The electrochemical reduction of α-nitrocumene in a protic and basic medium on large surface area (porous) electrodes: Electronation-protonation or electrocatalytic hydrogenation?

Article abstract of DOI:10.1139/v99-035

The electrochemical reduction of α-nitrocumene (1) has been investigated under controlled potential, at a mercury pool cathode and at Raney metal (Raney nickel, Raney cobalt and Devarda copper) and fractal nickel electrodes in basic aqueous ethanol. A comparison of the product distribution from the reduction at Hg (electronation-protonation (EP) mechanism) and that from the reduction at Raney metals and fractal nickel has shown that both the electrocatalytic hydrogenation (ECH) and EP mechanisms can be involved at large surface area (porous) transition metal electrodes in a basic protic medium. Cyclic voltammetry was used to determine the reduction potential of 1 at Hg and at bright polycrystalline nickel, cobalt, and copper cathodes in the same medium.

Full text of DOI:10.1139/v99-035

6
87
The electrochemical reduction of ␣-nitrocumene
in a protic and basic medium on large surface
area (porous) electrodes: electronation–
protonation or electrocatalytic hydrogenation?
Elisa Soazara Chan-Shing, Denys Boucher, and Jean Lessard
Abstract: The electrochemical reduction of α-nitrocumene (1) has been investigated under controlled potential, at a
mercury pool cathode and at Raney metal (Raney nickel, Raney cobalt and Devarda copper) and fractal nickel
electrodes in basic aqueous ethanol. A comparison of the product distribution from the reduction at Hg (electronation–
protonation (EP) mechanism) and that from the reduction at Raney metals and fractal nickel has shown that both the
electrocatalytic hydrogenation (ECH) and EP mechanisms can be involved at large surface area (porous) transition
metal electrodes in a basic protic medium. Cyclic voltammetry was used to determine the reduction potential of 1 at
Hg and at bright polycrystalline nickel, cobalt, and copper cathodes in the same medium.
Key words: electrocatalytic hydrogenation, electroreduction, 2-nitro-2-phenylpropane, Raney metal cathodes, mecha-
nism.
Résumé : La réduction électrochimique, à potentiel controlé, de l’α-nitrocumène (1) a été étudiée, en milieu
hydroéthanolique basique, sur Hg, sur des électrodes de métaux de Raney (nickel de Raney, cobalt de Raney et cuivre
de Devarda) et sur des électrodes de nickel fractal. Une comparaison de la distribution de produits obtenue par
réduction sur Hg (mécanisme d’électronation–protonation (EP)) avec celles obtenues par réduction sur les électrodes de
métaux de Raney et sur l’électrode de nickel fractal démontre que les deux mécanismes, celui d’électronation–
protonation (EP) et celui d’hydrogénation électrocatalytique (HEC), interviennent sur les électrodes de métaux de
transition à grande surface spécifique (poreuses) en milieu basique protique. La voltamétrie cyclique a été utilisée pour
déterminer le potentiel de réduction de 1, dans le même milieu, sur des électrodes de Hg et des électrodes de nickel,
cobalt et cuivre polycristallins.
Mots clés : hydrogénation électrocatalytique, électroréduction, 2-nitro-2-phénylpropane, cathodes de métaux de Raney,
mécanisme. Chan-Shing et al. 694
Introduction
but are reducible in neutral protic media and the reduction
stops at the hydroxylamine (5, 6). In contrast, the electro-
chemical reduction of nitro compounds at Raney metal elec-
trodes (Raney nickel (R-Ni), Raney cobalt (R-Co), and
Devarda copper (D-Cu)), in basic and neutral aqueous meth-
anolic solutions, gives the corresponding amine in high yields
and current efficiencies, both for nitroaryl (7–9) and nitro-
alkyl (10) compounds. Therefore, in basic and neutral protic
media, the electrochemical reduction, at Raney metal elec-
trodes, of arylhydroxylamines and alkylhydroxylamines to
the corresponding amines must involve an electrocatalytic
hydrogenation (ECH) mechanism, that is, reaction with
chemisorbed hydrogen, M(H) (reactions [2]–[4]), generated
by reduction of water (reaction [1]) (7–10).
Unprotonated hydroxylamines are not reducible by elec-
tron transfer (1). As a consequence, the electrochemical re-
duction of aromatic nitro compouds (2–4) and tertiary
nitroalkanes (5, 6) at a mercury electrode in neutral and ba-
sic (pH > 5) protic media, which proceeds by successive
electron and proton transfer steps (electronation–protonation
(
EP) mechanism), stops at the hydroxylamine. At a mercury
electrode, primary and secondary nitroalkanes are not reduc-
ible at pH > 13 because they exist as nitronate anions which
are not reducible by electron transfer (EP mechanism) (1)
Received November 3, 1998.
+
–
This paper is dedicated to Jerry Kresge in recognition of his
many achievements in chemistry.
[1]
2 H O(H O ) + 2e + M
2 3
Volmer
   2M(H) + 2OH (H O)
–
1
2
E.S. Chan-Shing, D. Boucher, and J. Lessard. Centre de
Recherche en Électrochimie et Électrocatalyse, Département
de Chimie, Université de Sherbrooke, Sherbrooke,
PQ J1K 2R1, Canada.
[2]
[3]
[4]
RNHOH + M  M(RNHOH)
M(RNHOH) + 2M(H)  M(RNH ) + H O
1Author to whom correspondence may be addressed.
Telephone: (819) 821–7091. Fax: (819) 821–8017.
e-mail: jlessard@courrier.usherb.ca
2
2
M(RNH )  RNH + M
2
2
Can. J. Chem. 77: 687–694 (1999)
© 1999 NRC Canada

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88
Can. J. Chem. Vol. 77, 1999
Scheme 1.
EP MECHANISM
RNO2
ECH MECHANISM
-
-
n M(H) + n OH
n H O + n e + M
2
M
M(RNO2)
-
e
2M(H)
·
RNO2
+
-
+
H , e , H
M
M
M
RN(OH)2
M[RN(OH)2]
-
H O
-H2O
2
2
M(H)
RNO
M(RNO)
(- H O)
2
-
+
2
e , 2H
2M(H)
M(RNHOH)
RNHOH
Not reducible by e.t.
2M(H)
(
-H O)
2
M(RNH2)
RNH + M
2
Since the reduction potential of water to chemisorbed
hydrogen M(H) is very close to that of nitro groups, their
area (porous electrodes) and in basic hydroorganic solutions,
both the EP and ECH mechanisms can participate and com-
pete.
reduction to the corresponding hydroxylamines (RNO 
2
RNHOH), at Raney metal electrodes and in neutral and ba-
sic media, could involve an ECH or an EP mechanism or
both (see Scheme 1) (7–10). The electrochemical reduction
of primary and secondary nitroalkanes at Raney metal elec-
trodes and pH > 13 must involve the reaction of the nitro-
nate anion with chemisorbed hydrogen (ECH mechanism)
Experimental
1H and 13C Nuclear Magnetic Resonance (NMR) spectra
were obtained on a Brucker AC-300 MHz instrument. IR
spectra were taken with a Perkin Elmer 1600 series FTIR.
Mass spectra and exact masses were obtained with a ZAB-
2F model VG instrument.
(
Scheme 2) (10). Interestingly, at Raney metal electrodes, if
dehydration of the intermediate dihydroxylamine is slower
than its reaction with chemisorbed hydrogen, the hydroxyl-
amine could be formed directly by electrocatalytic hydro-
genolysis of the dihydroxylamine which could be formed by
an EP or an ECH mechanism (see Schemes 1 and 2).
α-Nitrocumene (1) (Scheme 3) was prepared as described
by Kornblum et al. (12) and its H NMR
1
spectrum
was iden-
–
1
tical with that reported. IR (CHCl ) cm : 1543 and 1349
3
1
3
(
NO2); C NMR (CDCl ) δ: 140.5 (C3), 129.0 (C4, C5),
3
One way to distinguish between an EP and an ECH mech-
anism for the electrochemical reduction of nitro compounds
to hydroxylamines at Raney metal electrodes would be to
use a molecule, the reduction of which would give a differ-
ent product distribution depending on the reduction mecha-
nism. Coté et al. (11) reported that 2-iodonitrobenzene was
not effective to distinguish between the EP and ECH mecha-
nisms, since the product distribution was similar for its elec-
trochemical reduction at mercury (EP mechanism), for its
catalytic hydrogenation at Raney metals (involving reaction
with chemisorbed hydrogen as in ECH) and for its electro-
chemical reduction at Raney metal electrodes.
125.2 (C6), 90.0 (C2), 27.5 (C1); HRMS, m/z: 119.0863;
+
calcd. for C H (M – NO ): 119.0861.
9
11
2
α-Aminocumene (3) was prepared by preparative electrol-
ysis of 1 at a Raney nickel (R-Ni) electrode in EtOH/H O
2
40:60 w/w + KOH 0.15 M or preparative electrolysis of
α-azidocumene at a R-Ni electrode in MeOH/H O 93:7 w/w
2
–
1
1
+ NaCl 0.2 M. IR (CHCl ) cm : 3446 and 3621 (NH ); H
3
2
NMR (CDCl ) δ: 7.4 (m, 4H, arom), 1.5 (s, 6H, CH ), 1.25
3
3
(s, 2H, NH ) (lit. (12): 7.1–7.2 (m, 5H), 1.41 (s, 8H));
2
+
HRMS, m/z: 136.1129; calcd. for C H N (MH) : 136.1126.
9
14
Bicumyl (6) was prepared by preparative electrolysis at a
mercury pool in EtOH/H O 40:60 w/w + KOH 0.15 M: mp
2
1
1
(
17–118°C, white powder (lit. (13): 115–117°C); H NMR
CDCl ) δ: 7.1 (m, 10H, arom), 1.3 (s, 12H, CH ); C NMR
In this paper, we report that, in the electrochemical reduc-
tion of α-nitrocumene (1), at electrodes with a large surface
1
3
3
3
©
1999 NRC Canada

Chan-Shing et al.
689
Scheme 2.
-
O
O
+
N
-
-
+
-
CH
O
+
+
OH
C = N O
+ H O
2
Not reducible by e.t.
-
-
O
O
+
-
C = N
O
2 M(H)
CH
N
OH
-
- OH )
(
2
(
M(H)
-
-OH )
CH N = O
-
-
CH N = O
+
OH
C = N
O
+ H O
2
Not reducible by e.t.
-
-
C = N O
+
+
2 M(H)
CH NH
O
Not reducible by e.t.
-
-
CH NH
O
2 M(H)
CH NH2
+ OH
(
4
CDCl ) δ: 146.4 (C3), 129.0 (C5), 127.0 (C6), 125.6 (C4),
3.4 (C2), 30.0 (C1); HRMS, m/z: 238.1709; calcd. for
by side were used. The same reference electrode as above
was used and positioned near the surface of the mercury
pool or, for the Raney metal electrodes, near the surface of
the electrode facing the counter electrode.
The R-Ni alloy (Ni-Al 50:50 w/w) powder was purchased
from Aldrich, the R-Co alloy (Co-Al 31:69 w/w) powder
from Alfa AESAR, the Devarda copper (D-Cu) alloy
(Cu-Al-Zn 50:45:5 w/w) powder from BDH, and Co and Cu
powders were purchased from Fisher. The Raney alloy pre-
cursory electrodes were prepared by pressing, at room tem-
perature, 5 g of R-Ni alloy powder (R-Ni electrodes) or 5 g
of a mixture of Raney metal alloy powder (4 g) and particles
3
C H : 238.1721.
1
8
22
Distilled ethanol and distilled water were used to prepare
solutions of KOH 0.15 M in ethanol–water 40:60 w/w. Solu-
tions for all voltammetric analyses and preparative electroly-
ses were deoxygenated by bubbling through dry and oxygen-
free argon saturated with ethanol. An argon atmosphere was
maintained throughout the experiments which were carried
out at room temperature.
The cyclic voltammetry experiments were performed us-
ing a Princeton Applied Research (PAR) 173 potentiostat
and a single-compartment cell with three electrodes contain-
(
1 g) of the corresponding transition metal (Co or Cu for R-
–
4
Co and D-Cu electrodes, respectively) at a pressure of 14.5
×
ing 5 mL of a 5 × 10 M solution of α-nitrocumene (1). A
carbon rod was used as counter-electrode. An Ag/AgCl/Cl
7
–2
–
10 kg m . The pressed electrodes were activated by
leaching in 10% aqueous NaOH for 2 h at 50°C. If not used
immediately, the electrode was kept in a 0.1 M aqueous so-
lution of NaOH. The fractal nickel powder was purchased
from INCO (filamentary nickel powder type 255), and the
electrodes were prepared by pressing 5 g of powder as
above.
The controlled-potential electrolyses at R-M electrodes were
run at the so-called zero-current potential. The procedure
was as follows. After leaching, the electrode was washed with
electrode whose potential was adjusted to that of an SCE
served as reference. The working electrode was a hanging
mercury drop or a metal disc (polycrystalline Ni, Co, or Cu)
2
having a geometric area of 3.14 mm . The solid electrodes
were polished with 0.05 mm micropolish alumina (Buehler)
before each use. The controlled-potential preparative elec-
trolyses were carried out using an Electrosynthesis Company
(
ESC) 410 potentiostat with an ESC 640 coulometer and an
amperometer connected in series within the auxiliary circuit.
For the electrolyses at a mercury pool, a vertical glass cell
with two concentric compartments (cathodic compartment:
distilled water and rinsed with EtOH/H O 40:60 w/w and
2
placed in the cathodic compartment containing the deox-
ygenated electrolyte. A steady current of 100 mA was main-
tained until a charge of 100 C was passed (hydrogen
evolution). Then the potential was increased slowly until the
current reached zero (zero-current potential: –1.1 V for R-Ni
and for R-Co electrodes and –1.2 V for D-Cu electrodes).
The substrate (33 mg, 0.2 mmol, 5 mM solution) was added,
4
0 mL; anodic compartment: 10 mL) and a platinum grid as
counter electrode were used. For the electrolyses at a Raney
metal (R-M) electrode, a two-compartment glass H-cell
(
40 mL for each compartment) with a Nafion-324 (E-I du
Pont de Nemours and Co.) membrane as separator and a
counter electrode consisting of two carbon rods placed side
©
1999 NRC Canada

6
90
Can. J. Chem. Vol. 77, 1999
Scheme 3.
·
-
NO2
NO
4
2
-
-
-
+
+
e
- NO2
e , H
EP
v
e
5
7
1
4
M(H)
ECH
coupling
(-H O)
2
v
H
NHOH
1/2
6
2
2
M(H)
(
-H O)
2
NH2
3
the current increased suddenly and then decreased steadily
and slowly until reaching a near-zero value when the sub-
strate was almost completely consumed. Aliquots of 0.3 mL
were taken and analyzed by VPC using a DB-5 column
ated solution. The substrate was added under a hydrogen
atmosphere which was maintained throughout the reaction.
Results and discussion
(
30 m, 0.25 µ) with a Hewlett Packard (HP) 5790 chroma-
tograph equipped with an FID detector and an HP 3390A in-
tegrator. The products were identified by comparing their
retention times with those of authentic samples. When the
electrolysis was completed, the electrode was rinsed (EtOH
then ether) and the products were extracted from the catholyte
and the mass balance and yield of products were determined
by VPC using the internal standard method.
Cyclic voltammetry was used to determine the reduction
potential of α-nitrocumene (1) in basic (KOH 0.15 M) aque-
ous ethanol (ethanol–water 40:60 w/w). At a hanging mer-
cury drop electrode (HMDE), two reduction peaks were
observed on the cathodic sweep, the first peak at –1.13 V vs.
SCE, corresponding to the one-electron reduction to the rad-
ical anion 4, and the second cathodic peak at –1.45 V, to the
reduction of 4 to the hydroxylamine 2 (see Scheme 3 for the
structures of 4 and 2). On the reverse scan: (i) no anodic
peak corresponding to the oxidation of the radical anion 4
appeared even when the potential was inverted immediately
after the first cathodic peak and the sweep rate was in-
creased from 100 to 500 mV s , thus showing that the
cleavage and (or) the protonation of the radical anion were
faster than its oxidation to 1 on the voltammetric time scale
as observed before in aprotic medium (13); and (ii) there
was one oxidation peak at –0.46 V corresponding to the oxi-
dation of hydroxylamine 2 to nitrosocumene (not shown in
Scheme 3), the reduction of which gave a cathodic peak at
–0.60 V on the second cathodic scan. At a polycrystalline
bright Ni electrode, no cathodic peak was observed because
Catalytic hydrogenations on nonpolarized electrodes were
performed in the same cell as that used above. The electrode
(
catalyst) was placed in the cathodic compartment contain-
ing the deoxygenated solution. Its potential was lowered in
order to reach a steady current of 100 mA and was kept at
this value until 100 C had passed. Then, the circuit was
opened and the electrode put to ground in order to depolar-
ize it. The counter and reference electrodes were removed.
The organic substrate was added, and aliquots of 0.3 mL
were taken for analysis as above. Catalytic hydrogenations
were also performed with the Raney metal powder obtained
by leaching the commercial alloy powder. After leaching,
the Raney metal particles were washed with water and trans-
ferred into a round-bottomed flask containing the deoxygen-
–
1
©
1999 NRC Canada

Chan-Shing et al.
691
Table 1. Preparative electroreduction of α-nitrocumene 1 (5 mM) at Hg and room temperature in basic ethanol–water (40:60 w/w)
solutions.
Yield of products (%)c
pHa
13
E (V)b
Conversion (%)
Bicumyl 6
Cumene 7
Current efficiency (%)
>
–1.13
–1.15
100
98
85
75
2
2
71
56
1
0
a
pH >13, KOH 0.15 M; pH = 10: Britton–Robinson buffer.
b
c
Reference electrode: Ag/AgCl/Cl¯ with E = ESCE; counter electrode: platinum.
Determined by gas chromatography using an internal standard. The other products were α-methylstyrene and α-hydroxycumene (see text). The material
balance was 94% at pH >13 and 96% at pH 10.
Table 2. Preparative electrolysis of α-nitrocumene 1 (5 mM) at porous metal electrodes and room temperature in basic (KOH 0.15 M,
pH > 13) ethanol–water (40:60 w/w) solutions.
Yield of products (%)c
Entry
Cathodea
R-Ni
Potential (V)b
Conversion (%)
Amine 3
Dimer 6
Current efficiency (%)d
1
2
3
4
a
–1.1e
–1.2
–1.1e
–1.2e
100
97
100
100
97
75
66–73
19
2
20
24 –31
76
72
19
26–58
9
R-Co
D-Cu
See Experimental for electrode preparation. The electrode was charged with chemisorbed hydrogen by applying a current of 100 mA until a charge of
00 C was passed.
1
b
Reference electrode: Ag/AgCl/Cl¯ with E = ESCE; counter electrode: vitreous carbon.
c
Determined by gas chromatography using an internal standard. Average value from two to three experiments. The range is indicated for variations
larger than ±2%.
d
Computed current efficiencies do not take into account the initial charge 100 C used for charging the electrode.
e
Zero-current potential (see Experimental).
the reduction of nitrocumene (1) was hidden by the reduc-
tion of water (hydrogen evolution) at ca. –1.1 V. At poly-
crystalline bright Co and Cu electrodes, only one cathodic
peak appeared in the cathodic sweep at –1.3 and –1.15 V,
respectively. No anodic peak was seen on the reverse scan at
any of the polycrystalline metal electrodes. Thus, the reduc-
tion of nitrocumene (1) occurs at similar potentials at mer-
cury and at polycrystalline metal electrodes, and these
reduction potentials are also similar to the so-called zero-
current potential (potential at which no current flows at an
electrode charged with chemisorbed hydrogen, see Experi-
mental) used for the electrolyses at Raney metal and fractal
nickel electrodes (–1.07 to –1.2 V vs. SCE). Thus, electron
transfer to (electronation of) α-nitrocumene (1) generating
the radical anion 4 could occur at Raney metal and fractal Ni
electrodes at the zero-current potential in basic medium, and
it does occur, as will be seen below.
radical anion 4 at pH 10 competing with its cleavage. The
resulting neutral radical (not shown in Scheme 3) was then
reduced rapidly to the hydroxylamine 2 which is unstable
2
and decomposed to α-methylstyrene and cumyl alcohol (14).
The results of Table 3 clearly show that reduction of α-
nitrocumene (1) by electron transfer in basic medium gives
bicumyl (6) as the main product. The current efficiencies
were lower than 100% and were lower at pH 10 than at
pH > 13. This is most probably due to the reduction of water
competing with the reduction of 1.
The results of the controlled-potential reduction of α-nitro-
cumene (1) at Raney metal electrodes in basic (pH > 13)
aqueous ethanol are summarized in Table 2. As shown in en-
try 1, at a R-Ni electrode and at the zero-current potential
(–1.1 V), which corresponds to the potential of formation of
radical anion 4 at a Hg cathode as already pointed out,
α-aminocumene (3) was formed in a 97% yield (Table 2, en-
try 1). Only 2% of bicumyl (6) was formed. This shows that,
under the conditions used, both the reduction of water to
chemisorbed hydrogen (M(H)) and the reaction of α-nitro-
cumene (1) with M(H) were about 50 times faster than the
electronation of 1 (see Scheme 1). Therefore, at the zero-
current potential and at pH > 13, 98% of the electrochemical
reduction of α-nitrocumene (1) at a R-Ni electrode proceeds
via an ECH mechanism.
The results of bulk electroreduction of α-nitrocumene (1)
at a mercury pool and at ca. –1.15 V vs. SCE in basic (pH >
1
3 and pH 10) ethanol–water (40:60 w/w) solutions are re-
corded in Table 1. The main product was bicumyl (dimer 6),
resulting from the cleavage of radical anion 4 to cumyl radi-
cal (5) and nitrite anion followed by coupling of two cumyl
radicals (5) (Scheme 3). Cumene (7), the product of reduc-
tion of radical 5, was formed in small amounts (-2%). The
other products not shown in Scheme 3 were α-methylstyrene
At a potential more negative than the zero-current poten-
tial (–1.2 V, see Table 2, entry 2), the amount of dimer (6)
increased to 20%, the amount of amine having decreased ac-
cordingly to 75%. This can be readily explained by an in-
(
7% at pH > 13, 1% at pH 10) and cumyl alcohol (<1% at
pH > 13, 19% at pH 10). The yield of dimer 6 was higher at
pH > 13 than at pH 10. This is due to a faster protonation of
2
Details of a study, as a function of pH, of the electroreduction of α-nitrocumene, p-cyano-α-nitrocumene, and p-methoxy-α-nitrocumene at
mercury and in aqueous ethanol will be reported in a subsequent paper.
©
1999 NRC Canada

6
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Can. J. Chem. Vol. 77, 1999
Table 3. Results of catalytic hydrogenation of α-nitrocumene 1 (5 mM) at Raney Ni, Raney Co, and Devarda Cu cathodes at room
temperature in basic (KOH 0.15 M, pH > 13) ethanol–water (40:60 w/w) solutions.
Yield of products (%)a
Entry
Catalyst
R-Ni
Conditions
Conversion (%)
Amine 3
Dimer 6
Time (h)b
c,d
1
2
3
4
5
6
7
8
9
Electrode
Electrode
100
100
100
100
100
100
0
100
100
0
91–86
88
91
98
100
79
7–4
5
4
2
0
7
0
31–20
10
0
20
24
19
16
13
22
24
23
20
24
e
Electrodef
Powder/H2g
h
Powder/H2
c,d
R-Co
D-Cu
Electrode
h
Powder/H2
0
c,d
Electrode
57–54
63
0
c,i
Electrode/H2
Powder/H2h
1
0
a
See footnote c of Table 2.
b
c
Reaction time.
Electrode: the electrode was prepared as for the ECH experiments except that it was connected to the ground after being polarized at a current of 100
mA for 100 C.
d
Electrode activated by leaching in 10% NaOH at 50°C.
e
Electrode activated by leaching in 10% NaOH at 50°C for 1 h but not polarized before adding the substrate.
f
Same conditions as in footnote c, except the time of leaching was 2 h.
g
Powder activated by leaching in 10% NaOH at 50°C for 1 h.
h
Powder activated by leaching in 30% NaOH at 70°C for 8 h.
i
Same conditions as in footnote d but a hydrogen atmosphere was maintained in the cathodic compartment instead of an argon atmosphere.
a
Table 4. Preparative electrolysis of α-nitrocumene 1 (5 mM) at a fractal nickel electrode and room temperature in basic (KOH 0.15
M, pH > 13) ethanol–water (40:60 w/w) solutions.
Yield of products (%)c
Entry
Potential (V)b
Conversion (%)
Amine 3
Dimer 6
Current efficiency (%)
1
2
a
–1.07d
–1.2
96
97
53
15
–2
38
56
26
5
4
Fractal nickel from INCO Type 255 (5 g of powder pressed at p = 14.5 × 10 kg m ) washed and kept in ethanol–water (40/60 w/w) before use.
See footnote b of Table 2.
b
c
See footnote c of Table 2.
d
Zero-current potential (see Experimental).
crease of the rate of electronation of α-nitrocumene (1), ve
with M(H) was now slower than its electronation: (v /v )
H
e D-Cu
(
Scheme 3), due to an increase of k , as the potential be-
≈ 0.25. It is noteworthy that, as the ECH process becomes
e
comes more negative, the rate of reaction of 1 with M(H), vH
less efficient with respect to the EP process (v /v becomes
H
e
(
Scheme 3), increasing less rapidly.
At a R-Co electrode (Table 2, entry 3), the electrochemical
smaller), the desorption of M(H) to give molecular hydrogen
reaction [5] and (or) reaction [6]), which competes with its
reaction with the adsorbed organic substrate, becomes more
important as shown by the decrease in current efficiency
from 72% with a R-Ni electrode to about 10% with a D-Cu
electrode. The electrochemical desorption of M(H) (reaction
(
reduction of α-nitrocumene (1) at the zero-current potential,
under the same conditions as those used for reduction at R-
Ni, gave a much larger proportion of dimer 6 (24–31%) than
the reduction at R-Ni (2%, entry 1) and less amine 3 accord-
ingly (73–66%). Thus, on a R-Co electrode, the reaction of
α-nitrocumene (1) with M(H) (ECH mechanism) competes
less effectively with its electronation than on a R-Ni elec-
[
(
5]) competes also with the electronation of α-nitrocumene
1). So, the situation is quite complex.
Heyrovsky
–
–
[
[
5]
M(H) + H O + e    H + M + OH
2 2
trode: (v /v )
≈ 48.5 >> (v /v ) ≈ 2.5 (Scheme 3).
H
e R-Ni
H
e R-Co
This is in agreement with the fact that R-Co is less efficient
than R-Ni as catalyst for catalytic hydrogenation (CH)
(
Tafel
M(H) + M(H)    H + M
6]
2
15–17). Copper catalysts are still less efficient than cobalt
←
− − − −
catalysts for CH (17), and as result, the electrochemical re-
duction of α-nitrocumene (1) at a D-Cu electrode, again at
the zero-current potential and under the same solvent and
pH conditions, gave the dimer 6 as the major product
The hydrogenation steps in the ECH mechanism shown in
Scheme 1 are exactly the same as in the classical catalytic
hydrogenation (CH) (reaction of the adsorbed organic sub-
strate with M(H) followed by desorption of the product(s)
(7–9)). The main differences between ECH and CH are two-
(
75–77%) and the amine 3 as the minor product (15–20%).
Thus, at a D-Cu electrode, the reaction of α-nitrocumene (1)
©
1999 NRC Canada

Chan-Shing et al.
693
fold: (i) M(H) is generated in situ by reduction of water in
the former (reaction [1]) and by thermal dissociation of
molecular hydrogen in the latter (reverse of reaction [6]);
(6) was formed on the polarized catalysts (24–31 and 76%,
respectively; see Table 2, entries 3 and 4) than on the
nonpolarized catalyst (7 and 20–31%, respectively; see Ta-
ble 3, entries 6 and 8), shows that direct electron transfer
from a negatively polarized electrode was faster as would be
expected. It is noteworthy that, when there was no residual
aluminum present in the R-Co and D-Cu catalysts, not only
electronation of nitrocumene (1) did not occur but there was
no CH either (see Table 3, entries 7 and 10). This was not
the case with R-Ni free of residual aluminum (Table 3, entry
5). This suggests that in the the case of R-Co and D-Cu, the
presence of residual aluminum would favor the CH process
possibly by influencing the adsorption energies of hydrogen
and of the organic substrate. The catalytic activity of the cat-
alyst in CH, that is, the rate of reaction of the adsorbed or-
ganic substrate with M(H), must depend on the energy of
adsorption of hydrogen as well as on the energy of adsorp-
tion of the organic substrate (M-H and M-substrate bond en-
ergies), and the same should hold for the electrocatalytic
activity of large surface area electrodes in ECH of organic
substrates. These energies of adsorption depend on the metal
itself (Ni, Co, or Cu in this work), and for a given metal, for
instance, R-Ni and fractal Ni, on the nature of the adsorption
sites as noted by Bockris and Minevski (20) and Lasia (21).
The nature of the adsorption site depends on (i) the type of
crystallographic structure (the crystal plane, a step, or an
edge) exposed for adsorption; (ii) irregularities in the crys-
tallographic surface structure; (iii) impurities strongly bound
to the surface; (iv) a small amount of a second metal, such
as aluminum, forming alloy (17). For instance, the nature of
the M—H bond may vary from a nearly covalent bond (22,
23), an adsorbed state producing a large increase in the free
electron density in the metal surface (23), to a hydridic bond
(22). So, the knowledge of the various factors and forces in-
volved is as yet insufficient to be able to pinpoint the exact
reasons of the differences in electrocatalytic activity ob-
served in this work.
(
ii) the catalyst is negatively polarized in the former and is
not polarized in the latter. Since the catalyst is not polarized
in CH, the sole product formed by CH of α-nitrocumene (1)
should be the amine 3 assuming that the nonpolarized cata-
lyst cannot transfer an electron to nitrocumene (1). Then,
CH constitutes the best model of an ECH mechanism. We
therefore studied the CH of α-nitrocumene (1) under the
same solvent and pH conditions and on the same catalysts as
those used for the ECH experiments, that is on Raney metal
“
electrodes” which had been initially charged with chemi-
sorbed hydrogen by reduction of water in the absence of α-
nitrocumene (1) but then, after opening the circuit, had been
connected to the ground. These CH experiments were car-
ried out under an argon atmosphere as for the electrolyses.
The product distribution was compared with that obtained
by carrying CHs on a freshly leached Raney catalyst powder
under a H atmosphere at atmospheric pressure and room
2
temperature. The results are summarized in Table 3.
The most striking feature of Table 3 is the formation of
some bicumyl (6) in all the CH experiments with the non-
polarized electrodes as catalysts (entries 1–3, 6, and 8). Thus,
some electronation of α-nitrocumene (1) did occur on the
nonpolarized Raney metal electrodes and, in the case of D-
Cu electrode (entry 8), to a nonnegligible extent (20–31%).
Such electronation is not due to reduction of nitrocumene
(
1) by the transition metal (Ni, Co, or Cu) but to its reduc-
tion by the residual aluminum left after leaching (18). This
was confirmed by the following experiments. (i) α-Nitro-
cumene (1) was quantitatively recovered when treated with
fractal Ni particles or with a Cu metallic powder in the same
basic medium. (ii) No bicumyl (6) was formed when CH of
α-nitrocumene (1) was carried out on a Raney metal powder
which was prepared by extensive leaching of the Raney
alloy (leaching for 8 h at 70°C in 30% aqueous NaOH; see
Table 3, entries 5, 7, and 10). (iii) Bicumyl (6) was formed
only when residual aluminum was still present in the cata-
lyst (according to eq. [7], 0.6 mg of Al would reduce up to
The results presented and discussed so far show that α-
nitrocumene (1) can be used to evaluate the competition be-
tween the ECH and EP processes (the ratio v /v ) in
H
e
3
3 mg of 1), that is, in all experiments performed with the
Scheme 3) for the electrochemical hydrogenation at porous
electrodes. This competition can be used to compare the ac-
tivity of various electrode materials for the ECH process at
zero-current potential. The results of Table 2 show clearly
that the activity of Raney metals decreases in the order R-Ni
(entry 1) > R-Co (entry 3) > D-Cu (entry 4) as already dis-
cussed. The activity of fractal Ni was evaluated also. The re-
sults of entry 1, Table 2, show that fractal Ni (vH/ve ≈ 1.4) is
more active than D-Cu (vH/ve ≈ 0.25) but somewhat less ac-
tive than R-Co (vH/ve ≈ 2.53). Upon decreasing the potential
to –1.2 V (entry 2, Table 2), the rate of electronation (ve) in-
creases, and the ratio of vH/ve decreases accordingly (vH/ve ≈
0.27) as was observed with R-Ni electrodes (compare entry
1 (vH/ve ≈ 48.5) and entry 2 (vH/ve ≈ 3.8) and of Table 4).
Fractal Ni was found to be completely inactive for CH of
α-nitrocumene (1) in basic aqueous ethanol either using
fractal Ni particles under a hydrogen atmosphere or pressed
fractal Ni electrodes initially charged with M(H) and put to
ground. α-Nitrocumene (1) was quantitatively recovered as
in the attempted CH on R-Co and D-Cu catalysts free of re-
sidual aluminum.
–
–
[
7]
Al + 3RNO + 4OH  Al(OH) + 3RNO2
2 4
nonpolarized electrodes (entries 1–3, 6, 8, and 9) leached
under milder conditions (2 h at 50°C in 10% aqueous
NaOH), in order to effect leaching of the surface layers of
the disc only (more vigorous leaching conditions led to com-
plete disintegration of the disc). (iv) Finally, bicumyl (6) and
cumene (7) were formed in 28–39 and 1% yields, respec-
tively, together with 7–3% of amine 3 and 25–28% of prod-
ucts (α- methylstyrene and α-hydroxycumene) resulting from
decomposition of hydroxylamine 2 (15, 16) when α-nitro-
cumene (1) was reduced with metallic aluminum (powder)
always in the same basic medium. So, on a nonpolarized cat-
alyst containing residual aluminum, there is a competition
between reaction of adsorbed α-nitrocumene (1) with chemi-
sorbed hydrogen present on the catalyst (as in Scheme 1)
and its reduction by electron transfer from aluminum ac-
cording to eq. [7] (19). The fact that, with the less active hy-
drogenation catalysts R-Co and D-Cu, much more bicumyl
©
1999 NRC Canada

6
94
Can. J. Chem. Vol. 77, 1999
Conclusions
References
1
. H. Lund. In Organic electrochemistry. Edited by M.M.Baizer
and H. Lund. Marcel Dekker, Inc., New York. 1991. Chap. 9.
. F. Haber. Z. Electrochem. 4, 506 (1898).
In the electrochemical reduction of α-nitrocumene (1) in
basic aqueous ethanol, at solid electrodes with large specific
surface areas (porous electrodes), two mechanisms of electro-
hydrogenation can be operative: namely, an ECH mecha-
nism giving α-aminocumene (3) and an EP mechanism giving
products derived from the cumyl radical (5), itself result-
ing from a rapid elimination of the nitrite anion from the α-
nitrocumyl radical anion (4) (Scheme 3). The relative rates
of the ECH and EP processes depend on the potential im-
posed, the rate of electronation (EP process) increasing upon
decreasing the potential. These relative rates depend also on
the nature of the transition metal (Ni, Co, Cu) and, for a po-
rous metal, on the surface structure of the electrode (R-Ni,
fractal Ni). Thus, α-nitrocumene (1) can be used to evaluate
the electrocatalytic activity of an electrode for the ECH pro-
cess. The following order of electrocatalytic activity was
determinated at the so-called zero-current potential (poten-
tial at which no current flows at an electrode charged with
chemisorbed hydrogen): R-Ni > R-Co > fractal Ni > D-Cu.
It is noteworthy that R-Co, fractal Ni, and D-Cu were
completely inactive for catalytic hydrogenation of α-
nitrocumene (1) at an atmospheric pressure of molecular hy-
drogen. On the other hand, when residual aluminum was
present on the catalyst charged with chemisorbed hydrogen,
both catalytic hydrogenation at atmospheric pressure and re-
duction by metallic aluminum (electron transfer) occurred
on D-Cu in a ratio close to 70/30. This suggests that the
presence of residual aluminum favors the catalytic hydroge-
nation process.
2
3
. G. Pezzatini and R. Guidelli. J. Electroanal. Chem. 102, 205
(
1979).
. A. Cyr, E. Laviron, and J. Lessard. J. Electroanal. Chem. 263,
9 (1989).
4
6
5
6
7
. P.E. Iversen and H. Lund. Acta Chem. Scand. 19, 2303 (1965).
. P.E. Iversen and H. Lund. Tetrahedron Lett. 4027 (1967).
. A. Cyr, P. Huot, G. Belot, and J. Lessard. Electrochim. Acta,
3
5, 147 (1990).
8
. A. Vélin-Prikidanovics and J. Lessard. J. Appl. Electrochem.
20, 527 (1990).
9. J. M. Chapuzet, A. Lasia, and J. Lessard. In Electrocatalysis.
Edited by J. Lipkowski. Wiley–VCH, Inc., Pennington, N.J.
1998. pp. 155–196.
10. P. Delair, A. Cyr, and J. Lessard. In Electroorganic synthesis.
Festchrift for M.M. Baizer. Edited by R.D. Little and N.L.
Weinberg. Marcel Dekker, Inc. New York. 1991. p.129.
1
1. B.J. Côté, D. Després, R. Labrecque, J. Lamothe, J.M. Chapuzet,
and J. Lessard. J. Electroanal. Chem. 355, 219 (1993).
2. N. Kornblum, S.C. Carlson, J. Widmer, M.J. Fifolt, B.N. New-
ton, and R.G. Smith. J. Org. Chem. 43, 1394 (1978).
3. A.K. Hoffmann, W.G. Hodgson, D.L. Maricle, and W.H. Jura.
J. Am. Chem. Soc. 86, 631 (1964).
4. E.S. Chan-Shing. Ph.D. thesis. Université de Sherbrooke,
Sherbrooke. 1999.
5. B.V. Aller. J. Appl. Chem. 8, 163 (1959).
6. B.V. Aller. J. Appl. Chem. 8, 492 (1958).
7. R.L. Augustine. Catalytic hydrogenation. Marcel Dekker, Inc.,
New York. 1965. pp. 23–56.
1
1
1
1
1
1
1
1
8. Y. Choquette, L. Brossard, and H. Ménard. J. Appl. Electro-
chem. 20, 855 (1990).
9. U. Birkenstock, R. Holm, B. Reinfandt, and S. Storp. Cataly-
sis, 93, 55 (1985).
Acknowledgements
We gratefully acknowledge financial support from the
Agence Canadienne de Developpement International (ACDI)
20. J.O’M. Bockris and Z.S. Minevski. Electrochim. Acta, 39, 1471
(
E.S. Chan-Shing), the Natural Sciences and Engineering
(1994).
Research Council of Canada (NSERC), the Fonds pour la
Formation de Chercheurs et l’Aide à la Recherche (FCAR)
du Québec and l’Université de Sherbrooke. We are thankful
to Dr. Jean Marc Chapuzet for helpful discussions and to
Mr. Gaston Boulay for the mass spectra.
21. A. Lasia. Can. J. Chem. 75, 1615 (1997).
22. D.J. Barclay. Electroanal. Chem. Interfacial Electrochem. 44,
47 (1973).
2
3. R.J. Nichols and A. Bewick. J. Electroanal. Chem. 243, 445
1988).
(
©
1999 NRC Canada