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Electrochemical hydrogenation
Electroanalytical Chemistry and Interracial Electrochem&try, 50 (1974) 417 -419
417
© Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
PRELIMINARY
NOTE
Electrochemical hydrogenation
J.M. SAVI~ANTand SU KHAC BINH Laboratoire d'Electrochimie de l'Universitd de Paris VII, 2 place Jussieu, 75221 Paris Cedex 05 [France)
(Received 21st January 1974)
A number of electrochemical reductions of organic compounds imply the exchange o f an equal number o f electrons and protons. They can therefore be formally considered as hydrogenations by atomic or molecular hydrogen. This is true, for example, for electrohydrodimerizations: :X+le-+IH+-+
1/zHX
{
I XH
(X being, e.g., an oxygen atom or an activated methylene group such as = C - C - N , , = C - COOR, = C - CO - R); for electrohydrocyclizations:
+ 2e- + 2H + -+
X
XH
(X having the same meaning as above); for electrohydrogenations of double bonds: "/ = X + 2 e - + 2 H
+ -~
H ]
XH
In a divided cell the cathodic compartment tends thus to become more and more basic as the electrolysis is carried out. This can lead to t h e decomposition of the solvent and/or the supporting electrolyte unless there is continuous addition o f an acid. This phenomenon has been frequently observed using acetonitrile (ACN) or dimethylformamide as solvents (see, e.g., refs. 1 - 4 ) . A means for avoiding these difficulties is to generate the required amount of protons by the anode reaction using an undivided cell. So the acidity level o f the medium would remain that chosen for selectivity requirements in the cathodic reaction. Additional advantages of such a procedure are related to the removal of the diaphragm s : (i) less electrical energy transformed into heat due to the lowering of the cell resistance; (ii) simplification o f the cell construction; (iii) avoidance o f the problems involved in the instability of the diaphragm. The anode reaction must correspond to the cathodic one so that an equal number of electrons and protons is exchanged. On the other hand the oxidation product must not
418
PRELIMINARY NOTE
be more easily reduced than the substrate o f the cathodic reaction. This happens, e.g., if the anodic reaction is simply the oxidation o f ACN with NEt4 C104 as supporting electrolyte. In such conditions an unlimited amount o f electricity flows through the cell corresponding to the oxidation of the solvent and/or the supporting electrolyte initiated by the partial reducVion of the substrate. Lastly the reduction product must not be oxidized at a more positive potential than that where the anodic reaction occurs. The anodic oxidation of molecular hydrogen may be considered as meeting the above requirements provided it occurs in conditions not too far from reversibility. The overall reaction would then be an electrochemical hydrogenation by molecular hydrogen. It is the purpose of this communication to present some preliminary experiments showing the feasibility of such reactions on the laboratory scale. The anode was a classical platinized platinum electrode and the test system was the hydropinacolization of acetophenone in ACN with NEt4C104 as supporting electrolyte. The cell employed is represented schematically in Fig. 1. The hydrogen electrode was a platinized circular platinum grid of 10.1 X 2 = 20.2 cm 2 surface area, with a mesh L I!I
L
..............
--.I
///
5 ....
8--
I
6 L - -
---
~/
3
I I
i
i
I
Fig.1. Electrolysis cell. (1) Mercury pool cathode, (2) platinum wire, (3) reference electrode, (4) hydrogen outlet, (5) mercury, (6) platinized platinum anode, (7) hydrogen inlet, (8) flitted disk, (9) magnetic stirrer. size of about 0.5 mm z . The platinization was carried out in another cell in 240 cm 3 water with 8 g H2 PtC16 and 64 mg (CH3 CO2)2 Pb at constant current 6 . The current intensity was maintained successively at 2 A for 2 min, 1 A for 4 min and 0.5 A for 8 min. The platinized electrode was then rinsed with distilled water and ACN and kept under ACN 24 h before use. In the electrolysis cell pure hydrogen is bubbled through two fritted disks as shown in the figure. Under these conditions the hydrogen oxidation occurs at 0 V vs. SCE as seen in experiments where the anode potential was controlled. At this potential the maximum current available is 1.2 A, i.e., a current density of 57 mA c m - 2 . The working electrode was a mercury pool of 50 cm 2 surface area and the reference electrode was a saturated calomel electrode. The potentiostat used was a Tacussel ASA 100-1C.
PRELIMINARY NOTE
419
Typical results are shown in Table 1. It is seen that the number o f moles o f electrons per mole o f acetophenone has the correct value corresponding to the formation o f the pinacol. However as shown b y v.p.c, analysis (Carbowax 20 M) a few percent of the alcohol C6 H s - C H O H - C H 3 were formed together with the pinacol in each run. This may be attributed to insufficient washing o f the hydrogen electrode after platinization in an acid aqueous medium. On the other hand the solution did not exhibit any tendency to decompose in contrast to what happens when a divided cell is used where the solution becomes progressively yellow, orange and dark brown. This shows again that the system works satisfactorily. It was found, however, that the mechanical and/or chemical de-activation o f the hydrogen electrode occurs easily and this implies frequent re-platinizations. TABLE 1 Acetophenone/ mmol l-~
Electrolysis potential E/V vs. $6~"
Initial current Ii/mA
Final current If/mA
Apparent number moles of electrons per mole
20 21 51
2.2 -2.3 2.0 -2.1 1.95-2.0
100 60 40
10 17 2.5
1.01 1.12 1.06
Defective platinization led to the flow o f more than 1 or even 2 moles o f electrons per mole and to the appearance of new cathodic waves replacing the two waves of acetophenone. Work is now in progress aiming at using higher concentrations and current densities and to devise a less sensitive hydrogen electrode. A cknowledgemen t
Work was supported in part by the CNRS (Equipe de Recherche Associ6, No. 309, Electrochimie Organique). REFERENCES 1 2 3 4 5 6
C.P. Andrieux and J.M. Savdant, J. Electroanal. Chem., 28 (1970) App. 12. L. Nadjo and J.M. Say'ant, J. Electroanal. Chem., 30 (1971) 41. C.P. Andrieux and J.M. Savdant, Bull. Soc. Chim. Fr., (1972) 3281. E. Lamy, L. Nadjo and J.M. Savdant, Z Electroanal. Chem., 42 (1973) 189. F. Beck, J. Appl. Electrochem., 2 (1972) 59. G. Chariot, J. Badoz-Lambling and B. Tremillon, Les rdactions dlectrochimiques, Mdthodes dlectrochimiques d'analyse, Masson, Paris, 1959, p. 147.
417
© Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
PRELIMINARY
NOTE
Electrochemical hydrogenation
J.M. SAVI~ANTand SU KHAC BINH Laboratoire d'Electrochimie de l'Universitd de Paris VII, 2 place Jussieu, 75221 Paris Cedex 05 [France)
(Received 21st January 1974)
A number of electrochemical reductions of organic compounds imply the exchange o f an equal number o f electrons and protons. They can therefore be formally considered as hydrogenations by atomic or molecular hydrogen. This is true, for example, for electrohydrodimerizations: :X+le-+IH+-+
1/zHX
{
I XH
(X being, e.g., an oxygen atom or an activated methylene group such as = C - C - N , , = C - COOR, = C - CO - R); for electrohydrocyclizations:
+ 2e- + 2H + -+
X
XH
(X having the same meaning as above); for electrohydrogenations of double bonds: "/ = X + 2 e - + 2 H
+ -~
H ]
XH
In a divided cell the cathodic compartment tends thus to become more and more basic as the electrolysis is carried out. This can lead to t h e decomposition of the solvent and/or the supporting electrolyte unless there is continuous addition o f an acid. This phenomenon has been frequently observed using acetonitrile (ACN) or dimethylformamide as solvents (see, e.g., refs. 1 - 4 ) . A means for avoiding these difficulties is to generate the required amount of protons by the anode reaction using an undivided cell. So the acidity level o f the medium would remain that chosen for selectivity requirements in the cathodic reaction. Additional advantages of such a procedure are related to the removal of the diaphragm s : (i) less electrical energy transformed into heat due to the lowering of the cell resistance; (ii) simplification o f the cell construction; (iii) avoidance o f the problems involved in the instability of the diaphragm. The anode reaction must correspond to the cathodic one so that an equal number of electrons and protons is exchanged. On the other hand the oxidation product must not
418
PRELIMINARY NOTE
be more easily reduced than the substrate o f the cathodic reaction. This happens, e.g., if the anodic reaction is simply the oxidation o f ACN with NEt4 C104 as supporting electrolyte. In such conditions an unlimited amount o f electricity flows through the cell corresponding to the oxidation of the solvent and/or the supporting electrolyte initiated by the partial reducVion of the substrate. Lastly the reduction product must not be oxidized at a more positive potential than that where the anodic reaction occurs. The anodic oxidation of molecular hydrogen may be considered as meeting the above requirements provided it occurs in conditions not too far from reversibility. The overall reaction would then be an electrochemical hydrogenation by molecular hydrogen. It is the purpose of this communication to present some preliminary experiments showing the feasibility of such reactions on the laboratory scale. The anode was a classical platinized platinum electrode and the test system was the hydropinacolization of acetophenone in ACN with NEt4C104 as supporting electrolyte. The cell employed is represented schematically in Fig. 1. The hydrogen electrode was a platinized circular platinum grid of 10.1 X 2 = 20.2 cm 2 surface area, with a mesh L I!I
L
..............
--.I
///
5 ....
8--
I
6 L - -
---
~/
3
I I
i
i
I
Fig.1. Electrolysis cell. (1) Mercury pool cathode, (2) platinum wire, (3) reference electrode, (4) hydrogen outlet, (5) mercury, (6) platinized platinum anode, (7) hydrogen inlet, (8) flitted disk, (9) magnetic stirrer. size of about 0.5 mm z . The platinization was carried out in another cell in 240 cm 3 water with 8 g H2 PtC16 and 64 mg (CH3 CO2)2 Pb at constant current 6 . The current intensity was maintained successively at 2 A for 2 min, 1 A for 4 min and 0.5 A for 8 min. The platinized electrode was then rinsed with distilled water and ACN and kept under ACN 24 h before use. In the electrolysis cell pure hydrogen is bubbled through two fritted disks as shown in the figure. Under these conditions the hydrogen oxidation occurs at 0 V vs. SCE as seen in experiments where the anode potential was controlled. At this potential the maximum current available is 1.2 A, i.e., a current density of 57 mA c m - 2 . The working electrode was a mercury pool of 50 cm 2 surface area and the reference electrode was a saturated calomel electrode. The potentiostat used was a Tacussel ASA 100-1C.
PRELIMINARY NOTE
419
Typical results are shown in Table 1. It is seen that the number o f moles o f electrons per mole o f acetophenone has the correct value corresponding to the formation o f the pinacol. However as shown b y v.p.c, analysis (Carbowax 20 M) a few percent of the alcohol C6 H s - C H O H - C H 3 were formed together with the pinacol in each run. This may be attributed to insufficient washing o f the hydrogen electrode after platinization in an acid aqueous medium. On the other hand the solution did not exhibit any tendency to decompose in contrast to what happens when a divided cell is used where the solution becomes progressively yellow, orange and dark brown. This shows again that the system works satisfactorily. It was found, however, that the mechanical and/or chemical de-activation o f the hydrogen electrode occurs easily and this implies frequent re-platinizations. TABLE 1 Acetophenone/ mmol l-~
Electrolysis potential E/V vs. $6~"
Initial current Ii/mA
Final current If/mA
Apparent number moles of electrons per mole
20 21 51
2.2 -2.3 2.0 -2.1 1.95-2.0
100 60 40
10 17 2.5
1.01 1.12 1.06
Defective platinization led to the flow o f more than 1 or even 2 moles o f electrons per mole and to the appearance of new cathodic waves replacing the two waves of acetophenone. Work is now in progress aiming at using higher concentrations and current densities and to devise a less sensitive hydrogen electrode. A cknowledgemen t
Work was supported in part by the CNRS (Equipe de Recherche Associ6, No. 309, Electrochimie Organique). REFERENCES 1 2 3 4 5 6
C.P. Andrieux and J.M. Savdant, J. Electroanal. Chem., 28 (1970) App. 12. L. Nadjo and J.M. Say'ant, J. Electroanal. Chem., 30 (1971) 41. C.P. Andrieux and J.M. Savdant, Bull. Soc. Chim. Fr., (1972) 3281. E. Lamy, L. Nadjo and J.M. Savdant, Z Electroanal. Chem., 42 (1973) 189. F. Beck, J. Appl. Electrochem., 2 (1972) 59. G. Chariot, J. Badoz-Lambling and B. Tremillon, Les rdactions dlectrochimiques, Mdthodes dlectrochimiques d'analyse, Masson, Paris, 1959, p. 147.