NO742803L - - Google Patents

Description

Process for the electrochemical production of pinacols.

The present invention relates to a new and particularly advantageous method for the production of pinacols.

Organic carbonyl compounds, especially aldehydes and ketones, can, as is well known, dimerize to so-called pinacols, i.e. to derivatives of ethylene glycol.

This hydrodimerization is only successful electrochemically with a cathode with a not too small hydrogen overvoltage, or with a suitable reducing agent, but it is not successful under catalytic hydrogenation conditions. Photochemical synthesis is not satisfactory, especially in terms of energy yield. In the electrochemical synthesis of pinacol, hydrogen is supplied by protons of the solvent, or by an added acid.

As is also known, the formation of pinacol from aromatic or aromatic/aliphatic carbonyl compounds takes place in moderate yields, while only medium or poor pinacol yields are obtained with purely aliphatic compounds.

This is related to the stability of radical intermediates.

by starting from acetone, the tetramethylglycol known as "pinacol" is obtained. This compound is transferred by acid-catalyzed cleavage of one or two hydrogen aroalkyls to pinacol, respectively to 2,3-dimethylbutadiene.

These direct by-products of pinacol, as well as pinacol itself, represent interesting intermediates for the synthesis of polymers, pharmaceuticals and pesticides. A wide application of these products was, however, hindered by the httil

very unsatisfactory process for the production of pinacol.

A method for producing pinacol is e.g. that one reacts acetone with amalgams of aluminium, magnesium or sodium. Using this method, pinacol 1 is also produced today on a smaller scale. This results in a large amount of isopropanol as a by-product, and the degree of utilization of the metal is relatively low, so that you get high costs. In addition, the formed salts have a disruptive effect as ballast. The latter disadvantage is indeed avoided by the direct electrolytic reduction with cathodes of lead, lead-copper alloy or lead-tin alloy in sulfuric acid or alkaline electrolytes, but the process was not technically used due to several disadvantages, thus the formation of highly toxic lead organyls ("lead oils") cannot be avoided at the cathode as a by-product. You also need a divided cell, otherwise acetone and pinacol decompose oxidatively at the Pb/<p>b02 anode. Slan also only gets low current yields. In addition, the electrolyte must be neutralized before processing in order to prevent the acid-catalyzed splitting of water into pinacols, respectively into dimethylbutadiene, whereby large amounts of electrolyte are obtained. It is also unfortunate that part of the acetone is reduced to the worthless by-product isopropanol, and that the solutions contain a lot of water, which during processing must partly

evaporates.

The task of the present invention is to provide a direct electrochemical method for the production of pinacols from carbonyl compounds, which eliminates the above-mentioned disadvantages. The invention solves this task.

According to the method according to the invention, pinacols are produced by electrolytic hydrolysis of carbonyl compounds in undivided cells in such a way that for electrolysis a mixture is used which contains 5 to 75 percent by weight of the carbonyl compound, 5 to 90 percent by weight of the alcohol corresponding to the carbonyl compound, 0.1 to 3% by weight of a quaternary ammonium salt and 0 to 30% by weight of water.

As carbonyl compounds, e.g. considering aliphatic aldehydes and ketones. Preferably, carbonyl compounds with formalent are used as starting materials for the pinacol synthesis

R' - CO - R,

in which R means a hydrogen atom or a hydrocarbon residue with 1 to 6 carbon atoms and R<1>a hydrocarbon residue with 1 to 6 carbon atoms.

The hydrocarbon residues can be straight-chain or branched and saturated or unsaturated. For example, the following hydrocarbon residues can be mentioned: methyl, ethyl, propyl, butyl, isopropyl, isobutyl, hexyl and cyclohexyl.

Suitable carbonyl compounds are e.g. acetone, acetaldehyde, methyl ethyl ketone, dethyl ketone, methyl isopropyl ketone and methyl cyclohexyl ketone. The use of acetone and the corresponding alcohol isopropanol is of particular technical interest.

The mixture subjected to electrolysis contains 3 to 75, preferably 10 to 40 percent by weight of said carbonyl compound. It also contains 5 to 90, preferably 20 to 30 percent by weight of the alcohol corresponding to the carbonyl compound, i.e. in the case of the preparation of pinacol of the above-mentioned formula IX, the corresponding alcohol of the formula

Suitable as quaternary ammonium salts, of which the mixture to be subjected to electrolysis must contain 0.1 to 3, preferably 0.5 to 1 percent by weight, are, for example, compounds of the formula t

in which the radicals R mean alkyl radicals, such as alkyl radicals with 1

to 6 c atoms, e.g. methyl, ethyl, n-propyl, i-propyl, n-butyl, aryl esters, such as the phenyl residue, and aralkyl residues, such as the benzyl residue, and Xe means an anion, e.g. a sulfate, alkyl sulfate, phosphate, carbonate, aryl sulfonate such as tosylate, tetrafluoroborate, hexafluorosilicate and perchlorate anion.

Tetraethylammonium methylsulphate, tetraethylammonium sulphate and tetrabutylammonium tetrafluoroborate are particularly suitable as such conducting salts. The concentration of these salts in the specified area must be kept as low as possible to achieve a simple isolation of pinacol and to avoid an anodic decomposition thereof.

Since an electrolytic synthesis can also be carried out at minimal salt concentrations with economical current and voltage values if cells with a small electrode distance are used, the so-called capillary gap cell is particularly suitable for the method according to the invention, which, for example** is described in DT-OS 1 804 809, and in J.Appl. Electrochem. 2, 59 (1972). In the present case, the electrode distance in these cells is suitably set to 0.1 to 100 ram, preferably 0*2 to 0.5 mm.

In principle, all metals with a medium or high hydrogen overvoltage, i.e. Cu, Ag, Cd, Zn, Sn, can be used as cathode material. Pb, Tl, Eg, 1 form of pure metals or in the form of their alloys. However, particularly advantageous cathode materials are graphite, coal or graphite-filled plastic. As examples, here can be mentioned the commercially available electrode carbon of the type LEK or EJOSf from the firm Conradty, Nuremberg, or of the type DIABON N, BS 70 or p 127 from the firm Sigri, Meitingen, henh. BASCO DUR

from the company Raschig, Ludwigshafen. Appropriately, the cathodes are carefully cleaned before the electrolysis, e.g. by flushing with concentrated hydrochloric acid and/or by brushing with pure quartz powder. It is advantageous to store the usually porous carbon electrodes in

distilled water. You can achieve an increase in the current yield of pinacol if you deposit certain metals, such as Hg, Pb, Cu, Ag, Au, in a very thin layer, namely between 1 and 1000 atomic layers, preferably between 30 and 100 atomic layers, before the electrolysis on the cathode, individually or in a mixture. The electrodes are then completely immersed in it

similar-built cell in the dilute, acidified aqueous solution of corresponding metal salts, such as Pb(N03)2,HgSO^,CuSO^, AgN03#AuCl-, and deposits under agitation of the solution at current densities of 0.1 to 1 A/dm 2 during the estimated time. The metals are probably distributed as islands in specific places on the surface, and they do not form a continuous layer.

Lead dioxide is primarily considered as anode material, preferably as a composite electrode on base surfaces of graphite, coal, lead, titanium or tantalum. Also other oxide anodes, such as Fe 3 o 2 , Mn 2 , T 1 2 O 3 or RuO 2 (on Ti) can be used, as well as gold, graphite, coal and platinum metals. The use of graphite anodes is particularly advantageous. As graphite cathodes are preferably also used, the bipolar electrodes conveniently consist of simple disks or plates of graphite, coal or graphite-filled plastic. At pH values above 7, especially above 10, passivated metals, such as Fe, Co, Ni, chrome-nickel steel, can also be used as anode material. A preferably used bipolar electrode construction consists of graphite or carbon plates which are optionally coated on the anode side with lead dioxide (100 to 500 microns) or by means of graphite-filled adhesive coated with thin foils, e.g. of Ten or Nine.

The current density is not critical in the method according to the invention, and it constitutes e.g. 0.1 to 100 A/dm, preferably 5 to 25 A/<to<2>.

The temperature is suitably kept between 0 and 50°C. Low temperatures do indeed increase the power yield, but they require additional technical work. Temperatures of 20 to 35°C are therefore preferred.

The pH value proved to be without particular impact when using carbon cathodes, and it can be set between 1 and 14. Without special pH regulation The pH value is set during the electrolysis between 2 and 4.

Bath movement is beneficial and if a capillary gap cell is used it is necessary. Good convection is achieved by pumping the electrolyte. The flow rate parallel to the electrodes is preferably set at 1 to 30 cm per second.

The electrolytic hydrodimerization preferably lasts until final cone concentrations (according to stationary concentrations in continuous operation) of pinacol of 1 to 30 percent by weight, preferably 5 to 20 percent by weight, have set in. The separation of the unreacted carbonyl compound from the product is not associated with any major problems.

Recommended for processing electrolysis products

that you first set the pH value to 7 by adding a little caustic soda to avoid the acid-catalyzed rearrangement to pinacol

1 during the processing. The unreacted acetone and isopropanol are removed, e.g. under reduced pressure. The remaining solution is cooled, possibly after adding water, with stirring to 0°c. This crystallizes e.g. pinacol as pinacol hexahydrate, which is filtered off and washed with a little ice water. The mother liquor contains the quaternary ammonium salt and can be returned to the electrolysis, optionally after a prior extraction with ether or methylene chloride to remove water-soluble by-products. With the method according to the invention, power yields of pinacol of over 50% can be achieved. Particularly high current yields are obtained by adding 1 to 30 percent by weight, preferably 3 to 10 percent by weight, of dioxane as co-solvent. These current yields significantly exceed the values of 37% previously stated in the literature for pinacol synthesis (US-PS 2 435 258) acc. 44% (US-PS 2,422,468)• Addition of methanol to the reaction mixture in a concentration of 5 to 70 percent by weight, preferably 10 to 40 percent by weight, is particularly advantageous when using graphite anodes.

The method according to the invention can be carried out discontinuously as well as continuously. In the continuous mode of operation, the reaction mixture is continuously pumped in a circuit through the cell (and suitably through a heat exchanger).

The method according to the invention is carried out in an undivided cell. Hereby, the preferred anode process consists in a dehydration of alcohol, e.g., according to the formula

CH3-CHOH-CH3■—> CH3-CO-CH3 + 2H+ + 2 e. At the anode occurs

that is, from alcohol a part of the ketone that is converted at the cathode. If the pinacol formation occurs cathodically with a high yield, which i

and in itself is desired, part of the ketone to be converted is used in the method according to the invention, in the form of the corresponding alcohol electron react ion. In the case of a high cathodic additional yield of alcohol, however, it is sufficient that only a small amount of alcohol is present. In each case, the net production of this unwanted by-product can be completely stopped, by the presence of alcohol at the anode in sufficient concentration, the anodic backsplitting of pinacol to the carbonyl compound is surprisingly prevented,

which is an anode reaction that normally finds stod.

In addition to the high current yield, the method according to the invention still has other advantages compared to known methods, thus salt formation is eliminated due to the neutralization of acids. Due to the low concentration of ammonium salts in the mixture, no solubilization effect will occur for pinacol. As the solutions contain relatively little water, only little energy is needed to concentrate the electrolysis product.

Example 1

As a cello, a capillary slit cello is used, which consists of a stack of circular horizontal plates of electrode carbon DIABON N (firma Sigri) with a diameter of 117 mm and a thickness of 10 mm. The plates are connected bipolar 1 series. The plate's anode is provided with a 300 micron thick, from lead nitrate anodic deposition layer of PbO_. the inside of the plate stack is provided with a 30 mm bore, so that the effective electrode surface amounts to 1 dm. The Mel losa plates are held with the help of radially placed polyetheratrips at a distance of 0.25 mm. The plate stack is suspended on the collo lid. The power supplies take place at the ends of the bipolar plate stack over an isolated intermediate axis (to the lower end), respectively directly to the upper end. The reaction solution is pumped in the center of the plate stack over a connecting piece at the lid, it flows through the capillary slits radially outside, and it is led over a heat exchanger back to the center of the plate stack. The cell, which is also equipped with a thermometer, a glass electrode and an exhaust line, is described in DOS 1 804 809.

At the beginning of the electrolysis, 1 kg of a mixture consisting of 50 weight percent acetone, 39.5 weight percent isopropanol, 10 weight percent water and 0.5 weight percent tetraethyl-amronium ethyl sulfate is introduced into the cell. while pumping the electrolyte (7.5 l/min) is conducted at a current density of 10 A/dm<2>, a temperature of 20°C, a pH value of 2 to 3 which maintains itself, on otrØiaaangdQ. of a total of 138.6 ampere-hours through the system. 1 Since the stabalon consists of 6 ©loctrodo pairs, ©this lasts © for an ©electrical time of 2.30 tisnar. In this way, the acetone used is theoretically converted to 60%. during the electrolysis the voltage of 55 volts (9.2 volts per electrode © pair) remains practically constant.

After the electrolysis is finished, the liquid is colorless. After taking a sample for acetone determination (with NH2OH.HCl) and setting the pH value to 7.0 by adding 4.3 ml of ln NaOH, acetone and isopropanol are introduced at 40°C and 100Torr in a rotary evaporator. The residue is cooled after the addition of a further SO g of water to 0°c. In this way pinacolhydrate crystallizes out

which is quickly sucked off and washed with a little ice water. One obtains

98.5 g of a pure white, crystalline product containing 53% pinacol, which therefore has approximately the composition of a pinacol hexahydrate. A further 2.5 g of pinacol can be extracted from the mother liquor with ether. Altogether, the plastic yield amounts to 52.2 + 2.5 / 54.7 g. This corresponds to an electricity yield of 20.3%. From the acetone balance

one can calculate, under the assumption of an anodic dehydration of isopropanol with 100% current yield, a current yield for the cathodic formation of acetone of 70.4%. as a high-boiling by-product, 6.6 g of 2-methyl-pentandlol-(2,4)- can be isolated from the above-mentioned ether extract. The secondary yield of isopropanol does not occur with the method according to the invention. Pinacol hexahydrate isolated as the main fraction has a melting point of 44°c (Lit. 45.4°c).

Example 2

The electrosynthesis described in example 1 was repeated, but with a change in the ratio of acetone to isopropanol, and to

partly also of the water content in the introduced products, d the following table is the concentration of the components, the current amounts, current yields of pinacol denoted as "SA", and the cell voltages (for 6 electrode pairs) are denoted as "U The results show that opti- mal current yields on pinacol are obtained at low acetone and high isopropanol concentrations.in all experiments the current density was?

with 10A/dm , the temperature with 20 C, and the lead salt concentration with 0.5% totraoyl ammonium methyl sulfate («SSEt^BtSO^ constant.

From both of the two leaner mixtures at the beginning of the table, a secondary yield of 2-methylpentanediol-(2,4)

of 26.3 g and 22.8 g respectively.

Example 3 In the test conditions mentioned in example 1, the temperature and the current density were varied. The table shows the results

It can be seen that current yields rise significantly with decreasing temperature and with increasing current density.

Example 4

The influence of dioxane as a co-solvent was investigated in more detail in the following series of experiments. The solutions contained 40% acetone, 2.5% water, 1% NEt^EtSO^ and the amounts of dioxane indicated in the following table, while the remainder was isopropanol. In the last column, the initial voltage is compared with the final voltage. During the experiments, a slightly increasing tendency and the formation of small amounts of an acidic by-product is observed, however, at the end of the experiment, no covering layer can be observed on the electrodes. Per kg of mixture, 138.7 amp/hour was used, which corresponds to a theoretical current conversion of 60%.<*>The current density, temperature and other electrolysis conditions were the same as in example 1.

Example 5

To investigate the influence of cathode material, the cathode materials compiled in the first column and the following table were used. Otherwise, electrolysis conditions and the cell were identical to those of example 1.

In some experiments, the cathode was coated with foreign metals before the electrolysis. Hg was deposited from a solution of 100 g HgSO 4 and 60 g B^SO^pr. Oxygen at 0.5 amp/dm while pumping the solution. At 100 atomic layers of Hg, a deposition time of 6 seconds had to be kept, if one assumes that an atomic layer contained 15 2

where 10<*>^ atoms per cm. The lead was deposited from an acidic lead tetrafluoroborate bath at the same current density.

A comparison of non-impregnated graphite grades shows that BASCODUR and DIABON N are more favorable than LEK or BS 70*Coating LEK with Hg, however, leads to better current yields than coating DIABON N.

Example 6

The following experiments compiled in the table are carried out in the cell described in example 1. The electrolysis conditions were the same as in this example" Only the lead salt and the water-concentration ion were varied. in water-poor and practically water-free experiments, 2-methyl-pentanediol-2,4 again occurred as a by-product with the yields indicated in the table. It appears from the results that the best current yields on pinacol are obtained in the presence of tetrabutylammonium tetrafluoroborate (»NBu4BF4) with 2.5% water. under these conditions relatively little of the by-product 2-methylpentane^diol-2,4 was formed.

Example 7

1 kg of a reaction mixture with the composition 50% methyl ethyl ketone, 39.5% sec. butanol, 10% water and 0.5% NEt 4 SO 4 was introduced into the capillary gap cell described in example 1 consisting of four electrode pairs. As in Example 5, DU\BON-N cathodes were coated for the experiment with "100 atomic layers" of mercury. The electrolysis was carried out at 10 A/dm and 20 CA for such a long time that a current of 111.8 amp/hour passed through the cell, corresponding to a theoretical current of 60%. the cell voltage increased during the electrolysis from 50.5 to 55 volts. The pH value at the end of the electrolysis was 5.0. For processing, the lowest boiling components of the electrolyte were distilled off under reduced pressure. 77.2 g of a brown crude product remained, which was examined directly by gas chromatography. It contained 12.3% => 9.5 g of pinacol (1,2-diraethyl-1,2-diethylglycol) corresponding to a current yield of 3.1%.

Example 8

The capillary gap element described in example 1 consists of

2

of a stack of DZABON N discs (6x1 dm , d = 250 microns). In this experiment, the anode is also composed of graphite, at the beginning of the electrolysis, 1 kg of the reaction mixture A is introduced into the cell, acc. B.

These reaction mixtures were reacted as in example 1, at 10 A/dm 2 , a pH value of 3 to 4, and a current amount of 138.6 A/hour (corresponding to one electrolysis time of 2.3 hours and a theoretical power turnover of 60%).

In the case of A, a voltage of 44 volts is set for the sauna. During the electrolysis, the anodic graphite is slightly attacked, and the electrolyte acquires a black color due to dispersed graphite particles. After electrolysis, graphite dust can be isolated in a total amount of 4.5 g. The power yield of pinacol is 21.5%, the energy requirement for pinacol is 15.5 KWh/kg.

In case B, the voltage is only 37 volts (at the six electrode pairs). during the electrolysis, anodic graphite is almost not attacked, and the liquid remains almost clear^The graphite dust filtered off after the electrolysis weighs 0.1 g. Pinacol is formed with an efa ötrom yield© of 19.0%. The energy requirement for pinacol is 14.7 KwfiAg.

Claims (4)

1. Method for the production of pinacols by electrolytic hydrodimerization of carbonyl compounds in undissolved cells, characterized by using for electrolysis a mixture containing 5 to 75 percent by weight of the carbonyl compound, 5 to 90 percent by weight of the alcohol corresponding to the carbonyl compound, 0 .1 to 3 percent by weight of a quaternary ammonium salt and 0 to 30 percent by weight of water. 2. Method as stated in claim 1, characterized by using as carbonyl compound a compound with the formula where R means a hydrogen atom or a hydrocarbon residue with 1 to 6 carbon atoms, and R' means a hydrocarbon residue with 1 to 6 carbon atoms. 3. Method as set forth in claim 1, characterized in that the mixture used for electrolysis also contains 1 to 30 percent by weight diocara" and/or 5 to 70 percent by weight methanol. 4. Method as specified in claim 1, characterized in that acetone is used as the carbonyl compound.