the possibility that the explanation for the dependence of the relative radical concentrations on the alkene structure lies in the relative rate constants for attack of 'OH and SO;-, respectively, on individual alkenes. Though the electrophilic sulphate radical-anion reacts rela- tively slowly with electron-deficient alkenes such as fumaric acidI4 (k d lo7 dm3 mol-' s- ' , cf: a rate constant of ca. lo9 dm3 mol-' s- ' for 'OH)," the rate constants for attack of both 'OH and SO;- on electron-rich alkenes (such as Me,C=CH,) are more nearly comparable (2 lo9 dm3 mol- ' s-') and hydroxyl adducts of the latter would have been expected if 'OH is indeed f~rrned.'~. ' '

We considered instead the likelihood that the structure of the first-formed radical-adduct might influence the ratio. Thus it has been observed that alkyl radicals (even those lacking an or-oxygen substituent) can be readily oxidized by peroxides (including, especially, S,O; - and HOOSO,) and that the ease of oxidation increases with increase in the number of alkyl substituents at the radical centre (Bu" > Pr" > Et' > Me'), as would be expected on the basis of the radicals' ionization potentials.2-'6 Thus a radical such as 'CMe ,CH,OH, with two alkyl (+I) substituents might be expected to be more readily oxidized by peroxomonosulphate than a radical of higher ionization potential, like 'CH(C0 ,H)CH(OH)CO,H. A mechanism is therefore pro- posed (scheme 1) for the formation of the sulphate radical- adduct (at high [Cu"]) in which first-formed hydroxyl adducts of relatively low ionization potential are oxidized by peroxomonosulphate in a one-electron transfer step which generates SO;- to react with the alkene (and to an extent which varies with the structure of the alkene).

(5 )

Cu' + HOOSO, -+ Cu" + HO' + SO;- (7)

(12) HO' + R'R2C=CR3R4 -+ 'CR'R2-CR3R40H

'CR 'R2-CR3R40H + HOOSO; -+

+CR'R2-CR3R40H + HO- + SO;- (13)

SO;- + R'R2C=CR3R4 + 'CR 'R2-CR3R40S0, (14)

Scheme 1.

In order to verify this proposal, experiments were carried out in which [HOOSO;] was varied (for fixed, high values of [Cu"]) and the effect on the relative concentrations of the two radical adducts was monitored. Results for but-2-ene- 1,4- diol are shown in fig. 3, from which it can be seen that the ratio of the two adducts is dramatically dependent upon the peroxide concentration, and that, though the 'OH adduct is formed almost exclusively at low peroxide concentrations, the sulphate adduct predominates under conditions of high [peroxide] when radical destruction via oxidation [reaction (1 3)] is encouraged. Similar but even more pronounced effects were observed for 2-methylprop-2-ene- 1-01. These findings, and a kinetic analysis given below, provide strong support for an overall mechanism involving hydroxyl radical formation and adduct oxidation, as shown in scheme 1.

Publ

ishe

d on

01

Janu

ary

1990

. Dow

nloa

ded

by L

aure

ntia

n U

nive

rsity

on

29/1

0/20

13 0

5:30

:46.

View Article Online

3264 J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86

[HOOSO,]/l 0-2 mol dm-3

Fig. 3. Variation in the relative radical concentrations of HO' (0) and SO:- (0) adducts to but-2-ene-1,4-diol with [HOOSO;], in experiments with Ti"'/HOOSO;/Cu" at pH 2. ([Ti"'], 1.7 x [Cu"] 3 x mol drn-j, [but-2-ene-l,4-diol] 2% v/v).

Reactions of Other Substrates in the Cu'/HOOSO, System

Experiments were performed with substrates, other than alkenes, which are known to exhibit markedly different reac- tivity towards SO;- and 'OH. For example, ethanoic acid and propanone are relatively unreactive towards SO;- whereas 'OH reacts at a much faster rate with both sub- s t r a t e ~ ; ' ~ * ' ~ furthermore, the major product of the reaction between ethanoic acid and SO;- is the methyl radical whereas reaction with 'OH gives 'CH2C02H [see e.g. ref.

The reaction of ethanoic acid with Ti"'/HOOSO, at pH ca. 2.5 led to the detection of weak signals from Me', evi- dently formed from the reaction of the substrate with SO;-, via oxidative decarboxylation (as noted previously' ') :

SO;- + MeCO, -+ SO:- + MeCO; --+ Me' + CO, (15)

However, on addition of Cu", at a concentration of rnol dm- 3, signals from %H ,CO,H were observed instead. This is entirely consistent with the production of 'OH in the Cu'/HOOSO, system, followed by the reaction

(16) (n.b. formation of Cu"' would not be expected to lead to hydrogen-atom abstraction in this way).

No signal was observed from the reaction of propanone in the Ti"'/HOOSO, system (which is as expected since SO;- is particularly unreactive towards such C-H sites on account of the presence of the electron-withdrawing carbonyl group). However, on addition of Cu", signals from 'CH,COCH, were clearly observed, as expected from the occurrence of the reaction

( 1 7 ) ~

MeC0,H + 'OH + H ,O + 'CH,CO,H

Me2C0 + 'OH + H,O + %H,COMe. (17)

Effect of the Addition of Cu" to the Reaction between Fe"-EDTA and HOOSO;

When either prop-2-ene- 1-01 or 2-methylpropene were employed as substrates in flow experiments with Fe"-EDTA and HOOSO,, signals from the corresponding sulphate adducts were detected, as expected on the basis of generation of SO;- from the Fenton-type reaction. Addition of Cu", up to mol dm-3 in concentration caused only a small reduction in the concentration of sulphate radical adducts : no hydroxyl-radical adducts were detected. This is exactly as would have been expected, since we have established that Cu' is not formed in flow-system experiments with Fe"-EDTA (an

observation which is attributed either to a low rate of reac- tion between Cu" and Fe"-EDTA and/or to complexation of Cu" by EDTA).4

Kinetic Analysis of the Reaction of Copper(1) with the Peroxymonosulphate Anion

A kinetic analysis of the Cul'/Ti"'/HOOSO~ system was undertaken in order to test the validity of the mechanism proposed in scheme 1 (i.e. to test the assertion that the hydroxyl radical is first generated and that SO;- is formed from the subsequent reaction of the hydroxyl-radical adduct with HOOSO,) and to determine the rate constants for the reaction of Cu' and the hydroxyl-radical adducts with the per oxide.

Analysis of the initiating reaction between Cu' and HOOSO; was based on the result of experiments in which [Cu"] was varied with 2-buten-1,4-diol as substrate at a fixed (low) peroxide concentration of 5.0 x loF3 mol dm-3 (to reduce the extent of oxidation of the hydroxyl radical-adduct by HOOSO,). The basis for this kinetic analysis is sum- marked in scheme 2, in which R; and R; represent the sulphate-radical adduct and the hydroxyl-radical adduct, respectively. The scheme proposed does not include the reduction and oxidation of the radical adducts by Cu' and Cu", respectively. The rates of these reactions are not known but, if the rates are similar for each adduct (as expected because of their similar structure) then these reactions will have little effect on the ratio of the two adducts. These reac- tions should also have little effect on [Cu'], as the copper ion concentration is much greater than that of the radicals.

Ti1'] + Cu" Ti'V + cu ' ( 5 )

Ti"' + HOOSO, &Ti'" + HO- + SO;- (3)

Cu' + HOOSO, Cu" + HO' + SO;- (7)

(18) SO;- + RH 5 R;

H O * + R H AR; (19) 2k R;+R;-)

non-radical products (20)

R;+ R; - Scheme 2.

It was also initially assumed that SO;- and the hydroxyl radical react with the alkene at the same rate (ka). Applying the steady-state principle for [SO;-], [HOI, [R;] and [R;] leads to the equations

k3[Ti'1'],[HOOS0~]o = k,[RH][SOk-] (21)

~ , [CU' ] , [HOOSO~]~ = ka[RH][HOl (22)

kaCRHICSOk-I = 2ktCR;ICR'I T (23)

kaCRHI CHOI = 2k tCR;1 CRl T (24)

where [RIT denotes the total radical Concentration and the concentration of peroxide in the cavity after time t , [HOOSO;], is assumed to be equal to the initial peroxide concentration [HOOSO;l0 (as the concentration of peroxide employed is typically much greater than that of the metal ions).

Publ

ishe

d on

01

Janu

ary

1990

. Dow

nloa

ded

by L

aure

ntia

n U

nive

rsity

on

29/1

0/20

13 0

5:30

:46.

View Article Online

J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86 3265

Dividing eqn (22) by (21) and (24) by (23), respectively, leads to the equation

Applying a steady-state analysis for Cu' leads to the equa-

(26) and as [Cu"], = [Cu"], - [Cu'], then equation (27) can be derived.

[CU'],(~,[HOOSO,]~ + k,[Ti"']) = k,[Ti"'],[Cu"], (27)

tion

k , [ H OOSO 3 ] , [ Cu'] = k , [Ti"'] ,[ Cu"] ,

Substituting for [Cut], in eqn (25) gives the equation:

As [Ti"'], + [Cu"], then [Ti"'It would not be expected to vary significantly as a function of [Cu"], and hence a plot of the ratio of the two radical adducts [RJCR;] against [Cu"], should be linear with a gradient of k , k5/k3(k,[HOOS0;], + k,[Ti"'],).

A good straight-line plot of [R;]/[R ;] against [Cu"], was obtained from results of experiments with but-2-ene-1,4-diol (fig. 4) and it is believed that the assumptions made are indeed valid. From the gradient of the plot (1.6 x lo4 dm3 mol-' s-') and using k , = 1 x lo6 dm3 mol-' s-', a value for k , for the reaction between Cu' and HOOSO, of 2 2.0 x lo4 dm3 mol- ' s- ' is calculated.7

Computer simulations were also performed based on the reactions given in scheme 2. Rate constants of 750 and lo6 dm3 mol-' s- ' were incorporated for the reactions between HOOSO, with Ti"' and for Ti"' with Cut', respectively, while diffusion-controlled rates (with k = lo9 dm3 mol- ' s- ') were assumed for the addition of the hydroxyl radical and SO:- to the alkene and for the termination reactions of R; and R;. Optimum agreement between the observed experimental behaviour (for variation of [Cu"]) and the calculated steady- state radical concentrations was obtained when a value for a rate constant between Cu' and HOOSO, of 2 x lo4 dm3 mol - ' s - ' was employed.

The simulation led to the prediction of somewhat higher overall radical concentrations than observed at high [Cu"], presumably because no reactions had been incorporated for the removal of radicals by Cu' and Cu". Inclusion of reduction of both radical adducts by Cu' (for example, with k = lo8 dm3 mol-' s-') and oxidation by CU" (for example k = 6 x lo6 dm3 mol-' s-') was found not to alter the ratio of the concentrations of the two radical adducts, and the (observed) overall decrease in radical concentration was pre- dicted at high [Cu"]. Good agreement with experimental results was observed for a value of k , of 2 x lo4 dm3 mol-' s- '.

In order to test the proposal that the hydroxyl-radical adduct is converted into the sulphate-radical adduct via oxi- dation by HOOSO, at high peroxide concentrations, simula- tions were also performed on the effect of altering the concentration of HOOSO;. The kinetic scheme used for these calculations was as above (scheme 2, with k7 = 2 x lo4 dm3 mol-' s-'), but with the inclusion of an oxidation step in which both radical adducts are oxidised by HOOSO; to generate SO;- :

(29)

Good agreement was obtained between the calculated radical concentrations and those observed for experiments with but-2-ene- 1,4-diol in which the peroxomonosulphate concen- tration was varied; a rate constant of 1.0 x lo5 dm3 mol-' s- ' was employed for the oxidation of both R; and R; by HOOSO, (see fig. 5 : n.b. values of 1.2 x lo5 and 3.8 x lo5 dm3 mol-' s-' have been measured for the oxidation of Me' and Et', respectively, by peroxomonosulphate).2 Similarly, for the more electron-rich substrates for which greater relative concentrations of sulphate adducts compared to hydroxyl adducts were observed at a fixed value of [HOOSO,] (see table l), rate constants for the oxidation of adducts were estimated to be 9 x lo5 dm3 mol-' s- ' (for HOCH,CMe=CH,) and 210, dm3 mol-' s- ' (for Me,C=CH,).

The good agreement between the experimental results and the simulated behaviour is believed to provide further support for the conclusions reached above that reaction of Cu' with the peroxomonosulphate anion leads directly to the production of the hydroxyl radical and that the hydroxyl-

R; + HOOSO, + R: + HO- + SO;-.

t This is believed to be a lower limit; the rate constant would be greater than this if the concentration of Cu' is less than that assumed.

0 *+ 0 1 2 3 Lc

[Cu"],/l 0 - 5 mol dm-3

Fig. 4. Variation in the relative concentrations of the HO' and SO:- adducts of but-2ene-1,4-diol with [Cu"], in steady-state EPR flow experiments involving Ti"' and HOOSO; ([Ti"'], 1.7 x [HOOSO;], 5.0 x lop3 mol dm-3, [substrate], 2% v/v, pH 2).

[HOOSO;]/l 0-2 mol dm-3

Fig. 5. Calculated variation with [HOOSO;], of the steady-state concentration of the OH' (0) and SO:- (u)adducts of but-2-ene-1, 4-diol (see fig. 3), with k(Cu* + HOOSO;) 2 x loo dm3 mol-' s - l

and k(R' + HOOSO;) 1 x lo5 dm3 mol-' s-' . For other kinetic parameters see text.

Publ

ishe

d on

01

Janu

ary

1990

. Dow

nloa

ded

by L

aure

ntia

n U

nive

rsity

on

29/1

0/20

13 0

5:30

:46.

View Article Online

3266 J. CHEM. SOC. FARADAY TRANS., 1990, VOL. 86

radical adducts of alkenes thus produced are rapidly oxidised by HOOSO, itself to give SO;-.

Conclusions Two notable features emerge if the reactions of Cu' with a series of peroxides ( H 2 0 2 , HOOSO, , S 2 0 i - ) are compared with those of the corresponding reactions of Fe" and Ti"'. The first is that for both Fe" and Ti"' the increase in the number of sulphate moieties in the peroxide is accompanied by a decrease in the rate constant for the (overall) electron- transfer process [see reactions [(l),M"+ = Fe",Tirr'], ( 2 ) and ( 3 ) and the results summarized in table 2 ) . Although compa- rable data on the one-electron reduction potentials of the peroxides are not available, it would be expected'* that peroxomono- and peroxodi-sulphates would be more power- ful oxidants via outer-sphere one-electron transfer than H 2 0 2 (we have established that the rates of oxidation of radicals with HOOSO, increase with decrease in the ionization potential of the appropriate radicals, as expected for an outer-sphere type of electron-transfer process governed by Marcus theory).2 Our conclusion is then that reaction of Fe" and Ti"' involves an inner-sphere reaction process in which the dominant feature is the possession in the peroxide of an hydroxyl group, which largely governs the nature and rate of attack. The most favourable transition state (which leads to the production of SO;-, in contrast to the thermo- dynamically-controlled production of HO' for the reaction of e.9 itself) is evidently one in which the dominant interaction is that between the relatively small, charged metal cation and the electron pair on the hydroxyl group [see e.g. reaction ( 3 ~ .

Ti"' + HOOSO, 4 Ti'V(OH)3+ + SO;- (30)

This proposal would also be in accord with the principle of hard and soft acids and bases:19 thus Ti"' would be expected to be a hard acid (acceptor atom of high positive charge, small size, and without easily polarized electrons) and the hydroxyl group is a hard base (donor atom of low polariza- bility, high electronegativity and with empty orbitals of high energy), leading to effective Ti-0 bonding. Similar argu- ments would apply to Fe".

In contrast, the rates of reaction of the peroxides with Cu' show a marked increase with the number of sulphate groups [and it is also notable that the product of electron transfer is the hydroxyl radical, reaction (7)]. It is then possible that their reaction is an outer-sphere process and that it is largely governed by energetic factors. This would account for the for- mation of 'OH rather than SO;- ( c j the reaction of the hydrated electron3) and the observation that the per- oxosulphates react faster than H 2 0 2 . [If this is the case then it follows that the oxidation of alkyl radicals by HOOSO,,

Table 2. Rate constants for the decomposition of some peroxides by transition-metal ions (M"') in aqueous solution'

peroxide Ti"' ref. Cu' ref. Fe"-EDTA ref.

H,O, 2.2 x lo3 1 ca. lo3 10, 11 1.0 x lo4 1 HOOSO; 7.0 x 10, 2 2 x lo4 this work 3.0 x lo4 2 s,o; - 2.0 x lo2 4 1 x lo6 4

which gives SO;-, must be considered an inner-sphere electron-transfer process, presumably involving attack of the (nucleophilic) radical centre on the hydroxyl oxygen (6 +), with displacement of the sulphate-radical anion]. On the other hand, the results for Cu' could also be explained in terms of an inner-sphere electron-transfer process in which the dominant feature in the affinity of Cu' (a relatively soft acid, being an acceptor atom of low positive charge, large size and easily excitable electrons) for the sulphate moiety in a transition state involving sulphate bridging,

( 3 1 ) Cu' + -03SOOH + Cu"S0;- + HO'.

We thank the S.E.R.C. and Interox Chemicals Ltd for support and Mr W. R. Sanderson and Professor M.C.R. Symons for helpful discussions.

References 1

2

3 4

5

6 7 8

9

10

11

12

13

14

15

16

17

18 19

B. C. Gilbert and M. Jeff, in Free Radicals: Recent Developments in Lipid Chemistry, Experimental Pathology and Medicine, ed. T. Dormandy and C. Rice-Evans (The Richelieu Press, 1988), p. 25; M. Fitchett, B. C. Gilbert and M. Jeff, Philos. Trans. R . SOC., London, 1985,311,517. B. C. Gilbert and J. K. Stell, J. Chem. Soc., Perkin Trans. 2, 1990, in press. P.Maruthamuthu and P. Neta, J. Phys. Chem., 1977,81,937. B. C . Gilbert, J. K. Stell and M. Jeff, J. Chem. Soc., Perkin Trans. 2, 1988, 1867. G. V. Buxton, J. C. Green, R. Higgins and S. Kanji, J. Chem. SOC., Chem. Commun., 1976, 158; G. V. Buxton and J. C. Green, J. Chem. Soc., Faraday Trans. 1, 1978,74,697. R. J. Faber and G. K. Fraenkel, J. Chem. Phys., 1967,47,2462. R. Livingston and H. Zeldes, J. Chem. Phys., 1966,44, 1245. M. J. Davies and B. C. Gilbert, J. Chem. SOC., Perkin Trans. 2, 1984, 1809. Magnetic Properties of Free Radicals, Group ZI Landolt- Biirnstein New Series (Springer-Verlag, Berlin, 1977), vol. 9, part b. S. Goldstein and G. Czapski, Znorg. Chem., 1985, 24, 1087; M. J. Nicol, S. Afr. J. Chem., 1982, 35, 77; M. Marsawa, H. Cohen, D. Meyerstein, D. L. Hickman, A. Bakac and J. H. Espenson, J . Am. Chem. Soc., 1988,110,4293. G. R. A. Johnson, N. B. Nazhat and E. A. Saadalla-Nazhat, J. Chem. Soc., Chem. Commun., 1985,407. P. Dobson, J. A. Norman and C. B. Thomas, J. Chem. SOC. Perkin Trans. 2, 1986, 1209. See e.g. G. Koltzenburg, E. Bastian and S. Steenken, Angew. Chem. Znt. Ed. Engl., 1988,27, 1066 and references therein. See e.g. A. B. Ross and P. Neta, Rate Constants for the Reactions of Inorganic Radicals in Aqueous Solution, National Standards Reference Data Systems (National Bureau of Standards, Wash- ington, 1988). Farhataziz and A. B. Ross, Selected SpeciJic Rates of Reaction of Transients from Water in Aqueous Solution ZII, Hydroxyl Radical and Perhydroxyl Radical and their Radical Zons, Nation- al Standard Reference Data Systems (National Bureau of Stan- dards, Washington, 1977). M. J. Davies, B. C. Gilbert and R. 0. C. Norman, J . Chem. SOC., Perkin Trans. 2, 1984, 503. M. J. Davies, B. C. Gilbert, C. B. Thomas and J. Young, J. Chem. SOC., Perkin Trans. 2, 1985, 1199. L. Eberson, Ado. Phys. Org. Chem., 1982,18,79. R. G. Pearson and J. Slangstad, J. Am. Chem. Soc., 1967,89, 18; T-L. Ho, Chem. Rev., 1975, 75, 1; R. G. Pearson, J. Chem. Ed., 1968,45, 58 1.

' pH ca. 2, room temperature. Paper 0/01414A; Receiued 30th March, 1990

Publ

ishe

d on

01

Janu

ary

1990

. Dow

nloa

ded

by L

aure

ntia

n U

nive

rsity

on

29/1

0/20

13 0

5:30

:46.

View Article Online