Electrolytic manufacture of alkyl-substituted hydroquinones
United States Patent 3909376
Alkylhydroquinones with from 1 to 3 alkyl radicals, which can contain from 1 to 4 carbon atoms, are produced by electrolytic oxidation of the corresponding alkylphenols which are unsubstituted in the para-position to the hydroxyl group, followed by electrolytic reduction of the resulting alkylquinones, wherein the electrolysis is carried out in the presence of a non-oxidizing aqueous mineral acid and in the presence of a water-soluble ketone.
US Patent References:
ELECTROCHEMICAL CONVERSION OF PHENOL TO HYDROQUINONE
Covitz et al. - October 1971 - 3616324
PROCESS FOR THE PRODUCTION OF HYDROQUINONE
Fremery et al. - March 1973 - 3721615
ELECTROCHEMICAL CONVERSION OF PHENOL TO HYDROQUINONE
Covitz et al. - October 1971 - 3616324
PROCESS FOR THE PRODUCTION OF HYDROQUINONE
Fremery et al. - March 1973 - 3721615
Application Number:
05/529284
Publication Date:
09/30/1975
Filing Date:
12/04/1974
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Export Citation:
Assignee:
BASF Aktiengesellschaft (Ludwigshafen, Rhine, DT)
Primary Class:
International Classes:
C25B3/00; C25B3/02; C25B3/04
Field of Search:
204/72,73,78
Primary Examiner:
Andrews R. L.
Attorney, Agent or Firm:
Johnston, Keil, Thompson & Shurtleff
Claims:
We claim
1. A process for the electrochemical manufacture of an alkylhydroquinone of the general formula ##SPC3##
2. a process as claimed in claim 1, wherein the anodic oxidation and the cathodic reduction are carried out at room temperature or at a temperature of up to 40°C.
3. A process as claimed in claim 1, wherein sulfuric acid of from 1 to 20 percent strength by weight is used as the non-oxidizing aqueous mineral acid.
4. A process as claimed in claim 1, wherein a current density of 5 to 20 amperes per dm2 is maintained during the electrolysis.
5. A process as claimed in claim 1, wherein acetone, methyl ethyl ketone or diethyl ketone is used as the water-soluble ketone.
6. A process as claimed in claim 1, wherein the electrolysis mixture contains from 20 to 60 percent by weight of acetone.
7. A process as claimed in claim 1, wherein the electrolysis mixture contains from 1 to 10 percent by weight of alkylphenol.
8. A process as claimed in claim 1, wherein an alkylphenol of the formula II, in which R is methyl, is used.
9. A process as claimed in claim 1, wherein 2-methylphenol, 2,6-dimethylphenol, 2,3,6-trimethylphenol or 2,3,5-trimethylphenol is used as the alkylphenol.
1. A process for the electrochemical manufacture of an alkylhydroquinone of the general formula ##SPC3##
2. a process as claimed in claim 1, wherein the anodic oxidation and the cathodic reduction are carried out at room temperature or at a temperature of up to 40°C.
3. A process as claimed in claim 1, wherein sulfuric acid of from 1 to 20 percent strength by weight is used as the non-oxidizing aqueous mineral acid.
4. A process as claimed in claim 1, wherein a current density of 5 to 20 amperes per dm2 is maintained during the electrolysis.
5. A process as claimed in claim 1, wherein acetone, methyl ethyl ketone or diethyl ketone is used as the water-soluble ketone.
6. A process as claimed in claim 1, wherein the electrolysis mixture contains from 20 to 60 percent by weight of acetone.
7. A process as claimed in claim 1, wherein the electrolysis mixture contains from 1 to 10 percent by weight of alkylphenol.
8. A process as claimed in claim 1, wherein an alkylphenol of the formula II, in which R is methyl, is used.
9. A process as claimed in claim 1, wherein 2-methylphenol, 2,6-dimethylphenol, 2,3,6-trimethylphenol or 2,3,5-trimethylphenol is used as the alkylphenol.
Description:
This application discloses and claims subject matter described in German Pat. application No. P 23 60 494.6, filed Dec. 5, 1973, which is incorporated herein by reference.
The invention relates to a process for the manufacture of alkylhydroquinones by electrolytic oxidation of alkylphenols and subsequent electrolytic reduction of the quinones thus obtained.
The anodic oxidation of phenols to the corresponding benzoquinones in a compartmented cell, and the electrochemical synthesis of hydroquinones in a non-compartmented cell, have been known for a considerable time (cf. Berichte der deutschen Chemischen Gesellschaft, Vol. 47, p. 2,003 (1914), Helv. Chim. Acta, Vol. 2, p. 583 (1919), Vol. 8, p. 74(1925), Vol. 10, p. 40 (1927) and Vol. 10, p. 102 (1927). However, in this process neither the product yield nor the current efficiency are very high, and furthermore a number of undesired byproducts are formed. These by-products necessitate expensive purification operations.
German Published application No. 1,643,558 has disclosed converting unsubstituted phenol into hydroquinone in 90 per cent yield through appropriate choice of the electrolysis parameters such as current density, depolarizer and electrolyte concentration. The current efficiencies are above 60%. If attempts are made to apply these conditions to the anodic oxidation of alkyl-substituted phenols, the corresponding hydroquinones are obtained, but the yields are very low and the current efficiencies even drop below 20%.
Further, Chemical Communications 1971, pp. 1,643, et seq. discloses that dimethylphenol can be oxidized electrolytically to dimethylquinone in the presence of acetonitrile as the solvent. However, if attempts are made to reduce an alkylquinone, thus obtained, to the corresponding hydroquinone, the yields obtained leave much to be desired. An essential disadvantage is that the hydroquinones obtained are insufficiently pure for further conversion and thus necessitate an expensive purification process.
I have found that alkylhydroquinones of the general formula ##SPC1##
In which R is alkyl of 1 to 4 carbon atoms and n is an integer from 1 to 3 can be manufactured particularly advantageously in an electrolytic process wherein alkylphenols of the formula ##SPC2##
In which R and n have the above meanings and which are unsubstituted in the para-position to the hydroxyl group, are oxidized anodically in a non-oxidizing aqueous mineral acid in the presence of a watersoluble ketone and the reaction mixture containing the corresponding alkylquinones is reduced cathodically.
It is an advantage of the new process that both the yields and the current efficiences are good. A decisive advantage of the new process is that the hydroquinones as obtained are more than 90 per cent pure and require no additional purification before further processing.
In the preferred alkylphenols of the formula II, R is methyl. The para-position, where oxidation is to take place, in the initial phenol II must of course always be unsubstituted. Examples of suitable phenols are 2-methylphenol, 2,6-dimethylphenol, 2,3,6-trimethylphenol and 2,3,5-trimethylphenol.
2,6-Dimethylphenol and 2,3,6-trimethylphenol have acquired particularly great importance in industry.
The electrolysis is carried out in the presence of an aqueous solution of a non-oxidizing mineral acid, especially sulfuric acid. In general, the concentration of the mineral acid used is from 1 to 20 percent by weight, especially from 5 to 10 percent by weight.
During the anodic oxidation and the cathodic reduction, the temperature is maintained at its ambient value or slightly above, for example at from 20° to 40°C. Under no circumstances should the boiling points of the solvents used be exceeded.
It is advantageous to maintain current densities greater than 5 amperes per dm 2 during the electrolysis. In general, the range of current densities used is from 5 to 20 amperes per dm 2 .
The anodes used are lead dioxide, or electrodes coated with lead dioxide, or electrodes of noble metals, such as, for example, platinum, platinized titanium or gold. Lead dioxide anodes are preferred.
Cathodes which can be used are lead, mercury, cadmium, tin, zinc, copper, nickel, silver amalgam and lead amalgam electrodes. Lead electrodes have acquired particular importance.
The anodic oxidation of the phenols is preferably carried out in a compartmented anode chamber and the alkylquinones obtained are then reduced in a subsequent stage, in a cathode chamber which is also compartmented. It has proved particularly convenient to use a compartmented cell, carry out the oxidation of the alkylphenols II in the anode chamber, pass the solution thus obtained into the cathode chamber and there carry out the electrochemical reduction to the corresponding hydroquinone.
An essential feature of the invention is the use of a water-soluble ketone as the solvent. Examples of suitable water-soluble ketones are acetone, methyl ethyl ketone and diethyl ketone. Acetone has proved a particularly suitable solvent. The electrolysis mixture used advantageously contains from 20 to 80 percent by weight, especially from 40 to 60 percent by weight, of acetone.
The initial electrolysis mixture preferably contains from 1 to 10 percent by weight of alkylphenols of the formula II.
The alkylhydroquinones obtained as end products are generally isolated by evaporating off the solvent. They can also be extracted with a suitable water-immiscible solvent and be isolated therefrom by conventional methods, for example precipitation or fractional crystallization.
Alkyl-substituted hydroquinones manufactured by the process of the invention can be used for the manufacture of plant protection agents, dyes or biologically active materials, for example vitamin E (cf. S.F. Dyke; The Chemistry of the Vitamins, pp. 256 et seq., Interscience Publishers 1965).
Alkylhydroquinones can also be used as polymerization inhibitors (cf. Belgian Pat. No. 779,388).
The Examples which follow illustrate the process of the invention.
EXAMPLE 1 ______________________________________ Anodic oxidation of 2,6-dimethylphenol. Apparatus: compartmented cell with cation exchange membrane Anode: PbO 2 electrode; surface area: 0.66 dm 2 Anolyte: 24.4 g (0.2 mole) of 2,6-dimethylphenol 550 ml of H 2 O 450 ml of acetone 49 g of concentrated H 2 SO 4 Catholyte: 1N H 2 SO 4 Cathode: Pb electrode Charge Q: 0.8 F Current I: 10 A ______________________________________
On completion of the electrolysis, a sample of the anolyte was taken, the acetone was distilled off and the residue was repeatedly extracted with ether. After distilling off the ether, the 2,6-dimethyl-p-benzoquinone and unconverted starting material are determined by gas chromatography. According to these determinations, the yield of 2,6-dimethyl-o-benzoquinone (based on 2,6-dimethylphenol converted) is 86.2% and the current efficiency is 54.8%.
EXAMPLE 2 ______________________________________ Electrochemical synthesis of 2,6-dimethylhydroquinone Apparatus Anode Anolyte As in Example 1 Cathode I and Q Catholyte; the anolyte from Example 1. ______________________________________
After completion of the electrolysis, the acetone is distilled off and the residue is steam-distilled to remove unconverted 2,6-dimethylphenol. On cooling the residue from the steam distillation, 2,6-dimethylhydroquinone precipitates. The yield is 65%, based on 2,6-dimethylphenol converted. The 2,6-dimethylhydroquinone obtained is 90% pure.
EXAMPLE 3 ______________________________________ Anodic oxidation of 2,3,6-trimethylphenol Apparatus: compartmented cell with cation exchange membrane Anode: PbO 2 ; surface area: 0.66 dm 2 Anolyte: 27.2 g (0.2 mole) of 2,3,6-trimethylphenol 500 ml of H 2 O 500 ml of acetone 49 g of concentrated H 2 SO 4 Catholyte: 1N H 2 SO 4 Cathode: Pb Q: 0.8 F; I: 5 A . ______________________________________
On working up a sample of the material leaving the electrolysis, analogously to Example 1, trimethyl-p-benzoquinone is obtained in 84.6 percent yield (based on 2,3,6-trimethylphenol converted). The current efficiency is 54%.
EXAMPLE 4 ______________________________________ Electrochemical synthesis of trimethylhydroquinone Apparatus Anode Anolyte As in Example 3 Cathode I and Q Catholyte: the anolyte from Example 3. ______________________________________
On completion of the electrolysis, the catholyte is worked up analogously to Example 2. This gives trimethylhydroquinone which is over 90 percent pure. The yield (based on 2,3,6-trimethylphenol converted) is 77.4%.
COMPARATIVE EXAMPLE
If the procedure described in Examples 3 and 4 is followed but acetonitrile is used instead of acetone, the yield of 2,3,6-trimethylhydroquinone is 45 - 55% and the purity is only 70 - 75%.
The invention relates to a process for the manufacture of alkylhydroquinones by electrolytic oxidation of alkylphenols and subsequent electrolytic reduction of the quinones thus obtained.
The anodic oxidation of phenols to the corresponding benzoquinones in a compartmented cell, and the electrochemical synthesis of hydroquinones in a non-compartmented cell, have been known for a considerable time (cf. Berichte der deutschen Chemischen Gesellschaft, Vol. 47, p. 2,003 (1914), Helv. Chim. Acta, Vol. 2, p. 583 (1919), Vol. 8, p. 74(1925), Vol. 10, p. 40 (1927) and Vol. 10, p. 102 (1927). However, in this process neither the product yield nor the current efficiency are very high, and furthermore a number of undesired byproducts are formed. These by-products necessitate expensive purification operations.
German Published application No. 1,643,558 has disclosed converting unsubstituted phenol into hydroquinone in 90 per cent yield through appropriate choice of the electrolysis parameters such as current density, depolarizer and electrolyte concentration. The current efficiencies are above 60%. If attempts are made to apply these conditions to the anodic oxidation of alkyl-substituted phenols, the corresponding hydroquinones are obtained, but the yields are very low and the current efficiencies even drop below 20%.
Further, Chemical Communications 1971, pp. 1,643, et seq. discloses that dimethylphenol can be oxidized electrolytically to dimethylquinone in the presence of acetonitrile as the solvent. However, if attempts are made to reduce an alkylquinone, thus obtained, to the corresponding hydroquinone, the yields obtained leave much to be desired. An essential disadvantage is that the hydroquinones obtained are insufficiently pure for further conversion and thus necessitate an expensive purification process.
I have found that alkylhydroquinones of the general formula ##SPC1##
In which R is alkyl of 1 to 4 carbon atoms and n is an integer from 1 to 3 can be manufactured particularly advantageously in an electrolytic process wherein alkylphenols of the formula ##SPC2##
In which R and n have the above meanings and which are unsubstituted in the para-position to the hydroxyl group, are oxidized anodically in a non-oxidizing aqueous mineral acid in the presence of a watersoluble ketone and the reaction mixture containing the corresponding alkylquinones is reduced cathodically.
It is an advantage of the new process that both the yields and the current efficiences are good. A decisive advantage of the new process is that the hydroquinones as obtained are more than 90 per cent pure and require no additional purification before further processing.
In the preferred alkylphenols of the formula II, R is methyl. The para-position, where oxidation is to take place, in the initial phenol II must of course always be unsubstituted. Examples of suitable phenols are 2-methylphenol, 2,6-dimethylphenol, 2,3,6-trimethylphenol and 2,3,5-trimethylphenol.
2,6-Dimethylphenol and 2,3,6-trimethylphenol have acquired particularly great importance in industry.
The electrolysis is carried out in the presence of an aqueous solution of a non-oxidizing mineral acid, especially sulfuric acid. In general, the concentration of the mineral acid used is from 1 to 20 percent by weight, especially from 5 to 10 percent by weight.
During the anodic oxidation and the cathodic reduction, the temperature is maintained at its ambient value or slightly above, for example at from 20° to 40°C. Under no circumstances should the boiling points of the solvents used be exceeded.
It is advantageous to maintain current densities greater than 5 amperes per dm 2 during the electrolysis. In general, the range of current densities used is from 5 to 20 amperes per dm 2 .
The anodes used are lead dioxide, or electrodes coated with lead dioxide, or electrodes of noble metals, such as, for example, platinum, platinized titanium or gold. Lead dioxide anodes are preferred.
Cathodes which can be used are lead, mercury, cadmium, tin, zinc, copper, nickel, silver amalgam and lead amalgam electrodes. Lead electrodes have acquired particular importance.
The anodic oxidation of the phenols is preferably carried out in a compartmented anode chamber and the alkylquinones obtained are then reduced in a subsequent stage, in a cathode chamber which is also compartmented. It has proved particularly convenient to use a compartmented cell, carry out the oxidation of the alkylphenols II in the anode chamber, pass the solution thus obtained into the cathode chamber and there carry out the electrochemical reduction to the corresponding hydroquinone.
An essential feature of the invention is the use of a water-soluble ketone as the solvent. Examples of suitable water-soluble ketones are acetone, methyl ethyl ketone and diethyl ketone. Acetone has proved a particularly suitable solvent. The electrolysis mixture used advantageously contains from 20 to 80 percent by weight, especially from 40 to 60 percent by weight, of acetone.
The initial electrolysis mixture preferably contains from 1 to 10 percent by weight of alkylphenols of the formula II.
The alkylhydroquinones obtained as end products are generally isolated by evaporating off the solvent. They can also be extracted with a suitable water-immiscible solvent and be isolated therefrom by conventional methods, for example precipitation or fractional crystallization.
Alkyl-substituted hydroquinones manufactured by the process of the invention can be used for the manufacture of plant protection agents, dyes or biologically active materials, for example vitamin E (cf. S.F. Dyke; The Chemistry of the Vitamins, pp. 256 et seq., Interscience Publishers 1965).
Alkylhydroquinones can also be used as polymerization inhibitors (cf. Belgian Pat. No. 779,388).
The Examples which follow illustrate the process of the invention.
EXAMPLE 1 ______________________________________ Anodic oxidation of 2,6-dimethylphenol. Apparatus: compartmented cell with cation exchange membrane Anode: PbO 2 electrode; surface area: 0.66 dm 2 Anolyte: 24.4 g (0.2 mole) of 2,6-dimethylphenol 550 ml of H 2 O 450 ml of acetone 49 g of concentrated H 2 SO 4 Catholyte: 1N H 2 SO 4 Cathode: Pb electrode Charge Q: 0.8 F Current I: 10 A ______________________________________
On completion of the electrolysis, a sample of the anolyte was taken, the acetone was distilled off and the residue was repeatedly extracted with ether. After distilling off the ether, the 2,6-dimethyl-p-benzoquinone and unconverted starting material are determined by gas chromatography. According to these determinations, the yield of 2,6-dimethyl-o-benzoquinone (based on 2,6-dimethylphenol converted) is 86.2% and the current efficiency is 54.8%.
EXAMPLE 2 ______________________________________ Electrochemical synthesis of 2,6-dimethylhydroquinone Apparatus Anode Anolyte As in Example 1 Cathode I and Q Catholyte; the anolyte from Example 1. ______________________________________
After completion of the electrolysis, the acetone is distilled off and the residue is steam-distilled to remove unconverted 2,6-dimethylphenol. On cooling the residue from the steam distillation, 2,6-dimethylhydroquinone precipitates. The yield is 65%, based on 2,6-dimethylphenol converted. The 2,6-dimethylhydroquinone obtained is 90% pure.
EXAMPLE 3 ______________________________________ Anodic oxidation of 2,3,6-trimethylphenol Apparatus: compartmented cell with cation exchange membrane Anode: PbO 2 ; surface area: 0.66 dm 2 Anolyte: 27.2 g (0.2 mole) of 2,3,6-trimethylphenol 500 ml of H 2 O 500 ml of acetone 49 g of concentrated H 2 SO 4 Catholyte: 1N H 2 SO 4 Cathode: Pb Q: 0.8 F; I: 5 A . ______________________________________
On working up a sample of the material leaving the electrolysis, analogously to Example 1, trimethyl-p-benzoquinone is obtained in 84.6 percent yield (based on 2,3,6-trimethylphenol converted). The current efficiency is 54%.
EXAMPLE 4 ______________________________________ Electrochemical synthesis of trimethylhydroquinone Apparatus Anode Anolyte As in Example 3 Cathode I and Q Catholyte: the anolyte from Example 3. ______________________________________
On completion of the electrolysis, the catholyte is worked up analogously to Example 2. This gives trimethylhydroquinone which is over 90 percent pure. The yield (based on 2,3,6-trimethylphenol converted) is 77.4%.
COMPARATIVE EXAMPLE
If the procedure described in Examples 3 and 4 is followed but acetonitrile is used instead of acetone, the yield of 2,3,6-trimethylhydroquinone is 45 - 55% and the purity is only 70 - 75%.