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The anodic oxidation of absolute methanol and ethanol
ELECTROANALYTICALCHEMISTRYAND INTERFACIALELECTROCHEMISTRY Elsevier Sequoia S.A.; Lausanne - Printed in The Netherlands
SHORT
265
COMMUNICATION
The anodic oxidation of absolute methanol and ethanol
Introduction The simple aliphatic alcohols have been widely used as solvents for electroorganic oxidations 1,2, but apart from a note by Parker 3 on the oxidation of absolute methanol containing sulphuric acid, there seems to be no report on the anodic oxidation of the solvents themselves. A study of the anodic oxidation of methanol and ethanol using as supporting electrolytes sodium alkoxide, sodium perchlorate and tetrabutylammonium tetrafluoroborate (TBAF) was therefore undertaken. Experimental Absolute methanol and ethanol were prepared according to a standard method 4. Sodium perchlorate was prepared from A.R. HC104 and A.R. NaeCO 3. It was recrystallized from water, dried in vacuum at 100°C and stored in a desiccator. TBAF was prepared according to the method proposed by Eberson and Olofson 5. Solutions of sodium alkoxide were made up by dissolving the appropriate amount of sodium metal in the alcohol. The chemicals needed as standards in the analyses were supplied by B.D.H. All electrochemical experiment s were carried out using a Chemical Electronics valve potentiostat and waveform generator. In the three-compartment cell the working and secondary electrodes were stationary platinum electrodes separated by a glass flit. The Ag/10- 2 M Ag + reference electrode was separated from the working electrode by a Luggin capillary and tap. In the preparative runs performed at a constant potential, 100-150 C were passed through a cell of capacity 10 ml. The solutions were analysed by v.p.c. (Pye 104 chromatograph). The qualitative analysis was performed by comparing the retention times and mass spectra with those of authentic specimens (Pye 104 chromatograph-AEI MS12 mass spectrometer). Quantitative analysis was performed on a 2.7 m 20 % polypropylene glycol column at 50-90°C. Formaldehyde was determined gravimetrically as the 2,4-dinitrophenylhydrazone. Results and discussion Current/potential curves were measured for solutions of 0.2 M NaC104 and N a O C H 3 in methanol and 0.2 M NaC104 and NaOCeH5 in ethanol (Fig. 1). These curves were obtained by applying a potential/time square wave profile to the working electrode so that every 3 s the electrode spent 0.3 s at 0.0 V. The current was read at the end of the anodic pulse. The use of this pulse sequence gave more reproducible i/E curves. A similar pulse profile was used for the preparative runs. The i/E curves for 0.2 M TBAF in both methanol and ethanol were found to be the same as in the case of J. Electroanal. Chem.,31 (1971)265-267
266
SHORT COMMUNICATION limA cm-2
_0°/6 ,/
~,~
,oo
olo _/ _~-~o - o _ o ~
t/
¢+
I× /
~ ,~ S
J //
.io
?2"/^ ?
I
~/x
/ /
S
+ 1
/ ¢/ ?
o;s
,.
I
,
v.,Ag,A "
i
,
I;5
,
2,
Fig. 1. Current/potential curves of the followingsolns. : (O) 0.2 M NaOCH3 in CH3OH, (O) 0.2 M NaC10 4 in CH3OH, (x) 0.2 M NaOC2H 5 in C2HsOH, (+) 0.2 M NaC10~ in C2H5OH. NaC104 indicating that it is the solvent which is oxidised in the region before the final current rise and not the base electrolytes 6,7. Preparative scale electrolysis at the beginning of the limiting current region were carried out for each of the electrolyte solutions and the results are shown in Table 1. In the case of the oxidation of NaOC2H5 TABLE
1
PRODUCTS FROM THE ANODIC OXIDATION OF METHANOL AND ETHANOL AT +
1.20 V EXPRESSED
AS CURRENT
YIELDS
Solution/0.2 M
Product
Yield/°,,o
NaC10~/CH3OH TBAF/CH3OH NaOCH3/CH3OH NaC104/C2HsOH TBAF/CzHsOH NaOC2Hs/C2HsOH
1,1-Dimethoxymethane 1,3-Dimethoxymethane Formaldehyde 1,1-Diethoxyethane 1,1-Diethoxyethane Acetaldehyde
58 70 74 86 92
in ethanol the yield of acetaldehyde could not be quantitatively determined, because the product undergoes rapid condensation in the alkaline solution. The formation of the acetals can be accounted for by the following reaction mechanism : +
R C H 2 O H ----}R C H O H + 2 e + H + OCHzR +
RCH2OH
RCHOH
I +
OCH2R RCH20H
> RCHOH --H +
(1)
I
~ RCH -H20
(2)
[
OCH2R The formation of the carbonium ion in step. (1) could involve the primary formation J. Electroanal. Chem.,
31 (1971)265-267
SHORT COMMUNICATION
267
of a radical, R C H O H . T h e f o r m a t i o n of such radicals in s o l u t i o n has been s t u d i e d by D i x o n a n d N o r m a n s. T h e c h e m i c a l r e a c t i o n between an a l d e h y d e a n d an a l c o h o l is acid c a t a l y z e d a n d is believed to occur via the s a m e i n t e r m e d i a t e as p r e d i c t e d in step (1) 9. F o r the s o d i u m a l k o x i d e s o l u t i o n s the e x p e r i m e n t a l results suggest t h a t the m a i n reacting species is the a l k o x i d e ion, R C H 2 0 - : RCH20-
-~ RCHO+2e+H
+
(3)
T h e results of the a n o d i c m e t h o x y l a t i o n of o r g a n i c c o m p o u n d s 2 suggest that r e a c t i o n (3) m a y involve the initial f o r m a t i o n of the a l k o x y radical, (RCH20)'. Finally, it m u s t be p o i n t e d out t h a t the a n o d i c f o r m a t i o n of acetals r e p o r t e d here for e t h a n o l a n d m e t h a n o l is n o t a general r e a c t i o n since a n o d i c o x i d a t i o n of the higher a l i p h a t i c a l c o h o l s leads to a r u p t u r e of the c a r b o n skeleton, i.e. it was found t h a t n - p r o p a n o l gives a b o u t equal a m o u n t s of 1 , 1 - d i p r o x y p r o p a n e a n d 1,1-dipropoxymethane. Chemistry Department, The University, Southampton S 0 9 5 N H (Great Britain)
G. S u n d h o l m *
1 N. L. WEINBERGAND H. R. WEINBERG,Chem. Rev., 68 (1968) 449. 2 C. K. MANNAND K. K. BARNES,Electrochemical Reactions in Nonaqueous Systems, M. Dekker, New York, 1970, p. 163. 3 V. D. PARKER,Chem. Ind. (London), (1968) 1363. 4 A. I. VOGEL,A Textbook of Practical Oroanic Chemistry, Longmans, London, 3rd edn., 1961, p. 167. 5 L. EaERSONAND B. OLOFSON,Acta Chem. Scand., 23 (1969) 2355. 6 A. J. BARD (Ed.), Electroanalytical Chemistry, Vol. 3, M. Dekker, New York, 1969, p. 61. 7 M. FLEISCHMANNAND D. PLETCHER, Tetrahedron Lett., (1968) 6255. 8 W. T. DIXONAND R. O. C. NORMAN,J. Chem. Soe., (1963) 3119. 9 I. L. FINAR,Organic Chemistrv, Vol. 1, Longmans, London, 4th edn., 1964, p. 162. R e c e i v e d 30th D e c e m b e r , 1970 * On leave from the Technical University, Helsinki, Finland, under the Royal Society European Exchange Programme. J. Electroanal. Chem., 31 (i971) 265~67
SHORT
265
COMMUNICATION
The anodic oxidation of absolute methanol and ethanol
Introduction The simple aliphatic alcohols have been widely used as solvents for electroorganic oxidations 1,2, but apart from a note by Parker 3 on the oxidation of absolute methanol containing sulphuric acid, there seems to be no report on the anodic oxidation of the solvents themselves. A study of the anodic oxidation of methanol and ethanol using as supporting electrolytes sodium alkoxide, sodium perchlorate and tetrabutylammonium tetrafluoroborate (TBAF) was therefore undertaken. Experimental Absolute methanol and ethanol were prepared according to a standard method 4. Sodium perchlorate was prepared from A.R. HC104 and A.R. NaeCO 3. It was recrystallized from water, dried in vacuum at 100°C and stored in a desiccator. TBAF was prepared according to the method proposed by Eberson and Olofson 5. Solutions of sodium alkoxide were made up by dissolving the appropriate amount of sodium metal in the alcohol. The chemicals needed as standards in the analyses were supplied by B.D.H. All electrochemical experiment s were carried out using a Chemical Electronics valve potentiostat and waveform generator. In the three-compartment cell the working and secondary electrodes were stationary platinum electrodes separated by a glass flit. The Ag/10- 2 M Ag + reference electrode was separated from the working electrode by a Luggin capillary and tap. In the preparative runs performed at a constant potential, 100-150 C were passed through a cell of capacity 10 ml. The solutions were analysed by v.p.c. (Pye 104 chromatograph). The qualitative analysis was performed by comparing the retention times and mass spectra with those of authentic specimens (Pye 104 chromatograph-AEI MS12 mass spectrometer). Quantitative analysis was performed on a 2.7 m 20 % polypropylene glycol column at 50-90°C. Formaldehyde was determined gravimetrically as the 2,4-dinitrophenylhydrazone. Results and discussion Current/potential curves were measured for solutions of 0.2 M NaC104 and N a O C H 3 in methanol and 0.2 M NaC104 and NaOCeH5 in ethanol (Fig. 1). These curves were obtained by applying a potential/time square wave profile to the working electrode so that every 3 s the electrode spent 0.3 s at 0.0 V. The current was read at the end of the anodic pulse. The use of this pulse sequence gave more reproducible i/E curves. A similar pulse profile was used for the preparative runs. The i/E curves for 0.2 M TBAF in both methanol and ethanol were found to be the same as in the case of J. Electroanal. Chem.,31 (1971)265-267
266
SHORT COMMUNICATION limA cm-2
_0°/6 ,/
~,~
,oo
olo _/ _~-~o - o _ o ~
t/
¢+
I× /
~ ,~ S
J //
.io
?2"/^ ?
I
~/x
/ /
S
+ 1
/ ¢/ ?
o;s
,.
I
,
v.,Ag,A "
i
,
I;5
,
2,
Fig. 1. Current/potential curves of the followingsolns. : (O) 0.2 M NaOCH3 in CH3OH, (O) 0.2 M NaC10 4 in CH3OH, (x) 0.2 M NaOC2H 5 in C2HsOH, (+) 0.2 M NaC10~ in C2H5OH. NaC104 indicating that it is the solvent which is oxidised in the region before the final current rise and not the base electrolytes 6,7. Preparative scale electrolysis at the beginning of the limiting current region were carried out for each of the electrolyte solutions and the results are shown in Table 1. In the case of the oxidation of NaOC2H5 TABLE
1
PRODUCTS FROM THE ANODIC OXIDATION OF METHANOL AND ETHANOL AT +
1.20 V EXPRESSED
AS CURRENT
YIELDS
Solution/0.2 M
Product
Yield/°,,o
NaC10~/CH3OH TBAF/CH3OH NaOCH3/CH3OH NaC104/C2HsOH TBAF/CzHsOH NaOC2Hs/C2HsOH
1,1-Dimethoxymethane 1,3-Dimethoxymethane Formaldehyde 1,1-Diethoxyethane 1,1-Diethoxyethane Acetaldehyde
58 70 74 86 92
in ethanol the yield of acetaldehyde could not be quantitatively determined, because the product undergoes rapid condensation in the alkaline solution. The formation of the acetals can be accounted for by the following reaction mechanism : +
R C H 2 O H ----}R C H O H + 2 e + H + OCHzR +
RCH2OH
RCHOH
I +
OCH2R RCH20H
> RCHOH --H +
(1)
I
~ RCH -H20
(2)
[
OCH2R The formation of the carbonium ion in step. (1) could involve the primary formation J. Electroanal. Chem.,
31 (1971)265-267
SHORT COMMUNICATION
267
of a radical, R C H O H . T h e f o r m a t i o n of such radicals in s o l u t i o n has been s t u d i e d by D i x o n a n d N o r m a n s. T h e c h e m i c a l r e a c t i o n between an a l d e h y d e a n d an a l c o h o l is acid c a t a l y z e d a n d is believed to occur via the s a m e i n t e r m e d i a t e as p r e d i c t e d in step (1) 9. F o r the s o d i u m a l k o x i d e s o l u t i o n s the e x p e r i m e n t a l results suggest t h a t the m a i n reacting species is the a l k o x i d e ion, R C H 2 0 - : RCH20-
-~ RCHO+2e+H
+
(3)
T h e results of the a n o d i c m e t h o x y l a t i o n of o r g a n i c c o m p o u n d s 2 suggest that r e a c t i o n (3) m a y involve the initial f o r m a t i o n of the a l k o x y radical, (RCH20)'. Finally, it m u s t be p o i n t e d out t h a t the a n o d i c f o r m a t i o n of acetals r e p o r t e d here for e t h a n o l a n d m e t h a n o l is n o t a general r e a c t i o n since a n o d i c o x i d a t i o n of the higher a l i p h a t i c a l c o h o l s leads to a r u p t u r e of the c a r b o n skeleton, i.e. it was found t h a t n - p r o p a n o l gives a b o u t equal a m o u n t s of 1 , 1 - d i p r o x y p r o p a n e a n d 1,1-dipropoxymethane. Chemistry Department, The University, Southampton S 0 9 5 N H (Great Britain)
G. S u n d h o l m *
1 N. L. WEINBERGAND H. R. WEINBERG,Chem. Rev., 68 (1968) 449. 2 C. K. MANNAND K. K. BARNES,Electrochemical Reactions in Nonaqueous Systems, M. Dekker, New York, 1970, p. 163. 3 V. D. PARKER,Chem. Ind. (London), (1968) 1363. 4 A. I. VOGEL,A Textbook of Practical Oroanic Chemistry, Longmans, London, 3rd edn., 1961, p. 167. 5 L. EaERSONAND B. OLOFSON,Acta Chem. Scand., 23 (1969) 2355. 6 A. J. BARD (Ed.), Electroanalytical Chemistry, Vol. 3, M. Dekker, New York, 1969, p. 61. 7 M. FLEISCHMANNAND D. PLETCHER, Tetrahedron Lett., (1968) 6255. 8 W. T. DIXONAND R. O. C. NORMAN,J. Chem. Soe., (1963) 3119. 9 I. L. FINAR,Organic Chemistrv, Vol. 1, Longmans, London, 4th edn., 1964, p. 162. R e c e i v e d 30th D e c e m b e r , 1970 * On leave from the Technical University, Helsinki, Finland, under the Royal Society European Exchange Programme. J. Electroanal. Chem., 31 (i971) 265~67