EP0219367B1 - Organic electrolysis cell with a consumable electrode - Google Patents

Abstract

The present invention relates to a cell for the organic electrosynthesis of organic or organometallic compounds, containing two electrodes (2) and (4) of which only one (4) is sacrificed by the electrochemical reaction of which it forms the seat. The sacrificial electrode (4) consists of at least one solid metal block and is applied under the influence of its own weight against the other electrode (2) from which it is separated by an electrical insulating material (5) which allows the passage of the electrolytic solution (6) and of which the shape and the dimensions enable the active substances of the two electrodes (2) and (4) to remain parallel. The active surface of the electrode (2) has a constant inclination relative to a direction D (9) forming an angle less than 45 degrees with the vertical on the one hand, and an inclination less than 45 degrees relative to the vertical on the other. Any straight line in direction D (9) passing through any point on the electrode (4) passes through the active surface of the electrode (2).

Description

The present invention relates to an electrolysis cell for electrosynthesis, in an organic medium, of organic or organometallic compounds, comprising two electrodes, one and only one of which is consumed during electrosynthesis by the electrochemical reaction of which it is the seat.

The patents US-3,573,178 and US-3,141,841 describe the synthesis of tetraethyl lead in an electrolysis cell comprising an anode constituted by lead balls separated from the cylindrical cathode by a porous insulating wall. Balls are added during electrolysis to replace those which are consumed. However, this device does not work well for very reducing metals such as magnesium, aluminum, zinc, titanium, which are covered with an insulating oxide layer which considerably increases the contact resistance between grains. Furthermore, the presentation in granules of these metals is sometimes expensive. In addition, metallic sludge and dust often form, which disrupts operation.

South African patent No. 6806413 describes the synthesis of tetraethyl lead in an electrolysis cell comprising a consumable anode in the form of a metallic strip which passes between two cathodes in the form of discs. This system has a number of drawbacks. The thickness of the anode must in particular be small so that the interelectrode space remains constant; the speed of advance of the anode must therefore be rapid, and, to avoid rupture of the ribbon, the device requires a relatively complicated mechanical system.

FR-1 412 239 describes an electrolysis cell designed to operate in an aqueous medium and comprising 2 compartments, cathodic and anodic, separated by a diaphragm. The wedge-shaped anode is made of graphite.

SU-1,046,022 describes an anode device for obtaining metal powders by electrolysis of aqueous solutions using consumable anodes, comprising an anode chamber in the form of a triangular prism and soluble anode elements, in the form of plates arranged horizontally. The wall of the chamber oriented towards the cathode is in the form of a grid composed of insulated fins and inclined with respect to the horizontal plane. This device makes it possible to maintain a constant distance between the electrodes during electrolysis.

Also known are several mechanical devices, often very complicated, allowing the distance between the electrodes to be controlled in order to keep it constant or allowing replacement of the used anodes. German patent 2107305 describes for example such a device.

Electrolysis cells comprising a consumable anode have already been described for the electrosynthesis of oxalic acid from carbon dioxide, on the one hand with aluminum in Chim. lnd. (Milan) 55. (1973) 156 and on the other hand with zinc in J. Appl. Electrochem. 11 (1981) 743, for the electrocarboxylation of ethylene (Tetrahedron Lett. 1973,3025) and for that of thioethers (Patent of the Democratic Republic of Germany No. 203537).

These cells are without diaphragm and generally have a coaxial cylindrical symmetry. In some cases the central electrode acts as a consumable anode (metal bar for example); in others, it acts as a cathode (graphite for example). These laboratory cells do not lend themselves to industrial use, in particular continuously, because on the one hand they require frequent and inconvenient renewal of the anode and on the other hand the distance between the 2 electrodes varies over time.

The present invention aims to provide an electrolysis cell allowing simple continuous industrial use, having the advantages of the above-mentioned industrial cells, namely in particular maintaining a constant gap between the electrodes, without having the disadvantages.

• - l'électrode consommable est constituée d'au moins un bloc métallique massif et s'applique, sous l'effet de son propre poids, contre l'autre électrode dont elle est séparée par un matériau isolant électrique laissant passer la solution d'électrolyse et dont la forme et les dimensions permettent aux surfaces actives des deux électrodes de rester parallèles au cours de l'électrosynthèse,
• -et en ce que la surface active de l'électrode non consommable est conique et présente en tous ses points une inclinaison inférieure à 45 degrés par rapport à la verticale.

The electrolysis cell according to the invention, for electrosynthesis in organic medium, of organic or organometallic compounds, comprising two electrodes, only one of which is consumed during electrosynthesis by the electrochemical reaction of which it is the seat, is characterized in that:

• - the consumable electrode consists of at least one solid metal block and is applied, under the effect of its own weight, against the other electrode from which it is separated by an electrical insulating material allowing the electrolysis solution to pass and whose shape and dimensions allow the active surfaces of the two electrodes to remain parallel during electrosynthesis,
• -and in that the active surface of the non-consumable electrode is conical and has at all points an inclination less than 45 degrees relative to the vertical.

The inclination, at a point of a surface, with respect to the vertical is conventionally considered to be the angle formed by the plane of tangency to the surface at this point and by the vertical passing through this point.

The cell according to the invention has many advantages. First of all, it allows a constant and preferably small gap (less than 5 mm) to be maintained between the two electrodes during the entire duration of the electrolysis, which is very important in an organic conductive medium, in order to avoid excessive power consumption and overheating by Joule effect.

One of the two electrodes being progressively consumed during the electrochemical reaction, a means is necessary necessarily allowing the distance between the two electrodes to be kept constant, which is obtained in the context of this invention, thanks to the particular design and geometry of the cell. It is also necessary to be able to easily replace the consumable electrode as soon as it is completely consumed, or better, for continuous processes, as it is consumed, without stopping and disturbing the electrolysis.

The cell according to the invention allows very easy replacement of the consumable electrode, without stopping the electrolysis, by superposition of one (or more) another block on the solid metal block (s) constituting the consumable electrode, which is a considerable advantage when implementing continuous processes. In addition, the entire electrode is consumed, without falling or loss. The cell according to the invention also allows the use of massive consumable electrodes, therefore not very bulky for a given mass, and of various shapes. This fact is economically very interesting.

Another advantage is the fact that, taking into account the geometry of the cell and in particular the inclination of the non-consumable electrode, the space requirement on the ground is very reduced, which allows a saving of space which is economically very appreciable.

In many cases the consumable electrode is the anode (anodic oxidation) as for the examples which follow but sometimes the consumable electrode is the cathode as is the case for the electrosynthesis of tetramethyl lead in acetonitrile medium from methyl bromide with lead cathode according to HE. Ulery JECS 116, 1201, 1969:

The consumable electrode consists of at least one solid metal block. Preferably, the metal is chosen from the group consisting of magnesium, aluminum, zinc and their alloys, namely any alloy containing at least one of the three aforementioned metals. Many other metals are also suitable, such as copper, nickel, and lead. The choice of metal depends inter alia on the compound which one wants to synthesize. In the case of electrosynthesis of organometallic derivatives, the consumable electrode is for example constituted by the corresponding metal or an alloy based on this metal.

In the case of electrosynthesis of carboxylic acids by reduction of organic halides in the presence of C0 _2, magnesium is preferred. For the electrosynthesis of alcohols by electrochemical reduction of organic halides in the presence of carboxylated derivatives as well as for the electrosynthesis of ketones and aldehydes by electrochemical reduction of organic halides in the presence of organic acid anhydrides, a metal chosen from the group formed by magnesium, zinc, aluminum and their alloys.

The solid metal blocks can be, for example, ingots whose cross section is square, or rectangular, or trapezoidal, or circular, or in any other form. They can optionally be machined before use so that their geometry is adapted to that of the non-consumable electrode. Preferably, but without this having an imperative character, such machining is carried out to facilitate the start of electrolysis.

According to a preferred variant, the consumable electrode consists of solid metal blocks stacked, each layer of the stack comprising only one block. According to another variant, at least one layer of the stack comprises several blocks arranged side by side.

The consumable electrode is applied under the effect of its own weight, by gravity, against the other non-consumable electrode. According to a preferred variant, the consumable electrode is applied against the other electrode under the sole effect of its own weight. According to another variant, the consumable electrode is applied against the other electrode under the effect, in addition to its own weight, of that of an inert charge resting on the consumable electrode. Preferably, the inert charge is electrically conductive and also serves to ensure the electrical supply of the consumable electrode.

According to another variant, the consumable electrode is applied against the other electrode under the effect, in addition to its own weight, of the force produced by a compressed spring between the upper part of the consumable electrode and a wall of the cell.

The non-consumable electrode is made of a conductive material. Without limitation, mention may be made of metals such as iron, aluminum and nickel, alloys such as stainless steel, metal oxides such as Pb0 ₂ and Ni0 ₂ , graphite. Preferably, it is made of a metal chosen from the group consisting of nickel and stainless steel.

Preferably, the distance between the active surfaces of the two electrodes is less than 5 mm. This distance is conventionally measured on a common perpendicular, between the two parallel surfaces;

The two electrodes are separated by an electrical insulating material allowing the electrolysis solution to pass and whose shape and dimensions allow the active surfaces of the 2 electrodes to remain parallel during electrosynthesis. Of course, this electrical insulating material must have sufficient mechanical strength to support the consumable electrode which rests on this material.

Preferably, the electrical insulating material is a plastic material in the form of a grid whose thickness is less than 5 mm and whose mesh consists of two networks of parallel wires, these two networks being superimposed, crossed, fixed one on the other at the contact points of the wires, the thickness of the wires of each network being the same. In general, the two networks are fixed to each other by welding and the wires of the two networks have the same thickness.

As an indication, the distance between the wires of each network is between a few millimeters and a few centimeters.

The wires in each network may not be parallel; their thickness may not be constant provided that after assembling the networks, the mesh has a constant maximum thickness at several points, less than about 5 mm.

The cross section of the wires can be arbitrary, for example square, rectangular, circular, elliptical, trapezoidal.

The plastic material can be, for example, polypropylene, polyethylene or polytetrafluoroethylene.

Such plastic gratings have on the one hand a high vacuum rate, which allows easy circulation of the electrolysis solution between the two electrodes and on the other hand a relatively small contact surface with the electrodes, which avoids an excessive reduction in their active surface.

As other materials separating the two electrodes, it is possible to use, within the framework of the present invention, a fabric, a canvas or a porous material of constant thickness such as for example a ceramic or a felt.

The renewal of the electrolysis solution between the electrodes can, for example, be ensured by mechanical agitation or by forced circulation using a pump, for example.

During electrolysis, the active surface of the consumable electrode opposite the active surface of the other electrode dissolves. The consumable electrode therefore descends gradually, by gravity, under the simple effect of its own weight. Furthermore, the dissolution being stronger at the locations closest to the non-consumable electrode, the consumable electrode tends to conform as best as possible to the shape of the non-consumable electrode, which limits the risks of irregular dissolution.

• - La figure 1 représente une vue de face d'un mode de réalisation d'une cellule d'électrolyse selon l'invention,
• - La figure 2 représente, en coupe droite selon IV-IV, la cellule représentée figure 1.
• - La figure 3 représente une vue en perspective d'une matière plastique en forme de grillage utilisable comme matériau isolant électrique entre les deux électrodes, et
• - La figure 4 représente un schéma synoptique d'une installation complète d'électrolyse.

The following description of a particular embodiment of the invention illustrates the invention, without limiting it.

• FIG. 1 represents a front view of an embodiment of an electrolysis cell according to the invention,
• - Figure 2 shows, in cross section along IV-IV, the cell shown in Figure 1.
• FIG. 3 represents a perspective view of a plastic material in the form of a grid usable as an electrical insulating material between the two electrodes, and
• - Figure 4 shows a block diagram of a complete electrolysis installation.

The electrolysis cell shown in Figures 1 and 2 comprises a tank whose bottom wall is formed by the cathode 12, which is not consumable. The active surface of the non-consumable electrode 12 is conical, the tip of the cone being directed downwards. This active surface has at all points a constant inclination of 15 degrees relative to the direction 19 which is that of the axis of the cone. For the cell shown in Figures 1 and 2 this axis is vertical.

The upper wall 21 of the tank is cylindrical and extends the cone so that the cylinder and the cone have the same axis, the diameter of the cylinder being the same as that of the base circle of the cone.

A horizontal circular wall 20, of diameter equal to that of the cylinder, closes the tank at its upper part.

The walls 20 and 21 are made of an electrical insulating material or internally covered with an electrical insulator 11, for example a paint or any other electrically insulating coating.

The anode 14 consists of a stack of cylindrical solid metal ingots whose diameter is slightly less than that of the cylindrical wall 21 of the tank. It is applied under the sole effect of its own weight against the cathode 12 which, alone, ensures the maintenance of the anode 14.

Any straight line 19 passing through any point of the consumable anode 14 crosses the active surface of the non-consumable cathode 12.

The anode 14 and the cathode 12 are separated by a plastic material 15 in the form of a mesh such as that shown in FIG. 3. The mesh consists of two networks of parallel wires A ₁ B ₁ C ₁ ... N ₁ of part and A ₂ 8 ₂ C _Z ... N ₂ on the other hand. The wires of these two networks are cylindrical, with a diameter of 1 mm. The distance between the wires is 1 cm. The two networks are superimposed, crossed at right angles and welded to the contact points of the wires.

The electrolysis solution 16 circulates from bottom to top in the cell. Pipes 18 allow the arrival and the exit of this solution 16, in the direction of the arrows 17.

The inlet pipe extends the tip of the cathode 12 along the axis of the cell. The electrodes 12 and 14 are supplied with electric current by a DC voltage source, not shown in FIGS. 1 and 2.

When the axis of the cell is rotated by an alpha angle around the tip of the cone, the active surface of the non-consumable electrode 12 always has at all its points a constant inclination of 15 degrees relative to the direction D represented by the axis of the cell and any straight line of direction D passing through any point of the consumable anode 14 crosses the active surface of the non-consumable cathode 12. First of all alpha must be less than 45 degrees in the context of the present invention. Furthermore, relative to the vertical, the inclination of the active surface of the non-consumable electrode 12 is between (15 + alpha) and 115-alphal.

In the context of the present invention, the inclination relative to the vertical must be less than 45 degrees, that is to say for this particular embodiment, that alpha must be less than 30 degrees. Otherwise, there may be significant anomalies in the functioning of the cell.

The upper wall 20 of the electrolysis cell according to the invention is removable or has a part removable so as to allow the introduction of massive metal blocks.

A complete installation allowing the continuous electrolysis of a solution is schematically represented in FIG. 4.

It consists of a closed circuit comprising a jacketed reactor 51 allowing the loading and recovery of the products, an electrolysis cell 52 and a pump 53 allowing the circulation of the electrolysis solution in the circuit. The lower part of the reactor 51 is connected to the lower part (inlet) of the cell 52 and the outlet of the cell 52 is connected to the upper part of the reactor 51.

The jacketed reactor 51 is cooled by a circulation of water, symbolized by the arrows 54.

The direction of circulation of the electrolysis solution previously defined is symbolized by the arrows 55.

The cell 52 shown diagrammatically in FIG. 4 is that shown in FIGS. 1 and 2.

The present invention also relates to the use of the new electrolysis cells described above, provided with an anode consumable in a metal chosen from the group formed by magnesium, zinc, aluminum and their alloys for electrosynthesis. in an organic solvent medium of organic compounds chosen from the group consisting of carboxylic acids, alcohols, ketones and aldehydes by electrochemical reduction of organic halides.

According to a first variant, an electrolysis cell is used provided with an anode consumable in a metal chosen from the group formed by magnesium and its alloys for the electrosynthesis of carboxylic acids by electrochemical reduction of organic halides in the presence of carbon dioxide.

It has been found that, completely unexpectedly, very high yields are obtained with little or no by-products, while the implementation is very simple and does not require one or more catalysts. This particular use applies to the electrosynthesis of a very large number of carboxylic acids, both aliphatic and aromatic. As the aliphatic chain, mention may, for example, be made of alkyl or cycloalkyl chains, substituted or not, unsaturated or not, containing from 1 to 21 carbon atoms.

As aromatic chains, mention may, for example, be made of substituted or unsubstituted phenyl, thiophene, furan and pyridine rings. The carboxyl group can as well be linked to an aliphatic carbon as to a carbon of an aromatic ring.

The use of a magnesium anode provides the best results. In particular, tests have been carried out with an anode either of aluminum or of zinc, all other conditions identical elsewhere. The yields are then lower than those obtained with the magnesium anode. The organic solvents used are the low protein solvents usually used in organic electrochemistry, such as hexamethylphosphorotriamide (HMPT) letetrahydrofuran (THF), N-methylpyrolidone (NMP), dimethylformamide (DMF).

The organic solvent conventionally contains an indifferent electrolyte such as tetrabutylammonium tetrafluoroborate (BF ₄ NBu ₄ ) or lithium perchlorate.

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

According to a second variant, an electrolysis cell is used provided with an anode consumable in a metal chosen from the group formed by magnesium, zinc, aluminum and their alloys for the electrosynthesis of alcohols, by electrochemical reduction. of organic halides having an atom or a functional group stabilizing carbanions attached to the carbon carrying the halogen, in the presence of carbonyl derivatives.

The latter can be aldehydes as well as ketones; the yields are high and the implementation relatively simple.

The organic halides have at least one atom or a functional group stabilizing carbanions, attached to the carbon carrying the halogen, that is to say located in the alpha position relative to the halogen.

The atoms and functional groups which stabilize carbanions are well known to those skilled in the art. Mention may be made, for example, of halogens, ester, ketone, allyl, benzene, alkoxy or nitrile groups.

• - un groupement benzylique, substitué ou non substitué
  
  Ar représentant un groupement aromatique)
• - un groupement allylique, substitué ou non substitué
  
• - un groupement alpha monohalogéné (-C-X), alpha dihalogéné
  
  ou alpha trihalogéné (CX₃)
• - un groupement alpha ester
  
• - un groupement alpha cétonique
  

Preferably, the organic halides which can be used in the context of the present invention correspond to the general formula RX in which X represents a halogen atom and R represents:

• - a benzyl group, substituted or unsubstituted
  
  Ar representing an aromatic group)
• - an allylic group, substituted or unsubstituted
  
• - an alpha monohalogenated group (-CX), alpha dihalogenated
  
  or trihalogenated alpha (CX ₃ )
• - an alpha ester group
  
• - an alpha ketone group
  

By way of illustration and without limitation, mention may be made, for example, of benzyl chloride, benzyl bromide, allyl chloride, 3-chloro 2 methyl propene, 3-chloro 1 butene, 1-chloro 1-methyl ethyl acetate, carbon tetrachloride, dichlorophenylmethane, 1-phenyl 3-chloro propene and 1-methyl 3-chloropropene.

dans laquelle R, et R₂, identiques ou différents, représentent:

• - un atome d'hydrogène,
• - une chaîne aliphatique ou cycloaliphatique, substituée ou non substituée, saturée ou non saturée,
• - un groupement aryle, substitué ou non substitué,

According to a particular embodiment, the carbonyl derivatives correspond to the general formula

in which R, and R ₂ , identical or different, represent:

• - a hydrogen atom,
• - an aliphatic or cycloaliphatic chain, substituted or unsubstituted, saturated or unsaturated,
• - an aryl group, substituted or unsubstituted,

or alternatively, R, and R ₂ , together with the carbon atom to which they are attached, form a ring, saturated or unsaturated, substituted or unsubstituted, optionally comprising one or more heteroatoms such as nitrogen, oxygen , phosphorus or sulfur. By way of illustration and without limitation, mention may, for example, be made of acetone, cyclohexanone, methyl ethyl ketone, acetaldehyde, benzophenone and dichlorobenzophenone.

dans laquelle R, R, et R₂ ont la signification précitée.According to a preferred variant, the alcohols obtained according to the process which is the subject of the present invention correspond to the general formula

wherein R, R, and R ₂ have the above meaning.

Particularly preferably, when the carbonyl derivatives are ketones, that is to say when R 1 and R ₂ are different from hydrogen, tertiary alcohols are obtained.

In general, to carry out the present invention, it is quite obvious to the skilled person that the carbonyl derivative must be more difficult to reduce than the organic halide and none of the substituents carried by R, and R ₂ must not be more electrophilic. than the carbonyl group itself.

The organic solvents and the indifferent electrolytes used are the same as those mentioned above for the synthesis of carboxylic acids. Preferably, DMF is used as solvent and the electrolysis is carried out at a temperature between -20 ° C. and + 30 ° C.

According to a third variant, an electrolysis cell is used provided with an anode consumable in a metal chosen from the group formed by magnesium, zinc, aluminum and their alloys for the electrosynthesis of ketones and aldehydes by electrochemical reduction of organic halides in the presence of organic acid anhydrides. The implementation is simple and the mass and faradaic yields high.

• - une chaîne aliphatique ou cycloaliphatique, substituée ou non substituée, saturée ou non saturée,
• - un groupement aryle, substitué ou non substitué,
• - un hétérocycle aromatique, substitué ou non substitué, comme par exemple le cycle thiophène, furanne ou pyridine.

According to a particular embodiment, the organic halides correspond to the general formula R ₃ X in which X represents a halogen chosen from the group consisting of chlorine, bromine and iodine and R ₃ represents:

• - an aliphatic or cycloaliphatic chain, substituted or unsubstituted, saturated or unsaturated,
• - an aryl group, substituted or unsubstituted,
• - an aromatic heterocycle, substituted or unsubstituted, such as for example the thiophene, furan or pyridine ring.

Preferably, R ₃ has an aliphatic chain substituted with at least one aromatic group, for example in benzyl chloride, benzyl bromide, 1-phenyl 1-chloro ethane and 1-phenyl 1-chloro propane.

In general, R ₃ can carry non-electro-reducible functions or more difficult to reduce than the R ₃ -X bond, under the experimental conditions of electrosynthesis. Such non-electroreducible functions are, for example, the cyano, ether, sulfide or ester functions.

• - un atome d'hydrogène,
• - une chaîne aliphatique ou cycloaliphatique, substituée ou non substituée, saturée ou non saturée,
• - un groupement aryle, substitué ou non substitué, ou
• - un hétérocycle aromatique, substitué ou non substitué, comme par exemple le cycle furanne, thiophène ou pyridine,
  et R_s représente:
• - une chaîne aliphatique ou cycloaliphatique, substituée ou non substituée, saturée ou non saturée,
• - un groupement aryle substitué ou non substitué,
• - un hétérocycle aromatique, substitué ou non substitué, comme par exemple le cycle furanne, thiophène ou pyridine, ou
• - un groupement OR₆ dans lequel R₆ représente:
• ^* une chaîne aliphatique ou cycloaliphatique, substituée ou non substituée, saturée ou non saturée,
• ^* un groupement aryle, substitué ou non substitué,
• ^* un hétérocycle aromatique, substitué ou non substitué, comme par exemple le cycle furanne, thiophène ou pyridine,
  ou bien encore R₄ et R_s forment au moins un cycle, substitué ou non substitué, comme c'est le cas par exemple pour l'anhydride phtalique ou l'anhydride succinique.

According to another particular embodiment, the anhydrides of organic acids correspond to the general formula in which, R ₄ represents:

• - a hydrogen atom,
• - an aliphatic or cycloaliphatic chain, substituted or unsubstituted, saturated or unsaturated,
• - an aryl group, substituted or unsubstituted, or
• - an aromatic heterocycle, substituted or unsubstituted, such as for example the furan, thiophene or pyridine ring,
  and R _s represents:
• - an aliphatic or cycloaliphatic chain, substituted or unsubstituted, saturated or unsaturated,
• - a substituted or unsubstituted aryl group,
• - an aromatic heterocycle, substituted or unsubstituted, such as for example the furan, thiophene or pyridine ring, or
• - an OR ₆ group in which R ₆ represents:
• ^* an aliphatic or cycloaliphatic chain, substituted or unsubstituted, saturated or unsaturated,
• ^* an aryl group, substituted or unsubstituted,
• ^* an aromatic heterocycle, substituted or unsubstituted, such as for example the furan, thiophene or pyridine ring,
  or alternatively R ₄ and R _s form at least one ring, substituted or unsubstituted, as is the case for example for phthalic anhydride or succinic anhydride.

When R _s represents a group OR _s , the corresponding anhydrides are then mixed anhydrides of carboxylic acids and carbonic acid. In the other cases, they are anhydrous carboxylic acids.

lorsque les halogénures organiques répondent à la formule générale:

When R ₄ represents a hydrogen atom, aldehydes are obtained. In this case, when the organic halides correspond to the general formula R ₃ X previously defined, the aldehydes obtained correspond to the general formula R ₃ CHO. In the other cases, when R ₄ does not represent a hydrogen atom, ketones are obtained. These ketones correspond to the general formula

when the organic halides correspond to the general formula:

In general, R ₄ and R ₅ can carry non-electro-reducible functions, or more difficult to reduce than the bond R _3― X, under the experimental conditions of electrosynthesis, and none of the functions carried by R ₃ or R ₄ does must be more electrophilic than the anhydride function itself.

Preferably, R ₄ and R ₅ represent a linear or branched alkyl chain.

Also preferably, R ₄ and R _s are identical.

In a particularly preferred manner, R ₄ and R _s are identical and represent an alkyl chain, linear or branched, as is the case for example for acetic anhydride.

The organic solvents and the indifferent electrolytes used are the same as those mentioned above for the synthesis of carboxylic acids. Preferably, DMF is used as solvent.

As a general rule, when using an electrolysis cell according to the invention for electrosynthesis, in organic medium, of organic or organometallic compounds and in particular for the electrosynthesis of the abovementioned organic derivatives, the direction D is to preferably the vertical direction. The following examples illustrate the invention without limiting it.

Example 1. Synthesis of dimethylbenzylcarbinol

An electrolysis cell such as that shown in FIGS. 1 and 2 is used. The cathode 12, made of stainless steel, is a cone with a height of 100 mm and a base diameter of 53 mm. The other walls of the tank are made of stainless steel and are internally covered with an inert electrical insulating coating 11.

The anode 14 consists of a stack of cylindrical aluminum blocks with a diameter of 50 mm and a height of 100 mm.

The plastic material 15 in the form of a mesh is polypropylene. This mesh is just placed on the active surface of the cathode 12, the shape of which it matches, before the introduction of the anode 14.

The complete installation is as shown in Figure 4.

For the first electrolysis, the lower aluminum block was machined so that it is approximately in the form of a cone with a height of 100 mm and a base diameter of 50 mm, which is easily achieved from of a cylindrical block having these dimensions.

After positioning the plastic material 15 in the form of a grid on the active surface of the cathode, the machined block which introduces the shape of the cathode is introduced, then several other blocks are stacked on this lower block up to the top of the cell.

After having mixed 200 g of benzyl chloride (1.58 mol), 20 g of tetrabutylammonium iodide, 280 g of DMF and 1500 g of acetone in the reactor, the solution thus obtained is circulated in the installation.

For the first electrolysis, we first operate at a constant intensity of 1A. As soon as the lower aluminum block reaches the bottom of the cell, a constant intensity of 2.5 A is maintained. The electrolysis voltage then remains stable, at around 15 V, which proves the proper functioning of the cell. It stabilizes very quickly, due to the machining of the first block. The electrolysis is stopped after 42 h.

After stopping the electrolysis, the dimethylbenzylcarbinol formed is isolated, and identified according to the usual methods, well known to those skilled in the art. The alcohol formed was isolated after hydrolysis of the solution using an aqueous solution of ammonium chloride and extraction with ether. After evaporation of the ether, the crude alcohol was purified by distillation. The pure alcohol thus isolated (purity verified by CPG) is identified by its NMR and IR spectra.

The yield of distilled dimethylbenzylcarbinol obtained is 60% (purity greater than 95%).

To then carry out other electrolyses, the intensity of the current is fixed at the start at 2.5 A since the optimal operating conditions are then already met, the anode being in the optimal position relative to the cathode.

Example 2. Synthesis of dimethylbenzylcarbinol

The same test is carried out as that of Example 1 but without machining the lower block before the first electrolysis. The same result is obtained but the operating equilibrium is much longer to reach.

Claims (12)

1. Electrolysis cell for the electrosynthesis, in an organic medium, of organic or organometallic compounds, containing two electrodes (12 and 14) of which only one (14) is sacrificed during the electrosynthesis by the electrochemical reaction of which it forms the seat, characterized in that:

the sacrificial electrode (14) consists of at least one solid metal block and is applied under the influence of its own weight against the other electrode (12) from which it is separated by an electrical insulating material (15) which allows the passage of the electrolytic solution (16) and of which the shape and the dimensions enable the active surfaces of the two electrodes (12 and 14) to remain parallel during the electrosynthesis,

and in that the active surface of the unsacrificial electrode (12) is conical and has an inclination less than 45 degrees relative to the vertical.

2. Electrolysis-cell according to Claim 1, characterized in that the sacrificial electrode (14) consists of stacked solid metal blocks, each layer of the stacking containing only a single block.

3. Electrolysis cell according to any one of the preceding claims, characterized in that the unsacrificial electrode (12) is made of a metal chosen from the group consisting of nickel and stainless steel.

4. Electrolysis cell according to any one of the preceding claims, characterized in that the distance between the active surfaces of the two electrodes (12 and 14) is less than 5 mm.

5. Electrolysis cell according to any one of the preceding claims, characterized in that the sacrificial electrode (14) is applied against the other electrode (12) under the influence, in addition to that of its own weight, of that of an inert load, which rests on the sacrificial electrode (14).

6. Electrolysis cell according to Claim 5, characterized in that the inert load is an electricity conductor and serves for ensuring the electricity supply to the sacrificial electrode (14).

7. Electrolysis cell according to any one of Claims 1 to 4, characterized in that the sacrificial electrode (14) is applied against the other electrode (12) under the sole influence of its own weight.

8. Electrolysis cell according to any one of the preceding claims, characterized in that the electrical insulating material (15) is a plastic material in the form of a grid of which the thickness is less than 5 mm and the meshwork consists of two parallel wire networks, these two networks being superimposed, crossed and joined to each other at the points of contact between the wires, the thickness of the wires of each network being the same.

9. Use of an electrolysis cell equipped with a sacrificial anode made of a metal chosen from the group consisting of magnesium, zinc, aluminium and their alloys according to any one of the preceding claims for the electrosynthesis, in an organic solvent medium, of organic compounds chosen from the group consisting of carboxylic acids, alcohols, ketones and aldehydes, by the electrochemical reduction of organic halides.

10. Use, according to Claim 9, of an electrolysis cell equipped with a sacrificial anode made of a metal chosen from the group consisting of magnesium and its alloys for the electrosynthesis of carboxylic acids by the electrochemical reduction of organic halides in the presence of carbon dioxide.

11. Use, according to Claim 9, of an electrolysis cell equipped with a sacrificial anode made of a metal chosen from the group consisting of magnesium, aluminium, zinc and their alloys for the electrosynthesis of alcohols by the electrochemical reduction of organic halides which have a carbanion-stabilizing functional group or atom bound to the halogen-carrying carbon in the presence of carbonyl derivatives.

12. Use, according to Claim 9, of an electrolysis cell equipped with a sacrificial anode made of a metal chosen from the group consisting of magnesium, aluminium, zinc and their alloys for the electrosynthesis of ketones or aldehydes by the electrochemical reduction of organic halides in the presence of organic acid anhydrides.

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