in the presence of 150
a
Table 1 Sonovoltammetric data for the reduction of 10 ml insoluble organic compound emulsified in 20 ml aqueous 1.0 m MgCl
2
2
2
W cm ultrasound
3
10 Dsolb/
9
10 Dappc/
9
E
1/2
/
10 ctotal
m
/
I
mA
lim
/
2
21
2
21
Compound
V vs. SCE
n
m s
m s
Diethylfumarate
Dimethylmaleate
Diethylmaleate
Dibutylmaleate
Diethylacetylene
dicarboxylate
21.16
21.34
21.23
21.24
2
2
2
2
3.0
4.0
3.1
2.2
1500
2500
1050
180
0.61
0.73
0.61
0.50
0.58
0.73
0.39
0.09
21.28
21.58
4
2
3.0
5.2
ca. 500
ca. 1700
0.60
0.83
0.20
0.38
Cyclohex-2-en-1-one
a
Voltammograms obtained at 25 °C, scan rate 0.1 V s21, at a 3 mm diameter glassy carbon disc electrode. E1/2 corresponds to the half wave potential for
b
6 c
sigmoidal responses. An empirical expression introduced by Wilke and Chang allows the diffusivity, Dsol, to be estimated to within ± 10% accuracy. The
apparent diffusivity Dapp has been calculated assuming a homogeneous solution based on eqn. (1) and using the approximate diffusion layer thickness
d = 1.6 mm.
5
electrode process. In the case of diethylmaleate at E1/2 = 21.23
V vs. SCE also a well defined response can be observed,
observed for diethyl-fumarate and -maleate. Due to the more
negative reduction potential diethylacetylene dicarboxylate is
reduced directly in a 4e process to diethylsuccinate. After
2
although now the voltammetric wave is more drawn out (E3/4
2
E
1/4 = 100 mV). For comparison, the electrochemical reduction
passing two- to three-fold excess charge the products were
isolated by simple extraction with chloroform and analysed by
NMR spectroscopy. Yields of the isolated compounds are
typically 50–70% based on the starting material and increase
with the amount of charge passed. Side-products due to
processes such as hydrodimerization or polymerization have not
been observed. The process responsible for the loss of faradaic
efficiency appears to be hydrogen evolution.
Although details of the mechanism for sonoelectrochemical
redox processes in emulsions remain to be studied, it is clear
that the effect of ultrasound, to generate microscopically small
droplets whilst ensuring extremely high rates of mass transport,
considerably extends and facilitates the possibilities in emulsion
electrochemistry.
of diethylmaleate dissolved in N,N-dimethylformamide de-
7
scribed by Bard et al. involves the cis–trans isomerization of
the radical anion formed as a short-lived intermediate in the
vicinity of the electrode surface. It was also shown that in N,N-
dimethylformamide solution the reduction of diethylmaleate
occurs at potentials approximately 0.2 V more negative of those
for the reduction of the fumarate. In regard of the contrasting
voltammetric features in water and in organic media, for the
reduction of diethylmaleate in aqueous media an extremely fast
rate of the cis–trans isomerization possibly due to the solvent
polarity may be predicted.
The comparison of the electrochemical reduction of di-
methyl-, diethyl-, and dibutyl-maleate emulsions in aqueous
2
1.0 m MgCl demonstrates the effect of the substrate solubility
on the cathodic limiting currents (Table 1). The increase in the
chain length of the carbon residue significantly alters the
magnitude of the apparent diffusivity in agreement with the
mechanism generally applied for electrochemical processes in
emulsion systems [eqn. (2)–(4)].
Footnotes
*
†
E-mail: compton@ermine.ox.ac.uk
The ultrasound generator employed was a VCX400 model sonic horn
1
(Sonics and Materials, USA) equipped with a stepped titanium microtip
(
electrically insulated) of 3 mm diameter emitting 20 kHz sound with a
A
oil " Asolution
(2)
(3)
(4)
2
2
power level set to 150 W cm
(calorimetrically determined). For
2
+
A
solution + 2 e + 2 H ? Bsolution
5
electrochemical experiments a thermostatted cell, and a PAR 173 (EG&G)
potentiostat controlled by an Oxford Electrodes programmer and connected
to a Lloyd P3 plotter were used. Electrodes used were a 3 mm diameter
glassy carbon disc (BAS), a graphite rod as a counter, and a saturated
calomel reference electrode. Chemical reagents and electrolyte salt
Bsolution " Boil
Under ultrasound condition the oil microdroplets penetrate
into the Nernst diffusion layer at the electrode surface and act as
a local source of the depolarizer [eqn. (2)]. After the product,
(
‡
Aldrich) were of highest commercially available quality.
Bulk electrolysis experiments were performed typically with 200 mg
substrate in a 20 ml electrolyte volume at a large surface area gold wire
B
solution, has been generated at the electrode surface the reaction
step (4) is essential for maintaining an active electrode surface
by re-absorbing the product. Depending on the kinetic regime of
the process the equilibrium concentration of the depolarizer in
the aqueous electrolyte or the rate of dissolution in step (2)
govern the limiting current.
2
2
electrode and 150 W cm ultrasound intensity.
References
In order to analyse the nature of the observed reduction
process bulk electrolyses‡ of diethylfumarate, diethylmaleate,
and diethylacetylene dicarboxylate emulsified in aqueous 1.0 m
1
H. Feess and H. Wendt, in Technique of Electroorganic Synthesis, ed. N.
L. Weinberg and B. V. Tilak, Wiley, New York, 1982, Part III, p. 81f.
2 Electrochemistry in Colloids and Dispersions, ed. R. A. Mackay and
J. Texter, VCH, Weinheim, 1992.
3 D. J. Walton and S. S. Phull, Adv. Sonochem., 1996, 4, 205.
2
MgCl were performed. The reduction of emulsions of diethyl-
fumarate and -maleate assisted by additives and special
hydrophobic electrodes in the absence of ultrasound has been
4
T. J. Mason and J. P. Lorimer, Sonochemistry: Theory, Applications and
Uses of Ultrasound in Chemistry, Ellis Horwood, Chichester, 1988.
H. A. O. Hill, Y. Nakagawa, F. Marken and R. G. Compton, J. Phys.
Chem., 1996, 100, 17395.
C. R. Wilke, P. Chang, AIChE J., 1955, 264.
A. J. Bard, V. J. Puglisi, J. V. Kenkel and A. Lomax, Faraday Discuss.,
8
reported recently by Nonaka and coworkers. Not surprisingly,
5
the scale-up of the sonoelectrochemical process involves a non-
simple dependence of the observed limiting current for the
reduction reaction on the amount of reactant present. On glassy
carbon electrodes a tenfold increase in the amount of organic
phase resulted in sonovoltammetric responses with decaying
limiting currents characteristic for electrode-blocking proc-
esses. However, a clean reduction process at a large surface area
gold electrode following a two-electron–two-proton reaction
pathway with reduction of the alkenic double bond was
6
7
1
973, 56, 353.
8 Y. Kunugi, P.-C. Chen, T. Nonaka, Y.-B. Chong and N. Watanabe,
J. Electrochem. Soc., 1993, 140, 2833.
Received in Exeter, UK, 5th March 1997; Com. 7/01642E
996
Chem. Commun., 1997