GB2160547A - Electrosynthes of carboxylic acids - Google Patents

Abstract

In the preparation of carboxylic acids by subjecting an aromatic organic halide to electrochemical reduction in an electrolysis cell containing an organic solvent medium containing an inert electrolyte and in the presence of carbon dioxide, a cell having an anode formed of magnesium is used. Improved yields are obtained.

Description

SPECIFICATION Process for the electrosynthesis of carboxylic acids The invention is concerned with a process for the electrosynthesis of carboxylic acids by electrochemical reduction of organic halides, particularly aromatic organic halides, in the presence of carbon dioxide gas, the process being carried out in an electrolysis cell in an organic solvent medium containing an inert electrolyte.

Carboxylic acids are widely used in the chemical industry, for example as synthesis intermediates, as plant protection compounds or anti-inflammatory compounds, or as precursors in the synthesis of penicillins.

Various processes for the electrosynthesis of carboxylic acids by electrochemical reduction of organic halides in the presence of carbon dioxide gas are known: BAIZER (J.Org. Chem., Vol. 37, No. 12, 1972, pages 1951-1960) describes the eelctrochemical reduction of benzyl and allyl halides at a controlled potential in the presence of carbon dioxide gas, in a two-compartment electrolysis cell, in a dimethylformamide (DMF) medium containing tetraethylammonium chloride as the inert electrolyte. The cathode consists of mercury and the anode of platinum. The products obtained are the corresponding esters.

WAWZONEK and SHRADEL (Journal of the Electrochemical Society, March 1979, Vol. 126, No.3, pages 401-403) describe the electrochemical reduction of phenyldichloromethane at a controlled potential and at ambient temperature in the presence of carbon dioxide gas in a cell with separate compartments, in a DMF medium containing tetrabutylammonium bromide at a concentration of 0.2 M as the inert electrolyte. The cathode consists of mercury and the anode of platinum. The phenyl-dichloromethane concentration in the DMF is 0.13 M. The maximum yield of mandelic acid obtained is 9.8%.

MATSUE, KITAHARA and OSA (DenkiKagaku, 1982, Vol.50, No. 9, pages 732-735) describe the electrochemical reduction of phenyl halides at a controlled potential in the presence of carbon dioxide gas in a cell with separate compartments, in a DMF medium containing tetraethylammonium perchlorate at a concentration of 0.1 M as the inert electrolyte. The cathode consists of mercury and the anode of platinum.

The phenyl halide concentration is varied from 10 to 50 x 10-3 M according to the particular example. The degree of conversion of the phenyl halides varies from 49 to 87%, depending on the particular example.

Yields of the corresponding acids, varying from 0 to 40% are obtained depending on the halogen. This paper also teaches that under these conditions an acid is not obtained if n-butyl bromide is used as the starting material.

TROUPEL, ROLLIN, PERICHON and FAUVAROUE {Nouveau Journal de Chimie, Vol. 5, No.12,1981, pages 621-625) describe the electrosynthesis of aromatic acids by electrochemical reduction of aromatic halides at ambient temperature in the presence of carbon dioxide gas and of a catalyst based on nickel, in a two-compartment cell, in a tetrahydrofuran (THF)/hexamethyl-phosphorotriamide (HMPT) medium containing an inert electrolyte (LiC104, NBu4-ClO4). The anode consists of platinum and the cathode of mercury or graphite. The electrochemical reduction of the nickel complexes NiX2L2 (L = PPh3 and X = Cl or Br) in the presence of aromatic halides (ArX) and of an excess of L gives stable complexes ArNiXL2.In the presence of CO2, such complexes can be reduced electrochemically and give the arylcarboxylates ArCOO- in yields of 40 to 90%, depending on the nature of Ar.

Starting from benzyl chloride, only dibenzyl, but no phenylacetic acid, is obtained. Starting from mixtures of ArX + RX (aliphatic halides), ArCOO and the esters ArCOOR, but no RCOO-, are obtained.

These latter experiments clearly show the limits to applicability of this technique.

French Patent No. 2,530,266 describes a process for the preparation of arylacetic and arylpropionic acids from the corresponding halides. Electrochemical reduction of the halides is carried out under a carbon dioxide gas atmosphere in the presence of a catalyst comprising at least one ligand-bonded metal halide of the formula MY2L, where M is palladium or nickel, Y is a halogen, and Lisa ligand of the formula PR2-(CH2)n - PR2, in which P is phosphorus and R is phenyl or an aliphatic group, and n is an integer less than or equal to 3. The cell comprises two separate compartments, the cathode consisting of mercury and the anode of lithium. The yields of carboxylic acids vary from 19 to 51% depending on the particular experiment.

To the best of our knowledge no simple process for the electrosynthesis of carboxylic acids by the electrochemical reduction of organic halides, particularly aromatic organic halides, in the presence of carbon dioxide gas in an electrolysis cell, in an organic solvent medium containing an inert electrolyte, has been described or is currently available which gives good yields and is of general applicability, that is to say is applicable to the electrosynthesis of the majority of conventional carboxylic acids.

We have now found that by using magnesium as the anode in such a process, excellent yields of a wide range of acids can be obtained.

According to the present invention, therefore, there is provided a process for the preparation of a carboxylic acid, which comprises subjecting an aromatic organic halide to electrochemical reduction in an electrolysis cell containing an organic solvent medium containing an inert electrolyte and in the presence of carbon dioxide, the cell having an anode formed of magnesium.

We have found that this process enables quite unexpectedly high yields with little or no by-products to be obtained, and at the same time 1) the process is very simple to carry out; it can, contrary to all the processes described in the prior art, be carried out in a single-compartment electrolysis cell without either diaphragm or frit, which is very important, particularly on an industrial scale, and it can be carried out without employing one or more catalysts; and 2) the process is of general applicability and is applicable to the electrosynthesis of a large number of carboxylic acids.

The cell used in carrying out the process may be any conventional single compartment cell.

In general, it is preferred to use an aromatic organic halide, that is to say, by definition, an organic halide in which the halogen is directly bonded to a carbon atom of an aryl group or of an aromatic heterocyclic group.

Suitable organic halides for use in the process are, for example, compounds of the formula XAX', in which: Xis chlorine, bromine or iodine, X' is hydrogen, chlorine, bromine or iodine, and, A is a substituted or unsubstituted, saturated or unsaturated, aliphatic or cycloaliphatic group, R1, containing from 1 to 21 carbon atoms, or an aryl group R2 which may optionally be substituted by one or more R1 groups and/or by one or more functional groups R3 which cannot be electro-reduced, or an aromatic heterocyclic group.

By functional groups which cannot be electro-reduced there are to be understood groups which cannot be electro-reduced under the conditions of the electrosynthesis.

As already indicated, it is preferred to use an aromatic organic halide, which means that A is preferably an aromatic heterocyclic group, or an aryl group which may optionally be substituted by an aliphatic or cycloaliphatic group, R1, which is itself optionally substituted and/or unsaturated and which contains from 1 to 21 carbon atoms, or by one or more functional groups R2 which cannot be electro-reduced.

If X' is halogen, X and X' are preferably carried on the same carbon atom or by carbon atoms in the a, ss or X position.

If A is an aromatic heterocyclic group, it is preferably thiophenyl, furan or pyridine.

If the R1 group is substituted, the substituents are preferably one or more aromatic groups or one or more functional groups R3 which cannot be electro-reduced.

The functional groups R3 which cannot be electro-reduced are preferably carbonyl, cyano, ether, sulphide, ester ortertiary amine groups or fluorine.

In another embodiment of the process of the invention, the process is carried out using a mixture of organic halides comprising at least one aromatic halide.

The process according to the invention is characterised by the fact that the anode is formed of magnesium.

The anode can have any desired shape and, in particular, any of the conventional shapes of metallic electrodes which are well known to those skilled in the art (twisted wire, flat bar, cylindrical bar, renewable bed, balls, gauze, grid, etc.). A cylindrical rod having a diameter adapted to the dimensions of the cell is preferably used. For example, for a cell with a total volume of 45 cm3, the diameter of the bar is suitabiy about 1 cm.

Before use it is preferred to clean the surface of the anode chemically (for example with diluted HCI) or mechanically (for example with a file or emery cloth), in particular so as to remove the magnesium oxide which is often present on the surface of the metal. The purity of the magnesium is not an important parameter and industrial grades are suitable.

As far as we have been able to ascertain no metal other than magnesium enables all the results obtainable by means of the present invention, to be obtained. In particular, tests have been carried out with anodes formed of lithium, aluminium or zinc, all other conditions of the process being identical to those employed when the anode is formed of magnesium and these have shown that the yield of carboxylic acid is less than that obtained with the magnesium anode (see Examples 43 to 45 below).

The cathode may be formed of any metal, for example stainless steel, nickel, platinum, gold or copper, or of graphite. It preferably consists of a cylindrical grid or plate arranged concentrically around the anode. For economic reasons, stainless steel is preferably used.

The electrodes are supplied with direct current via a stabilised supply unit.

Suitable organic solvents for use in the process are low protic solvents such as are conventionally used in organic electrochemistry. Suitable solvents include, for example, hexamethylphosphorotriamide (HMPT), tetrahydrofuran (THF), THF-HMPT mixtures, N-methylpyrrolidone (NMP), tetramethylurea (TMU), dimethylformamide (DMF), acetonitrile and acetone.

The inert electrolyte used may be any of those conventionally used in organic electrochemistry. Suitable electrolytes include, for example, tetrabutylammonium tetrafluoroborate (NBu4BF4), lithium perchlorate (LiC104), tetrabutylammonium chloride (NBu4CI), tetraethylammonium chloride (NEt4CI), and tetrabutylammonium perchlorate (NBu4C104). The concentration of inert electrolyte in the organic solvent is preferably from 10-2 M to 2 x 10- M and the concentration of organic halide or halides in the organic solvent is preferably from 10-2 M to 1 M. The concentration of halide can therefore be relatively high, which is unusual in electrosynthesis and of value from an economic point of view.

The preferred manner of carrying out the process is, in detail, as follows: 1 ) The temperature is preferably from 0 C to 60"C, more preferably ambient temperature (about 20"C) for convenience.

2) The current density at the magnesium anode which is preferably from 10-1 to 100 mAlcm2, more preferably from 5 to 50 mA/cm2. In general, a constant current is employed, but it is also possible to work at constant voltage, at a controlled potential, or with both variable current and variable potential.

3) The carbon dioxide gas pressure in the cell is preferably from 10-1 to 50 bar, more preferably atomospheric pressure for convenience. At atmospheric pressure, for example, the gas can be bubbled through the solution by means of a dip tube.

4) The solution is preferably stirred, for example by means of a bar magnet.

After an amount of electricity corresponding to 2 Faradays (2 x 96,500 C) per mole of monohalogenated derivative (2n Faraday per mole or n-halogenated derivative) has been passed: 1. the amount of residual organic halide is measured by gas phase chromatography (GPC), in order to determine the degree of conversion, 2. the amount of carboxylate formed is determined by acid-base titration in order to calculate the yield of product relative to the initial amount of organic halide, and 3. the carboxylic acid formed is isolated in order to characterise it and calculate the yield of isolated product.

The yields obtained in respect of the carboxylate formed are high, very ofter greater than 99%. The yields of carboxylic acid isolated vary from 70 to 90% of the yield of carboxylate formed.

In the case of aromatic chlorinated derivatives of the type XAX' in which the chlorine atom is directly bonded to the aromatic nucleus, where X s chlorine, X' is hydrogen or chlorine, and A is an aryl group R2 as defined above, (for example chlorobenzene), better yields are obtained when the current density is low, the temperature is below 20"C (for example 5"C), and the solvent is DMF.

In order that the invention may be more fully understood, a suitable electrolysis cell for use in the process according to the invention will now be described, by way of example, with reference to the accompanying drawing, in which the single Figure is a schematic elevation of the cell.

The cell has only one compartment. The upper part 5 is made of glass and is provided with 5 necks for the carbon dioxide gas inlet and outlet, the electrical leads, and access for sampling of the solution during the electrolysis (neck 4). The lower part 6 consists of a sleeve provided with a base 3 and screwed on to the glass part 5.

The anode lisa cylindrical rod of magnesium and the cathode 2 is a cylindrical stainless steel gauze arranged concentrically around the anode 1.

The arrows A and B denote the carbon dioxide gas inlet and outlet respectively.

In order that the invention may be more fully understood the following examples are given by way of limitation only.

Example 1 The cell used was of the type described above with reference to the Figure. The total volume of the cell was 45 cm3. The anode was a cylindrical magnesium rod of 1 cm diameter. It was introduced into the cell through the central neck and immersed in the solution to a depth of 2.5 cm. The initial working surface of this electrode was 8 cm2.

35 ml of anhydrous NMP, 17.4 millimoles of benzyl chloride (C6H5CH2CI) and 1.75 millimoles of tetrabutylammonium tetrafluoroborate (NBu4BF4) were introduced into the cell. CO2 was bubbled into the solution by means of a tube dipping into the solution. The CO2 was at atmospheric pressure.

The solution was stirred by means of a bar magnet and kept at ambient temperature (about 20"C).

The electrodes were supplied with direct current from a stabilised supply unit; a constant current of 400 mA, corresponding to a current density of 50 mA/cm2, was applied to the magnesium electrode.

After 2 Faradays had been passed per mole of benzyl chloride, the latter had been completely converted to phenyl acetate (C6HsCH2CO2), as shown by GPC (determination of C6H5CH2Cl) and by carboxylate determination. The latter was carried out as follows: 2 ml of solution were treated with 1 M NaOH and then with diethyl ether. The aqueous phase was separated off, acidified with 1 M H2SO4, and extracted twice with ether. Water was added to the ether phase, the mixture was rendered basic and the determination of carboxylate was carried out by titration with H2SO4.

When the same procedure was carried out on the whole of the solution, but stopped after the extraction of the acid medium with ether, and ether was then evaporated, pure phenylacetic acid (identified by its melting point of 76"C, against a theoretical value of 77"C, and by its NMR and IR spectra) was obtained in a yield of 90% relative to the initial C6H5CH2CI.

Examples 2 to 7 The following experiments were carried out under conditions similar to those of Example 1, but using different current densities at the magnesium anode.

Example Current density Degree of Yield of No. at the anode conversion (%) CH5CH2CO2 (mA/cm2) of C6H5CH2Cl (%) at2 Faradaylmole 2 6.25 > 99 > 99 3 12.5 > 99 > 99 4 25 > 99 > 99 5 31.25 > 99 > 99 6 37.5 > 99 > 99 7 125 90 80 The yields of C6H5CH2CO2- are those found by acid-base titration.

In the case of Example 7 it should be noted that 7% of C6H5CH2CH2C6H5 and 3% of C6H5CH3 were also formed.

Examples 8 to 13 The following experiments were carried out under conditions similar to those described in Example 1, but with different concentrations of the electrolyte NBu4BF4 or with other electrolytes. The current density at the magnesium anode was kept constant at 31.25 mA/cm2.

Example Nature and con- Degree ofcon- Yield of No. centration of version(%) of C6H5CH2CO2 the electrolyte C6H5CH2Cl (%) (molell) at 2 Faradaylmole 8 NBu4BF4(0.02) > 99 > 99 9 NBu4BF4(0.2) > 99 > 99 10 NBu4CI (0.05 > 99 > 99 11 NEt4CI (0.05) 90 70 12 LiClO4 (0.05) 95 92 In Examples 11 and 12, toluene was obtained as another reduction product. This was due to the use of hydrated salts.

Examples 13 to 17 The following experiments were carried out under conditions similar to those of Example 1, but with solvents other than NMP and with varying current densities, as specified for each example.

Example Solvent Current density Degree ofcon- Yield of No. at the anode version (%) of C6H5CH2C02 (mAlcm) C6H5CH2Cl at (%) 2Faradaylmole 13 Dimethyl- 50 > 99 95 formamide 14 Tetrahydro- 37.5 > 99 > 99 furan 15 Tetrahydro- 6.25 > 99 > 99 furan (75% by volume) + hexa methylphosphoramide (25% by volume) 16 Acetone 31.25 > 99* 80 17 Acetonitrile 27.5 > 99** > 99 In Example 14, the low solubility of the carboxylate formed, which in part deposited on the magnesium anode, should be noted.

* In the case of Example 16, the electrolysis could not be taken to completion (2 Faraday/C6H 5CH2CI) because of an increase in the viscosity of the solution, which became converted to a gel. The electrolysis was stopped after 7.5 millimoles of C6H5CH2CI, representing 43% of the initial amount (17.4 millimoles) had been consumed.

** In the case of Example 17, the electrolysis was stopped after 1 Faraday per mole of C6H5CH2CI had been passed. The yield of C6H5CH2 COO shown was calculated on this basis.

Examples 18 to 20 The following experiments were carried out under conditions similar to those of Example 1, but in the presence of varying amounts of benzyl chloride and with a current density of 50 mA/cm2 at the magnesium anode.

TABLE 4 Example Initial C6H5CH2Cl Yield of No. C6H5CH2CI converted C6H5CH2CQ (mllllmoles) (millimoles) o) 18 8.7 8.7 > 99 19 34.8 25 > 99 20 73.9 30 95 Under these experimental conditions, it is not, in practice, possible to produce concentrations of C6H5CH2CO2-in excess of about 0.7 M. Above this concentration, the solutions are too viscous for electrolysis to continue.

In Examples 19 and20, the electrolyses were stopped after 1.4 and 0.8 Faradays respectively had been passed per mole of C6H5CH2CI.

Examples 21 to 41 The following examples were carried out under the same general conditions as those described in Examples 1 to 20. The halogenated derivatives set out in Tables I and II below were used in place of C6H5CH2CI TABLE I - Working conditions Example Organic halide Solvent Current density Temperature No. at the Mg anode (mA/cm2) 21 p-Bromofluorobenzene THF-HMPT 12.5 ambient 22 p-Bromofluorobenzene NMP 25 40"C 23 p-Bromofluorobenzene DMF 25 40 C 24 p-Bromofluorobenzene DMF 31.25 5"C 25 2-Chlorothiophene THF-HMPT 6.25 ambient 26 3-Bromofuran THF-HMPT 6.25 ambient 27 -Bromostyrene (mixture THF-HMPT 6.25 ambient of the cis and trans isomers) 28 p-Bromoacetophenone THF-HMPT 6.25 ambient 29 1-Bromodecane THF-HMPT 6.25 ambient 30 1-Chloroethylbenzene THF-HMPT 6.25 ambient (phenethyl chloride) 31 3-Chloro-1-phenyl- THF-HMPT 6.25 ambient propene 32 Bromobenzene THF 15 ambient 33 Ethyl- chloroacetate THF 6.25 ambient 34 Bromobenzene DMF 37.5 5"C TABLE I (continuation) Example Organic halide Solvent Current density Temperature No. at the Mg anode (mA/cm2) 35 Methylene chloride THF 6.25 ambient 36 1-Bromododecane DMF 18.75 ambient (12.3 mmol) 37 1-Bromooctadecane THF-HMPT 6.25 ambient (6 mmol) 38 Monochloroacetone DMF 31.25 5 C (12.4 mmol) 39 1,3-Dichloroacetone DMF 37.5 5 C (9.5 mmol) 40 1,2-Dichloroethane DMF 37.5 5 C (12.6 mmol) 41 Chlorobenzene(l9mmoI) DMF 12.5 5 C TABLE II - Results Example Organic halide % of halide Yield of Yield of carboxy- Melting point of the No. converted at carboxylate lic acid isolated acid isolated, in C 2 Faradaylmole (in %) (in % of carboxylate formed) Experimental Theory result 21 p-Bromofluoro- > 99 > 99 85 184 185 benzene 22 p-Bromofluoro- 98 50 80 184 185 benzene 23 p-Bromofluoro- 78 67 80 184 185 benzene 24 p-Bromofluoro- > 99 91 80 184 185 benzene 25 2-Chlorothiophene > 99 > 99 80 126 129 26 3-Bromofuran > 99 95 78 121 122-123 27 ss-Bromostyrene > 99 > 99 80 106 trans: 135 (mixture of the cis: 42-56 cis and trans isomers 28 p-Bromoacetophenone > 99 > 99 82 206 210 29 1-Bromodecane > 99 > 99 75 28 28.6 30 1-Chloroethyl- > 99 > 99 80 not measured not benzene (phenethyl recorded chloride) 31 3-Chloro-1-phenyl- > 99 > 99 85 85 87 propene 32 Bromobenzene > 99 90 85 122 122.4 33 Ethyl chloroacetate > 99 90 not determined - 34 Bromobenzene > 99 81 85 122 122.4 35 Dichloromethane not determined mixture com- not determined - prising ClCH2CO2- and CH2(CO2)2'= not determined 36 1-Bromododecane > 99 60 not determined - (12.3 mmol) 37 1-Bromooctadecane > 99 50 not determined (6 mmol) 38 Monochloroacetone > 99 > 99 not determined - (12.4 mmol) 39 1,3-Dichloro- > 99 > 99 not determined - caetone (9.5 mmol) (after passing 4F/mole) 40 1,2-Dichlorethane > 99 > 99 not determined - (12.6 mmol) (after passing 4F/mole) 41 Chlorobenzene 78 66 not determined - (19 mmol) (+ 12% benzene) In Examples 33 and 35 to 41, the acids were not isolated. In all cases, the yields were determined by acid-base titration of the carboxylate formed. The yields of isolated acid were, for Examples 21 to 32 and 34, between 70 and 90% of the amount found by carboxylate determination. The isolated acids were identified by their NMR and IR spectra as well as by their melting point.

Example 42 The following experiment was carried out under conditions similar to those described in Example 1, except that a mixture of C6HsCH2CI and p-BrC6H4F was electrolysed in dimethylformamide using a current density at the anode of 31.25 mA/cm2, at a temperature of 5"C.

Number of Residual Residual C6H5CH2Co2 Faradaysper C6H5CH2CI p-BrC6H4F mole of (mmol) (mmol) p-FC6H4CO2 mixture formed (mmol) 0 9 9 0 1 4 4.5 not determined 2 0 0.5 16.7(+1.3of C6H5F) Example 43 The following experiment was carried out under conditions similar to those described in Example 1, but using a current density of 31.25 mA/cm2 and with the magnesium anode replaced by a zinc anode of identical shape and diameter.

After passing 2 Faradays per mole of C6H5CH2CI, only 60% of the latter had been converted to a mixture of C6H5CH3 (90%) and traces of dibenzyl and C6H5CH2CO2-.

Example 44 The following experiment was carried out under conditions similar to those described in Example 1, but in a mdeium of DMF containing 5 x 10-2 M of LiClO4, at 5"C, using a current density of 31.25 mA/cm2 and with the magnesium anode replaced by a lithium anode of identical shape and diameter.

After passing 2 Faradays per mole of C6H5CH2CI, 40% of the latter had been converted to dibenzyl ( > 99%).

Example 45 The following experiment was carried out under conditions similar to those described in Example 34, but in a DMF medium containing 4 x 10-2M of NBu4BF4, at 5 C, using a current density of 20 mA/cm2, and with the magnesium anode replaced by an aluminium anode of identical shape and diameter.

After passing 2 Faradays per mole of C8H5Br, 50% of the latter had been converted to a plurality of products, including benzene and benzoic acid, the latter compound being the major constituent and representing 60% of the products formed.

Example 46 An electrolyser having an anode consisting of a magnesium rod centred along the axis of a stainless steel tube which served as the cathode, was used. The solution to be electrolysed, contained in a thermostatically controlled reactor, was circulated between these two electrodes by means of a pump. This solution consisted of 150 g of DMF containing 1 g of NBu4BF4 and 4 g of 1-(6-methoxy-naphth-2-yI)-1 -chloroethane.

The CO2 pressure was 1 bar.

After passing 1.2 times the theoretical amount of electricity, i.e. 2.4 Faradays per mole of chloride (the electrolysis being carried out at a constant current of 500 mA), followed by evaporation of the solvent, neutralisation of the mixture and extraction, the corresponding acid (naproxen) was isolated in a yield of 80%. The product was identified by NMR spectrometry and mass spectrometry.

Example 47 The experiment was carried out in the electrolyser used in Example 46, but the solution consisted of 100 g of DMF containing 2 g of NBu4BF4 and 7 g of meta-phenoxyphenethyl chloride. After passing twice the theoretical amount of electricity, i.e. 4 Faradays per mole of chloride (the electrolysis being carried out at a constant current of 1A),80% of the corresponding acid (fenoprofen) was obtained, identified by mass spectrometry and N MR spectrometry, together with 18% of unconverted starting material.

Example 48 The experiment was carried out under conditions similar to those described in Example 45, but with the aluminium anode replaced by a zinc anode of identical shape and diameter. After passing 2 Faradays per mole of CeH5Br, 15% of the latter had been converted to a plurality of products, including 80% of benzene and traces of benzoic acid.

Example 49 The experiment was carried out under the same conditions as those of Example 48, but the DMF was replaced by THF. After passing 2 Faradays per mole of C6H5Br, 10% of the latter had been converted to a plurality of products, including 90% of benzene and traces of benzoic acid.

Example 50 The experiment was carried out under the same conditions as Example 47, but the metaphenoxyphenethyl chloride was replaced bypara-trifluoromethylchlorobenzene and a CO2 pressure of 4 bar instead of 1 bar was used.Purepara-trifluoromethylbenzoicacid, identified by NMR spectrometry and mass spectrometry, was isolatred in a yield of 89%.

Example 51 The experiment was carried out under the same conditions as Example 50, but the paratrifluoromethylchlorobenzene was replaced bypara-bromodiphenyl ether. Purepara-phenoxybenzoic acid, identified by NMR spectrometry and mass spectrometry, was isolated in a yield of 68%.

Example 52 The experiment was carried out under the same conditions as Example 50, but the paratrifluoromethylchlorobenzene was replaced by ortho-dichlorobenzene. Ortho-chlorobenzoic acid, identified by NMR spectrometry and mass spectrometry, was isolated in a yield of 66%.

Example 53 The experiment was carried out under the same conditions as Example 52, but the ortho-dichlorobenzene was replaced by 1,2,4-trichlorobenzene and only 2 Faradays per mole of trichlorobenzene were passed. After extraction, a mixture of dichlorobenzoic acids, composed of 90% of 2,4-dichlorobenzoic acid and 10% of 2,5-dichlorobenzoic acid, was isolated in a yield of 50%.

Example 54 The experiment was carried out under the same conditions as Example 53, but the 1,2,4-trichlorobenzene was replaced by 2,5-dichlorotoluene. A mixture of chlorotoluic acids was obtained in a yield of 90%. This mixture was composed of 89% of 3-chloro-4-methyl-benzoic acid and 11% of 1 -methyl-4-chlorobenzoic acid.

Claims (15)

1. A process for the preparation of a carboxylic acid, which comprises subjecting an aromatic organic halide to electrochemical reduction in an electrolysis cell containing an organic solvent medium containing an inert electrolyte and in the presence of carbon dioxide, the cell having an anode formed of magnesium.

2. A process according to claim 1, in which the aromatic organic halide is a compound of the formula XAX', in which: Xis chlorine, bromine or iodine, X'is hydrogen, chlorine, bromine or iodine, and A is an aromatic heterocyclic group or an aryl group which may optionally be substituted by an aliphatic or cycloaliphatic group R1 which is itself optionally substituted and/or unsaturated and which contains 1 to 21 carbon atoms, or by one or more functional group R3 which cannot be electro-reduced.

3. A process according to claim 2, in which in the aromatic halide XAX', A is thiophen, furan or pyridine.

4. A process according to claim 2, in which the aliphatic group R1 is substituted by one or more aromatic groups or by one or more functional groups R3 which cannot be electro-reduced.

5. A process according to claim 2 or 4, in which the functional group R3 which cannot be electro-reduced is a carbonyl, cyano, ether, sulphide, ester or tertiary amine group or fluorine.

6. A process according to any of claims 1 to 5, in which the starting material is a mixture of organic halides containing at least one aromatic organic halide.

7. A process according to any of claims 1 to 6, in which the organic solvent is hexamethylphosphorotriamide (HMPT), tetrahydrofuran (THF), a HMPT-THF mixture, acetone, acetonitrile, N-methylpyrrolidone (NMP), dimethyl formamide (DMF) ortetramethylurea (TMU).

8. A process according to any of claims 1 to 7, in which the inert electrolyte is tetrabutylammonium tetrafluoroborate, tetrabutylammonium perchlorate, or lithium perchlorate.

9. A process according to any of claims 1 to 8, in which the conventration of aromatic organic halide in the organic solvent is from 10-2M to 1 M and the concentration of inert electrolyte in the organic solvent is from 109M and 2.10-1M.

10. A process according to any of claims 1 to 9, which is carried out at a temperature of about 20or.

11. A process according to any of claims 1 to 10, in which the carbon dioxide gas pressure is atmospheric pressure.

12. A process according to any of claims 1 to 11, in which the electrochemical reduction is carried out at constant current.

13. A process according to any of claims 1 to 12, in which the cathode of the cell is formed of stainless steel.

14. A process for the preparation of a carboxylic acid according to claim 1, substantially as herein described in any of the Examples.

15. Carboxylic acid when prepared by the process claimed in any of the preceding claims.