NO743223L - - Google Patents

Description

"Procedure for the electrochemical preparation of pinacol'i

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

Organic carbonyl compounds, in particular aldehydes and ketones, can, as is known, be dimerized by hydration to so-called pie-cols, i.e.

derivatives of alkylene glycol. penne hydrodimerization only succeeds electrochemically .. • at a cathode with not too little hydrogen overvoltage, combined with a suitable reducing agent, it does not, however, succeed under the conditions of catalytic hydrogenation. Photochemical synthesis is not satisfactory, especially with regard to energy yield. During the electrochemical synthesis of pinacol, hydrogen is made available by

protons of a solvent, acc. of an added acid

As is also known, the pinacoldane synthesis from aromatic or aromatic/aliphatic carbonyl compounds occurs with high yields, while with purely aliphatic compounds one can only expect moderate to poor pinacol yields.

This fact is related to the stability of radical intermediates.

Starting from acetone, the tetramethylglycol known as "pinacol" is obtained. This compound is transferred by acid-catalyzed cleavage of a two water molecules to the pinacol, henh. 2,3-dimethylbutadiene,

Both these direct by-products of pinacol and pinacol itself are interesting intermediates for the synthesis of polymers, pharmaceutical products and pesticides. However, an extensive application has so far been hindered by the fact that only unsatisfactory processes for the production of pinacol were available.

One method for producing pinacol consists, for example, of

in that acetone is reacted with amalgams of aluminium, magnesium or sodium. Pinacol is still produced today on a smaller scale using this method. 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 resulting salts have a disturbing effect as ballast, the latter disadvantage is admittedly avoided by direct electroreduction on cathodes of lead, lead-copper alloy or lead-tin alloy in sulfuric acid or alkaline electrolytes, however, this method could not be carried out technically due to several disadvantages. Thus, the formation of highly toxic lead organelles ("lead oils") as a by-product at the cathode cannot be avoided.

Only small current yields are also achieved. Furthermore, the electrolyte must be neutralized before processing, in order to prevent the acid-catalysed water splitting to pinacol acc. dimethylbutadiene, whereby large amounts of salt are obtained. also disadvantageous is that part of the acetone is reduced to the worthless by-product isopropanol, and that

the solutions contain a lot of water, which must partly evaporate during processing.

The purpose of the present invention is to develop a direct electrochemical method for the production of pinacol from acetone, which makes it possible to avoid the above-mentioned disadvantages. The task is solved using the method according to the invention.

According to the method, pinacol is produced by electrolytic hydrodimerization of acetone in a split cell by using for electrolysis a catholyte containing 10 to 90% by weight of acetone, 1 to 60% by weight of water and 1 to 50% by weight of a quaternary ammonium salt.

As a cell, a split cell is used. Porous diaphragms, particularly cation exchange membranes of the sulphonated, cross-linked polystyrene type, are suitable as a partition wall.

As anode, lead dioxide-coated lead, graphite or titanium is used, especially in connection with 1 to 10% sulfuric acid as anolyte.

The catholyte contains 10 to 90, preferably 40 to 80% by weight of acetone, 1 to 60, preferably 5 to 30% by weight of water and 1 to 50, preferably 5 to 20% by weight of a quaternary ammonium salt.

As quaternary ammonium salts, e.g. compounds with the formula

in which residues R are alkyl residues, such as alkyl residues with 1 to 6 C atoms, e.g. methyl, ethyl, n-propyl, i-propyl, n-butyl, aryl residues, such as the phenyl residue, and aralkyl residues, such as the benzyl residue, and 2fe is an anion, e.g. a sulfate, alkyl sulfate, phosphate, carbonate, aryl sulfonate such as tosylate, tetrafluoroborate, hexafluorosilicate, halide, and perchlorate anion.

Tetraethylammonium ethylsulfate, tetraethylammonium p-toluenesulfonate, tetraethylammonium sulfate and tetrabutylammonium tetrafluoroborate are particularly suitable as such conducting salts. The concentration of these salts must be kept as low as possible in the specified range in order to achieve a simple isolation of pinacol and to prevent its anodic decomposition.

As cathode materials, in principle, all metals with an average or high hydrogen overvoltage can be used, i.e. Cu, Ag, Cd, Zn, Sn, Pb, Ti, Hg, as well as pure metals which, in the form of their alloys, are particularly advantageous cathode materials .however, graphite, coal or graphite-filled plastic. For example, the commercially available electrode carbons of the LEK or EXN type can be mentioned here

of fa. conradty, Ntlrnberg, or of the DIABON N, BS 70 or P 127 type of fa. Sigri, Meitingen, respectively of the BASCODUR type from fa. Raschig, Ludwigshafen. It is expedient to clean the carbon cathodes carefully 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. An increase in the current yield of pinacol can only be achieved if certain metals, such as Hg, Pb, Cu, Ag, Au, before the electrolysis are deposited individually or in mixture on the cathode in a very thin layer, namely between 1 and 1000 atomic layers, preferably between 30 and 100 atomic layers. For this purpose, the electrodes in the fully assembled cell are immersed in a diluted, acidified aqueous solution of corresponding metal salts, such as pbfNQ^^* Hg-JO^,CuSO^, AgBTC^, Aucl^, and deposited, while stirring the solution, by current densities of n 0.1 to 1 amp/dm 2 during the calculated time. The metals are probably distributed as islands in certain places of the surface and do not form a continuous layer. ;The current density is not critical In the method according to the invention, and it lies e.g. at 0.1 to 100amp/dra<2>, preferably at 5 to 25 anp/dm 2. ;Keep the temperature appropriately between 0 and 50°C<, Low temperatures do increase the current yield, but they require technical aids. Temperatures of 20 to 35°C are therefore preferred. With the use of carbon cathodes, the pH value turned out to be of little importance, and it can be set to between 1 and 14. A pH regulation in the catholyte takes place by adding dilute acid. ;One good convection of the catholyte is beneficial. It is achieved, for example, by re-pumping. The flow speed parallel to the cathode is hereby preferably set to values between 0.1 and 50 cm/sec. ;When working up the catholyte, the following arbéids method is recommended ;<p>first you set the pH value of the catholyte to approx. 7» Unreacted acetone and the formed isopropanol are stripped together with part of the water under reduced pressure, the remaining sump is cooled, possibly after the addition of water, to 0°C with stirring. This crystallizes pitacol hexahydrate which can easily be filtered off. or centrifuged. It is washed with a little ice water. The mother liquor ex- . triturate with ether or methylene chloride to remove water-soluble by-products, e.g. the unsymmetrical dimer 2-methylenepentanediol-(2,4). The remaining lead salt solution can be returned to the electrolysis. ;For continuous turnover of the catholyte, the catholyte enriched with pinacol is continuously removed from the cell and processed as described above. At the same time, the acetone-rich starting solution is continuously supplied to the cell. , ;According to the method, current yields of pinacol of over 65% can be obtained. These current yields significantly exceed the values of 37% stated so far in 'lit tér a tur on for pinacol synthesis; (US-PS 2 485 258) acc. 44% (US-*>S 2,422,468).

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

In addition to the high current yield, the method according to the invention also has other advantages compared to known methods. Thus, you do not get the precipitation of salts that. inevitable, obtained by neutralizing the acids. As the solution only contains relatively little water, only little energy is needed for the concentration of electrolysis products.

Bksempol 1

A rectangular plate and frame cell consists of a cathode plate, a cation exchange frame, a lead dip-oxide-coated anode end plate of lead, the two electrolyte frames made of polypropylene (the cathode frame carries at the upper and lower narrow sides connections for insertion and abduction of catholyte) and.seals of PERBUWAH. The cell components are assembled according to the pattern of a filter press. The catholyte is pumped in a circuit over a heat exchanger. Apart from the cathode surface, the catholyte must not come into contact with metals anywhere, as traces of foreign substances, such as Fe, Cu, Hi, Cr, are detrimental to the product yield. The anolyte is cooled in the anode compartment by means of a cooling coil. The catholyte is connected to the outside by means of a lake-cooled exhaust line. The free cathode surface in the present cell is 2.5 dm, and the distance between the cathode surface and the membrane surface is 0.6 cm.

At the start of the electrolysis, the anode compartment is filled with 5% sulfuric acid. In the catholyte circuit, 1 kg of a mixture of 80% by weight technical acetone, 13% distilled water and 7% tetraethylamraonium ethylsulphate (NEt^EtSO^) is introduced. The catholyte is re-pumped with a glass centrifugal pump, corresponding to a speed at the cathode of 8 cm/sec. Joule heat that occurs during the electrolysis is carried away by means of a water-cooled cooler, so that the temperature is kept at 25°C. The electrolysis is carried out at a current of 25 A resulting in a current density of lo A/dm.

The cell voltage rises during electrolysis from 19 to 24 volts. The catholyte is kept slightly auric by the addition of slightly diluted sulfuric acid. Portion-by-portion distilled water is introduced into the anode compartment to compensate for water loss by electroosmosis. After the passage of 129.5 ampere-hours, corresponding to an electrolysis time of 5.18 hours and a theoretical current conversion of 35%, the electrolysis is interrupted.

After stripping the low-boiling components in the rotary evaporator at room temperature and water jet vacuum, the residue is treated with 100 g of water and cooled to 0°C with stirring. The crystallisate is washed with a little water and weighed as "raw crystallisate". In the case of water supply Pinacol analysis Pinacol is determined by Karl-Pischer titration or by gas chromatography. The composition of the crystallisate corresponds approximately to a pinacol hexahydrate (theoretically 52.3% pinacol). An N/S analysis shows that the crude crystallisate always contains less than 1% of the lead salt used.

The mother liquor is extracted in a continuous extractor with ether.

The ether extract (average 5 to 30g) contains besides pinacol (average 10 to 20%) especially 2-methylpentanediol-(2,4) (average 20 to 70%), but no pinacolon. Pinacol is taken into account when calculating current yields. An N/S analysis shows that less than 0.5% of the used lead salt passes into the ether extract. N/S analysis of the extracted mother liquor shows that more than 90% of the used conductive salt is found there. The isopropanol formed is determined directly in the catholyte by gas chromatography.

In table z, the obtained current yields are compiled in dependence on the cathode material.

The results show that pinacol is obtained by the method according to the invention with current yields of 70 to 80%. Surprisingly, a polished copper plate forms a favorable cathode, whereas a pre-coppered graphite plate does not. The two tests given for the sake of comparison with sulfuric acid electrolyte (7% H2S04 instead of 7% NEt4EtS04) show that under these known reaction conditions the current yield drops very strongly.

Example 2

In the plate and frame cell described in example 1, the catholyte described there is reacted with 7% NEt^Etso^ in 1.5 kg charger under the conditions described there at a DIABON-N cathode. The duration per charge amounts to 7 hours and 47 minutes at 35% theoretical power turnover. The cathode is not cleaned between the individual charges, the cathode compartment is filled with distilled water in the approx. 16-hour breaks between electrolysis processes.

In Table II, the results for the individual chargers are listed in chronological order.

The activity of the cathode therefore decreases only slightly with time. An analogous series of experiments with a leaded copper cathode (see Table I) similarly showed only a slight time dependence of the results. Here, the cathode compartment was under acetone between experiments to prevent corrosion of the lead. In contrast, the activity of the same cathode in sulfuric acid electrolyte drops significantly already within 24 hours, cf. US-PS 2,422,468, 2,485,258 and 0;C.Slotterbeck,< Trans,©lectrochem. Soc. 92, 377 (1947).

Claims (3)

1. Process for the production of pinacol by electro- lytic ti<y>drbdimerization of acetone in a split cell, characterized by being used for electrolysis a catholyte containing 10 to 90 weight percent acetone, 1 to 60 weight percent water, and 1 to 50 weight percent of a quaternary ammonium salt. 2. Procedure as stated in claim 1, characterized in that the catholyte contains 40 to 80 weight percent acetone, 5 to 30 weight percent water and 5 to 20 weight percent of a quaternary ammonium salt. 3. Method as stated in claim 1, characterized in that the cathode consists of graphite, coal or graphite-filled plastic.