Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/20—Processes
  • C25B3/29—Coupling reactions
  • C25B3/295—Coupling reactions hydrodimerisation
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/01—Products
  • C25B3/07—Oxygen containing compounds

Definitions

• ABSTRACT Pinacols are prepared by electrolytic hydrodimerization of carbonyl compounds in non-compartmented cells using a mixture of from 5 to 75% by weight of the carbonyl compound, from 5 to 90% by weight of the alcohol corresponding to the carbonyl compound, from 0.1 to 3% by :weight of a quaternary ammonium salt and from 0 to 30% by weight of water.
• the invention is concerned with a new and particu larly advantageous process for the electrochemical manufacture of pinacols.
• organic carbonyl compounds especially aldehydes and ketones
• pinacols that is to say to derivatives of the alkylene glycol
• pinacols from aromatic or aromatic/aliphatic carbonyl compounds gives high yields, whilst only moderate to poor yields of the pinacol are to be expected with purely aliphatic compounds.
• This situation is related to the stability of the radical intermediates.
• pinacol tetramethylethylene glycol
• This compound is converted into pinacolene or into 2,3- dimethylbutadiene by acid-catalyzed elimination of one or two molecules of water, respectively.
• pinacol One process for the manufacture of pinacol consists, for example, in reacting acetone with amalgams of aluminum, magnesium or sodium. This process is still being used to manufacture pinacol on a small scale. The process produces a great deal of isopropanol as a by-product and the degree of utilization of .the metal is relatively low, so that the resulting costs are high. Furthermore, the salts produced are an objectionable ballast. While the lastmentioned disadvantage is avoided in direct electro-reduction on lead, lead-copper alloy or lead-tin alloy cathodes in electrolytes containing sulfuric acid or in alkaline electrolytes, this process has not found industrial acceptance, because it suffers from various disadvantages.
• pinacols of the formula II in which R is hydrogen or a hydrocarbon radical of one to 'sixcarbon atoms and R is a hydrocarbon radical of one to six carbon atoms are manufactured by electrolytic hydrodimerization of carbonyl compounds of the formula I in non-compartmented cells, using for the electrolysis a mixture which contains from 5 to by weight of the carbonyl compound, from 5 to 90% by weight of the alcohol corresponding to the carbonyl compound, of the formula from 0.1 to 3% by weight of a quaternary ammonium salt and from O to 30% by weight of water.
• the hydrocarbon radicals can be straight-chain or branched radicals and can be saturated or unsaturated. Methyl, ethyl, propyl, butyl, isopropyl, isobutyl, hexyl and cyclohexyl may be mentioned as examples of hydrocarbon radicals.
• Suitable carbonyl compounds are acetone, acetaldehyde, methyl ethyl ketone, diethyl ketone, methyl isopropyl ketone and methyl cyclohexyl ketone.
• acetone and of the corresponding alcohol isopropanol is of particulr interest in industry.
• the mixture to be subjected to electrolysis contains from 5 to 75% by weight, preferably from 10 to 40% by weight, of the above carbonyl compound. It also contains from 5 to 90% by weight, preferably from 20 to by weight, of the alcohol corresponding to the carbonyl compound.
• Suitable quaternary ammonium salts are, for example, compounds of the formula in which the radicals R are alkyl, such as alkyl of one to six carbon atoms, for example methyl, ethyl, npropyl, i-propyl or n-butyl, aryl, such as phenyl, or am]- kyl, such as benzyl, and X is an anion, for example a sulfate, alkylsulfate, phosphate, carbonate, arylsulfonate such as tosylate, tetrafluoroborate, hexafluosilicate or perchlorate anion.
• Particularly suitable conducting salts of this type are tetraethylammonium ethyl-sulfate, tetraethyl- 3 ammonium sulfate and tetrabutylammonium tetrafluoborate.
• concentration of these salts should be kept as low as possible to simplify the isolation of the pinacol and avoid its anoidic degradation.
• the electrode spacing in these cells in suitably from 0.1 to 1.0 mm, preferably from 0.2 to 0.5 mm.
• cathode material any metal of medium or high hydrogen overvoltage, that is to say Cu, Ag, Cd, Zn, Sn, Pb, Tl and Hg, as the pure metals or in the form of their alloys.
• particularly advantageous cathode materials are graphite, carbon and graphite-filled plastics. Examples which may be mentioned are the commercially available electrode carbons of type LEK or EXN as supplied by Conradty, Nuremberg, West Germany, or of type DIABON N, BS 70 or P 127 from Sigri, Meitingen, West Germany or BASCODUR from Raschig, Ludwigshafen, West Germany.
• the carbon cathodes should preferably be cleaned carefully before the electrolysis, for example by rinsing them with concentrated hydrochloric acid and/or by brushing them with pure quartz powder.
• the carbon electrodes which are usually porous, should preferably be stored in distilled water.
• the current efficiency can be increased by depositing a very thin layer, namely from 1 to 1,000 atom layers, preferably from 30 to 100 atom layers, of certain metals, such as Hg, Pb, Cu, Ag or Au, individually or as mixtures, on the cathode prior to the electrolysis.
• the electrodes in the readyassembled cell, are dipped into a dilute acidified aqueous solution of the corresponding metal salt such as Pb(NO HgSO CuSO AgNO or AuCl and the metal is deposited at current densities of from 0.1 to l A/dm for the calculated period of time, whilst circulating the solution.
• the metals are more probably distributed over the surface as islands at selected points than as a coherent layer.
• a suitable anode material is above all lead dioxide, preferably as a composite electrode on base surfaces of graphite, carbon, lead titanium or tantalum.
• Other oxide anodes such as Fe O MnO T1 or RuO (on Ti) can also be employed, as can gold, graphite, carbon and the platinum metals.
• the use of graphite anodes is particularly advantageous. Since graphite cathodes are also preferred, the bipolar electrodes are thus conveniently simple discs or plates of graphite, carbon or graphite-filled plastics.
• metals which can be passivated such as Fe, Co, Ni or chrome nickel steel, as the anode material in the process according to the invention.
• a preferred bipolar electrode consists of graphite plates or carbon plates which have optionally been coated with lead dioxide (100 to 500 p.) on the anode side, or to which thin foils of, for example, Ti or Ni have been glued by means of a graphite-filled adhesive.
• the current density used in the process according to the invention is not critical and is, for example, from 0.1 to 100 A/dm preferably from 5 to 25 A/dm".
• the temperature is suitably maintained at from 0to 50C. Whilst low temperatures increase the current yield, they entail technical complications. For this reason, temperatures of from 20to 35C are preferably used.
• the pH proves to have little influence and can be selected to be from 1 to 14. If the pH is not regulated externally, it assumes a value of from 2 to 4 during the electrolysis.
• Movement of the bath is advantageous, and is essential when using a capillary gap cell.
• Good convection is achieved by circulating the electrolyte by means of a pump.
• the rate of flow parallel to the electrodes is preferably set to values of from 1 to 30 cm per second.
• the process according to the invention can be carried out batchwise or continuously.
• the reaction mixture is circulated continuously through the cell (and, preferably, through a heat exchanger).
• the process of the invention is carried out in a noncompartmented cell.
• the preferred anode process is the dehydrogenation of the alcohol, for example in accordance with the equation
• a part of the ketone which is converted at the cathode is produced from the alcohol at the anode.
• a part of the ketone to be converted is introduced in the form of the corresponding alcohol in the process according to the invention.
• the acetone is consumed at the cathode, is a single-electron reaction, more rapidly than it can be replaced from the isopropanol, at the anode, in a twoelectron reaction.
• the process according to the invention has yet other advantages over the known processes.
• the production of salts which is unavoidable when the acids are neutralized, does not arise.
• the low concentration of the ammonium salts in the mixture there is no solubilizing effect of the pinacol. Since the solutions contain relatively little water, only little energy is required to concentrate the material issuing from the electrolysis.
• the cell used is a capillary gap cell consisting of a stack of circular horizontal plates of DIABON N (Messrs. Sigri) electrode carbon, the discs being of l 17 mm diameter and mm thickness.
• the plates are wired bipolar in series.
• the anode side of the plates is provided with a 300 ,0. thick layer of PbO anodically deposited from lead nitrate solution.
• the interior of the stack of plates bears a 30 mm hole, so that the effective electrode surface is 1 dm A spacing of 0.25 mm is maintained between the plates by radially applied polyester strips.
• the stack of plates is suspended from the cover of the cell.
• the current is supplied at the ends of the bipolar stack of plates, via an insulated middle axial in the case of the bottom end, or directly in the case of the upper end.
• the reaction solution is pumped through a union on the cover of the cell into the center of the stack of plates, flows radially outward through the capillary gaps and is returned into the center of the stack of plates via a heat exchanger.
• the cell which is further provided with a thermometer, a glass electrode and an off-gas pipeline, is described German laid-open sae'e'ifieauon No. 1,804,809.
• the liquid After termination of the electrolysis, the liquid is colorless.
• the acetone with NH OH.HC1
• the pH to 7.0 by adding 4.3 ml of 1 N NaOH solution
• the acetone and the isopropanol are stripped off in a rotary evaporator at 40C under 100 mm Hg.
• a further 50 g of water are added to the residue and the mixture is cooled to 0C.
• pinacol hydrate crystallizes out and is filtered off quickly and rinsed with a little ice water. 98.5 g of a pure white crystalline product containing 53% of pinacol, and thus having a composition close to that of pinacol hexahydrate, are obtained.
• pinacol can be extracted from the mother liquor by means of ether. Accordingly, the total pinacol yield is 52.2 2.5 54.7 g. This corresponds to a current efficiency of 20.3%. Assuming anodic dehydrogenation of isopropanol with 100% current efficiency, a current efficiency of 70.4% for its cathodic formation can be calculated from the acetone balance. 6.6 g of 2-methy1- pentane-2,4-diol were isolated as a high-boiling byproduct from the above ether extract. The subsidiary yield of isopropanol had no nett effect in the process according to the invention. The pinacol hexahydrate isolated as the main fraction melts at 44C (literature value: 454C).
• EXAMPLE 2 The electrosynthesis described in Example 1 was repeated, varying the acetone 1 isopropanol ratio and in some cases also varying the water content in the batches.
• the table which follows lists the concentrations of the components, the amounts of current, the current efficiencies based on pinacol, designated CE, and the cell potentials (for six electrode pairs), designated U.
• the results show that optimum current efficiencies based on pinacol are obtained at low acetone concentrations and high isopropanol concentrations.
• the current density of 10 A/dm the temperature of 20C, and the conducting salt concentration of 0.5% of tetraethylammonium ethyl-sulfate NEt .EtSO.,) were kept constant.
• EXAMPLE 4 The influence of dioxane as a co-solvent was examined in more detail in the series of experiments which follows.
• the solutions contained 40% of acetone, 2.5% of water, 1% of NEt .EtSO and the amounts of dioxan listed in the table which follows, the remainder consisting of isopropanol.
• the penultimate column compares the initial potential with the final potential.
• the experiments showed a slight tendency for the potential to rise, and indicated the formation of small amounts of an acid by-product, but no covering layers were detectable on the electrodes at the end of the experiment. 138.7 ampere hours were used per kg of batch, corresponding to a theoretical conversion of 60%.
• the current density, temperature and other conditions of electrolysis were the same as in Example 1.
• Hg was deposited from a solution of 100 g of HgSO and g of H per liter at 0.5 A/dm", whilst circulating the solution. This required a deposition time of 6 seconds for atom layers of Hg, if it is assumed that one atom layer contains 10 atoms per cm
• the lead was deposited from an acid lead tetrafluoborate bath, using the same current density.
• EXAMPLE 7 1 kg of a reaction mixture composed of 50% of methyl ethyl ketone, 39.5% of sec. butanol, 10% of water and 0.5% of l ⁇ lEt.,.SO was introduced into the capillary gap cell described in EXample l, but consisting of four electrode pairs.
• the DlABON-N cathodes were coated with 100 atom layers of mercury as in Example 5 prior to the experiment.
• the electrolysis was carried out at A/dm and C until an amount of current of 111.8 ampere hours had been passed through, corresponding to a theoretical conversion of 60%. Accordingly, the electrolysis time was 2.8 hours. During the electrolysis, the cell potential rose from 50.5 to 55 volt.
• the pH at the end of the electrolysis was 5.0.
• the low-boiling constituents of the electrolyte were distilled off under reduced pressure. 77.2 g of a brown crude product were left; this was examined directly by gas chromatography..lt contained 12.3% 9.5g of pinacol (1,2-dimethyl-l ,2- diethyl glycol), corresponding to a current efficiency of 3.1%.

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• 1973
  • 1973-08-25 DE DE2343054A patent/DE2343054C2/de not_active Expired
• 1974
  • 1974-08-02 NO NO742803A patent/NO742803L/no unknown
  • 1974-08-19 US US498447A patent/US3899401A/en not_active Expired - Lifetime
  • 1974-08-21 SE SE7410641A patent/SE7410641L/ not_active Application Discontinuation
  • 1974-08-23 BE BE147844A patent/BE819123A/xx unknown
  • 1974-08-23 NL NL7411282A patent/NL7411282A/xx unknown
  • 1974-08-23 FR FR7429021A patent/FR2241631B3/fr not_active Expired
  • 1974-08-26 JP JP49097142A patent/JPS5052010A/ja active Pending

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