Mechanism of electroreduction of allyl alcohol at platinized platinum electrode in acidic aqueous solution

Article abstract of DOI:10.1021/jo0012288

The electroreduction of allyl alcohol to form propene at the platinized platinum electrode in acidic aqueous solution has been studied using CV plots, IR, ESR, and MS spectra, and a semiempricial MO method (MOPAC7/AM1, PM3). From the determinations of charge-transfer coefficients, reaction orders and apparent activation energy for the given reaction, the detection of the intermediates such as C₃H₅⁺, C₃H₅·, and C₃H₅^- species, and PM3 calculations of charge distribution and frontier orbital energies of the reaction species C₃H₅OH and C₃H₅⁺, the authors suggest that in acidic aqueous solution the production of propene via reductive splitting of the C-OH bond situated in the allyl position of allyl alcohol obeys a carbonium ion-carbanion mechanism.

Full text of DOI:10.1021/jo0012288

J . Org. Chem. 2001, 66, 4487-4493
4487
Mech a n ism of Electr or ed u ction of Allyl Alcoh ol a t P la tin ized
P la tin u m Electr od e in Acid ic Aqu eou s Solu tion
Huang Shukun,*,†,‡ Song Youqun,† Zhang J indong,§ and Sun J ian|
College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, China,
Department of Chemistry, Changsha Electric Power Institute, Changsha, 410077, China, and
Institute of Chemistry, Academia Sinica, Beijing, 100080, China
yzliang@hunu.edu.cn
Received August 11, 2000
The electroreduction of allyl alcohol to form propene at the platinized platinum electrode in acidic
aqueous solution has been studied using CV plots, IR, ESR, and MS spectra, and a semiempricial
MO method (MOPAC7/AM1, PM3). From the determinations of charge-transfer coefficients, reaction
orders and apparent activation energy for the given reaction, the detection of the intermediates
•
such as C3H5+, C3H5 , and C3H5- species, and PM3 calculations of charge distribution and frontier
orbital energies of the reaction species C3H5OH and C3H5+, the authors suggest that in acidic
aqueous solution the production of propene via reductive splitting of the C-OH bond situated in
the allyl position of allyl alcohol obeys a carbonium ion-carbanion mechanism.
In tr od u ction
In recent years, analysis of the electroreduction prod-
ucts of allyl alcohol (AA) at a platinized platinum
electrode in perchloric acid aqueous solutions using MS
and IR spectroscopy has been performed by Chen J ian
et al.1 and Pastor et al.,2 respectively, who point out that
in acidic aqueous solution propene and propane are the
main products. To elucidate the production of the prod-
ucts, the electroreduction mechanism concerned is sug-
gested by Chen J ian et al.1 and Hora´nyi and Torkos.3
According to them, at the electrode surface, propene is
•
formed via the allyl radical C3H5 and propane via
catalytic hydrogenation of propene. Nevertheless, they
•
hold different views about how the radical C3H5 and
propene can be formed during the electroreduction of AA.
Chen J ian et al. assume that the radical C3H5• is formed
from the dissociation of the anion radical C3H5OH•- and
propene from the hydrolytic action of the carbanion
C3H5-. In contrast, Hora´nyi and Torkos suppose that the
•
+
radical C3H5 is formed via the carbonium ion C3H5
+
dissociated from the oxonium ion C3H5OH2 and pro-
pene via a coupled reaction between the radical C3H5• and
the atom H. We have studied the electroreduction reac-
tion from AA to propene at the platinized platinum
electrode in perchloric acid aqueous solution and put
forward a carbonium ion-carbanion mechanism to elu-
cidate the reaction process. It is of wide significance to
elucidate the mechanism to produce alkene or alkyne via
the electroreduction cleavage of the C-OH bonds situ-
ated in allyl or propargyl positions of some unsaturated
fatty alcohols, e.g., allyl alcohol, propargyl alcohol, 1,4-
butynediol, and 3-hexyne-2,5-diol in acidic aqueous solu-
tions.
F igu r e 1. CV plots of electroreduction of AA at variable sweep
rates. Test solution: 5.0 × 10-3 mol L-1 AA + 0.10 mol L-1
NaClO4 + 5.0 × 10-3 mol L-1 HClO4 + H2O. Sweep rates, mV
s
-1: (a) 26, (b) 39, (c) 52, (d) 78, (e) 157. Platinized platinum
electrode (ca. 0.02 cm2).
Resu lts a n d Discu ssion
† Hunan University.
‡ Fax: 86-0731-8824386.
Ch a r a cter istics of Electr or ed u ction of AA. The
electroreduction of AA in acidic aqueous solution ob-
served in Figure 1 takes place in a less negative potential,
-0.35 V (vs SCE, the same below), which is consistent
with the values reported by different authors.1-3 In the
electroreduction of organic compounds, it is seldom the
§ Changsha Electric Power Institute.
| Academia Sinica.
(1) Chen, J .; Liu, P. F.; Wang, H.; Zha, Q. X. Acta Chim. Sin. 1993,
51, 150 (Ch.).
(2) Pastor, E.; Wasmus, S.; Iwasita, T.; Are´valo, M. C.; Ganza´lez,
S.; Arvia, A. J . J . Electroanal. Chem. 1993, 353, 81.
(3) Hora´nyi, G.; Torkos, K. J . Electroanal. Chem. 1980, 111, 279.
10.1021/jo0012288 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/07/2001

4488 J . Org. Chem., Vol. 66, No. 13, 2001
Shukun et al.
Ta ble 1. Electr och em ica l P a r a m eter s of
Electr or ed u ction of AA
The broad reduction wave at -0.35 V is surely related
to the original electroreduction of the stable reactant AA
or protonated AA. Moreover, the final products propene
and propane could also be received at the potential as
such. It follows that the broad wave should be regarded
as two reduction waves merged with each other,8 which
result from the above-mentioned two one-electrom reac-
tions forming the products from the reactant. In addition,
the reduction wave at -0.35 V remains unchanged in its
wave shape and wave height even though the CV plot
was recorded repeatedly in a higher sweep rate. Conse-
quently, the reaction intermediates taking part in the two
one-electron reactions are all unstable. Therefore, to
elucidate in detail the mechanism of the electroreduction
of AA, it is necessary to detect these intermediates using
a variety of spectroscopic techniques.
apparent
activation
transfer coefficients
Rc Ra
0.49 ( 0.05a 1.14 ( 0.15a 0.72 ( 0.07a 1.13 ( 0.05a
reaction orders
energy, ∆Ea
ZAA
ZH
(kJ mol-1
18.0
)
+
a
Average error in two calculated values.
case that the reduction potential is more positive than
the potential, -0.42 V, of evoluting hydrogen in acidic
aqueous solution.4 Its electroreduction occurring with
ease was also proved by the fact that the apparent
activation energy of the electroreduction of AA obtained
from recording reduction current of AA at various tem-
perature was no more than 18.0 kJ mol-1 (Table 1).
Unlike the reversible electrode reaction of evoluting
hydrogen in which its peak potential is independent of
sweep rate and the difference between the cathodic and
anodic peak potentials is no more than 0.059 V,4 the
electrode reaction in the electroreduction of AA has some
irreversible characteristics in which its cathodic (or
anodic) peak potential shifts negatively (or positively)
with increasing sweep rate and the difference between
cathodic and anodic peak potentials is greater than 0.059
V (Figure 1). In addition, considering that there also
exists an anodic peak together with a cathodic peak in
its CV plot, it may be confirmed that the electrode
reaction studied is not a totally irreversible electrode
reaction but a partly irreversible or quasireversible
electrode reaction.
An a lysis of In ter m ed ia tes. Allyl Ca tion . AA is a
molecule with uneven charge distribution, where the
atom O has the largest negative charge density (Scheme
1, A) and will easily result in an attack of the electrophilic
reagent H+ and form the oxonium ion C3H5OH2+. In
addition, the protonation heat of AA may amount to 766.9
kJ mol-1 9a,b
As a result, like many alcohols, e.g., ethyl
.
alcohol, in acidic aqueous solution, AA will also partly
+
exist in the form of C3H5OH2 via the reaction C3H5OH
+ H+ ) C3H5OH2+. However, unlike C2H5OH2+, which is
+
unable to dissociate and form the ethyl cation C2H5
,
,
+
C3H5OH2+ may dissociate and form the allyl cation C3H5
as its dissociation energy was no more than 66.9 kJ
mol-1 9a Forming C3H5+ with ease is due to its an amount
.
of stabilty, which comes from the divergence of its
positive charge (Scheme 1, B) resulting from the overlap
of its π-orbit electron and empty p-orbit.
A strong nucleophilic reagent, e.g., sodium azide, might
be used as a trap-to-trap carbonium ion.10 The IR spectra
of AA in acidic aqueous solutions containing sodium azide
(Figure 2) can be explained by means of reaction scheme
2.
That the reaction order with respect to H+ is close to
1 (Table 1), which was found from the cathodic peak
currents of AA by changing the pH values in the studied
solutions, indicates there is a preceding chemical reaction
involving H+ in the studied solution. In other words, the
electroreduction of AA in acidic aqueous solution obeys
a CE model rather than an EC model.
The fact that the reaction order with respect to AA is
a fraction, 0.72 (Table 1), is a reflection of the difference
between the surface and bulk concentrations of AA.5 That
is, the reactant AA or protonated AA during its electrore-
duction is adsorbed at the electrode surface. It may be
further recognized that the reactant AA or protonated
AA has a weak adsorption at the electrode surface since
a small peak is absent at the back of the cathodic peak
at -0.35 V.6 Therefore, following the above-mentioned
preceding chemical step in the bulk solution, there is also
an adsorption step involving AA or protonated AA at the
electrode/solution interface prior to the charge transfer
steps of the partly irreversible electrode reaction.
On the basis of the data (Rc + Ra) > 1 and Ra > Rc (Table
1) and Tafel coefficient 2.3RT/(RcnaF) close to 0.118,7a it
may be deduced from the Butler-Volmer equation7b that
the electroreduction of AA is a two-electron reaction
involving two one-electron consecutive electrochemical
steps in which the first electrochemical step is the rate-
determining step (rds, the same below) of the electrore-
duction of AA.
reaction scheme 2
a. acidic aqueous solution of ethyl alcohol:
SN2
-
C2H5OH2+ + N3
8 C2H5N3 + H2O(aq) (1)
b. acidic aqueous solution of AA:
SN1
+
C3H5OH2
8 C3H5+ + H2O(aq)
(2)
C3H5+ + N3- f C3H5N3(aq)
c. acidic aqueous solution of AA being electrolyzed:
SN1
+
C3H5OH2
8 C3H5+ + H2O(aq)
C3H5+ + N3- f C3H5N3(aq)
(3)
•
C3H5+(aq) + eM- f C3H5 ads
A weak absorption of νC-N of the azide C2H5N3 at 1325
cm-1 is observed on line a, which is attributed to the SN2
+
-
reaction (1) between C2H5OH2 and N3 owing to the
(4) Breiter, M. Electrochim. Acta 1963, 8, 925.
(5) Theoretical Electrochemistry; Antropov, L., Wu, Z. D., et al., Eds.;
translated from English; Higher Education Press: Beijing, 1982;
Chapter 20 (Ch.).
(6) Wopschall, R. H.; Shain, I. Anal. Chem. 1967, 39, 1527.
(7) (a) An Introduction to Electrode Kinetics; Zha, Q. X., Ed.; Science
Press: Beijing, 1976; Chapter 4 (Ch.). (b) Bockris, J . O’M.; Damjanovic,
A. D. J . Electrochem. Soc. 1972, 119, 285.
(8) Plocyn, D. S.; Shain, I. Anal. Chem. 1966, 38, 370.
(9) (a) Luo, Y.-R.; Holmes, J . L. J . Phys. Chem. 1994, 98, 303. (b)
Moryl, J . E.; Greasy, W. R.; Farrar, J . M. J . Phys. Chem. 1983, 87,
1954.
(10) Harris, J . M.; Saber, D. J .; Hall R. E.; Schleyer, P. R. J . Am.
Chem. Soc. 1970, 92, 5729.

Electroreduction of C3H5OH at Platinized Pt Electrode
J . Org. Chem., Vol. 66, No. 13, 2001 4489
Sch em e 1
+
decrease the concentration of C3H5 and C3H5N3 in the
+
bulk solution. That is to say, the allyl cation C3H5 will
not only form from the dissociation of the oxonium ion
+
C3H5OH2 in the bulk solution will but also partcipate
in the electroreduction of AA as a reactive intermediate
at the electrode/solution interface.
Allyl Ra d ica l. A six-line spectrum having triplets-of-
doublets pattern is observed in Figure 3 (with the
asterisks). Hyperfine coupling constants of the spin
â
adduct concerned, RN ) 15.50 × 10-4 T, RH ) 21.60 ×
10-4 T, were obtained from the six-line spectrum. Obvi-
ously, not an oxyl radical but a carbon-centered radical,
which was trapped by the spin trap 5,5-dimethyl-1-
pyrroline-1-oxide (DMPO), was formed during the elec-
â
troreduction of AA because RH is greater than RN and
additional hyperfine splitting of γ-H is wanting in this
spectrum.11
In the first place, the hydroxylethyl radical CH3C˙ HOH
cannot be formed from the solvent ethyl alcohol through
abstracting its hydrogen by electrogenerated atom H at
the cathode or electrogenerated radical OH at the anode
separated by the cathode. It may be proved by the lack
of the six-line pattern in the spectrum of the electrolyzed
blank solution without AA. Similarly, the hydroxylallyl
radical CH2dCHC˙ HOH should not be formed from the
reactant AA, which might further be confirmed by the
fact that when the trap phenyl tert-butyl nitrone (PBN)
in lieu of DMPO was used in our previous work12 the six-
line spectrum with a large â-H splitting resulting from
the adduct CH2dCHC˙ HOH and PBN13 was not present
in the observed spectrum. Obviously, the above-men-
tioned carbon-centered radical trapped by DMPO does
not come from either the solvent C2H5OH or the reactant
AA. It is surely the carbon-centered radical formed in the
process of the electroreduction of AA.
F igu r e 2. IR spectra of various solutions in the presence of
sodium azide: (a) 5.0 × 10-2 mol L-1 HClO4 + 7.5 × 10-2 mol
L-1 NaN3 + 1:3 C2H5OH:H2O (ratio in volume); (b) 1.5 × 10-2
mol L-1 AA + a; (c) 1.5 × 10-2 mol L-1 AA + 5.0 × 10-2 mol
L-1 HClO4 + 12.5 × 10-2 mol L-1 NaN3 + 1:3 C2H5OH:H2O;
(d) 1.5 × 10-2 mol L-1 AA + a, Controlled potential: -0.40 V,
Tme of electrolysis: 30 min.
According to Chen J ian1 and Hora´nyi,3 the propyl
existence of the solvent ethyl alcohol in the studied
solution. A strong absorption of νC-N at 1350 cm-1 on line
b is due to the formation of C3H5N3 via the SN1 reaction
•
•
radical C3H7 and the allyl radical C3H5 were possibly
the radicals formed during the electroreduction of AA.
However, cis-hydrogenation of the two carbons on the
double bond of propene to produce propane occurs almost
+
-
(2) between C3H5OH2 and N3 rather than the SN2
reaction in between. This view was supported by the fact
•
simultaneously and thus C3H7 has a low yield at the
electrode surface. In addition, like propane, C3H7 has a
+
that the reaction velocity of the reaction, i.e., C3H5OH2
•
+ N3- f C3H5N3 + H2O, is independent of the concentra-
tion of N3-* (SN1 reaction ν ) k[C3H5OH]; SN2 reaction
ν ) k′[C3H5OH][N3-]) considering that the intensity of
νC-N on line c in comparison with that on line b remains
unchanged despite increasing the concentration of so-
very small solubility in alcohol-water solutions and thus
•
less possibility to be trapped. On the contrary, C3H5 , as
will be mentioned in reaction scheme 4, forms easily from
the discharge of C3H5+ at the electrode/solution interface.
Additionally, it has a definite solubility in the bulk
solution because of the polarization effect of its π-electron.
+
dium azide. Therefore, no doubt, C3H5 can be formed
+
through the dissociation of C3H5OH2 in AA acidic
•
•
Therefore, in comparison with C3H7 , C3H5 will be a
radical to be trapped and detected easily. Its formation
might also be proved from the decaying spectra lines in
our previous work,12 which should be ascribed to the
aqueous solution.
In the acidic aqueous solution of AA being electrolyzed,
the intensity of the absorption νC-N on line d is decreased
in comparison with that on line b or c. The difference
lies in that the electrode electron can act as “another
nucleophilic reagent” along with sodium azide in the bulk
solution. Under the circumstances, they will commonly
(11) J anzen, E. G.; Liu, J . I. J . Magn. Reson. 1973, 9, 510.
(12) Zhang, J . D.; Kuang, Y. F.; Huang, S. K.; Xie, N. X.; Wang, F.
Z.; Chen, D. W. Electrochemistry 1996, 3, 343 (Ch.).
+
combine with C3H5 via the reactions (3) and thus
(13) J anzen, E. G.; Lopp, I. G. J . Magn. Reson. 1972, 7, 107.

4490 J . Org. Chem., Vol. 66, No. 13, 2001
Shukun et al.
F igu r e 3. ESR spectrum of product solution during the electroreduction of AA in the presence of DMPO. Test solution: 1.0 ×
10-2 mol L-1 AA + 1.0 × 10-2 mol L-1 DMPO + 0.1 mol L-1 NaClO4 + 5.0 × 10-3 mol L-1 HClO4 + 1:3 C2H5OH:H2O. Controlled
potential: -0.40 V. Time of electrolysis: 10 min.
F igu r e 4. MS spectra of product solutions of electroreduction of AA: (a) 5.0 × 10-2 mol L-1 AA + 0.1 mol L-1 NaClO4 + 2.0 ×
10-2 mol L-1 HClO4 + 1:3 C2H5OH:H2O; (b) a + CO2(gas). Controlled potential: -0.40 V. Time of electrolysis: ca. 1 h.
•
•
formation of the less stable adduct PBN-C3H5 rather
is possibly formed via the combination of C3H5 with an
•
than that of the stable adduct PBN-C3H7 , as the π-type
electron. Considering that C3H5• has a great yield at the
electrode surface, Fermi electrons at the electrode surface
have a higher surface concentration and reaction activity,7a
radical C3H5• is more stable than the σ-type radical C3H7 .
•
It is known that a solvated electron is stable in liquid
ammonia and unstable in aqueous solution. The forma-
tion of C3H5• is highly improbable through the combina-
tion of the solvated electron with C3H5 in the bulk
solution. Consequently, the reaction intermediate C3H5
-
and in aqueous solution C3H5 neither forms via the
combination of C3H5• with the solvated electron nor exists
due to its reaction with H+ or H2O to produce propene,
it is certain that C3H5- is formed at the electrode surface
+
•
•
-
is sure to form at the electrode/solution interface via the
via the electron-transfer reaction C3H5 ads + eM
f
electron-transfer reaction C3H5+(aq) + eM f C3H5 ads
.
C3H5
.
-
•
-
ads
Allyl An ion . As compared to Figure 4a, the peaks of
the molecular ion C3H5COOH•+ (m/e ) 86) and its
fragment CH2dC(OH) (OH+) (m/e ) 60) arising from the
so-called McLafferty rearrangement are presented in
Figure 4b. In addition, the m/e ) 41 and m/e ) 69 peaks
have an increasing height, caused separately by the frag-
Mech a n ism of Electr or ed u ction of AA. Because in
acidic aqueous solution and low concentrations of AA
(below 20.0 × 10-3 mol L-1) AA exists in the form of a
monomeric molecule instead of a polymer15 and propene
is a stable product received from the electroreduction of
AA, the overall electroreduction reaction producing pro-
pene from AA may be represented as follows:
ments CH2dCHCH2 (M - 45) and CH2dCHCH2CtO+
+
(M - 17) produced from the fission of the molecular ion.
All these characteristic peaks of lower fatty acids indicate
3-butenoic acid, C3H5COOH, was formed in the electro-
lyzed AA test solution agitated with carbon dioxide.
C3H5OH(aq) + 2H+(aq) + 2eM- f
C3H6(g) + H2O(aq) (4)
-
Evidently, only the intermediate allyl anion C3H5 may
react with CO2 to form C3H5COOH via a nucleophilic
addition reaction, i.e., C3H5- + H2O + CO2 f C3H5COOH
+ OH-. It follows that C3H5- is likely to form during the
electroreduction of AA.
•
The formation of the allyl radical C3H5 should be
presupposed from the production of propene. In fact, it
(14) J arjis, H. M.; Khalil, S. M. J . Chem. Soc., Perkin Trans. 2 1986,
1701.
(15) Smith, P.; Wood, P. B. Can. J . Chem. 1967, 45, 649.
-
The allyl anion C3H5 possesses a greater resonance
-
stabilization than the allyl radical C3H5• 14 and thus C3H5

Electroreduction of C3H5OH at Platinized Pt Electrode
J . Org. Chem., Vol. 66, No. 13, 2001 4491
has been detected by ESR spectroscopy (Figure 3). Its
detection is an important step to elucidate the mecha-
nism of the electrogeneration of propene from AA. Reac-
tion 4 may be regarded as the combination of the two
reaction stages of forming C3H5• from AA and producing
occurs in a less negative potential -0.35 V, which is
consistent with the carbonium ion pathway with a lower
unoccupied level. Second, if C3H5OH•- was formed, like
the electroreduction of some unsaturated aromatic alco-
hols via anion radicals,18 n-propyl alcohol should be partly
received in the final products. However, the yield of
n-propyl alcohol was negligibly small.1,2 On the contrary,
•
propene from C3H5 on the basis of the formation of the
•
radical C3H5 . The reaction pathways of the two reaction
+
stages will be discussed separately.
the formation of the allyl cation C3H5 has been discov-
•
Rea ction s F or m in g C3H5 fr om AA (eqs 5-11).
ered in this work. Finally, if the formation of C3H5• from
AA could be ascribed to the anion radical pathway, the
electroreduction of AA should have an EC model as
indicated in reaction scheme 3. In fact, its electroreduc-
tion, as mentioned earlier, belongs to a CE model as
indicated in reaction scheme 4.
reaction: C3H5OH(aq) + H+(aq) + eM- f
•
C3H5 ads + H2O(aq) (5)
reaction scheme 3
anion radical pathway1: C3H5OH(aq) +
An anion radical is a common intermediate during the
electroreduction of organic compounds, indeed. But it is
not the case with regard to the electroreduction of AA in
acidic aqueous solution. As indicated by Hora´nyi, it
“seemed to be an interesting exception to the general
rule”.19 The “interesting exception”, as mentioned in this
eM- f C3H5OH•-
(6)
ads
C3H5OH•- f C3H5 ads
+
•
ads
+
work, lies in that the allyl cation C3H5 can be easily
OH-(aq) (7)
+
formed by means of the reductive splitting of the C-OH2
bond situated at the allyl position of the oxonium ion
C3H5OH2 which exists in AA acidic aqueous solution.
H
+ + OH- f H2O (aq) (8)
+
However, our further work17c also indicated that when
the electroreduction of AA takes place at the mercury
electrode and in nearly neutral aqueous solutions, its
electroreduction to produce propene will occur at a more
negative potential, e.g., -1.25 V. Under the circum-
reaction scheme 4
carbonium ion pathway3: C3H5OH + H+
f
C3H5OH2+(aq) (9)
•
stances, the anion radical pathway forming C3H5 from
C3H5OH2+ f C3H5
+
AA must be obeyed.
R ea ct ion s P r od u cin g P r op en e fr om C3H 5 (eq s
+
•
H2O(aq) (10)
12-18).
C3H5+(aq) +
•
reaction: C3H5 ads + H+(aq) + eM- f C3H6(g)
(12)
eM- f C3H5ads (11)
reaction scheme 5
coupling pathway3: H+(aq) + eM- f Hads
According to Chen J ian1 and Hora´nyi,3 two pathways
(13)
exist to form C3H5 from AA. Like C3H5OH2+, C3H5OH•-
•
is also an easily dissociated species due to the formation
of more stable radical C3H5 , and thus the degree of the
•
Hads + C3H5 ads f C3H6(g) (14)
•
difficulty or ease of combination of the electrode electron
with C3H5OH or C3H5+ will play a main role in determin-
reaction scheme 6
carbanion pathway1: C3H5 ads + eM- f C3H5
•
ing the pathway for forming C3H5 . Fukui16 pointed out
•
-
ads (15)
that under the given electrode and solvent conditions the
reduction potential of any organic compound is linearly
related to the energy of the lowest unoccupied orbit. The
higher its lowest unoccupied level, the more negative its
reduction potential, and thus its electroreduction will be
rare. When C3H5OH•- is electrogenerated from the AA
molecule, an electrode electron will be required to fill into
the antibonding orbit of the molecule AA, where its
LUMO is 1.05 eV and the energy gap between LUMO
and HOMO is 11.36 eV, calculated by PM3. However, the
-
C3H5
+ H+(aq) f C3H6(g)
ads
(16)
-
or even more correctly, the hydration of C3H5
:
ads
-
C3H5
+ H2O(aq) f C3H6(g) + OH-(aq) (17)
ads
H
+ + OH- f H2O(aq)
(18)
•
+
electrogeneration of C3H5 from C3H5 will make the
+
electrode electron occupy the nonbonding orbit of C3H5
,
In organic electroreduction the reaction forming car-
banion and the coupled reaction are two kinds of common
reactions that produce hydrocarbon products from radical
where its NBMO is -8.28 eV, with the energy gap
between NBMO and HOMO 9.66 eV. As compared to the
AA molecule, the allyl cation C3H5+ obviously has a lower
level and energy gap. Therefore, the carbonium ion
pathway is surely preferred to the anion radical pathway.
The above-mentioned inference obtained from the
quantum-chemical calculation was supported by many
experimental facts. First, in the case of forming an anion
radical, the reduction potential should have a relatively
negative value.17a,b However, the electroreduction of AA
(16) Fukui, K.; Morokuma, K.; Kato, H.; Yonezawa, T. Bull. Chem.
Soc. J pn. 1963, 36, 217.
(17) (a) Huang, S. K.; Chen, D. W.; Huang, S. H. Acta Chim. Sin.
1996, 54, 783 (Ch.). (b) Piette, L. H.; LuDwig, P.; Adams, R. N.
Anal. Chem. 1962, 34, 916. (c) Huang, S. K.; Zhang, J . D., unpubilished
work.
(18) Lund, H.; Doupeux, H.; Michei, M. A.; Mousset, G.; Somonet,
J . Electrochim. Acta 1974, 19, 629.
(19) Hora´nyi, G. Electrochim. Acta 1986, 31, 1095.

4492 J . Org. Chem., Vol. 66, No. 13, 2001
Shukun et al.
intermediates. Whether one of the two reactions prevails
over another or they are of the same importance will
depend on the special characteristics of the two reactions
under discussion and the given reaction conditions.
As mentioned earlier, the electroreduction of AA at
-0.35 V is a two-electron electrode reaction involving two
consecutive electrochemical steps, where the first elec-
trochemical step is the rds of the electroreduction of AA.
On the one hand, obviously not reaction (13) but reaction
(15) is determined to be the consecutive electrochemical
step of the first electrochemical step, i.e., the reaction
two-electron electrode reaction. Prior to its electron-trans-
fer reaction, a homogeneous chemical equilibrium related
+
to the formation of the protonated AA, i.e., C3H5OH2
,
in the bulk solution and a heterogeneous surface trans-
+
formation related to the adsorption of AA or C3H5OH2
at the electrode/solution interface are involved in the
process of the electroreduction reaction. Some reaction
intermediates, i.e., the allyl cation C3H5+, the allyl radical
•
C3H5 , and the allyl anion C3H5-, which are all unstable,
are formed during the electroreduction. The overall
reaction process may be divided into the two stages of
•
•
•
C3H5+(aq) + eM- f C3H5 ads (11), since the product C3H5
forming C3H5• from AA and forming propene from C3H5
•
of reaction (11) is the reactant of reaction (15). On the
other hand, reaction (11) occurs at the electrode/solution
interface in which a definite amount of activation poten-
tial hill to detach the hydrated shell of C3H5+(aq) needs
to be satisfied,7b while reaction (15) is a rapid radical
quenching reaction at the electrode surface, and hence
reaction (11) as compared to reaction (15) will have a
slower reaction velocity and is determined to be the rds
of the electroreduction of AA when the two electrochemi-
cal steps are composed of reactions (11) and (15). There-
fore, the second electrochemical step in the electroreduc-
tion of AA, which is involved in the reactions producing
on the basis of the formation of C3H5 . Unlike the
electroreduction of ordinary organic compounds, the
•
+
formation of the radical C3H5 from AA or C3H5OH2 is
not via the dissociation of the anion radical C3H5OH•-
,
which is unable to form in acidic aqueous solution, but
via the discharge of the carbonium ion C3H5+ dissociated
+
from the oxonium ion C3H5OH2 and/or the direct dis-
charge of C3H5OH2+ itself because of its easy dissociation.
•
As for the formation of propene from C3H5 , there are two
possibilities, i.e., coupling of C3H5• with the atom H and
the hydration of the carbanion C3H5-, where the latter
is the main pathway at the ordinary reduction potential
of AA, -0.35 V. The electron-transfer reactions are com-
posed of the two consecutive electrochemical steps, i.e.,
•
propene from C3H5 , is not reaction (13) but reaction (15).
The hydration reaction (17) may also prevail over the
coupled reaction (14) due to a lower accumulation of the
atom H at the electrode surface when the electroreduction
of AA takes places in a less negative potential, e.g. -0.35
V. The above-mentioned view was supported by a higher
yield of propene in the final products. The yield of
propene may amount to 20-30%2,3, especially there is an
obvious increase in yield from 20 to 29% when the
reduction potential of AA was shifted negatively by a
small amount from -0.32 to -0.34 V, i.e., from -0.08 to
-0.10 V (vs SHE).19 That much propene is not likely to
desorb and evolute at the electrode/solution interface due
to the adsorption of propene with the help of its π-electron
polarization effect and its catalytic hydrogenation to form
propane at the electrode surface. A possible explanation
for the greater yield of propene is due to the existence of
reaction (17). Unlike propene, partly electrogenerated
•
+
+
the step of forming C3H5 from C3H5 and/or C3H5OH2
-
•
and the step of forming C3H5 from C3H5 , wherein the
former is the rds of the electroreduction of AA. The
overall electrode reaction and a carbonium ion-carbanion
mechanism to produce propene from AA may be repre-
sented as follows:
overall electrode reaction: C3H5OH(aq) +
2H+(aq) + 2eM- f C3H6(g) + H2O(aq)
partial reactions: C3H5OH + H+ ) C3H5OH2+(aq)
+
C3H5OH2+(aq) ) C3H5OH2
ads
C3H5OH2+ f C3H5+ + H2O(aq)
-
(at Helmholtz layer)
C3H5 ads, owing to its negative charge, will easily desorb
from the cathode surface with the negative charge and
enter the interface liquid layer, from where it will rapidly
react with H2O to produce propene, and finally propene
will escape from the interface liquid layer in the form of
a gas.
C3H5+(aq) + eM rds8 C3H5 ads
-
•
+
-
and/or
C3H5OH2
+ eM rds8
ads
•
C3H5 ads + H2O(aq)
Nevertheless, when the electroreduction of AA takes
place in at a more negative potential, e.g. -0.40 V, which
is close to the hydrogen evolution potential -0.42 V,4 a
large quantity of the atom H will be accumulated at the
platinum surface because the recombination of two atoms
H is the rds of evoluting hydrogen at platinum.5 The
obvious accumulation of the atom Hads has been proved
by the existence of the spin adduct H-DMPO having a
nine-line spectrum with a 1:1:2:1:2:1:2:1:1 pattern (Figure
3, peaks denoted ∆) at -0.40 V. A large amount of the
atom Hads allowed it to react with the allyl radical
•
-
C3H5 ads + eM- f C3H5
ads
-
C3H5
+ H2O(aq) f
ads
C3H6(g) + OH-(aq)
H+ + OH- f H2O(aq)
Exp er im en ta l Section
Electr och em ical Measu r em en t. The electrochemical mea-
surement of the electroreduction of AA was performed using
cyclic voltammetric (CV) plots or the electrolysis at a controlled
potential, -0.40 V. The electrolytic cell is an H-cell whose
cathode and anode were separated by a sintered glass disk,
where a special platinized platinum wire (apparent area ca.
0.02 cm2 in CV plot or 2 cm2 under the electrolysis of a
controlled potential), a platinum film with a large area, and a
SCE served as the working, counter, and reference electrode,
•
C3H5 ads and produce propene. Under the circum-
stances, the coupled reaction (14) will keep up with the
hydration reaction (17), i.e., there is the possibility of two
pathways to produce propene.
Mech a n ism of Electr or ed u ction of AA. The elec-
troredection of AA to produce propene at the platinized
platinum electrode in acidic aqueous solution is a complex

Electroreduction of C3H5OH at Platinized Pt Electrode
J . Org. Chem., Vol. 66, No. 13, 2001 4493
respectivly. Cleaning and activation treatments of the working
electrode at -0.60 V were carried out successively prior to
recording data. Of all the electrochemical measurement the
background currents without AA were subtracted.
perchloric acid contained, and finally, the extracted solutions
served as an MS analysis to detect the allyl anion C3H5
-
formed during the electroreduction of AA by comparing the
two MS spectral lines (Figure 4). To increase the sensitivity
of detecting C3H5-, a direct sampling was adopted. The
vaporization of the sample solutions was performed by the
programmed temperature rise from 20 to 100 °C at the rate
of 80° min-1. The spectra were recorded at the vaporization
temperature ca. 60 °C (ca. 0.5 min).
The cathodic and anodic transfer coefficients Rc and Ra of
the electroreduction of AA were found separately from the
correlations20 between peak potentials and the square roots
of sweep rates and/or peak potentials and the logarithms of
peak currents obtained in Figure 1. The reaction orders ZAA
+
of AA and ZH of the ion H+ in the electroreduction reaction
For the purpose of improving the detectable sensitivities for
•
-
the reaction intermediates C3H5+, C3H5 , and C3H5 entering
into the bulk solutions from the Helmholtz layer and/or the
electrode surface, some measures were applied, e.g., the
electrolysis at -0.40 V was performed, which is slightly more
negative than the potential of the electroreduction of AA -0.35
V, a given volume of the solvent ethyl alcohol was added to
the test solution, and the ratio of the apparent area of the
working electrode to the volume of the catholyte was enlarged.
Qu a n tu m -Ch em ica l Ca lcu la tion . The charge distribution
and frontier orbital energies of the reaction species C3H5OH
of AA were found separately from the correlations between
the logarithms of cathodic peak currents of AA and the
logarithms of the concentrations of AA and H+ in the studied
solutions, where the cathodic peak currents were measured
from the CV plots (plots omitted) at the sweep rate 4.0 × 10-2
V s-1 in the studied solutions separately containing 0-6 × 10-3
mol L-1 AA and 5.0 × 10-3 mol L-1 perchloric acid and
containing 5.0 × 10-3 mol L-1 AA and 2 × 10-4 to2 × 10-2 mol
L-1 perchloric acid. To find the apparent activation energy ∆Ea
of the electroreduction of AA, the electrolysis of AA in the
studied solution containing 1.0 × 10-2 mol L-1 AA, 0.1 mol
L-1 sodium perchlorate, and 5.0 × 10-3 mol L-1 perchloric acid
was carried out at -0.40 V and the reduction currents at
15-60 °C were determined; then ∆Ea was calculated according
to the Arrhenius formula.
+
and C3H5 were calculated by means of a semiempirical MO
method (MOPAC7/AM1, PM3).
Rea gen ts. AA was purified by distillation, the reagents
NaClO4, NaN3 and C2H5OH, purchased in China, were of
analytical quality, the solvent C2H5OH used in ESR spectrum
was pretreated by electrolysis, and DMPO was an Aldrich
product.
Sp ectr u m Deter m in a tion . IR Sp ectr a . A drop of various
AA test solutions containing sodium azide, either unelectro-
lyzed or electrolyzed, was dropped onto a compressed KBr disk,
+
and IR spectra were recorded to detect the allyl cation C3H5
Ack n ow led gm en t. We thank professor Yan Dayu
(Graduate School of the University of Science and
Technology of China) for the quantum-chemical calcula-
tions. This work was supported by State Key Laboratory
for Structural Chemistry of Unstable and Stable Species
(Institute of Chemistry, The Chinese Academy of Sci-
ence) and a grant from the National Natural Science
Foundation (Grant 29773012).
in the test solutions (Figure 2).
ESR Sp ectr u m . The AA test solution in the presence of
DMPO was electrolyzed under -0.40 V (ex-situ method). After
10 min, the electrolyzed test solution was sucked into a quartz
sampling tube, and immediately the tube was inserted into a
microwave cavity and the ESR spectrum was recorded (Figure
3) to detect the allyl radical C3H5• formed during the electrore-
duction of AA.
MS Sp ectr a . First, two kinds of AA test solutions, agitated
separately with nitrogen and carbon dioxide, were electrolyzed
at -0.40 V until only the background currents appeared.
Second, the electrolyzed test solutions were drawn from the
electrolytic cell and extracted with ethyl ether to eliminate the
Su p p or tin g In for m a tion Ava ila ble: The CV plots of the
electroreduction of AA for different concentrations, that for
different pH values and that at the mercury electrode and in
nearly neutral aqueous solution, and the charge and spin
densities of C3H5OH•-. This material is available free of charge
via the Internet at http://pubs.acs.org.
(20) Electrochemical Methods, Fundamentals and Applications;
Bard, A. J ., Faulkner, L. R., Eds.; J ohn Wiley & Son. Inc.: New York,
1980; Chapter 6.
J O0012288