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Electrodimerization
ELECTROANALYTICAL CHEMISTRY AND INTERFACIAL ELECTROCHEMISTRY i Elsevier Sequoia S.A., Lausanne Printed in The Netherlands
223
ELECTRODIMERIZATI ON II. R E D U C T I O N M E C H A N I S M OF I M M O N I U M CATIONS
C. P. A N D R I E U X AND J. M. SAVISANT* Laboratoire de Chimie de l'Ecole Normale Supdrieure, Paris (France) (Requ le 22 d6cembre 1969)
Electrodimerization is one of the most common processes in organic electrochemistry. The best known examples of accompanying dimerization reactions in the field of reduction processes are aromatic and ~, fi unsaturated aldehydes and ketones. Similarly, electrodimerization occurs in the reduction of ~, fl unsaturated esters and nitriles as shown by Baizer and coworkers. Of particular importance for synthetic purposes is the electrodimerization of acrylonitriie leading to adiponitrile. In all these cases, however, the exact analysis of the reduction mechanism is difficult owing to the interference, besides the dimerization processes, of acid-base type reactions such as protonation of the depolarizer or of the primary reduction product. For this reason we chose the reduction of immonium cations as a model example for which the acid-base type reactions are of little influence on the overall process, even in nonacidic solvents in which the remaining acidic impurities, mainly water, are in concentrations at least comparable to that of the depolarizer itself. The solvents used for the present study were acetonitrile and benzonitrile without drastic attempts to eliminate the traces of water present. Another advantageous feature of the immonium group over the parent carbonyl function for reduction studies is its easier reducibility owing to the positive charge. Even immoniums deriving from aliphatic ketones are reducible in the usual potential range. The main phenomenological features of the electrochemical reduction of a series of immonium cations have already been described in a preliminary communication1. The aim of the present paper is to present a complete analysis of the reduction mechanism using controlled potential electrolysis with identification of products, coulometry, detection of the radical species by electron paramagnetic resonance, polarography and linear and triangular sweep voltammetry. These two last techniques allow the mechanism to be studied through the analysis of the electrochemical kinetics of the overall process. For this purpose, the formal kinetic analysis presented in the preceding paper of this series 2, and the diagnostic criteria and rate determination procedures therein have been fully employed in the present study. As stated earlier 1, the polarograms of some of the immonium cations studied exhibit a second wave situated at rather negative potentials. This wave is strongly dependent on the content of acidic impurities, mainly water, of the solvent, whereas the first wave is not. The present analysis is restricted to the first reduction process for which dimerization occurs whereas the second reduction step is concerned with * To whom correspondence should be addressed : 24 rue Lhomond, 75 Paris 5, Frar~ce. J. Electroanal. Chem., 26 (1970) 223-235
224
c . P . ANDRIEUX, J. M. SAVI~ANT
the formation of the amine by a two-electron reduction. The general formula of immonium cation is: RI-._ _+,/R R 2/t2=N-. R ,
The main structural influence on the electrode process is by the R I and R 2 groups rather than by R and R'. FOr this reason structural variation in the series of cations studied has been mainly on the groups carried by the functional carbon atom. EXPERIMENTALL
Chemicals 1. Immonium salts. The immonium salts prepared and studied in the present work are listed in Table 2. Some of them have not been prepared before and are noted with an asterisk in Table 2. Salts I-VII, IX and X were prepared by the method of Leonard and Paukstelis a. Salts XIII-XV and XVIII-XXI were prepared by methylation of the corresponding N-methylated imine. The imines were obtained according to the method of Hauser and co-workers4. For the imines other than (C6H5)2C=NCH3, the method was slightly modified: ZnC12 was added to the reaction mixture as a dehydration catalyst and the imine was not separated from the unreacted ketone before the successive alkylation leading to the immonium salt. This alkylation was performed by adding a large excess of methyl iodide directly to the reaction mixture. After a few hours of reflux heating, the immonium salt precipitates ; it is then washed with methyl iodide and finally ether. In the case of salt XVI, the N-phenylated imine was prepared according to Reddelien 5, and the immonium salt obtained by the addition of methyl iodide in large excess. Salts VIII and XVII were obtained by bubbling anhydrous hydrogen chloride through an ethereal solution of the corresponding imine and precipitation of the immonium salt. Salts XI and XII were prepared according to Arnold 6'7. 2. Solvents and supporting electrolytes. Acetonitrile (Carlo Erba, spectrograde) and benzonitrile (Eastman-Kodak, spectrograde) were used without further purification. The supporting electrolytes were tetraethylammonium perchlorate (Carlo Erba, polarographic grade), tetrabutylammonium bromide and perchlorate. The resulting water content for a 0.4 M solution of tetraethylammonium perchlorate in acetonitrile was 0.03 ~.
Spectrometry The NMR analysis of the reduction product was carried out on an A60 Varian spectrometer. EPR~spectra were recorded on a model V 4502 Varian spectrometer using a flat electrolysis cell, with a platinum gauze working electrode, a platinum wire counter electrode and an aqueous saturated calomel reference electrode.
Controlled potential electrolysis. Coulometry The controlled potential electrolysis for isolation of the reduction products, J. Electroanal. Chem., 26 (1970) 223-235
REDUCTION OF IMMONIUMS
225
coulometric determination and radical generation in the EPR spectrometer cavity were performed by using a high power potentiostat (100 V-1 A, 200 V-0.5 A, Tacussel, model ASA 100-1). For coulometry an electronic integrator (Tacussel model IG3-100) was employed. The error for the coulometric measurements was about 5 ~. The working electrode for preparation of the r6duction products and coulometry was either a mercury pool of about 20 cm 2 surface area or a cylindrical platinum gauze electrode. The counter electrode was a platinum wire and the corresponding compartment was separated from the main one by two fine-porosity fritted glass disks. The reference electrode was either an aqueous saturated calomel electrode or an Ag/Ag ÷ electrode according to Pleskov 8 (10-2 M in acetonitrile). Polarography. Linear and triangular sweep voltammetry 1. Electronics. The polarograph was composed of a potentiostat (Tacussel model PRT 500), an electromechanical ramp generator (Tacussel model Servovit) and a galvanometric XY recorder (Sefram model Luxytrace). For linear and triangular sweep voltammetry an electronic function generator was used (Tacussel model GSTP). Recording of the polarization curves was on a storage oscilloscope (Tektronix model 564). For moderately fast sweeps (up to about 200 V s- 1) a conventional potentiostat (Tacussel model 20-2Z) was employed but was replaced for higher sweep rates (up to about 5 kV s-1) by a solid state operational amplifier potentiostat with IR drop compensation 9. 2. Cells and electrode. The electrolytic cell was equipped with a water circulation arrangement. The temperature was 20°C. The volume of the solution was 10 cm 3. The counter electrode was a mercury pool of about 3 cm 2 surface area. For polarography a forced drop device was used. The natural dropping time being about 20 s, the interval of forced dropping was varied between 2 and 5 s. With linear and triangular sweep voltammetry a long dropping time (60-100 s) capillary was employed. The reference electrode was either an aqueous saturated calomel electrode or an Ag/Ag + electrode (in acetonitrile). 3. Determination of peak characteristics. Errors. Peak potentials were determined under single linear sweep polarization in order to analyse the dimerization mechanism and to measure the rate constant of the dimerization reaction according to the procedures already described 2. For moderate sweep rates no special difficulties are encountered. For high sweep rates, the double-layer charging current tends to prevail over the faradaic current and the IR drop becomes more and more important. It was noted, using blank solutions, that in the potential region corresponding to the discharge of the immonium cations, the variation of the double-layer capacity was practically unaffected by the discharge process; the base line for measuring the faradaic current can therefore be chosen as the horizontal line starting from the foot of the voltammetric wave. Peak potentials were thus measured directly on the oscilloscope screen. Under these conditions, the accuracy on the peak potential measurements was evaluated as _+2.5 mV. A part of the ohmic drop was eliminated by using the potentiostat equipped with a positive feedback loop IR drop compensation. Nevertheless, for very high sweep rates this compensation does not reach the range of experimental errors. This effect was minimized by using shorter capillary rest times in the range of high sweep rates. The remaining IR drop was corrected by using the theoretical working curves calculated previously2. In this correction one of the imJ. Electroanal. Chem., 26 (1970) 223-235
226
C. P. ANDRIEUX, J. M. SAVEANT
monium salts featured by a reversible discharge process (XIV) was used to determine the value of the ohmic drop parameter assuming the diffusion coefficients to be the same. The determination of the dimerization rate constant was carried out according to the procedure already described 2, by measuring the sweep rate vi which corresponds, on the Ep-log v diagram, to the intersection of the horizontal straight line and the oblique straight line. The rate constant value is then deduced from this measurement and the theoretical intersection value 2i (either for D I M 1 or for D I M 2) knowing the value of the initial concentration C °. Using a least square method for evaluation of the accuracy of resulting rate constant value, it was considered that the standard deviation on vi is expressed by: A log vi = 2 ~ AEp/t7 where AEp is the standard deviation on peak potential measurements (here 2.5 mV) and cr the slope of the oblique line (here 19.4 mV for D I M 1). Thus A log vi = 0.18 The relative error on vi and consequently on k is thus about _+45 % for D I M 1. Assuming a relative uncertainty of 3 % on the initial concentration value, the total relative error on the dimerization rate constant is about 50 % (in the D I M 1 case). For the determination of the dimerization rate constant the asymmetrical triangular scan technique 2 was employed. The cathodic scan durations were reckoned from the middle point of the peak/half-peak interval. The ratio of asymmetry of the scan was 10 as a minimum. Under these conditions, the theoretical approximations lead to a negligible uncertainty on rate constant determination as shown earlier 2. The double-layer charging current was eliminated by subtracting from the photographic record of the polarization curve the photographic record corresponding to a blank experiment. The anodic sweep rate was selected at a value corresponding to a reversible pattern obtained with a symmetrical scan. The value of the purely diffusion controlled peak c u r r e n t (id)p was determined by means of the same experiment. The value of the kinetic parameter kC°Of is deduced from each value of the peak current ratio using the working curves corresponding either to D I M 1 or to D I M 2 (Fig. 7 in ref. 2). The error on the determination of k arises from the errors on the peak currents ratio (ia)p/(id)p, o n the cathodic scan duration Of and on the initial concentration value C °. These two last quantities can be estimated as follows : A0f/0f=2%,
AcO/C °=3%.
The main uncertainty on peak current measurements arises from noise and the remaining 50 Hz signal due to the supply of the various electronic devices. This absolute error can b e considered as approximatively constant in the whole scale of peak currents. For typical values of the purely diffusion-controlled peak current: A(ia)p/(id) p = A(id)p/(id) p = 4 %
Therefore, the error on the peak current ratio is
A[(ia)p/(id)p] =
I1 +
(ia)p/(id)p] × 4 %
This error was calculated for the various values of the peak, current ratio and is reported in Table 1. In the same table are reported the corresponding relative errors J: Electroanal. Chem., 26 (1970) 223-235
REDUCTION OF IMMONIUMS
227
TABLE 1 ERRORS IN THE DETERMINATION OF DIMERIZATION RATE CONSTANTS BY ASYMMETRICAL TRIANGULAR SWEEP VOLTAMMETRY
(G
kco
0,90 0,80 0.70 0.60 0.50 0.40 0.30 0.20 0.10
0.076 0.072 0.068 0,064 0.060 0.056 0.052 0.048 0.044
0.28 0.75 1.36 2.38 4 7.7 17 58 430
kco
100 65 48 32 36 42 52 72 138
r °
k
105 70 53 37 41 47 57 77 142
o n kC°Of, in the case of the D I M 1 reaction scheme, and finally the resulting relative error on the rate constant k as a function of the peak current ratio. This function is presented in Fig. 1. It is to be noted that the error is minimum for medium values of the peak current ratio and very large for extreme values of this quantity. Accordingly, only measurements performed in the range 0.25-0.80 have been retained as significant, corresponding to errors less than 70 ~ . The value of the rate constant k was then 10C
7~
/
5(2
~* 25 t. LO
.0
0.75
0.50
0.25
(ia)p (id)p Fig. 1. Determination of the dimerization rate constant by asymmetrical cyclic voltammetry. Error on the rate constant as a function of the peak currents ratio for the D I M 1 reaction scheme.
calculated as a weighted mean of all the determinations corresponding to the various values of the ratio of peak currents; the weight for each of these values was the inverse of the square of the absolute error on k. Under these conditions, the standard deviation for the resulting mean value is:
ak= {~l/(akl)2} ~ Aki b e i n g t h e a b s o l u t e e r r o r c o r r e s p o n d i n g
to each determination. J. Electroanal. Chem., 26 (1970) 223-235
c.P.
228
ANDRIEUX, J. M. SAVI~ANT
TABLE 2 POLAROGRApHIC ,HALFrWAVE
POTENTIALS AND
C Y C L I C V O L T A M M E T R Y R E V E R S I B I L I T Y OF T H E I M M O N I U M
CATIONS
Immonium cation
Anion
E~:/V
Degree of reversibility
I
(CH3)2C=N
ClOg
- 1.95
Irreversible
II
C6H5"C N/~ CH3/-=-"
ClOg
-1.53
--
C102
- 1.63
--
_ +/C6H5 (CH3)2C--N..cH3
II1~ IV
C6Hs"--'+/C6H5 CH3/U-NxCH3
C104
- 1.13
--
V*
(CH3)2C=N(C6Hs)2
C104
- 1.39
--
VI
C6Hs-CH=N
C102
- 1.21
--
VII*
C2H5/-
c6vI"c=~/~
ClO;
-
--
C1-
- 1.11
--
C104
- 1.18
--
C102
-0.97
CI-
-2.11
--
CiO2
- 1.99
--
I-
- 1.49
--
I-
- 1.17
Reversible
I-
- 1.17
--
I-
-0.89
--
C1-
- 0.84
I-
- 1.36
--
I-
- 1.51
P a r t i a l l y reversible
I-
-1.89
--
I-
-- 1.87
--
-. ~j
1.50
+
VIII*
(C6Hs)2C=NH-CH3
2-L
IX
..o/CH=N
X
C6H s _ C H = C H _ C H = ~ q ~/ j +
XI
+
(CHa)2-N-CH:N(CH3) 2 +
xii
(CH3)aN-CH=CH-CH=N(CH3)2 C6H5.." + CH3/C=N(CH3)2
XIII*
+
XIV
(C6Hs)2C=N(CH3)2 ..... +/CH3 ([£6tts)2IS=IN...C2H5
XV #
. . +/CH3 (C6H 5}2C=N...C6H5 + (C6Hs)2C=NHC6H 5
XVI XVII
+
XVIII*
[ ( p O C H 3 ) C 6 H 4 ] 2 C = N ( C H 3)2 C6Hs-. _ + _ _ (CH3)zCH/C-N(CH3)2
XIX* XX*
N
H3) 2 +
XXI*
[ ( C H 3 ) 2 C H ] 2 C = N ( C H 3)2
RESULTS A N D DISCUSSION
1. General features o f the reduction process The
polarographic
J. Electroanal. Chem., 26
analysis
(1970)223-235
performed
with
a 2 × 10-3M
concentration,of
REDUCTION OF IMMONIUMS
229
depolarizer shows a one-electron reduction wave for all the immoniums studied. The corresponding half-wave potentials referred to the Pleskov electrode are reported in Table 2*. Coulometric analysis at potentials corresponding to the polarographic plateau current were performed on samples of salts I, VI, XIV and XIX. In every case the number of the electrons exchanged was found to be 1. In some cases the first wave is followed by another wave the height and position of which depend on the content of acidic impurities in the solvent. This wave presumably corresponds to a reduction of the immonium group in the amine and will not be considered in detail here. Three kinds of immonium can-be distinguished according to their behaviour in cyclic (symmetrical) voltammetry (this behaviour is indicated in Table 2 for each salt): a. Complete irreversibility whatever the sweep rate (in the range 0.1-5000 V s - 1). In this case, the reduction product is the diamine resulting from an electrodimerization process. This point was verified by preparative electrolysis and isolation of the products for two examples: I and VI. For I, the N M R spectra of the isolated product correspond to 12 H (CH3) and 8 x 2 H (CH2):
~H3 ~H3 ~
showing no single H and no splitting of the methyl group peak as in the spectrum of the amine: N - - CH (CH3) 2
(which was prepared chemically by addition of isopropyl iodide to pyrrolidine). The hydrochloride of the diamine was isolated (m.p. : 195° C). In the case of VI, the diamine:
--C--CIN
was isolated and identified by its N M R spectrum. It is a white solid, m.p. : 120 ° C. The N M R spectrum exhibits two peaks for the H in e position of the amino group, the distance between them being independent of temperature. This is consistent with a stereochemical course of the electrodimerization involving the formation of a racemic mixture. However, the stereochemistry of the reaction does not allow a distinction to be made between the D I M 1 and the D I M 2 mechanisms, since the radical as well as the cation can be considered as plane. b. Complete reversibility. This behaviour is observed when the groups carried by the functional carbon are both aromatic. A stable free radical is obtained, in the absence of oxygen, by electrolysis on the plateau of the first wave. This radical is blueviolet in the case ofXIV, XV and XVIII and red in the case of XVI and XVII. A well * Polarography of XI and XII in dimethylformamidehas been previouslyreported1°. J. Electroanal. Chem., 26 (1970)223-235
230
c . P . ANDRIEUX, J. M. SAVI~ANT
resolved hyperfine structure of the E P R spectrum was observed only for XIV. A plausible set of hyperfine coupling constants is the following"
32 2 1 C-2(~ ~L.../,1
aN = an1 = all2 = an~ =
N/CH3 ~cH3
5.50 2.65 1.10 3.20
G G G G
2
5g
I
t
~
v
A
Fig. 2. EPR spectrum of the radical (C6Hs)2C-N(CHa) 2. (a) experimental, (b) calculated.
as shown by the comparison of the experimental and calculated spectra given on Fig. 2 (a Lorenzian shape was assumed with a band width of 0.215 G). The spectrum of XV exhibits an ill-defined hyperfine structure and is very similar to the spectrum of XIV with a larger band width. This is consistent with a small participation of the protons in the groups carried by the nitrogen atom for XIV and XV. Bubbling of oxygen rapidly destroys the radical, leading finally to benzophenone. In the first stage of the oxygen attack the process remains reversible: bubbling of nitrogen through the solution just after the disappearance of the blue colour regenerates the colour. c. Partial reversibility is observed with three immonium cations : XIX, XX and XXI. With a slow scan, the voltammetric pattern is completely irreversible whereas for high sweep rates reversibility is reached. In the intermediate range the anodic current is smaller than the cathodic one and varies with sweep rate. J. Electroanal. Chem., 26 (1970) 223-235
REDUCTION OF IMMONIUMS
231
The degree of reversibility of the electrodimerization process, and then the rate of dimerization, depends on two main structural factors:the aromaticity and the volume of the groups carried by the functional carbon atom. In this respect the substituents on the nitrogen atom are of far less importance. In every case the addition of a proton donor, e.9. phenol, to the solution does not influence the half-wave and peak potentials. This is a confirmation of our assumption concerning the negligible influence of the remaining acidic impurities, particularly water, on the first reduction process. In order to study the reoxidation process of the diamine produced by the electrodimerization, the mercury working electrode was replaced by a platinum one. An irreversible reoxidation wave is then obtained by using cyclic voltammetry. These experiments were carried out on samples of II, VI, IX, and X. For a sweep rate of 1 V s- 1, the anodic peak potentials are respectively : 0.80, 0.75, 0.00, 0.30 V vs. SCE. When a periodical triangular signal is applied to the working electrode, it can be seen that the system immonium-diamine is reversible, i.e. the anodic oxidation of the diamine regenerates the immonium cation. Accordingly, preparative oxidation of the diamine regenerates the immonium cation as observed, for example, with II, VI and XIX. The identification of the immonium cation was performed by its voltammetric pattern on mercury, the preparative oxidation of the diamine being on a platinum electrode. In the case of XIX this identification is particularly convincing since the same partial reversibility is found with cyclic voltammetry with the oxidation product of the diamine as with the original immonium cation. The oxidation wave of the diamine was found to be irreversible whatever the sweep rate, involving presumably the initial formation of an aminium cation radical followed by a fast and irreversible breaking of the carbon-carbon bond. It is to be noted that in the case of aldehydic immoniums (VI, X), cleavage occurs between the two carbons and not elimination of a proton. 2. M e c h a n i s m o f the electrodimerization
The mechanistic analysis of the electrodimerization concerns the salts of types a and c. For the other immoniums the reduction mechanism is reduced to a simple fast electron transfer in the time range of polarography and linear sweep voltammetry. The peak potentials of the cations of types a and c are given in Fig. 3 as functions of the logarithm of sweep rate. The medium was a 0.4 M solution of tetraethylammonium perchlorate in acetonitrile and the concentration of immonium was 10 -3 M. The peak potentials are measured vs. the aqueous saturated calomel electrode. These diagrams exhibit an oblique linear portion and, for the cations of type c, an horizontal linear portion corresponding to the normal potential of the redox couple immonium-radical. The slope of the oblique portion of the Ep-log v diagrams was calculated for each immonium considered (see Table 3). In the case of X an abnormal behaviour was noted in acetonitrile : for moderate values of the sweep rate the peak potential remains constant and shifts cathodically only for high sweep rates. This is presumably due to adsorption of reactants and resulting autoinhibition of the discharge process. This phenomenon disappears completely in benzonitrile with 0.1 M tetraethylammonium perchlorate as supporting electrolyte. The corresponding Ep-log v diagram is given in Fig. 4, together with the diagram corresponding to the reduction of XIX in this medium. The slope of the oblique J. Electroanal. Chem., 26 (1970) 223-235
232
Ep/V
c . P . ANDRIEUX, J. M. SAVI~ANT
SCE)
-0.95 -1o0C
i
-1.23 -1.2• -1.2! -1.2~ - 1,,5~ -1.58 -1.60
- 1.65
-l'BOi
!l
-1"B5t 0.05
0.5
5 log v (V s-1)
50
500
Fig. 3. Peak potential-sweep rate diagrams, in acetonitrile, 0.4 M tetraethylammonium perchlorate. Immonium concn. 10 -3 M. Temp. 20°C.
portion of these diagrams is given in Table 3. Comparison of the experimental slopes with the theoretical values corresponding to DIM 1 (19.4 mV), DIM 2 (29.2 mV) and DIM 3 (14.6 mV) at 20°C, shows that the electrodimerization process is a purely radical one for all the immonium cations studied. 3. Dimerization rate constants
Determination of the dimerization rate constant is possible for the cations XIX, XX and XXI. Two methods were used : determination of peak potentials with single linear sweep voltammetry and determination of the peak current ratios with an asymmetrical triangular scan. For XIX, a difficulty was encountered when using acetonitrile : the second wave is too close to the first one to allow the inversion of the potential scan according to the theoretical requirements. For this salt, the rate constant was therefore determined in benzonitrile (0.4 M in tetrabutylammonium bromide) by the asymmetrical scan method and in both media by the single linedr scan method. The determination of the dimerization rate constant by single linear sweep J. Electroanal. Chem., 26 (1970) 223-235
REDUCTION OF IMMONIUMS
233
TABLE 3 DETERMINATION OF THE DIMERIZATION MECHANISM: SLOPE OF THE PEAK POTENTIAL VS.
Immonium
log V DIAGRAM
Slope/mV
Acetonitrile ; 0.4 M C6H5 -._ +. I~i/C=N ~
E t 4 N C I 0 4 ;.
Conch. immonium, 10-3 M; t=20°C
17.3 211
CH 3/U=N[utt 3)2
18.1
C6Hs'-- + . . . . ((CH3)2CH)..tz-N[tsta3)2
22.0
+
((CHa)2CH)C=N(CH3)2
19.2
@ I~I(CH3)2
19.5
(CH3)zC=N
18.1 +
(CH3)2N-CH=N(CH3)2 Benzonitrile; 0.1 M Et4NCI04;
Conch. immonium 10-3 M; t=20°C
C6Hs~CH=CH-CH=N
19.9
C6H5-..
_+
(CH3)2cH/C-N(CH3)2
19.7
19.3
voltammetry was carried out for one depolarizer concentration: 10-3 M. The resulting k values are given in Table 4 with the corresponding error as evaluated previously. With the asymmetrical scan method, three concentrations of the i m m o n i u m salt were considered for XIX, the dimerization of which is moderately fast. F o r XX and X X I only two concentrations were dealt with since for 2 x 10- 3 M, dimerization is too fast to be quantitatively characterized. The dimerization rate constants are reported in Table 5 for each concentration. The uncertainty on these values was evaluated according to the method previously described. The resulting mean value for the dimerization rate constant is also given in this Table as well as the value of the sweep rate used in the anodic scan. TABLE 4 DIMERIZATION RATE CONSTANTS AS MEASURED BY THE PEAK POTENTIAL METHOD
Immoniurn (conen. 10-3 M)
Dimerization rate constant (20°C)/ s -1 rno1-1 1
XIX (in CH3CN) XIX (in C6HsCN) XX (in CH3CN) XXI (in CH3CN)
(5.70_+2.9) 104 (1.40_+0.70) 1 0 4 (2.80_+1.40) 10 6 (1.75_+0.88) 105 J. Electroanal. Chem., 26 (t970) 223-235
234
c.P.
ANDRIEUX, J. M. SAVI~ANT
TABLE 5 D I M E R I Z A T I O N R A T E C O N S T A N T S AS M E A S U R E D BY T H E P E A K C U R R E N T R A T I O M E T H O D
mmonium
XXI (in CH3CN)
~ n
X IX (in C6HsCN)
XX (in CH3CN)
0.5
(2.31+0.40) 104
(2.69_+0.51)
10 6
(2.41_+0.48) 105
(2.54_+0.48) 106
(2.64_+0.52) 105
(lO-~M)~ 1.0
(2.13_+0.36) 104
2.0
(2.31 _+0.39) 104
Mean value of the rate constant/s -1 tool - t 1
(2.25_+0.38) 104
(2.61 +'0.49) 106
(2.52_+0.50) 105
Anodic sweep rate/V s-1
135
5500
1400
E~lV (SCE) ,
iu
,i,.
ql,
--0.6~ IC6H~CH~CH-CH"N~
- -0.6~
(oH5 • '~ ~ C=N (CHa)a (O..~)2,CH- - -1.;
-
•
|
-!.24
0£)5 0.5 5 log v (V s -1) 50 Fig. 4. Peak potential-sweep rate diagrams, in benzonitrile, 0.1 M tetraethylammonium perchlorate. Immonium c o n c n . 1 0 - 3 M. Temp. 20°C.
The values of the dimerization rate constant determined by each method are in good agreement within the limits of the experimental uncertainty. ACKNOWLEDGEMENT
This work was supported in part by the CNRS (Laboratoire associ6 n ° 32, m6canismes reactionnels). SUMMARY
The electrochemical reduction of immonium cations in acetonitrile and benJ. Electroanal. Chem., 26 (1970) 223-235
REDUCTION OF IMMONIUMS
235
zonitrile leads either to a stable free radical or to a coupling diamine, according to the aromaticity and the volume of the groups carried by the functional carbon atom. In this last case, the reduction process offers a simple mechanistic example of an electrodimerization since acidic impurities present do not influence significantly the first electron transfer process. It is shown, using the diagnostic criteria previously stated, that in a broad series of immonium salts, the mechanism of the reductive coupling is a purely radical one. For some immoniums of the series the dimerization rate constant has been measured using either single linear sweep voltammetry or symmetrical cyclic voltammetry. REFERENCES 1 2 3 4 5 6 7 8 9 10
C. P. ANDRIEUX AND J. M. SAV~ANT,Bull. Soc. Chim. France, (1968) 4671. C. P. ANDRIEUX, L. NADJO AND J. M. SAVEANT, 26 (1970) 147. N. J. LBONARD AND J. V. PAUKSTELIS,J. Org. Chem., 28 (1963) 3021. C. R. HAUSER, R. M. MANYCK, W. R. BRASEN AND P. L. BAYLESS,J. Org. Chem., 20 (1955) 1119. E. REDELLIEN, Chem. Ber., 42 (1959) 4760. Z. ARNOLD, Collection Czech. Chem. Commun., 24 (1959) 760. Z. ARNOLD AND F. SORM, Collection Czech. Chem. Commun., 23 (1959) 452. V. A. PLESKOV, J. Phys. Chem. (USSR), 22 (1948) 351. D. GARREAUAND J. M. SAVI~ANT,to be published. A. HOLY, J. KRUPICKA AND Z. ARNOLD, Collection Czech. Chem. Commun., 30 (1965) 4127.
J. Electroanal. Chem., 26 (1970) 223-235
223
ELECTRODIMERIZATI ON II. R E D U C T I O N M E C H A N I S M OF I M M O N I U M CATIONS
C. P. A N D R I E U X AND J. M. SAVISANT* Laboratoire de Chimie de l'Ecole Normale Supdrieure, Paris (France) (Requ le 22 d6cembre 1969)
Electrodimerization is one of the most common processes in organic electrochemistry. The best known examples of accompanying dimerization reactions in the field of reduction processes are aromatic and ~, fi unsaturated aldehydes and ketones. Similarly, electrodimerization occurs in the reduction of ~, fl unsaturated esters and nitriles as shown by Baizer and coworkers. Of particular importance for synthetic purposes is the electrodimerization of acrylonitriie leading to adiponitrile. In all these cases, however, the exact analysis of the reduction mechanism is difficult owing to the interference, besides the dimerization processes, of acid-base type reactions such as protonation of the depolarizer or of the primary reduction product. For this reason we chose the reduction of immonium cations as a model example for which the acid-base type reactions are of little influence on the overall process, even in nonacidic solvents in which the remaining acidic impurities, mainly water, are in concentrations at least comparable to that of the depolarizer itself. The solvents used for the present study were acetonitrile and benzonitrile without drastic attempts to eliminate the traces of water present. Another advantageous feature of the immonium group over the parent carbonyl function for reduction studies is its easier reducibility owing to the positive charge. Even immoniums deriving from aliphatic ketones are reducible in the usual potential range. The main phenomenological features of the electrochemical reduction of a series of immonium cations have already been described in a preliminary communication1. The aim of the present paper is to present a complete analysis of the reduction mechanism using controlled potential electrolysis with identification of products, coulometry, detection of the radical species by electron paramagnetic resonance, polarography and linear and triangular sweep voltammetry. These two last techniques allow the mechanism to be studied through the analysis of the electrochemical kinetics of the overall process. For this purpose, the formal kinetic analysis presented in the preceding paper of this series 2, and the diagnostic criteria and rate determination procedures therein have been fully employed in the present study. As stated earlier 1, the polarograms of some of the immonium cations studied exhibit a second wave situated at rather negative potentials. This wave is strongly dependent on the content of acidic impurities, mainly water, of the solvent, whereas the first wave is not. The present analysis is restricted to the first reduction process for which dimerization occurs whereas the second reduction step is concerned with * To whom correspondence should be addressed : 24 rue Lhomond, 75 Paris 5, Frar~ce. J. Electroanal. Chem., 26 (1970) 223-235
224
c . P . ANDRIEUX, J. M. SAVI~ANT
the formation of the amine by a two-electron reduction. The general formula of immonium cation is: RI-._ _+,/R R 2/t2=N-. R ,
The main structural influence on the electrode process is by the R I and R 2 groups rather than by R and R'. FOr this reason structural variation in the series of cations studied has been mainly on the groups carried by the functional carbon atom. EXPERIMENTALL
Chemicals 1. Immonium salts. The immonium salts prepared and studied in the present work are listed in Table 2. Some of them have not been prepared before and are noted with an asterisk in Table 2. Salts I-VII, IX and X were prepared by the method of Leonard and Paukstelis a. Salts XIII-XV and XVIII-XXI were prepared by methylation of the corresponding N-methylated imine. The imines were obtained according to the method of Hauser and co-workers4. For the imines other than (C6H5)2C=NCH3, the method was slightly modified: ZnC12 was added to the reaction mixture as a dehydration catalyst and the imine was not separated from the unreacted ketone before the successive alkylation leading to the immonium salt. This alkylation was performed by adding a large excess of methyl iodide directly to the reaction mixture. After a few hours of reflux heating, the immonium salt precipitates ; it is then washed with methyl iodide and finally ether. In the case of salt XVI, the N-phenylated imine was prepared according to Reddelien 5, and the immonium salt obtained by the addition of methyl iodide in large excess. Salts VIII and XVII were obtained by bubbling anhydrous hydrogen chloride through an ethereal solution of the corresponding imine and precipitation of the immonium salt. Salts XI and XII were prepared according to Arnold 6'7. 2. Solvents and supporting electrolytes. Acetonitrile (Carlo Erba, spectrograde) and benzonitrile (Eastman-Kodak, spectrograde) were used without further purification. The supporting electrolytes were tetraethylammonium perchlorate (Carlo Erba, polarographic grade), tetrabutylammonium bromide and perchlorate. The resulting water content for a 0.4 M solution of tetraethylammonium perchlorate in acetonitrile was 0.03 ~.
Spectrometry The NMR analysis of the reduction product was carried out on an A60 Varian spectrometer. EPR~spectra were recorded on a model V 4502 Varian spectrometer using a flat electrolysis cell, with a platinum gauze working electrode, a platinum wire counter electrode and an aqueous saturated calomel reference electrode.
Controlled potential electrolysis. Coulometry The controlled potential electrolysis for isolation of the reduction products, J. Electroanal. Chem., 26 (1970) 223-235
REDUCTION OF IMMONIUMS
225
coulometric determination and radical generation in the EPR spectrometer cavity were performed by using a high power potentiostat (100 V-1 A, 200 V-0.5 A, Tacussel, model ASA 100-1). For coulometry an electronic integrator (Tacussel model IG3-100) was employed. The error for the coulometric measurements was about 5 ~. The working electrode for preparation of the r6duction products and coulometry was either a mercury pool of about 20 cm 2 surface area or a cylindrical platinum gauze electrode. The counter electrode was a platinum wire and the corresponding compartment was separated from the main one by two fine-porosity fritted glass disks. The reference electrode was either an aqueous saturated calomel electrode or an Ag/Ag ÷ electrode according to Pleskov 8 (10-2 M in acetonitrile). Polarography. Linear and triangular sweep voltammetry 1. Electronics. The polarograph was composed of a potentiostat (Tacussel model PRT 500), an electromechanical ramp generator (Tacussel model Servovit) and a galvanometric XY recorder (Sefram model Luxytrace). For linear and triangular sweep voltammetry an electronic function generator was used (Tacussel model GSTP). Recording of the polarization curves was on a storage oscilloscope (Tektronix model 564). For moderately fast sweeps (up to about 200 V s- 1) a conventional potentiostat (Tacussel model 20-2Z) was employed but was replaced for higher sweep rates (up to about 5 kV s-1) by a solid state operational amplifier potentiostat with IR drop compensation 9. 2. Cells and electrode. The electrolytic cell was equipped with a water circulation arrangement. The temperature was 20°C. The volume of the solution was 10 cm 3. The counter electrode was a mercury pool of about 3 cm 2 surface area. For polarography a forced drop device was used. The natural dropping time being about 20 s, the interval of forced dropping was varied between 2 and 5 s. With linear and triangular sweep voltammetry a long dropping time (60-100 s) capillary was employed. The reference electrode was either an aqueous saturated calomel electrode or an Ag/Ag + electrode (in acetonitrile). 3. Determination of peak characteristics. Errors. Peak potentials were determined under single linear sweep polarization in order to analyse the dimerization mechanism and to measure the rate constant of the dimerization reaction according to the procedures already described 2. For moderate sweep rates no special difficulties are encountered. For high sweep rates, the double-layer charging current tends to prevail over the faradaic current and the IR drop becomes more and more important. It was noted, using blank solutions, that in the potential region corresponding to the discharge of the immonium cations, the variation of the double-layer capacity was practically unaffected by the discharge process; the base line for measuring the faradaic current can therefore be chosen as the horizontal line starting from the foot of the voltammetric wave. Peak potentials were thus measured directly on the oscilloscope screen. Under these conditions, the accuracy on the peak potential measurements was evaluated as _+2.5 mV. A part of the ohmic drop was eliminated by using the potentiostat equipped with a positive feedback loop IR drop compensation. Nevertheless, for very high sweep rates this compensation does not reach the range of experimental errors. This effect was minimized by using shorter capillary rest times in the range of high sweep rates. The remaining IR drop was corrected by using the theoretical working curves calculated previously2. In this correction one of the imJ. Electroanal. Chem., 26 (1970) 223-235
226
C. P. ANDRIEUX, J. M. SAVEANT
monium salts featured by a reversible discharge process (XIV) was used to determine the value of the ohmic drop parameter assuming the diffusion coefficients to be the same. The determination of the dimerization rate constant was carried out according to the procedure already described 2, by measuring the sweep rate vi which corresponds, on the Ep-log v diagram, to the intersection of the horizontal straight line and the oblique straight line. The rate constant value is then deduced from this measurement and the theoretical intersection value 2i (either for D I M 1 or for D I M 2) knowing the value of the initial concentration C °. Using a least square method for evaluation of the accuracy of resulting rate constant value, it was considered that the standard deviation on vi is expressed by: A log vi = 2 ~ AEp/t7 where AEp is the standard deviation on peak potential measurements (here 2.5 mV) and cr the slope of the oblique line (here 19.4 mV for D I M 1). Thus A log vi = 0.18 The relative error on vi and consequently on k is thus about _+45 % for D I M 1. Assuming a relative uncertainty of 3 % on the initial concentration value, the total relative error on the dimerization rate constant is about 50 % (in the D I M 1 case). For the determination of the dimerization rate constant the asymmetrical triangular scan technique 2 was employed. The cathodic scan durations were reckoned from the middle point of the peak/half-peak interval. The ratio of asymmetry of the scan was 10 as a minimum. Under these conditions, the theoretical approximations lead to a negligible uncertainty on rate constant determination as shown earlier 2. The double-layer charging current was eliminated by subtracting from the photographic record of the polarization curve the photographic record corresponding to a blank experiment. The anodic sweep rate was selected at a value corresponding to a reversible pattern obtained with a symmetrical scan. The value of the purely diffusion controlled peak c u r r e n t (id)p was determined by means of the same experiment. The value of the kinetic parameter kC°Of is deduced from each value of the peak current ratio using the working curves corresponding either to D I M 1 or to D I M 2 (Fig. 7 in ref. 2). The error on the determination of k arises from the errors on the peak currents ratio (ia)p/(id)p, o n the cathodic scan duration Of and on the initial concentration value C °. These two last quantities can be estimated as follows : A0f/0f=2%,
AcO/C °=3%.
The main uncertainty on peak current measurements arises from noise and the remaining 50 Hz signal due to the supply of the various electronic devices. This absolute error can b e considered as approximatively constant in the whole scale of peak currents. For typical values of the purely diffusion-controlled peak current: A(ia)p/(id) p = A(id)p/(id) p = 4 %
Therefore, the error on the peak current ratio is
A[(ia)p/(id)p] =
I1 +
(ia)p/(id)p] × 4 %
This error was calculated for the various values of the peak, current ratio and is reported in Table 1. In the same table are reported the corresponding relative errors J: Electroanal. Chem., 26 (1970) 223-235
REDUCTION OF IMMONIUMS
227
TABLE 1 ERRORS IN THE DETERMINATION OF DIMERIZATION RATE CONSTANTS BY ASYMMETRICAL TRIANGULAR SWEEP VOLTAMMETRY
(G
kco
0,90 0,80 0.70 0.60 0.50 0.40 0.30 0.20 0.10
0.076 0.072 0.068 0,064 0.060 0.056 0.052 0.048 0.044
0.28 0.75 1.36 2.38 4 7.7 17 58 430
kco
100 65 48 32 36 42 52 72 138
r °
k
105 70 53 37 41 47 57 77 142
o n kC°Of, in the case of the D I M 1 reaction scheme, and finally the resulting relative error on the rate constant k as a function of the peak current ratio. This function is presented in Fig. 1. It is to be noted that the error is minimum for medium values of the peak current ratio and very large for extreme values of this quantity. Accordingly, only measurements performed in the range 0.25-0.80 have been retained as significant, corresponding to errors less than 70 ~ . The value of the rate constant k was then 10C
7~
/
5(2
~* 25 t. LO
.0
0.75
0.50
0.25
(ia)p (id)p Fig. 1. Determination of the dimerization rate constant by asymmetrical cyclic voltammetry. Error on the rate constant as a function of the peak currents ratio for the D I M 1 reaction scheme.
calculated as a weighted mean of all the determinations corresponding to the various values of the ratio of peak currents; the weight for each of these values was the inverse of the square of the absolute error on k. Under these conditions, the standard deviation for the resulting mean value is:
ak= {~l/(akl)2} ~ Aki b e i n g t h e a b s o l u t e e r r o r c o r r e s p o n d i n g
to each determination. J. Electroanal. Chem., 26 (1970) 223-235
c.P.
228
ANDRIEUX, J. M. SAVI~ANT
TABLE 2 POLAROGRApHIC ,HALFrWAVE
POTENTIALS AND
C Y C L I C V O L T A M M E T R Y R E V E R S I B I L I T Y OF T H E I M M O N I U M
CATIONS
Immonium cation
Anion
E~:/V
Degree of reversibility
I
(CH3)2C=N
ClOg
- 1.95
Irreversible
II
C6H5"C N/~ CH3/-=-"
ClOg
-1.53
--
C102
- 1.63
--
_ +/C6H5 (CH3)2C--N..cH3
II1~ IV
C6Hs"--'+/C6H5 CH3/U-NxCH3
C104
- 1.13
--
V*
(CH3)2C=N(C6Hs)2
C104
- 1.39
--
VI
C6Hs-CH=N
C102
- 1.21
--
VII*
C2H5/-
c6vI"c=~/~
ClO;
-
--
C1-
- 1.11
--
C104
- 1.18
--
C102
-0.97
CI-
-2.11
--
CiO2
- 1.99
--
I-
- 1.49
--
I-
- 1.17
Reversible
I-
- 1.17
--
I-
-0.89
--
C1-
- 0.84
I-
- 1.36
--
I-
- 1.51
P a r t i a l l y reversible
I-
-1.89
--
I-
-- 1.87
--
-. ~j
1.50
+
VIII*
(C6Hs)2C=NH-CH3
2-L
IX
..o/CH=N
X
C6H s _ C H = C H _ C H = ~ q ~/ j +
XI
+
(CHa)2-N-CH:N(CH3) 2 +
xii
(CH3)aN-CH=CH-CH=N(CH3)2 C6H5.." + CH3/C=N(CH3)2
XIII*
+
XIV
(C6Hs)2C=N(CH3)2 ..... +/CH3 ([£6tts)2IS=IN...C2H5
XV #
. . +/CH3 (C6H 5}2C=N...C6H5 + (C6Hs)2C=NHC6H 5
XVI XVII
+
XVIII*
[ ( p O C H 3 ) C 6 H 4 ] 2 C = N ( C H 3)2 C6Hs-. _ + _ _ (CH3)zCH/C-N(CH3)2
XIX* XX*
N
H3) 2 +
XXI*
[ ( C H 3 ) 2 C H ] 2 C = N ( C H 3)2
RESULTS A N D DISCUSSION
1. General features o f the reduction process The
polarographic
J. Electroanal. Chem., 26
analysis
(1970)223-235
performed
with
a 2 × 10-3M
concentration,of
REDUCTION OF IMMONIUMS
229
depolarizer shows a one-electron reduction wave for all the immoniums studied. The corresponding half-wave potentials referred to the Pleskov electrode are reported in Table 2*. Coulometric analysis at potentials corresponding to the polarographic plateau current were performed on samples of salts I, VI, XIV and XIX. In every case the number of the electrons exchanged was found to be 1. In some cases the first wave is followed by another wave the height and position of which depend on the content of acidic impurities in the solvent. This wave presumably corresponds to a reduction of the immonium group in the amine and will not be considered in detail here. Three kinds of immonium can-be distinguished according to their behaviour in cyclic (symmetrical) voltammetry (this behaviour is indicated in Table 2 for each salt): a. Complete irreversibility whatever the sweep rate (in the range 0.1-5000 V s - 1). In this case, the reduction product is the diamine resulting from an electrodimerization process. This point was verified by preparative electrolysis and isolation of the products for two examples: I and VI. For I, the N M R spectra of the isolated product correspond to 12 H (CH3) and 8 x 2 H (CH2):
~H3 ~H3 ~
showing no single H and no splitting of the methyl group peak as in the spectrum of the amine: N - - CH (CH3) 2
(which was prepared chemically by addition of isopropyl iodide to pyrrolidine). The hydrochloride of the diamine was isolated (m.p. : 195° C). In the case of VI, the diamine:
--C--CIN
was isolated and identified by its N M R spectrum. It is a white solid, m.p. : 120 ° C. The N M R spectrum exhibits two peaks for the H in e position of the amino group, the distance between them being independent of temperature. This is consistent with a stereochemical course of the electrodimerization involving the formation of a racemic mixture. However, the stereochemistry of the reaction does not allow a distinction to be made between the D I M 1 and the D I M 2 mechanisms, since the radical as well as the cation can be considered as plane. b. Complete reversibility. This behaviour is observed when the groups carried by the functional carbon are both aromatic. A stable free radical is obtained, in the absence of oxygen, by electrolysis on the plateau of the first wave. This radical is blueviolet in the case ofXIV, XV and XVIII and red in the case of XVI and XVII. A well * Polarography of XI and XII in dimethylformamidehas been previouslyreported1°. J. Electroanal. Chem., 26 (1970)223-235
230
c . P . ANDRIEUX, J. M. SAVI~ANT
resolved hyperfine structure of the E P R spectrum was observed only for XIV. A plausible set of hyperfine coupling constants is the following"
32 2 1 C-2(~ ~L.../,1
aN = an1 = all2 = an~ =
N/CH3 ~cH3
5.50 2.65 1.10 3.20
G G G G
2
5g
I
t
~
v
A
Fig. 2. EPR spectrum of the radical (C6Hs)2C-N(CHa) 2. (a) experimental, (b) calculated.
as shown by the comparison of the experimental and calculated spectra given on Fig. 2 (a Lorenzian shape was assumed with a band width of 0.215 G). The spectrum of XV exhibits an ill-defined hyperfine structure and is very similar to the spectrum of XIV with a larger band width. This is consistent with a small participation of the protons in the groups carried by the nitrogen atom for XIV and XV. Bubbling of oxygen rapidly destroys the radical, leading finally to benzophenone. In the first stage of the oxygen attack the process remains reversible: bubbling of nitrogen through the solution just after the disappearance of the blue colour regenerates the colour. c. Partial reversibility is observed with three immonium cations : XIX, XX and XXI. With a slow scan, the voltammetric pattern is completely irreversible whereas for high sweep rates reversibility is reached. In the intermediate range the anodic current is smaller than the cathodic one and varies with sweep rate. J. Electroanal. Chem., 26 (1970) 223-235
REDUCTION OF IMMONIUMS
231
The degree of reversibility of the electrodimerization process, and then the rate of dimerization, depends on two main structural factors:the aromaticity and the volume of the groups carried by the functional carbon atom. In this respect the substituents on the nitrogen atom are of far less importance. In every case the addition of a proton donor, e.9. phenol, to the solution does not influence the half-wave and peak potentials. This is a confirmation of our assumption concerning the negligible influence of the remaining acidic impurities, particularly water, on the first reduction process. In order to study the reoxidation process of the diamine produced by the electrodimerization, the mercury working electrode was replaced by a platinum one. An irreversible reoxidation wave is then obtained by using cyclic voltammetry. These experiments were carried out on samples of II, VI, IX, and X. For a sweep rate of 1 V s- 1, the anodic peak potentials are respectively : 0.80, 0.75, 0.00, 0.30 V vs. SCE. When a periodical triangular signal is applied to the working electrode, it can be seen that the system immonium-diamine is reversible, i.e. the anodic oxidation of the diamine regenerates the immonium cation. Accordingly, preparative oxidation of the diamine regenerates the immonium cation as observed, for example, with II, VI and XIX. The identification of the immonium cation was performed by its voltammetric pattern on mercury, the preparative oxidation of the diamine being on a platinum electrode. In the case of XIX this identification is particularly convincing since the same partial reversibility is found with cyclic voltammetry with the oxidation product of the diamine as with the original immonium cation. The oxidation wave of the diamine was found to be irreversible whatever the sweep rate, involving presumably the initial formation of an aminium cation radical followed by a fast and irreversible breaking of the carbon-carbon bond. It is to be noted that in the case of aldehydic immoniums (VI, X), cleavage occurs between the two carbons and not elimination of a proton. 2. M e c h a n i s m o f the electrodimerization
The mechanistic analysis of the electrodimerization concerns the salts of types a and c. For the other immoniums the reduction mechanism is reduced to a simple fast electron transfer in the time range of polarography and linear sweep voltammetry. The peak potentials of the cations of types a and c are given in Fig. 3 as functions of the logarithm of sweep rate. The medium was a 0.4 M solution of tetraethylammonium perchlorate in acetonitrile and the concentration of immonium was 10 -3 M. The peak potentials are measured vs. the aqueous saturated calomel electrode. These diagrams exhibit an oblique linear portion and, for the cations of type c, an horizontal linear portion corresponding to the normal potential of the redox couple immonium-radical. The slope of the oblique portion of the Ep-log v diagrams was calculated for each immonium considered (see Table 3). In the case of X an abnormal behaviour was noted in acetonitrile : for moderate values of the sweep rate the peak potential remains constant and shifts cathodically only for high sweep rates. This is presumably due to adsorption of reactants and resulting autoinhibition of the discharge process. This phenomenon disappears completely in benzonitrile with 0.1 M tetraethylammonium perchlorate as supporting electrolyte. The corresponding Ep-log v diagram is given in Fig. 4, together with the diagram corresponding to the reduction of XIX in this medium. The slope of the oblique J. Electroanal. Chem., 26 (1970) 223-235
232
Ep/V
c . P . ANDRIEUX, J. M. SAVI~ANT
SCE)
-0.95 -1o0C
i
-1.23 -1.2• -1.2! -1.2~ - 1,,5~ -1.58 -1.60
- 1.65
-l'BOi
!l
-1"B5t 0.05
0.5
5 log v (V s-1)
50
500
Fig. 3. Peak potential-sweep rate diagrams, in acetonitrile, 0.4 M tetraethylammonium perchlorate. Immonium concn. 10 -3 M. Temp. 20°C.
portion of these diagrams is given in Table 3. Comparison of the experimental slopes with the theoretical values corresponding to DIM 1 (19.4 mV), DIM 2 (29.2 mV) and DIM 3 (14.6 mV) at 20°C, shows that the electrodimerization process is a purely radical one for all the immonium cations studied. 3. Dimerization rate constants
Determination of the dimerization rate constant is possible for the cations XIX, XX and XXI. Two methods were used : determination of peak potentials with single linear sweep voltammetry and determination of the peak current ratios with an asymmetrical triangular scan. For XIX, a difficulty was encountered when using acetonitrile : the second wave is too close to the first one to allow the inversion of the potential scan according to the theoretical requirements. For this salt, the rate constant was therefore determined in benzonitrile (0.4 M in tetrabutylammonium bromide) by the asymmetrical scan method and in both media by the single linedr scan method. The determination of the dimerization rate constant by single linear sweep J. Electroanal. Chem., 26 (1970) 223-235
REDUCTION OF IMMONIUMS
233
TABLE 3 DETERMINATION OF THE DIMERIZATION MECHANISM: SLOPE OF THE PEAK POTENTIAL VS.
Immonium
log V DIAGRAM
Slope/mV
Acetonitrile ; 0.4 M C6H5 -._ +. I~i/C=N ~
E t 4 N C I 0 4 ;.
Conch. immonium, 10-3 M; t=20°C
17.3 211
CH 3/U=N[utt 3)2
18.1
C6Hs'-- + . . . . ((CH3)2CH)..tz-N[tsta3)2
22.0
+
((CHa)2CH)C=N(CH3)2
19.2
@ I~I(CH3)2
19.5
(CH3)zC=N
18.1 +
(CH3)2N-CH=N(CH3)2 Benzonitrile; 0.1 M Et4NCI04;
Conch. immonium 10-3 M; t=20°C
C6Hs~CH=CH-CH=N
19.9
C6H5-..
_+
(CH3)2cH/C-N(CH3)2
19.7
19.3
voltammetry was carried out for one depolarizer concentration: 10-3 M. The resulting k values are given in Table 4 with the corresponding error as evaluated previously. With the asymmetrical scan method, three concentrations of the i m m o n i u m salt were considered for XIX, the dimerization of which is moderately fast. F o r XX and X X I only two concentrations were dealt with since for 2 x 10- 3 M, dimerization is too fast to be quantitatively characterized. The dimerization rate constants are reported in Table 5 for each concentration. The uncertainty on these values was evaluated according to the method previously described. The resulting mean value for the dimerization rate constant is also given in this Table as well as the value of the sweep rate used in the anodic scan. TABLE 4 DIMERIZATION RATE CONSTANTS AS MEASURED BY THE PEAK POTENTIAL METHOD
Immoniurn (conen. 10-3 M)
Dimerization rate constant (20°C)/ s -1 rno1-1 1
XIX (in CH3CN) XIX (in C6HsCN) XX (in CH3CN) XXI (in CH3CN)
(5.70_+2.9) 104 (1.40_+0.70) 1 0 4 (2.80_+1.40) 10 6 (1.75_+0.88) 105 J. Electroanal. Chem., 26 (t970) 223-235
234
c.P.
ANDRIEUX, J. M. SAVI~ANT
TABLE 5 D I M E R I Z A T I O N R A T E C O N S T A N T S AS M E A S U R E D BY T H E P E A K C U R R E N T R A T I O M E T H O D
mmonium
XXI (in CH3CN)
~ n
X IX (in C6HsCN)
XX (in CH3CN)
0.5
(2.31+0.40) 104
(2.69_+0.51)
10 6
(2.41_+0.48) 105
(2.54_+0.48) 106
(2.64_+0.52) 105
(lO-~M)~ 1.0
(2.13_+0.36) 104
2.0
(2.31 _+0.39) 104
Mean value of the rate constant/s -1 tool - t 1
(2.25_+0.38) 104
(2.61 +'0.49) 106
(2.52_+0.50) 105
Anodic sweep rate/V s-1
135
5500
1400
E~lV (SCE) ,
iu
,i,.
ql,
--0.6~ IC6H~CH~CH-CH"N~
- -0.6~
(oH5 • '~ ~ C=N (CHa)a (O..~)2,CH- - -1.;
-
•
|
-!.24
0£)5 0.5 5 log v (V s -1) 50 Fig. 4. Peak potential-sweep rate diagrams, in benzonitrile, 0.1 M tetraethylammonium perchlorate. Immonium c o n c n . 1 0 - 3 M. Temp. 20°C.
The values of the dimerization rate constant determined by each method are in good agreement within the limits of the experimental uncertainty. ACKNOWLEDGEMENT
This work was supported in part by the CNRS (Laboratoire associ6 n ° 32, m6canismes reactionnels). SUMMARY
The electrochemical reduction of immonium cations in acetonitrile and benJ. Electroanal. Chem., 26 (1970) 223-235
REDUCTION OF IMMONIUMS
235
zonitrile leads either to a stable free radical or to a coupling diamine, according to the aromaticity and the volume of the groups carried by the functional carbon atom. In this last case, the reduction process offers a simple mechanistic example of an electrodimerization since acidic impurities present do not influence significantly the first electron transfer process. It is shown, using the diagnostic criteria previously stated, that in a broad series of immonium salts, the mechanism of the reductive coupling is a purely radical one. For some immoniums of the series the dimerization rate constant has been measured using either single linear sweep voltammetry or symmetrical cyclic voltammetry. REFERENCES 1 2 3 4 5 6 7 8 9 10
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J. Electroanal. Chem., 26 (1970) 223-235