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Eect of Sodium Ions on the Electrochemical
Reduction of Diethyl Fumarate in
Dimethylsulfoxide and Acetonitrile
Michael D. Ryan
Marquee University)'"& $(.4 *) .-1$8$$#1
Dennis H. Evans
University of Wisconsin - Madison
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NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be
accessed by following the link in the citation at the bottom of the page.
Journal of the Electrochemical Society, Vol. 121, No. 7 (July 1974): pg. 882-883. DOI. This article is © Electrochemical
Society and permission has been granted for this version to appear in e-Publications@Marquette. Electrochemical Society
does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express
permission from Electrochemical Society.
1
Effect of Sodium Ions on the
Electrochemical Reduction of Diethyl
Fumarate in Dimethylsulfoxide and
Acetonitrile
Michael D. Ryan
Department of Chemistry, University of Wisconsin
‐
Madison
Madison, WI
Dennis H. Evans
Department of Chemistry, University of Wisconsin
‐
Madison
Madison, WI
Childs et al. (1) studied the electrochemical reductive coupling
of diethyl fumarate (trans-C
2
H
5
O
2
CCH = CHCO
2
C
2
H
5
), 1, in N,N-
dimethylformamide (DMF). Their attention was focused on the
dimerization mechanism with tetra-n-butylammonium iodide as
supporting electrolyte. Using double potential step
chronoamperometry, these workers showed that the initially formed
radical anions reacted by a second-order, irreversible, dimerization
reaction in contrast to more complex schemes which had been
proposed earlier. This conclusion was confirmed for 1 and other
activated olefins by later studies using rotating ring-disk electrode
voltammetry (2), linear sweep voltammetry (3), and current-reversal
chronopotentiometry (4).
Keywords: electrohydrodimerization, cyclic voltammetry, ion pairs.
NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be
accessed by following the link in the citation at the bottom of the page.
Journal of the Electrochemical Society, Vol. 121, No. 7 (July 1974): pg. 882-883. DOI. This article is © Electrochemical
Society and permission has been granted for this version to appear in e-Publications@Marquette. Electrochemical Society
does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express
permission from Electrochemical Society.
2
Childs et al. (1) noted that the addition of sodium or lithium ions
to the supporting electrolyte caused a substantial increase in the rate
of dimerization of the radical anions of diethyl fumarate. Lithium also
had the effect of decreasing the importance of polymerization side
reactions as evidenced by an increase of the coulometric n-value to
unity. An analogous effect of sodium and lithium has been reported for
the reduction of some 𝛼, 𝛽-unsaturated ketones in dimethylsulfoxide
(5). The increased rate of dimerization was ascribed to ion pair
formation between the radical anions and the metal cations with the
neutral ion pairs reacting more rapidly than the anion radicals. A
quantitative study of the effect of sodium on the reduction of
phthalaldehyde in N,N-dimethylformamide has been reported by Lasia
(6) who treated his data in terms of the following reactions (A =
phthalaldehyde)
If the rate of the ion pairing reaction [2] is rapid enough so that
the reaction can be assumed to be at equilibrium, and if sodium is
present in excess, the observed dimerization rate constant, k
obs
, may
be related to K, k
1
, k
2
, k
3
, and C
Na
by the equation
where K is the formation constant for the ion pair (M
-1
) and C
Na
is the
molar concentration of sodium.
NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be
accessed by following the link in the citation at the bottom of the page.
Journal of the Electrochemical Society, Vol. 121, No. 7 (July 1974): pg. 882-883. DOI. This article is © Electrochemical
Society and permission has been granted for this version to appear in e-Publications@Marquette. Electrochemical Society
does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express
permission from Electrochemical Society.
3
This paper presents results for the reduction of 1 in the
presence of sodium in dimethylsulfoxide (DMSO) and acetonitrile (AN).
Though these solvents have similar dielectric constants, the tendency
for ion pair formation is much greater in AN than in DMSO. Fujinaga et
al. (7) noted that formation constants for the ion pairs of the radical
anions of naphthoquinones and metal ions are greater in AN than in
DMSO. As pointed out by Krygowski (8), this is consistent with the
concept of donicity (DN) defined by Gutmann (9). A high donicity
solvent such as DMSO (DN = 29.8) stabilizes the unpaired metal ions
by solvation in contrast to a lower donicity solvent such as AN (DN =
14.1) in which solvation of metal ions is weak and ion pair formation is
favored.
Experimental
Materials
Dimethylsulfoxide (DMSO) and acetonitrile (AN) were Matheson
Coleman and Bell Spectroquality solvents. Water content as
determined by gas chromatography was about 30 mM for each sol-
vent. Tetra-n-butylammonium perchlorate (Matheson Coleman and
Bell) was recrystallized from acetone-water and vacuum dried at room
temperature. Anhydrous sodium perchlorate (Matheson Coleman and
Bell) was found to contain less than 1 mole per cent (m/o) water by
determining loss of weight after vacuum drying at 200°C (10). Diethyl
fumarate (Aldrich) was used as received.
Apparatus
The voltammetric cell has been described elsewhere (11) as has
the digital data acquisition system and procedures for analysis of cyclic
voltammetric data (12). A silver reference electrode (SRE) comprising
a silver wire in contact with 0.10M tetra-n-butylammonium perchlorate
and 0.010M silver nitrate in DMSO was used for the DMSO studies and
an aqueous saturated calomel electrode (SCE) was used in AN. The
hanging mercury drop electrode area was 0.029 cm
2
.
Experimental procedures
Experiments were performed at a temperature of 24° ± 1°C. For
sodium perchlorate concentrations less than 0.10M, sufficient tetra-n-
NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be
accessed by following the link in the citation at the bottom of the page.
Journal of the Electrochemical Society, Vol. 121, No. 7 (July 1974): pg. 882-883. DOI. This article is © Electrochemical
Society and permission has been granted for this version to appear in e-Publications@Marquette. Electrochemical Society
does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express
permission from Electrochemical Society.
4
butylammonium perchlorate was added to assure an ionic strength of
0.1M.
Rate constants were determined using the method described by
Olmstead, Hamilton, and Nicholson (13) where the anodic peak
current, 𝑖
𝑎
′
, is measured from zero current rather than from an
extension of the cathodic peak. The theoretical results were extended
to switching potentials necessary for this research but not
encompassed in the work of Olmstead et al. by applying digital
simulation techniques (14). The rate law for dimerization of the
radicals, R, was written as -dC
R
/dt = 2kC
R
2
where k is the second-
order dimerization rate constant. Olmstead et ca. omitted the 2 in
their formulation of the rate law.
Results and Discussion
Peak potentials and observed rate constants, k
obs
, as a function
of sodium concentration for DMSO as solvent are presented in Table I.
The rate constants are averages of individual values obtained from at
least three different scan rates in the range of 0.5-50 V/sec. The
shapes of the voltammetric peaks, the peak current ratios, and the
dependence of the cathodic peak potential on scan rate were all
consistent with an irreversible dimerization following reversible elec-
tron transfer (13).
There are several constants which must be evaluated in Eq. [6].
The dimerization rate constant for the radical anions, k
1
, was taken as
k
obs
in the absence of sodium ions. The value found was
1.4 X 10
3
liter∙mole
-1
sec
-1
. The evaluation of K is based on the
dependence of the peak potential on sodium concentration. Two
factors cause the peak potential to move in the positive direction as
the sodium concentration increases. First, the ion pair formation
(reaction [2]) causes a positive shift in the potential. This would be
observed even if the radicals did not dimerize. Second, the chemical
reactions of the anion radicals, reactions [3] - [5], also cause a
positive shift in the peak potential. Since the two effects are additive,
we may correct the observed peak potentials for the effects of the
second factor by applying the theory for an irreversible dimerization
![Fig. 2. Observed rate constants for dimerization of radical anions of diethyl fumarate in DMSO as a function of sodium concentration. Concentration of diethyl fumarate, 0.96 mM. Curve is Eq. [6] with K = 5.8 M-1, k1 = 1.4 X 103, k2 = 5.0 x 104, and k3 = 3.1 X 105 liter∙mole-1sec-1.](/figures/fig-2-observed-rate-constants-for-dimerization-of-radical-2h3l4p0f.webp)
