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EC mechanisms: electrodimerization of benzophenone on mercury electrode
EC MECHANISMS: ELECTRODIMERIZATION OF BENZOPHENONE ON MERCURY ELECTRODE M.
BL~QUEZ,
J.
M. RODRIGUEZ-MELLADOand J. J. RUG?
Departamento de Quimica F&a, Facultad de Ciencias, Universidad de C6rdoba, 14005Cbrdoba, Spain (Received 2 April 1985) Abstract-Polarographic and kinetic studies of the first reduction wave. of benzophenone in buffered ethanol-water solutions have beencarried out. The effect of pH, benzophenane concentration, drop timeand ethanol content on the polarographic and kinetic parameters is shown. The results obtained herein and the theoretical studies on EC mechanisms reported in the literature are thoroughly discussed. Likewise, the inhibition effect ofpinacol on the electrodic process is shown. From the C-E curves obtained, the adsorption of benzophenone on a dme is shown to occur. The extent of adsorption decreases as the ethanol content increases. On the basis of these experimental data we can conclude that: (a) the electron transfer is reversible, (b) a protonation step precedes the electron transfer and (c) the process takes place either at the interface or in the reaction layer, depending on the experimental conditions.
INTRODUCTION There are many works in the literature dealing with the polarographic reduction of aromatic aldehydes and ketones in general[l-IO] and benzophenone in particular[l l-131. The reduction mechanisms of aromatic carbonyl compounds have been extensively studied and are assumed to occur through Scheme 1.
In their work, Suzuki and Elving conclude that the reduction wave is irreversible due to the chemical dimerization step and to the formation of an insoluble bcnzopinacol film. The comments of Mairanovskii and Laviron lead us to think it advisable to reinvestigate the electrodimerization process of benzophenone following the suggestions of these authors on the experimental conditions to employ.
Scheme 1.
To the elucidation of this mechanism has greatly contributed the derivation of explicit theoretical expressions for the waves carried out by Mairanovskii[7] and Laviron[I] on the basis of both the reaction layer theory and the equations proposed by Koutecky and Hanus[l4]. These equations have been applied to compounds such as benzaldehyde, acetophenone, benzaldehyde derivatives, aliphatic olefins etc. Likewise, the study of the influence of pinacol on the electrodic process by Laviron[8] has also contributed to the comprehension of this mechanism. However, these studies have not yet been applied to the reduction process of benzophenone. On the other hand both Mairanovskii and Laviron, in commenting on the latest and most comprehensive study on reduction of bcnzophenone due to Suzuki and Elving, indicate that the deviations from the general behaviour observed by these authors are due to the use of inappropriate experimental conditions and to analysis criteria inapplicable under these experimental conditions. * To whom all correspondence
should be addressed.
Therefore, our work aims to answer to questions such as the reversible or irreversible nature of the electron transfer, the homogeneous or heterogeneous character of the overall reaction, the effect of pinacol and the influence of the characteristics of the medium on the process, and finally, to propose a reduction mechanism accounting for the polarographic and kinetic parameters found.
EXPERIMENTAL All reagents used were Merck p.a. grade. Buffered ethanol-water solutions with the following components were used as supporting electrolyte: 0.05 M acetic acid; 0.05 M phosphoric acid. For pH < 1.8, solutions of sulphuric or perchloric acids were used. The pH and the ionic strength were adjusted with NaOH and NaNOa respectively. All measurements were made in a nitrogen atmosphere. dc polarographic curves were recorded automatically by means of a 465 Amel polarograph at 25°C. Measurements were carried out using a thermostatted
1527
1528
M. BLAZQUEZ, J. M. RODRIGUEZ-MEJUDO
AND
J. J. Rurz
Amel 494 ceil. The working electrode was a Radiometer B 4COmercury capillary with the following characteristics: m = 2.01 mgs-‘, t = 4.19 s, open circuit, in our buffer solution at pH = 1.94 and with h = 40 cm. The auxiliary electrode was an Ingold 4805 platinum electrode. All potentials were referred to a saturated calomel electrode (Ingold 303-N.%). Differential capacity-potential curves were recorded automatically by means of a system based on the application of a small triangular signal superimposed to a slowly varying dc potential[ 151.A Telequipment D-101 1 oscilloscope was also used. The i-t curveswere registered for each first drop to avoid depleting the solution. In all cases, i-z curves corresponding to the supporting electrolyte at the same potential were recorded and subsequently subtracted from those corresponding to the depolarizer.
b /
RESULTS General behaviour
Benzophenone shows one or two dc polarographic waves depending on the pH of the medium. Below pH 6, two waves with the same value of i, can be observed, but above this pH value both waves merge to yield only one, whose i, value is twice that corresponding to the separate waves. Above pH 8, the limiting current of the combined wave decreases as a new wave appears at more negative potentials, the sum of their corresponding limiting currents being pHindependent. All these waves are diffusion-controlled at potentials corresponding to the limiting current, with the exception of the combined wave, which is kinetically controlled above pH 8. In this work, only the first wave in the O-6 pH range is studied, since this is the one corresponding to an EC mechanism. Polarographic
Fig. 1. Variation ofE,,, withlog c; H,SO, 0.72 M; y0EtOH: (a) 0; (b) 10; (c) 80.
EI
(In”,
I
behaviour
Figure 1 shows the variation of El,1 with loge for different ethanol-water solutions. When the ethanol content in the medium is either very low or nil, these plots show a linear segment followed by a discontinuity, but when this content is greater than 50% in volume, the plot shows only a linear stretch with a slope of 20 mVdec_ ‘. The drop time influence of Eilz is similar to that of the concentration. Thus, when the ethanol content is low. the E, ,2uslog f plot (Fig. 2) shows a linear segment followed by a discontinuity, but for ethanol contents greater than 50 %, the plot is linear over the whole drop time range under study and has a slope of 20 mV. The logarithmic analysis of this wave at various benzophenone and ethanol concentrations has been carried out from E us log [i”/(io- i)] plots (Fig. 3). These plots are curves for a = 1 but when both the benzophenone concentration and ethanol content are very low, a linear plot is obtained for (I = l/2. Likewise, when the ethanol content is greater than 50”/ these plots are always linear for a = 2/3. Figure 4 shows the variation of i,/c with the electrolysis time on a hdme for solutions with ethanol contents of 0 and 80 %_As can be seen, the value of iD/c
-0.5
010
0:5
10st
(6)
Fig. 2. Variation of IT,,, with logl; H,SO, 0.72 M; (a) 10% EtOH, c = 6 x 1O-5 M; (b) 10% EtOH, c = 2 x lo-’ M; (c) 80% EtOH, c = 6 xxl$Mti, (d) 80 % EtOH, c = 2 for the ethanol-water solution is approx. twice that corresponding to the aqueous solution. i-t curves i-t curves of benzophenone at various i/i, values and at different ethanol contents have been recorded. The values of the slopes of the log i us log? plots obtained as functions of i/i, for an ethanol content of 80% are gathered in Table 1. The i-t recordings obtained when the ethanol content is very low are rather anomalous, showing some maxima and minima which result in nonlinear log i us log t plots.
EC mechanisms:
electrodimerization
of benzophenone
on mercury electrode
1529
0 0
Cl.1
0.c
O0
00
-0.5
0 0 0 0
-1.0
Fig. 4. Variation of idc with the electrolysis time at hdmp. % EtOH: (a) 0; (b) 80.
~ -1.5 -800
-90”
EWI
Fig. 3. Plot of log [F/(iD- i)] us E: (0) 10% EtOH, c = 2 x 1W4 M, a = l/i; (a) 10% EtOH, c = 6 x 1O-5 M, a = l/2; (A) SO”/, EtOH, c = 2 x lo-+ M, (I = 2/3; (A) 80% EtOH, c = 2 x lo+ M, 0 = 2/3.
C-E curves Figure
ing electrolyte
Table 1. Results of i-t curves for benzophenone;
i/i,
5 shows
the C-E
curves
H2S01 0.72 M; EtOH 80%
0.27
0.49
0.89
1.00
t
0.63
0.52
0.40
0.22
0.19
theoretical from equation
slope (11
0.58
0.45
0.34
0.20
0.17
theoretical from equation
slope (21
0.67
0.49
0.37
0.21
0.17
i/slog
-0.25
-0.50
-0.75
to the support-
(solid lines). As can be seen, when the
0.10
31og
for benzophenone
(dotted lines) and those corresponding
-0.25
-0.50
-0.75 E(V)
Fig. 5. C-E curves: (a)-HCl 0.1 M; --- (a) fbenzophenone 2.2 x lo-’ M; (b) - (a)+ 10% EtOH: --(b) + benzophenone 2.2 x lo-’ M; (c) - (a) + 50 % EtOH; ~~~ (c) + benzophenone 2.2 x IO-’ M; (d) - (a) +80x EtOH; ~~~ (d)+ benzophenone 2.2 x lo-’ M. Arrows indicate the initiation potential of the
M. BLAZQIm,J.
1530
I
ethanol content is very low, the different capacity between both solutions indicates that benzophenone is adsorbed on the mercury electrode at potentials close to those of its reduction. However, when the ethanol content is greater than 50% both curves overlap over the whole potential range, indicating that the extent of adsorption is either small or negligible. Kinetic
parameters
at the fool
J.1. RUE
M. RODRIGUEZ-MELLADOAND
of’ the waves
A study on the variation with pH, benzophenone concentration and ethanol content of the i-E polarization curves obtained at potentials corresponding to the foot of the wave has been carried out. The values of the Tafel slopes and reaction orders with respect to Hf ion and benzophenone found are gathered in Table 2. As can be seen, the values of the kinetic parameters depend on the ethanol content. Figure 6 shows the variation of the Tafel slope with the percentage of ethanol in the medium.
Fig. 6. Variation of theTafel slope with the ethanol content.
, RT i,-i E = E0+Flni”2+1Fln
if the dimerization and as:
DISCUSSION
E
=E,
0
The polarography of benzophenone has been extensively studied and many details of its behaviour have been clarified; the two well-defined waves appearing in acidic medium are due to the processes shown in Scheme 2. The combined wave observed in the 68 pH range is due to the two-electron reduction of benzophenone to carbinol.
Hb
bH
k,ccf,
~
K:i,
g J
70
(1)
step takes place at the interphase
+Elnin-i F
if, on the contrary, layer[l7, 181.
I
?‘3
RTln3htd, ~ 7i,Ki
3F
(2)
this takes place in the reaction
In these equations, k, is the rate constant of dimerization step, K, is the reciprocal of the equilibrium constant of reaction (a), c and cH are the concentrations of benzophenone and H+ ion, respect-
_ -C I
RT
I’dJ
OH
H:e I+-e-+
+-CH*H-+
OH
SC :me 2.
The decrease in the limiting current with pH (following the pattern of a titration curve) and the increase of its kinetic character indicate the occurrence of a protonation step prior to the electron transfer. Above pH 8 this chemical step is the rate-determining one at potentials corresponding to the limiting current. These facts allow us lo formulate the process corresponding to the first reduction wave of benzophenone as follows (Scheme 3).
For a process of this type in which reaction (a) is very fast (although very displaced towards the left), reaction (b) is reversible and reaction (c) is the rds, the i-E relationship can be expressed[ 16, 171 as:
ively, and Eb =E,f$lng a where ki and kc2 denote the direct and inverse rate constants of the electron transfer step at E = 0. In view of the Equations (1) and (2), and of the experimental results displayed in the previous section, benzo-
phenone can be said to show three types of electrochemica behaviour. In the zone where the ethanol content is low (CklOyJ and the benzophenone concentration is very low
EC mechanisms: electrectiirization
of benzophenone on mercury electrode
1531
( < 6 x lo-’ M), the experimental data seem to conform to Equation (1) roughly. Thus, the El,, us loge plots are linear, with slopes (30-40 mV), [close to the theoretical value (30 mV)] (Fig. 1) and the logarithmic analyses, ie the E us log [i’/‘/(iei)] plots are linear, with a slope of - 65 mV (also close to the theoretical value (- 59 mV)) (Fig. 3). However, the slope of the linear E, ,a us log t plot, 42 mV, is much greater than the theoretical value (15 mV). This divergence could be due to the fact that Equation (1) has been derived by assuming that the depolarizer is not adsorbed on the mercury electrode, whereas the C-E curves indicate that when the ethanol content is very low, benzophenone is indeed adsorbed on the electrode surface. This fact was also observed by Mairanovskii[ 193 in the reduction of N-alkyl-pyridinium salts. On the other hand, when the ethanol content is greater than SO%, the experimental data conform to Equation (2) perfectly. Thus, the El,, DSlog c and log t plotsare linear, with slopes of 18-20 mV (Figs 1 and 2), which coincide with the theoretical value (20 mV). Moreover, the E vs log [i2’3/(i, - i)] plots are linear, with slopes of - 58 mV. These data, together with the C-E curves, which show that for ethanol contents greater than 50% the adsorption of benzophenone is negligible, indicate that the reduction process takes place in the homogeneous phase. Finally, when the ethanol content is low and the benzophenone concentration is greater than 10m4 M, the experimental results show an anomalous behaviour (Figs 1 and 2). Thus, the El ,2 us log c and log t plots are linear up to a certain value, over which a discontinuity can be observed. Likewise, the logarithmic analyses are not linear (Fig. 3). These facts were observed by Laviron[8] in the reduction of some aromatic aldehydes and attributed by this author to the formation of an insoluble film layer of pinacol at the electrode surface in the course of the reduction process. This possibility was also considered by Suzuki and Elving[l l] in the reduction of benzophenone on a dme. This insoluble film layer exerts an inhibiting effect on the process by decreasing the available surface of the electrode. When the ethanol content increases, the critical values of c or r also increase due to the increasing solubility of benzopinacol. Thus, the transition concentrations are 5 x 10m5, 1.3 x 10m4 and > 8 x 1O-4 M for ethanol contents of 0, 10 and 80% respectively. Likewise, when the bcnzophenone concentration is 6 x lo-’ M, the transition drop times are 5 and > 6.3 s for ethanol contents of 10 and 80% respectively, whereas for a bcnzophenone concentration of 2 x 10m4 M , the corresponding time values are 1.6 and > 6.3 s, respectively. Evidence in favour of this interpretation is that the values of i& corresponding to the i,/c us t curve for a solution containing 80 “/, ethanol obtained with a hdm are twice those corresponding to an aqueous solution (Fig. 4). On the other hand, the anomalous shape of the i-t curves obtained when the ethanol content is very low is probably due to the effect of theinsoluble,and possibly adsorbed, film of benzopinacol. This fact, together with the characteristics of the C-E curves, which show an appreciable extent of adsorption of benzophenone on the electrode surface (even at potentials close to its
M. BLA~WEZ,J. M.
1532
RODRIGUEZ-MELLADO AND
reduction) indicate that, under these experimental conditions, the electroreduction process occurs in heterogeneous phase. However, when the ethanol content is greater than SO%, the plots of log i us log 1are linear, although the values of the slopes do not allow us to conclude whether the dimerization step takes place either at the interphase or in the reaction layer, since the theoretical values of these slopes, derived from Equations (I) and (2) are very similar (Table I). Finally, the theoretical values of the kinetic parameters derived from Equations (1) and (2) are: Tafelslope=
-F[i+&]lnlO
(3)
where x = i/i,; order with respect to benzophenone
= &
order with respect to Ht ion = F
(4) (5)
if the dimerization step occurs on the electrode surface, and Tafel slope=
= &
order with respect to H+ ion = s
(8)
+ H + G=PhW-Ph
I
OH
I OH
+ eG
I
Ph#ZOHXOH-Ph,.
OH When the ethanol content is very low, this process takes place on the electrode surface and the step
iPh1;-‘-Phi 0
* I,,7-Ph sol
1 0
ads
must be formulated as the previous one. Moreover, if the benzophenone concentration is high enough, benzopinacol can form an insoluble film layer, thus partially inhibiting the electrode process. On the other hand when the ethanol content is greater than 50 %, its molecules completely cover the electrode surface preventing the adsorption of benzophenone. Likewise, the solubility of benzopinacol in these media hinders the formation of the film layer at the electrode surface and for these reasons, the ekctroreduction process must be considered as homogeneous.
(7)
if it takes place in the reaction layer. The experimental values of these kinetic parameters determined at potentials corresponding to the foot of the wave confirm the conclusions drawn from the polarographic data, that is, they are in agreement with the above theoretical expressions. Thus, the value of Tafel slopes shifts from - 30 to - 39 mV as the ethanol content increases from 0 to 50% (Fig. 6). These limiting values coincide with those obtained from expressions (3) and (6), respectively, for x -Z 0.05. Likewise, the reaction orders with respect to benzophenone and H+ ion shift from 2.0 to 1.5 over the same ethanol percentage range, in agreement with those predicted by the Equations (4), (5), (7) and (8). From the polarographic and kinetic parameters and from their variation with pH, benzophenone concentration, ethanol content, etc. we can conclude that the electroreduction process corresponding to the first reduction wave of benzophenone occurs via the following reaction kinetic pathway:
Ph-&Ph
2~Ph~-Ph~~~
(6)
-F[i+&]lnl@
order with respect to benzophenone
Ph--C-Ph
J. J. RUIZ
PhW-Ph bH
..
REFERENCES 1.
M. Ashworth, Colln. Crech. them.
Commun. Engl. Edrt 13,
229 (1948).
2. R. Pasternak, He/o. chim. Acra 31, 753 (1948). 3. R. A. Day and J. J. Kirkland, J. Am. them. Sot. 72,2766 (1950). 4. H. J. Gardner, Chem. Ind. 819 (1951). 5, P. J. Elving and J. T. Leone, J. Am. them. Sot. 80, 1021 (1958). 6. S. G. Mairanovskii, Izu. Akod. Nauk SSSR Old. Khim. Nauk 12, 214 (1961). 7. S. G. Mairanovskii and V. N. Paulov, Zh. fiz. Khim. 38, 980 (1964). 8. E. Laviron, Colln. Czech. them. Commun. Engl. Edn 30, 4219 (1965). 9. E. Laviron and J. C. Lucy, Bull. Sot. chim. Fr. 2202 ( 1966). 10. P. Zuman, D. Barnesand A. Ryvolova-Kejharova, Dicuss. Faraday Sot. 45, 202 (1968). 11. M. Suzuki and P. I. Elvina. J. ph vs.Chem. 65.39 1( 1961). 12. S. G. Mairanovskii, L. i.Y&&skaya. V. Kond;atova and G. V. Kristhal, Elektrokhimiya 4(9), 1019 (1968). 13. A. Ryvolova-Kejharova and P. Zuman, Cohn Czech. them. Commun. Engl. Edn 35, 22
(I 970).
14. J. Koutecky and V. Hanus, Cohn Czech. Engl. Edn 20, 124
them. Commun.
( 1955).
15. E. Roldan, M. Dominguez and D. Gonzalez-Arjona, An. Quim. 78, 158 (1982). 16. R. Bonnaterre and G. Cauquis, J. electroanal. Chem. 32, 183 (1971). 17. L. &macho, J. electroanal. Chem 177, 59 (1984). 18. R. Brdicka, V. Hanus and J. Koutecky, Progress in Polarography, Vol. 1, p. 145. Inters&n&, New York (1962). 19. S. G. Mairanovskii, Dokl. Akad Nauk SSSR 110, 593 (1956).
BL~QUEZ,
J.
M. RODRIGUEZ-MELLADOand J. J. RUG?
Departamento de Quimica F&a, Facultad de Ciencias, Universidad de C6rdoba, 14005Cbrdoba, Spain (Received 2 April 1985) Abstract-Polarographic and kinetic studies of the first reduction wave. of benzophenone in buffered ethanol-water solutions have beencarried out. The effect of pH, benzophenane concentration, drop timeand ethanol content on the polarographic and kinetic parameters is shown. The results obtained herein and the theoretical studies on EC mechanisms reported in the literature are thoroughly discussed. Likewise, the inhibition effect ofpinacol on the electrodic process is shown. From the C-E curves obtained, the adsorption of benzophenone on a dme is shown to occur. The extent of adsorption decreases as the ethanol content increases. On the basis of these experimental data we can conclude that: (a) the electron transfer is reversible, (b) a protonation step precedes the electron transfer and (c) the process takes place either at the interface or in the reaction layer, depending on the experimental conditions.
INTRODUCTION There are many works in the literature dealing with the polarographic reduction of aromatic aldehydes and ketones in general[l-IO] and benzophenone in particular[l l-131. The reduction mechanisms of aromatic carbonyl compounds have been extensively studied and are assumed to occur through Scheme 1.
In their work, Suzuki and Elving conclude that the reduction wave is irreversible due to the chemical dimerization step and to the formation of an insoluble bcnzopinacol film. The comments of Mairanovskii and Laviron lead us to think it advisable to reinvestigate the electrodimerization process of benzophenone following the suggestions of these authors on the experimental conditions to employ.
Scheme 1.
To the elucidation of this mechanism has greatly contributed the derivation of explicit theoretical expressions for the waves carried out by Mairanovskii[7] and Laviron[I] on the basis of both the reaction layer theory and the equations proposed by Koutecky and Hanus[l4]. These equations have been applied to compounds such as benzaldehyde, acetophenone, benzaldehyde derivatives, aliphatic olefins etc. Likewise, the study of the influence of pinacol on the electrodic process by Laviron[8] has also contributed to the comprehension of this mechanism. However, these studies have not yet been applied to the reduction process of benzophenone. On the other hand both Mairanovskii and Laviron, in commenting on the latest and most comprehensive study on reduction of bcnzophenone due to Suzuki and Elving, indicate that the deviations from the general behaviour observed by these authors are due to the use of inappropriate experimental conditions and to analysis criteria inapplicable under these experimental conditions. * To whom all correspondence
should be addressed.
Therefore, our work aims to answer to questions such as the reversible or irreversible nature of the electron transfer, the homogeneous or heterogeneous character of the overall reaction, the effect of pinacol and the influence of the characteristics of the medium on the process, and finally, to propose a reduction mechanism accounting for the polarographic and kinetic parameters found.
EXPERIMENTAL All reagents used were Merck p.a. grade. Buffered ethanol-water solutions with the following components were used as supporting electrolyte: 0.05 M acetic acid; 0.05 M phosphoric acid. For pH < 1.8, solutions of sulphuric or perchloric acids were used. The pH and the ionic strength were adjusted with NaOH and NaNOa respectively. All measurements were made in a nitrogen atmosphere. dc polarographic curves were recorded automatically by means of a 465 Amel polarograph at 25°C. Measurements were carried out using a thermostatted
1527
1528
M. BLAZQUEZ, J. M. RODRIGUEZ-MEJUDO
AND
J. J. Rurz
Amel 494 ceil. The working electrode was a Radiometer B 4COmercury capillary with the following characteristics: m = 2.01 mgs-‘, t = 4.19 s, open circuit, in our buffer solution at pH = 1.94 and with h = 40 cm. The auxiliary electrode was an Ingold 4805 platinum electrode. All potentials were referred to a saturated calomel electrode (Ingold 303-N.%). Differential capacity-potential curves were recorded automatically by means of a system based on the application of a small triangular signal superimposed to a slowly varying dc potential[ 151.A Telequipment D-101 1 oscilloscope was also used. The i-t curveswere registered for each first drop to avoid depleting the solution. In all cases, i-z curves corresponding to the supporting electrolyte at the same potential were recorded and subsequently subtracted from those corresponding to the depolarizer.
b /
RESULTS General behaviour
Benzophenone shows one or two dc polarographic waves depending on the pH of the medium. Below pH 6, two waves with the same value of i, can be observed, but above this pH value both waves merge to yield only one, whose i, value is twice that corresponding to the separate waves. Above pH 8, the limiting current of the combined wave decreases as a new wave appears at more negative potentials, the sum of their corresponding limiting currents being pHindependent. All these waves are diffusion-controlled at potentials corresponding to the limiting current, with the exception of the combined wave, which is kinetically controlled above pH 8. In this work, only the first wave in the O-6 pH range is studied, since this is the one corresponding to an EC mechanism. Polarographic
Fig. 1. Variation ofE,,, withlog c; H,SO, 0.72 M; y0EtOH: (a) 0; (b) 10; (c) 80.
EI
(In”,
I
behaviour
Figure 1 shows the variation of El,1 with loge for different ethanol-water solutions. When the ethanol content in the medium is either very low or nil, these plots show a linear segment followed by a discontinuity, but when this content is greater than 50% in volume, the plot shows only a linear stretch with a slope of 20 mVdec_ ‘. The drop time influence of Eilz is similar to that of the concentration. Thus, when the ethanol content is low. the E, ,2uslog f plot (Fig. 2) shows a linear segment followed by a discontinuity, but for ethanol contents greater than 50 %, the plot is linear over the whole drop time range under study and has a slope of 20 mV. The logarithmic analysis of this wave at various benzophenone and ethanol concentrations has been carried out from E us log [i”/(io- i)] plots (Fig. 3). These plots are curves for a = 1 but when both the benzophenone concentration and ethanol content are very low, a linear plot is obtained for (I = l/2. Likewise, when the ethanol content is greater than 50”/ these plots are always linear for a = 2/3. Figure 4 shows the variation of i,/c with the electrolysis time on a hdme for solutions with ethanol contents of 0 and 80 %_As can be seen, the value of iD/c
-0.5
010
0:5
10st
(6)
Fig. 2. Variation of IT,,, with logl; H,SO, 0.72 M; (a) 10% EtOH, c = 6 x 1O-5 M; (b) 10% EtOH, c = 2 x lo-’ M; (c) 80% EtOH, c = 6 xxl$Mti, (d) 80 % EtOH, c = 2 for the ethanol-water solution is approx. twice that corresponding to the aqueous solution. i-t curves i-t curves of benzophenone at various i/i, values and at different ethanol contents have been recorded. The values of the slopes of the log i us log? plots obtained as functions of i/i, for an ethanol content of 80% are gathered in Table 1. The i-t recordings obtained when the ethanol content is very low are rather anomalous, showing some maxima and minima which result in nonlinear log i us log t plots.
EC mechanisms:
electrodimerization
of benzophenone
on mercury electrode
1529
0 0
Cl.1
0.c
O0
00
-0.5
0 0 0 0
-1.0
Fig. 4. Variation of idc with the electrolysis time at hdmp. % EtOH: (a) 0; (b) 80.
~ -1.5 -800
-90”
EWI
Fig. 3. Plot of log [F/(iD- i)] us E: (0) 10% EtOH, c = 2 x 1W4 M, a = l/i; (a) 10% EtOH, c = 6 x 1O-5 M, a = l/2; (A) SO”/, EtOH, c = 2 x lo-+ M, (I = 2/3; (A) 80% EtOH, c = 2 x lo+ M, 0 = 2/3.
C-E curves Figure
ing electrolyte
Table 1. Results of i-t curves for benzophenone;
i/i,
5 shows
the C-E
curves
H2S01 0.72 M; EtOH 80%
0.27
0.49
0.89
1.00
t
0.63
0.52
0.40
0.22
0.19
theoretical from equation
slope (11
0.58
0.45
0.34
0.20
0.17
theoretical from equation
slope (21
0.67
0.49
0.37
0.21
0.17
i/slog
-0.25
-0.50
-0.75
to the support-
(solid lines). As can be seen, when the
0.10
31og
for benzophenone
(dotted lines) and those corresponding
-0.25
-0.50
-0.75 E(V)
Fig. 5. C-E curves: (a)-HCl 0.1 M; --- (a) fbenzophenone 2.2 x lo-’ M; (b) - (a)+ 10% EtOH: --(b) + benzophenone 2.2 x lo-’ M; (c) - (a) + 50 % EtOH; ~~~ (c) + benzophenone 2.2 x IO-’ M; (d) - (a) +80x EtOH; ~~~ (d)+ benzophenone 2.2 x lo-’ M. Arrows indicate the initiation potential of the
M. BLAZQIm,J.
1530
I
ethanol content is very low, the different capacity between both solutions indicates that benzophenone is adsorbed on the mercury electrode at potentials close to those of its reduction. However, when the ethanol content is greater than 50% both curves overlap over the whole potential range, indicating that the extent of adsorption is either small or negligible. Kinetic
parameters
at the fool
J.1. RUE
M. RODRIGUEZ-MELLADOAND
of’ the waves
A study on the variation with pH, benzophenone concentration and ethanol content of the i-E polarization curves obtained at potentials corresponding to the foot of the wave has been carried out. The values of the Tafel slopes and reaction orders with respect to Hf ion and benzophenone found are gathered in Table 2. As can be seen, the values of the kinetic parameters depend on the ethanol content. Figure 6 shows the variation of the Tafel slope with the percentage of ethanol in the medium.
Fig. 6. Variation of theTafel slope with the ethanol content.
, RT i,-i E = E0+Flni”2+1Fln
if the dimerization and as:
DISCUSSION
E
=E,
0
The polarography of benzophenone has been extensively studied and many details of its behaviour have been clarified; the two well-defined waves appearing in acidic medium are due to the processes shown in Scheme 2. The combined wave observed in the 68 pH range is due to the two-electron reduction of benzophenone to carbinol.
Hb
bH
k,ccf,
~
K:i,
g J
70
(1)
step takes place at the interphase
+Elnin-i F
if, on the contrary, layer[l7, 181.
I
?‘3
RTln3htd, ~ 7i,Ki
3F
(2)
this takes place in the reaction
In these equations, k, is the rate constant of dimerization step, K, is the reciprocal of the equilibrium constant of reaction (a), c and cH are the concentrations of benzophenone and H+ ion, respect-
_ -C I
RT
I’dJ
OH
H:e I+-e-+
+-CH*H-+
OH
SC :me 2.
The decrease in the limiting current with pH (following the pattern of a titration curve) and the increase of its kinetic character indicate the occurrence of a protonation step prior to the electron transfer. Above pH 8 this chemical step is the rate-determining one at potentials corresponding to the limiting current. These facts allow us lo formulate the process corresponding to the first reduction wave of benzophenone as follows (Scheme 3).
For a process of this type in which reaction (a) is very fast (although very displaced towards the left), reaction (b) is reversible and reaction (c) is the rds, the i-E relationship can be expressed[ 16, 171 as:
ively, and Eb =E,f$lng a where ki and kc2 denote the direct and inverse rate constants of the electron transfer step at E = 0. In view of the Equations (1) and (2), and of the experimental results displayed in the previous section, benzo-
phenone can be said to show three types of electrochemica behaviour. In the zone where the ethanol content is low (CklOyJ and the benzophenone concentration is very low
EC mechanisms: electrectiirization
of benzophenone on mercury electrode
1531
( < 6 x lo-’ M), the experimental data seem to conform to Equation (1) roughly. Thus, the El,, us loge plots are linear, with slopes (30-40 mV), [close to the theoretical value (30 mV)] (Fig. 1) and the logarithmic analyses, ie the E us log [i’/‘/(iei)] plots are linear, with a slope of - 65 mV (also close to the theoretical value (- 59 mV)) (Fig. 3). However, the slope of the linear E, ,a us log t plot, 42 mV, is much greater than the theoretical value (15 mV). This divergence could be due to the fact that Equation (1) has been derived by assuming that the depolarizer is not adsorbed on the mercury electrode, whereas the C-E curves indicate that when the ethanol content is very low, benzophenone is indeed adsorbed on the electrode surface. This fact was also observed by Mairanovskii[ 193 in the reduction of N-alkyl-pyridinium salts. On the other hand, when the ethanol content is greater than SO%, the experimental data conform to Equation (2) perfectly. Thus, the El,, DSlog c and log t plotsare linear, with slopes of 18-20 mV (Figs 1 and 2), which coincide with the theoretical value (20 mV). Moreover, the E vs log [i2’3/(i, - i)] plots are linear, with slopes of - 58 mV. These data, together with the C-E curves, which show that for ethanol contents greater than 50% the adsorption of benzophenone is negligible, indicate that the reduction process takes place in the homogeneous phase. Finally, when the ethanol content is low and the benzophenone concentration is greater than 10m4 M, the experimental results show an anomalous behaviour (Figs 1 and 2). Thus, the El ,2 us log c and log t plots are linear up to a certain value, over which a discontinuity can be observed. Likewise, the logarithmic analyses are not linear (Fig. 3). These facts were observed by Laviron[8] in the reduction of some aromatic aldehydes and attributed by this author to the formation of an insoluble film layer of pinacol at the electrode surface in the course of the reduction process. This possibility was also considered by Suzuki and Elving[l l] in the reduction of benzophenone on a dme. This insoluble film layer exerts an inhibiting effect on the process by decreasing the available surface of the electrode. When the ethanol content increases, the critical values of c or r also increase due to the increasing solubility of benzopinacol. Thus, the transition concentrations are 5 x 10m5, 1.3 x 10m4 and > 8 x 1O-4 M for ethanol contents of 0, 10 and 80% respectively. Likewise, when the bcnzophenone concentration is 6 x lo-’ M, the transition drop times are 5 and > 6.3 s for ethanol contents of 10 and 80% respectively, whereas for a bcnzophenone concentration of 2 x 10m4 M , the corresponding time values are 1.6 and > 6.3 s, respectively. Evidence in favour of this interpretation is that the values of i& corresponding to the i,/c us t curve for a solution containing 80 “/, ethanol obtained with a hdm are twice those corresponding to an aqueous solution (Fig. 4). On the other hand, the anomalous shape of the i-t curves obtained when the ethanol content is very low is probably due to the effect of theinsoluble,and possibly adsorbed, film of benzopinacol. This fact, together with the characteristics of the C-E curves, which show an appreciable extent of adsorption of benzophenone on the electrode surface (even at potentials close to its
M. BLA~WEZ,J. M.
1532
RODRIGUEZ-MELLADO AND
reduction) indicate that, under these experimental conditions, the electroreduction process occurs in heterogeneous phase. However, when the ethanol content is greater than SO%, the plots of log i us log 1are linear, although the values of the slopes do not allow us to conclude whether the dimerization step takes place either at the interphase or in the reaction layer, since the theoretical values of these slopes, derived from Equations (I) and (2) are very similar (Table I). Finally, the theoretical values of the kinetic parameters derived from Equations (1) and (2) are: Tafelslope=
-F[i+&]lnlO
(3)
where x = i/i,; order with respect to benzophenone
= &
order with respect to Ht ion = F
(4) (5)
if the dimerization step occurs on the electrode surface, and Tafel slope=
= &
order with respect to H+ ion = s
(8)
+ H + G=PhW-Ph
I
OH
I OH
+ eG
I
Ph#ZOHXOH-Ph,.
OH When the ethanol content is very low, this process takes place on the electrode surface and the step
iPh1;-‘-Phi 0
* I,,7-Ph sol
1 0
ads
must be formulated as the previous one. Moreover, if the benzophenone concentration is high enough, benzopinacol can form an insoluble film layer, thus partially inhibiting the electrode process. On the other hand when the ethanol content is greater than 50 %, its molecules completely cover the electrode surface preventing the adsorption of benzophenone. Likewise, the solubility of benzopinacol in these media hinders the formation of the film layer at the electrode surface and for these reasons, the ekctroreduction process must be considered as homogeneous.
(7)
if it takes place in the reaction layer. The experimental values of these kinetic parameters determined at potentials corresponding to the foot of the wave confirm the conclusions drawn from the polarographic data, that is, they are in agreement with the above theoretical expressions. Thus, the value of Tafel slopes shifts from - 30 to - 39 mV as the ethanol content increases from 0 to 50% (Fig. 6). These limiting values coincide with those obtained from expressions (3) and (6), respectively, for x -Z 0.05. Likewise, the reaction orders with respect to benzophenone and H+ ion shift from 2.0 to 1.5 over the same ethanol percentage range, in agreement with those predicted by the Equations (4), (5), (7) and (8). From the polarographic and kinetic parameters and from their variation with pH, benzophenone concentration, ethanol content, etc. we can conclude that the electroreduction process corresponding to the first reduction wave of benzophenone occurs via the following reaction kinetic pathway:
Ph-&Ph
2~Ph~-Ph~~~
(6)
-F[i+&]lnl@
order with respect to benzophenone
Ph--C-Ph
J. J. RUIZ
PhW-Ph bH
..
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