Electroanalytical Chemistry and lnterfacial Electrochemistry, 62 (1975) 259-265 259 © Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

USE OF HOMOGENEOUS COMPETITION FOR OH RADICALS IN ELECTROCHEMICAL STUDIES OF H ABSTRACTION FROM ORGANIC MOLECULES*

G. B O T T U R A and B. BUBANI

lstituto Chimico "G. Ciamician" e Centro di Elettrochimica Teorica e Preparativa dell'Universita, Bologna (Italy)

G. C. BARKER

Atomic Energy Research Establishment, Harwell, Didcot, Oxon (England)

(Received 20th December 1974)

Photocurrent observed at mercury in contact with a solution containing N20 may be partly suppressed when an organic solute, unreactive to e~q such as a short chain aliphatic alcohol, is added to the solution 1~. This effect is caused in the alcohol case by homogeneous abstraction of H atoms from the alcohol molecules by OH radicals formed homogeneously after the capture of e~q by N20 the resulting alcohol radicals being, over much of the accessible potential range, heterogeneously oxidised after diffusion from the almost two-dimensional H ab- straction reaction zone to the electrode. Studies 3'4 of such suppression of the photo- current produced by u.v. light have provided kinetic data for H abstraction from aliphatic alcohols and information about the reactivity of radicals likely to be formed in the electrochemical oxidation of alcohols at more noble metals than mercury. This unusual approach to topics bridging the fields of radiation chemistry and electrochemistry has the not inconsiderable merit of requiring only simple, reliable and inexpensive apparatus.

OH radicals generally rupture C-H bonds and it might be thought, in the absence of inhibiting complications due to competition for e~q by the organic solute, that the photocurrent method would find wide application in the study of H abstraction processes and in the elucidation of electrochemical reactions involving the resulting radicals. However, in some instances the radical formed by H abstraction is highly unstable with respect to electrochemical reduction to the original molecule throughout much or all of the accessible potential range. This behaviour results in little or no change in the N20 photocurrent on introducing the organic solute into the solution and could be the cause of the anomalously low values for the H abstraction rate constants deduced from photocurrent measurements with and with- out phenol, or aniline, present 4. More explicitly, when the organic solute is present, the third step (c) in the normal N20 photocurrent mechanism:

e~ + N 2 0 -~ OH (hom.) (a)

e~q ~ e- (Hg) (het.) (b)

O H + e - ( H g ) ---, O H - (het.) (c)

* Dedicated to Prof. K. S. G. Doss on the occasion of his reaching his 70th year.

260 G. BOTTURA, B. BUBANI, G. C. BARKER

tends to be partly replaced by the reaction sequence:

O H + S k~ ~ + H 2 0 (hom.) (d)

~ + H 2 0 + e - ( H g ) ~ S + O H - (het.) (e)

where S is the added organic solute and ~ the organic radical formed by H abstraction from S. We assume here and elsewhere that the solution is neutral or slightly alkaline, that the presence of S in the system does not significantly in- fluence electron deposition in the solution and that S does not react at an appreciable rate with e~q or N20. Also that radical ~ is homogeneously stable in a solution containing N20. Clearly in a "steady state" experiment 2 using unmodulated light 5, light modulated at a low frequency (e.g., 200 Hz) 1 or a long (> 0.2 ms) light pulse 2 the partial replacement of (c) by (d) and (e) will leave the photocurrent unchanged though distinct changes in the coulostatic photopotential in an "unsteady state" experiment 6 are to be expected.

Despite insensitivity of the "steady state" photocurrent to the presence of S it may still be possible to determine the rate constant of (d) by adding to the solution a second homogeneous scavenger of OH radicals that gives a radical known to be oxidised by the electrode over an appreciable part of the accessible potential range. Two obvious contenders for this role that are sufficiently unreactive to e~q are methanol and ethanol, both of which form radicals that are fairly easily oxidised in neutral or slightly alkaline solution at a mercury electrode. The resulting competition for OH between the two organic solutes results in a decrease in photo- current dependent on the degree of competition. Obviously, in the absence of some complications to be touched on later, it should then be feasible to determine kd if the rate constant for H abstraction by OH from the competing solute is known.

THEORY

When OH radicals, formed by homogeneous capture by N20 of a small fraction of the electrons deposited in the solution, are destroyed homogeneously in a H abstraction reaction foil, the fraction of the radicals arriving at the electrode, is given by 2'7

fort = Q,/(Q, + Qo) (1)

where Q, and Qo respectively are the reciprocal distance constants 7 for the linear reaction of e~, with N20 and of OH with the organic solute. More explicitly Qn=(k . c . /DJ and Qo=(k,,c,,/Do) ½ where c. and Co are respectively the N20 and organic solute concentrations, k n and ko are respectively the rate constants for ejq capture and H abstraction, and D e and Do are respectively the diffusion coefficients of e~q and OH. In the derivation of (1) heterogeneous destruction of OH by adsorbed organic molecules is ignored.

If simultaneously OH is homogeneously destroyed by (d) and by reaction with alcohol A

A + O H k,_> A + H 2 0 (hom.) (f)

where A denotes the resulting alcohol radical (or radicals), (1) is still valid provided

H ABSTRACTION FROM ORGANIC MOLECULES 261

that the composite nature of Qo is recognised, this parameter now being defined by:

Qo = [(kdCs + kfca)/Do] ~ (2)

where kf refers to reaction (f), and cs and Ca are respectively the concentrations of solute S and alcohol A.

If A is fu l ly oxidised by an electrode reactiorr which formally may be written:

A --* A + + e - ( H g ) (het.) (g)

where A + denotes the oxidised (and not necessarily charged) form of A, it follows from (l) that the steady state photocurrent is:

ip = ie( l + Qn/(Q, + Qo) + [Qo/(Q, + Q o ) ] [ 4 9 - ( 1 - q~)]) (3)

OHT redn. g r~edn. ~ oxidn.

where ie is the current component due solely to homogeneous e~q destruction. The dimensionless parameter 4) is the fraction of the OH radicals consumed homo- geneously by (d) and (f) that are destroyed by reaction with S and

c~ = kd Cs/(kd Cs + kf Ca) (4)

From (3) it follows that the dimensionless current ratio Z = ip/( ipo- ip), where ipo is the unsuppressed photocurrent when the solution contains no organic solutes, is:

Z = 4~/(1 - 4~) + Q./[Oo( 1 - ~)] (5)

If this expression is applicable a plot of Z against c[ ~, Ca -½ or (Cs+Ca) -½ for constant potential and a constant ratio of Ca/Cs (i.e. constant 4)) should be linear. From its slope, or the intercept with the Z axis, ~b can be calculated and finally kd can be evaluated if kf is known. Unfortunately, in practice, it may not always be possible to ensure sufficiently complete oxidation of A to make (5) applicable.

If A is methanol or ethanol and unmodulated light, light modulated at a low frequency, or a long duration (20 ms) light pulse is employed, (5) will normally be applicable at potentials more positive than -1 .4 V vs. SCE in the absence of complications connected with marked specific adsorption of solute S or caused by the homogeneous reaction of g with A or of A with S. However, if u.v. light is supplied by a photographic type of xenon arc, the duration of the light pulse (~0.6 ms) may be such as to seriously restrict the range of potential in which the rate of oxidation of A virtually equals the rate at which A is formed 2. Errors then tend to arise when measurements of the peak value of the photocurrent at controlled potential are made. Errors of this type largely can be avoided by measuring (by electronic means) the integral of the photocurrent pulse over a sufficiently long time interval (e.g. 20 ms) and this expedient has the added advantage that error due to the interplay of slightly imperfect potential control and changes in interfacial impedance due to organic adsorption also is largely eliminated. However, the use of this expedient may not be feasible and an expression for Z taking account of slow oxidation of A will be needed if only measurements of the peak value of ip can be made. The relevant expression easily can be shown to be:

Z = 1 -~+~b( l+~) 2Q. (1 +~)(1-~b) + (1 +a)(1-q~)Q,, (6)

262 G. BOTTURA, B. BUBANI, G. C. BARKER

where ~ is the ratio at the time of current measurement of the rate of oxidation of A to the rate of formation of the same radical.

If, for constant potential, ~ shows no dependence on theorganic content of the solution, it can be seen from (6) that plots of experimental data in the co-ordinates Z-(organic concentration) -½ should be linear and that, in principle, from the slopes and intercepts with the Z axis both ~ and ~b can be calculated. Thus, incomplete oxidation of the alcohol radical may not prevent the evaluation of k d. Indeed, if c~ at constant potential is invariant, the analysis of the data may be refined with the aid of ~ values deduced from measurements made in the absence of S but with and without the alcohol present. In practice, however, the use of (6) may be seriously restricted by specific adsorption of S which usually will lead to a dependence of on the organic content of the solution in at least part of the accessible potential range. Also it has to be borne in mind that specific adsorption of S indirectly may introduce uncertain errors that are connected with (a) its influence on the rate of transfer of electrons to the solution 9 and with (b) the concomitant change in inter- facial impedance when the time constant for potential control is not very much smaller than the duration of the light pulse.

HYDROGEN ABSTRACTION BY OH RADICALS FROM PHENOL

To test the simple theoretical arguments advanced above we have studied the influence of a mixture of phenol and. methanol (Ca/C s = 5.6), of phenol alone, and of methanol alone on the peak value of the photocurrent for mercury (DME) in contact with 0.2 M KC1 saturated with 10~o N20/90~o argon at 23___ 2°C. The light source was the Rollei xenon flash source used previously a and measurements were made of the maximum value of the photocurrent at controlled potential using a slightly modified Southern Instrument pulse polarograph (A 3100) to control the potential of the DME, to amplify the photocurrent waveform and to reject back- ground current connected with non-faradaic and faradaic processes occurring at the DME prior to the light pulse. The pulse polarograph in modified form also was used to study the influence of phenol on the interracial impedance. Potentials were measured with respect to the SCE.

In Fig. 1 can be seen tracings of the photocurrent waveforms for 0.2 M KC1 saturated with 10~o N20, with and without 10- 2 M phenol present, and at various potentials between -0 .8 and - 1.5 V. From literature values of k d for H abstraction from phenol and (1), it can be shown that about 75~o of the OH radicals should be consumed homogeneously. The photocurrent waveforms reveal barely perceptible changes in current at potentials < - 1.3 V. However, at more positive values of the potential phenol produces a decrease in the peak value of the photocurrent of the order of 7~o. These slight decreases might be electrochemical artefacts connected with an increase in the effective high frequency value of the differential capacity of the electrode, or with a small decrease in the rate of deposition of eZq in the solution, as there is appreciable surface coverage with phenol at -0 .8 and -0 .9 V. However, the impedance measurements revealed virtually no change in interfacial impedance due to phenol adsorption at - 1.2 and - 1.1 V and hence there is some reason to believe that one of the three possible primary products of H abstraction from

H ABSTRACTION F R O M ORGANIC MOLECULES 263

Sen$ x 2

i -77-

NO Phenol

[ ! -0.8V

°09V

-1.0V

. • 1 • ¸ ] I

10 -2 N Phenol

-1.IV

-1.2V

-1.3V

-1.,'*V -1.5V

Fig. 1. Photocurrent waveforms at controlled potential for 0.2 M KC1 saturated with 10~ N20/90 % argon, with and without 10 2 M PhOH present. 200 #s/hor. divn.

phenol* is not fully reduced to phenol at the electrode at potentials > - 1.3 V, possibly because the radical in question undergoes a rapid stabilizing transformation in solution. Nevertheless, it is clear that no great error should result from the simplifying assumption, in the analysis of results of competitive experiments, of rapid reduction to phenol of all species formed by H abstraction from phenol.

Results for solutions containing methanol at various concentrations between 4.24 x 10- j and 8.42 x 10- 2 M gave values for k r of the order of magnitude of the one obtained earlier in experiments a using rectangularly-modulated 253.7 nm light. They also confirmed the occurrence at negative potentials of values of ct at the current peak significantly smaller than unity. The ct values deduced from the results were for - 1.0, - 1.2, - 1.3, - 1.4, a n d ' - 1.5 V respectively, ca. 1.0, 0.4o, 0.22, 0.15 and 0.11 for a neutral solution. Thus not until the potential is such that there tends to be appreciable adsorption of phenol does ~ attain the value needed to justify the use of eqn. (5) in the analysis of data for methanolic-phenolic solutions. At the more negative potentials incomplete oxidation of A makes it difficult to evaluate Z which is then very sensitive to errors in the measurement of ip and ipo. However, the presence of methanol in addition to phenol does produce the expected reduction of ip due to competition for OH between the two OH scavengers. In general Z for Ca/C s-- 5.6 and various values ofY. c = cs + c a between 5 x l03 and 10-1 M decreased progressively as the organic content of the solution was raised. There were some signs of linearity in the plots of Z against (Ec) -½ but the scatter in the points was large and there

* o- , m- and p-C6H4OH.

264 G. BOTTURA, B. BUBANI, G. C. BARKER

were, not unexpected, signs of effects due to the inconstancy of~ at certain potentials. Thus in the analysis of the data it was thought advisable to concentrate attention on (a) the values of Z for - 1.4 and - 1.3 V (when phenol adsorption is absent) using (6) and the values of ~ given above, and (b) values of Z for potentials between -0 .9 and - 1.1 V and E c = 5 × 10 -3 M, assuming ~= 1 for this potential range. This selective and slightly arbitrary procedure led to average values of ~ of ca. 0.48 ( - 0 . 9 to -1.1 V) and of ca. 0.40 ( -1 .4 , -1 .3 V), taking for Q, /Qo values calculated from measurements with no phenol present. Thus we conclude that ~b for t a l c s ~ - 5.6 has a value close to 0.45 and hence, taking k r - 6 × l0 s 1 mol- i s- l (ref. 3), it is found that kd --~ 2.8 × 1091 mol- 1 s- 1, a value in agreement with what seem to be the most reliable literature values ranging from 2.1× 109 to 5.1 × 109 1 mol- 1 s- 1 (ref. 10).

Though in this initial attempt to employ homogeneous competition for a secondary intermediate for kinetic purposes it has not proved possible to avoid complications caused by slow alcohol radical oxidation, by adsorption of the main organic solute, and by experimental error, the results show that the slight suppression of the N20 photocurrent by phenol 4 is due to the easy regeneration of phenol by reduction of most of the (~6H4OH radicals formed by hydrogen abstraction. It is possible, however, that one of three types of phenol radical may not be completely reduced throughout the entire potential range.

ACKNOWLEDGEM ENTS

This work was supported financially by the Laboratorio di Fotochimica e Radiazioni ad Alta Energia del C.N.R. di Bologna. We are grateful to Prof. G. Semerano for his support of the work and his interest in its progress. We are indebted to D. McKeown for taking the photographs on which Fig. 1 is based.

SUMMARY

The influence on the N20 photocurrent of homogeneous competition for OH radicals between two organic solutes which form (as the result of hydrogen ab- straction) radicals, one of which is reduced by the electrode, the other being oxidised, is considered theoretically. Such competition can be employed to investigate the kinetics of hydrogen abstraction in a case in which uncompetitive homogeneous destruction of OH radicals by a solute has no effect on the photocurrent. The influence of incomplete oxidation of alcohol radicals when a light pulse of short duration is employed is discussed, together with complications caused by adsorption of the organic solute. Competition for OH radicals between phenol and methanol points to a rate constant for H abstraction from phenol of ca. 2.8 x 10 9 1 mol- 1 s-X and to rapid heterogeneous reduction of most of the ~6H4OH radicals throughout the accessible potential range.

REFERENCES

1 G. C. Barker, A. W. Gardner and D. C. Sammon, J. Electrochem. Soc., 113 (1966) 1182. 2 G. C. Barker, Electrochim. Acta, 13 (1968) 1221.

H ABSTRACTION FROM ORGANIC MOLECULES 265

3 G. C. Barker and J. A. Bolzan, J. Electroanal. Chem., 49 (1974) 239. 4 V. V. Eletskii and Yu. V. Pleskov, Elektrokhimiya, 10 (1974) 179. 5 Yu. V. Pleskov and Z. A. Rotenberg, J. Electroanal. Chem., 20 (1969) 1. 6 G. C. Barker, D. McKeown, M. J. Williams, G. Bottura and V. Concialini, Discuss. Faraday Soc.,

56 (1973) 41. 7 G. C. Barker and J. A. Bolzan, J. Electroanal. Chem., 49 (1974) 227. 8 V. Concialini, O. Tubertini and G. C. Barker, J. Electroanal. Chem., 57 (1974)413. 9 S. B. Sheberstov and A. M. Brodskii, Elektrokhimiya, 6 (170) 1762.

10 M. Anbar and P. Neta, Intern. J. Appl. Radiation Isotopes, 18 (1967) 493.