Electrodimerization

J. Electroanal. Chem., 64 (1975) 1 4 3 - - 1 5 4 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

ELECTRODIMERIZATION XI. COUPLING MECHANISM OF AN ACTIVATED OLEFIN: p-METHYLBENZYLIDENE-MALONONITRILE AS STUDIED BY CONVOLUTION POTENTIAL SWEEP VOLTAMMETRY

L. N A D J O , J.M. SAVt~ANT and D. TESSIER Laboratoire d'Electrochlmte de l'Universit~ de Paris VII, 2 Place Jussieu, 75221 Paris Cedex 05 (France) (Received 14th April 1975)

ABSTRACT The m e c h a n i s m of the e l e c t r o h y d r o d i m e r i z a t i o n of an activated olefin, p - m e t h y l b e n z y l i dene-malononitrile, is studied by c o n v o l u t i o n potential sweep v o l t a m m e t r y aiming at confirm a t i o n of the conclusion derived f r o m the m e a s u r e m e n t s of the peak p o t e n t i a l shifts with sweep rate and initial c o n c e n t r a t i o n . It is s h o w n that CPSV is a m o r e p o w e r f u l t o o l than LSV for discriminating b e t w e e n the n u m e r o u s mechanistic possibilities, based on the fact that it uses m o r e of the i n f o r m a t i o n available along the current potential curves. The various logarithmic analyses corresponding to the possible reaction schemes are a p p h e d to the c o n v o l u t e d e x p e r i m e n t a l c u r r e n t - - p o t e n t i a l curves which allows the possibilities to be reduced to only three cases. The final diagnosis is m a d e on the basis o f a study o f the effect of water addition. It is c o n f i r m e d that the coupling is o f the radical--radical t y p e and the rate c o n s t a n t is determined.

INTRODUCTION

Originating in the commercial interest of the electrochemical synthesis of adiponitrile from acrylonitrile, a large amount of work has been devoted in the last ten years to the electrohydrodimerization of activated olefins [ 1 ]. Analysis of the mechanism of the reaction has been performed in non-aqueous solvents such as acetonitrile (ACN), DMF and DMSO and several methods have been employed: linear sweep voltammetry [2--4] chronoamperometry [5] rotating ring--disc voltammetry [6,7], chronopotentiometry [8] and also a non-electrical method in which the decay of the initial anion radical is followed by electron spin resonance spectrometry [9]. The main goal aimed at by all these studies was to discriminate between three mechanistic possibilities concerning the coupling step itself:

144

coupling (RRC) radical--substrate coupling (RSC) -- ion--substrate coupling (ISC). In order to accomplish this task one must consider that the coupling reaction is n o t merely associated with the initial charge transfer step b u t that it interferes in fact in a complex reaction sequence including eventually other electrode-electron transfers, solution-electron transfers and numerous protonation steps revolving the species produced by the above reactions. As a consequence, the number of kinetic cases to be considered for a rigorous analysis is large [ 10]. This is the reason w h y some of the possibilities, particularly solution-electron transfers and/or protonations, have been often explicitly or implicitly neglected. It has been shown however [3,4] that a complete analysis is possible using the variations in the observed kinetics with the initial substrate concentration and with the a m o u n t of water present in the aprotic solvent. The conclusion of this investigation was that the EHD of the activated olefins studied was of the RRC type. The aim of the present work was to confirm this conclusion, taking as example, one of the previously studied olefins: p-methylbenzylidene-malononitrile (MBM) [3]: --radical--radical --

CHa--C~H4--CH = C(CN)2 and using convolution potential sweep voltammetry (CPSV) instead of LSV. The fundamentals of CPSV have already been considered [ 11 ], in particular the principles of its application to electrochemical mechanisms involving follow-up homogeneous chemical reactions [11,12] and the method tested on a simple example of EHD: the pinacolization of acetophenone in acetonitrile [121. That a CPSV study of a mechanistic problem already analyzed b y LSV may result in actual re-inforcement and not merely a repetition of the conclusions, derives from the remark that CPSV uses the kinetic information available along the whole t--E curve and not only that contained in the LSV peak. PREPARATIVE SCALE ELECTROLYSIS

In the previous study of MBM [3], it was implicitly assumed that its reduction leads to the hydrodimer as for the other activated olefins [ 1 ].This point was checked prior to the present mechanistic study b y bulk electrolysis of 0.84 g of MBM in 300 ml of ACN with 0.1 MEt4NC104 at --1.80 V vs. Ag/ AgC104 0.01 M. (The electrolysis cell and potentiostat were the same as already described [ 3 ] ). The recovered product was composed of hydrodimers as checked by mass spectrometry. The yield after ethereal extraction and recrystallization in hexane was 84%. However, the n.m.r, analysis did not allow an accurate structure determination, the p r o d u c t being probably a mixture of isomers.

145 P R O C E D U R E F O R M E C H A N I S M D E T E R M I N A T I O N BY CPSV

As shown previously [11,12] the CPSV analysis of reaction mechanisms comprising Nernstian charge transfers coupled with homogeneous chemical reactions, involves the following steps: (i) Computation of the convoluted curve, I--E, from the experimental current--potential curve, i--E, according to:

1

= 1 ~ i(v.) 7r1/2 ~ (t - - 0 ) 1 / 2 dv

(ii) determination of the logarithmic analysis of the I--E curve that characterizes the expected mechanism in pure kinetic conditions; (iii) treatment of the I--E curve according to this logarithmic analysis in the range of values of the experimental parameters, particularly the sweep rate, where purely kinetic conditions are achieved; (iv) if the assumed mechanism is compatible with the experimental data the logarithmic analysis must result in a straight line with the proper slope; (v) the whole procedure must be repeated with the largest possible range of sweep rates and concentrations compatible with purely kinetic conditions. This procedure has already been applied to an electrodimerization process: the reductive pinacolization of acetophenone in acetonitrile in order to test the method rather than to bring new chemical information. The application was then particularly straightforward since the mechanism was known a priori and only verification was looked for. In the present case the situation is much more complex since one has to decide among a large number of mechanistic possibilities. It follows that the procedure has to be applied several times, i.e., for all the corresponding logarithmic analyses. The possible mechanisms of the three reaction types, RRC, RSC, ISC, have been already listed [10] according to a symbolic designation showing the reaction sequence and the rate determining step, and their main LSV characteristics have been given, i.e., the rates of variation of the peak potential with the sweep rate v and the initial concentration c o as well as the dimensionless equation of the LSV wave. (See particularly in Ref. 10, Tables 3 and for 4 on which the present discussion is based.) The logarithmic analyses pertaining TABLE 1 F o r m s o f t h e l o g a r i t h m i c analysis E = E k +

7(RT/F)ln f ( l l - - l , l , i )

Conventional numbering

1

2

3

4

5

6

f(Ii-- I,l,i )

I1--I t

I1--I i2/3

I1--I ill3i213

I1--1 i112

I1--I ill4i112

I1--I i2/3i2/3

146

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148

to each reaction scheme are easily deduced from these dimensionless equations. They are of the general form E = E k + " ) , ( R T / F ) l n f(I1-- I , I , D

i.e., at 22 ° C, which is the temperature of our experiments: E/mV

= Ek/mV + 58.5 7 log f(Iz--I,I,i)

where 58.5 ~t is a characteristic slope in mV and E k a characteristic potential depending on the rate and equilibrium constants of the chemical reactions. Although 32 reaction schemes have been determined, the function f assumes only 6 different forms {Table 1), the 32 reaction schemes being distributed over these 6 forms with identical or different 7 values. Table 2 shows the CPSV characteristics of each reaction scheme, i.e., the number of the corresponding logarithmic analysis, its slope, and the expression for the potential Ek {for the definition of symbols see ref. 10) together with the main LSV peak potential characteristics (variation with v and cO). EHD MECHANISM OF p-METHYLBENZYLIDENE-MALONONITRILE

The reduction of MBM was studied in acetonitrile with 0.1 M Et4NC104. The water content of the solution was 0.3% as determined by the Karl-Fischer method. (Note that in Refs. 3 and 4 the water content was erroneously reported as 0.05%; it was actually 0.5%.) The temperature was 22°C. Eight sweep rates were used ranging from 0.055 to 185 Vs -1 every half-decade. The t--E curves were recorded for these sweep rates at 3 concentrations: 0.5, 1.2 and 3 mmol 1-1. The working electrode was a mercury droplet hanging on a gold disc of about 4 mm 2 surface area. The reference electrode was an Ag/AgC104 0.01 M electrode in acetonitrile. The cell, controlling and measuring instrument, analog to digital converter as well as the convolution procedure and correction for ohmic drop {residual resistance 2 1 0 ~2) and sphericity (D1/2/ro ---0.08) were the same as already described [ 12,13 ]. The experimental I - - E curves were first treated according to logarithmic analysis 2 which corresponds to the most probable reaction scheme [3,4], and also to that corresponding to pure diffusion control (E = E ° + ( R T / F ) l n [ (I l - I } / I]), in order to evaluate the range of sweep rates where pure kinetic conditions are achieved and to eventually determine the standard potential of the substrate/anion radical couple. c o = O. 5 m m o l 1-1

The slopes obtained for these two analyses are given in Fig. 1 as a function of sweep rate. It is seen that pure diffusion control is achieved at the highest

149

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N 6O

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+ ~e.~+ j+

4C

Fig. 1. S l o p e s o f d i f f u s i o n and logarithmic analyms 2 at c ° = 0.5 m m o l 1-1.

sweep rate (184 V s-l). The intersection of the logarithmic straight line then provides the value of the standard potential E ° = --1485 mV vs Ag/AgC104 0.01 M At the slowest sweep rates along one decade, pure kinetic conditions are -8

m ¢ iN A O

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i

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t

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-1.45

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0.54

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0.19

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0054

i

i

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-1.50 E / V

Fig. 2. c ° = 0.5 m m o ] 1-1. Logarithmic analysis 2 for v = 0 . 0 5 , 0 . 1 9 , 0 . 5 4 V s - 1 .

150

achieved: the slope for analysis 2 remains close to 58.6. The results of the logarithmic analysis 2 in the pure kinetic zone, i.e., for the 3 lowest sweep rates, are shown in Fig. 2. c o = 1.2 mmol

1-1

As seen in Fig. 3 it is no longer possible to reach pure diffusion control at the highest sweep rate. The sweep rate where pure kinetic conditions are achieved ranges now over 1.5 decades. The corresponding logarithmic analyses 2 are shown in Fig. 4. c o = 3 mmol

l-]

Pure kinetic conditions are achieved now in a larger range of sweep rates: 2.5 decades (Figs. 5 and 6). It follows from these data that, in the pure kinetic zone, logarithmic analysis 2 leads to a straight line with a slope close to 58.6 mV and in any case clearly different from the two other possible values: 87.8 and 43.9 mV. The slopes obtained by treating the data by the other logarithmic analyses are shown in Table 3 for c o = 1.2 mmol 1-1. It is clearly seen that the only loga-

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151 -8

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Fig. 4. c ° = 1 . 2 m m o l 1-1. L o g a r i t h m i c a n a l y s i s 2 f o r e = 0 . 0 5 , 0 . 1 9 , 0 . 5 4 , 1 . 8 3 V s - 1 .

8C

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E

0

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50

40

Fig. 5. S l o p e s o f d i f f u s i o n a n d l o g a r i t h m i c

a n a l y s i s 2 a t c °= 3 m m o | | - - I .

152 -8 *W ~m o

Sweep *

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c o t e / V s -1 18.B 5A2 183

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Fig. 6. c ° = 3 m m o l l - I . Logarithmic analysis 2 for v = 0.05, 0.19, 0.54, 1.83, 5.4, 18.8 V s--1.

rithmic analysis compatible with the experimental data is the second one with a slope of 58.6 mV. It follows (see Table 2), taking into account t h a t the medium is unbuffered, that the mechanism is one of the following three: e--rrc--P--p e--RRC--p--p e--rsc--d--P--p It is worth noting that CPSV appears to be more powerful than LSV as concerns mechanism discrimination. Indeed, in the same kind of analysis based on the v and c o variations of the LSV peak potential, five mechanisms appeared to be compatible with the experimental data, before study of the effect of water addition [4]; the two additional possibilities were: e--rsc--p--D--p and e--p--rsc--D--p. This is particularly important in the case of the last one since even after the water addition study, it was eliminated not on the basis of the kinetic data, but by a chemical argument based on the observed absence of the polymer formation that this mechanism would have implied. It remains now to discriminate between the three possibilities, particularly between the two of the RRC type and that of the RSC type. For this it was TABLE3 Logarithmic analysis: theoretical and experimental slopes Logarithmic analysis

1

2

3

4

5

6

Possible values of the theoretical slopes mV

29.3 58.6

43.9 58.6 87.8

43.9 58.6 87.8

58.6 78.1

78.1

43.9

Experimental slopes mV

52.1

59.5

48.2

64.1

53.9

40.5

153 TABLE 4 V a r i a t i o n of LSV peak p o t e n t i a l w i t h H 2 0 c o n c e n t r a t i o n H20%

v = 180 V s - - 1 ; c O = 0.2 m m o 1 - 1 - - E p / m V

v=l.8Vs -1. c o = 1 m m o l 1-1 - - E p / m V

AEp/mV

--E°/mV

0.3 2.3 5.3 10.3

1539.9 1526.7 1516.9 1506.5

1489.6 1476.3 1466.9 ]455.8

50 3 50.4 50.0 50.7

1510.9 1497.7 1487.9 1477.5

necessary to have recourse to a study of the effect of water addition. This was done through an analysis of the variations of the LSV peak potential with the H 2 0 concentration. The results are shown in Table 4 where the values of the peak potential obtained respectively for c ° = 0.2 m m ol 1-1 and v = 180 V s-1, and for c o = 1 mmo l 1-1 and v = 1.8 V s-1, are c ompared for 4 water concentrations. The first series corresponds to pure diffusion control and thus provides the values o f the standard potential E ° (fifth column} of the MBM/MBM- couple as a function o f the water concentration. The second series corresponds to pure kinetic conditions. The f o u r t h column AEp is the difference bet w een the second and the first ones and thus reflects in a logarithmic form the variations of the rate constant of the rate determining step with the water concentration. It is seen th at these variations are negligible and t herefore t hat the only possible reaction scheme is e--RRC--p--p, i.e.: MBM + 1 e - ~ MBM 2 M BM- ~ (MBM) 2(MBM) 2- + 2 H 2 0 ~ (MBMH) 2 + 2 O H the radical--radical coupling reaction being the rate determining step. The rate constant is most conveniently determined from the CPSV data obtained with no water added and with c o = 0.5 m m o l 1-1 for which E ° and Ek can be measured in the same e x p e r i m e n t by varying the sweep rate: Ek - - E ° = 190 mV and thus using the expression of Ek given in Table 2 {second line, last column} k¢ = 1.3 10 v mo1-11 s-1 It is seen th at the coupling rate is practically insensitive to the water concentration in the range 0.3--10%, whereas this was n o t the case with e.g. acetophenone which has been recently studied in this respect by the same technique [12]. This may be interpreted as due to the difference in bulkiness of the at om groups where the negative charge on the anion-radical is mostly located; (C(CN)2 here and O in the o th e r case), which results in a smaller solvation and a smaller influence o f the solvation on the coupling rate. The values of E ° figured in the first co lu mn o f Table 3 exhibit variation with the water c o n c e n t r a t i o n which is a b o ut ten times less than with a c e t o p h e n o n e [12].

154 CONCLUSION

The conclusion of this CPSV study of the EHD mechanism of MBM is thus a confirmation of the previous statement that it occurs through a radical--radical coupling of two anion radicals followed by the protonation of the dimeric specms. ACKNOWLEDGEMENTS

The work was supported in part by the C.N.R.S. (Equipe de Recherche Associ~e No. 309: Electrochimie Organique). Prof. M. Hulin, Universit~ de Paris VI, is thanked for the permission to use the 1130 IBM c o m p u t e r of the " G r o u p e de Physique des Solides de l'Ecole Normale Sup~rieure, Paris".

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