same buffer and by coulometry on the plateau of the wave (napp= 1.07). Benzo- phenone and p-phenylbenzophenone exhibit a single two-electron wave as ...
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Electroanalytical Chemistry and Interfacial Electrochemistry, 47 (1973) 146-149 © Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
S H O R T COMMUNICATION
Linear sweep voltammetry: kinetic control by charge transfer and/or secondary chemical reactions II. Reduction of carbonyl compounds
F. AMMAR, L. NADJO and J. M. SAVI~ANT
Laboratoire d'Electrochimie de l'Universit~ de Paris VII, 2 Place Jussieu, 75221 Paris Cedex 05 (France) (Received 30th January, 1973)
A previous study ~ of the reduction of aromatic carbonyl compounds in alkaline ethanolic buffer (pH range 13.6-18.9) has shown that their behaviour can be mainly characterized as follows: (i) For two groups of these compounds the reduction mechanism is particularly simple. In the first one, a typical representative of which is benzaldehyde, two separated one-electron waves are observed in the pH range considered, the first wave corresponding to the hydrodimerization into pinacol. In the second group, an example of which is benzophenone, two one-electron waves are also observed in the most alkaline medium, whereas, when the pH is decreased they merge into a single two-electron wave. The first wave is reversible and is not related to dimerization. The two-electron wave is related to hydrogenation into the alcohol. Other carbonyl compounds, such as acetophenone, show an intermediate behaviour: hydrodimerization along the first wave in the most alkaline medium, hydrogenation along the single two-electron wave in more acidic media. (ii) Hydrodimerization occurs through a rate determining coupling of a ketyl anion radical with the neutral ketyl preceded by a fast and equilibrated protonation and followed by the neutralization of the pinacolate. (iii) The mechanism of the hydrogenation involves in most cases a rate determining solution electron transfer between the ketyl anion and the neutral ketyl again preceded by a fast and equilibrated protonation and followed by a neutralization of the alcoholate. Only at the most acidic extremity of the pH range considered and for low buffer concentration does the first protonation step become rate determining. In this last situation it is not known whether the second electronation step occurs at the electrode (e.c.e. type mechanism) or in the solution (disp. 1 type mechanism). (iv) The first electron transfer appears fast enough not to interfere kinetically. This is in accordance with the observation that electron transfer to large organic molecules is fast (exchange rate constants uncorrected for the diffuse double layer effect lie in the range 0.5-5 cm s-~ for e.g. aromatic nitrocompounds2). However, it might well be that decreasing further the pH, the resulting
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acceleration of the consecutive chemical process would lead the charge transfer to interfere kinetically and to become ultimately the rate determining step. It is the purpose of the present communication to discuss this point for the examples of benzaldehyde, benzophenone and p-phenylbenzophenone in benzoic acid buffer. The discussion will be based on the formal kinetic analysis of mixed kinetic control by charge transfer and chemical reactions given in the first paper of this series a for first order deactivation, dimerization, e.c.e, and disproportionation mechanisms. Polarography, coulometry and linear sweep voltammetry (l.s.v.) devices, procedures for elimination of the ohmic drop in l.s.v., and preparation of the ethanolic buffers from the acid and a tetrabutylammonium hydroxide solution were the same as previously 1. Various concentrations of buffer were used, from 0.01 to 1.5 mol 1-1. In each case, another supporting electrolyte, tetrabutyla .m~monium iodide, was added to the solution at a concentration of 0.1 mol 1-1. The pH of the benzoic acid buffer is 10.1 according to the literature 4. The first polarographic wave of benzaldehyde involves the exchange of 1 e per tool, as shown by comparison with the two-electron wave of fluorenone in the same buffer and by coulometry on the plateau of the wave (napp= 1.07). Benzophenone and p-phenylbenzophenone exhibit a single two-electron wave as checked by comparison with fluorenone. In all cases the cyclic voltammetry pattern is completely irreversible, indicating a fast and irreversible follow-up chemical process. The kinetics of the system were therefore investigated by means of the variations of the l.s.v, peak potential with the sweep rate v and the initial depolarizer concentration c °. Repeated experiments showed that the accuracy of peak potential determination is about + 3 mV. The Ep-log v plots constructed from a variation of about two decades in v displayed a linear behaviour in all cases. Their slopes were therefore used as characteristic quantities in the discussion. The temperature of the experiment was 25 ° C.
H ydrodimerization ( benzaldehyde) The Ep-log v slope for c°= 1 mmol 1-1 and a buffer concentration of 0.1 tool I-1 was 43 mV. The same value was also found using the acetic acid buffer (pH--- 10.4 instead of 10.1) showing that no specific effect occurs with the benzoic acid-benzoate system. The slopes obtained with the same depolarizer concentration and various buffer concentrations are shown in Table 1. It is seen that the slopes do not depend significantly upon the buffer concentration. The variations of Ep with the depolarizer concentration in the range 0.5-5 mmol 1-1 observed with a buffer concentration of 0.5 mol 1-1 are small. At the TABLE 1 BENZALDEHYDE IN BENZOIC ACID BUFFER Depolarizer concentration, 1 mmol 1-t Buffer conch. ~tool l- 1 -t3Ev/~ loo v
/mV
0.01
0.05
0.10
0.50
1.00
1.50
43.5
42
43
45
40
41
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lowest sweep rate (0.07 V s-1) E, shifts anodically by about 10 mV for a tenfold increase in co. Raising the sweep rate, the shift diminishes and falls rapidly into the range of uncertainty. Both the variations with the sweep rate and the initial concentration are consistent with a mixed kinetic control by the charge transfer and the dimerization process (see Figs. 9 and 10 in ref. 3). The cause of this variation in the kinetic control when the pH decreases from 13.6 to 10.4 is clearly the acceleration of the dimerization process. However, this is not sufficient to render the charge transfer the only rate determining step. The pK A of benzophenone ketyl determined by Porter and Wilkinson 5 in a water-isopropanol mixture is 9.2. It is therefore probable that the neutral ketyl of benzaldehyde predominates over the ketyl anion at equilibrium in the ethanolic benzoic acid buffer. Since, on the other hand, the coupling between two neutral ketyls is likely to be faster than between the neutral ketyl and the ketyl anion, the most probable reaction scheme for the hydrodimerization is: C6Hs-CHO
+ e- ~ C 6 H s ~ H O -
C6Hs-(~HO- + TH ~ C 6 H s - ~ H O H + T - (TH/T- : proton donor couple) 2 C6H 5 CHOH ~ C6Hs-CHOH-CHOH-C6H 5 The lack of variation of the slopes with the buffer concentration and the small but definite variation of Ep with c o at slow sweep rate show, as regards the dimerization process itself, that the coupling reaction remains the rate determining step, while the protonation of the ketyl anion remains at equilibrium. This conclusion would not be affected if, in contrast with the above, the coupling between the neutral and charged ketyls had been assumed to still play a significant role in the benzoic acid buffer.
Hydrogenation ( benzophenone and p-phenylbenzophenone) The slopes are now dependent upon the buffer concentration as seen in Table 2. For 0.01 mol 1-1 the value of the slope indicates that the kinetic control is by the chemical process alone as in the veronal buffer 1, whereas by augmenting the buffer concentration the charge transfer increasingly interferes kinetically. Change in depolarizer concentration has a negligible effect on Ep. Even if the neutral ketyl predominates at equilibrium over the ketyl anion the disproportionation reaction, if occurring~ is more likely to proceed through electron TABLE 2 VALUES OF -(dEp/8 log v) IN mV FOR BENZOPHENONE AND p-PHENYLBENZOPHENONE AS FUNCTION OF THE BUFFER CONCENTRATION Depolarizer concentration, 1 mmol 1-1 Buffer concn. /mol l- 1 C6Hs-CO-C6H ~ P-C6Hs-C6H4-CO-C6Hs
0.01
0.10
0.50
29 31
32 41
38 --
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transfer between the ketyl anion and the neutral ketyl than through hydrogen atom exchange between two neutral ketyls. It follows that the most probable reaction scheme for hydrogenation is, as in the more alkaline medial: R-CO-C6H 5 + e - ~ R-(~O--C6H s R - ( 7 0 - ~ 6 H 5+ T H ~ R - C O H - C 6 H 5+ T R--~O---C6H5 + R-~OH-C6H
s --. R - C O - C 6 H
5 + R-CHO--C6H
s
(disp.)
R--~OH-C6H 5 + e - ~ R-CHO---C6H s (e.c.e.) R - C H O - - C 6 H 5 + TH ~ R - C H O H - C 6 H s The lack of variation of Ep with c o as well as the significant dependence of the slope on the buffer concentration show, as concerns the chemical process itself, that the kinetic control is by the protonation reaction of the ketyl anion. From the present data it is not possible to know whether this results in an e.c.c, or a disp. 1 mechanism, or in a mixed character mechanism. The trend towards a kinetic control by the first charge transfer although clearly apparent is less pronounced than for reductive hydrodimerization of benzaldehyde.
Acknowledgement This work was supported in part by the C.N.R.S. (Equipe de Recherche Associ6e no. 309: Electrochimie Organique). REFERENCES 1 2 3 4
L. Nadjo and J. M. Sav6ant, J. Electroanal. Chem., 33 (1971) 419. M. E. Peover and J. S. Powell, J. Electroanal. Chem., 20 (1969) 427. L. Nadjo and J. M. Sav6ant, J. Electroanal. Chem., 44 (1973) 327. G. Charlot and B. Tr6millon, Les ROactions Chimiques dans les Solvants et les Sels Fondus, Gauthier-Villars, Paris, 1963, p. 303. 5 G. Porter and F. Wilkinson, Trans. Faraday Soc., 57 (1961) 1686.
