JPH01121233A - Production of 2-chloropropionaldehyde - Google Patents

Abstract

PURPOSE:To obtain the title compound which is used as an intermediate of chemicals, agrochemicals and medicines with high catalyst activity efficiently by conducting the reaction between vinyl chloride, carbon monoxide and hydrogen in the presence of a rhodium compound and a base under specific conditions. CONSTITUTION:The reaction between vinyl chloride, a carbon monoxide and hydrogen is carried out in the presence of a rhodium compound and a base such as hydride carbonyl tristriphenylphosphine rhodium [RhH(CO)(PPh3)3], in a solvent which is insoluble or slightly soluble in water, such as a saturated hydrocarbon such as hexane or an aromatic hydrocarbon such as benzene, as extraction with an aqueous medium of pH0.5-7 is effected to give the subject compound. As the aqueous medium, is preferably used a carboxylic acid and the concentration is preferably 0.01-1mol. per 1l of the medium.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is directed to the production of 2-chloropropion produced from vinyl chloride, carbon monoxide and hydrogen according to the following reaction formula (1). This invention relates to a method for producing aldehydes. 2-Chloropropionaldehyde can be used as a useful intermediate for chemicals, agricultural medicines, etc.

(Prior Art) A method for producing 2-chloropropionaldehyde using vinyl chloride, carbon monoxide and hydrogen as raw materials is known, for example,
French Patent No. L397,779, Helvetica Kimiriki Acta (formerly HELVETICA C) ITCAA
CTA), Vol. 48, No. 5, pp. 1151-1157. All of these methods use cobalt carbonyl as a catalyst and are described, for example, in the French patent no.
According to No. 397,779, the reaction is carried out for 90 minutes at a reaction temperature of 110° C. and a reaction pressure of 200 atm.
Reaction results were obtained with a conversion rate of vinyl chloride of 57.4% and a selectivity of 2-chloropropionaldehyde of 86.2%.

(Problems to be Solved by the Invention) However, these methods using cobalt carbonyl as a catalyst have extremely low catalytic activity per cobalt, and therefore require a large amount of cobalt carbonyl and a high reaction pressure of 160 to 200 atmospheres. In addition, the reaction temperature is 75~
A method is used in which the reaction is carried out at 125° C. for 90 to 120 minutes.

The desired product, 2-chloropropionaldehyde, is a thermally unstable substance, and at such reaction temperatures and times, a considerable proportion of it is consumed in sequential reactions, reducing the reaction yield. This method has poor reproducibility, and hydrogen chloride is produced as a by-product due to this sequential reaction or other side reactions.
This has the problem that it not only severely corrodes the material of the reactor, but also that it reacts with the cobalt carbonyl catalyst to form cobalt chloride, which hinders the reuse of the catalyst.

(Means and effects for solving the problems) The present inventors conducted detailed research to solve these problems. As a result, when vinyl chloride, carbon monoxide, and hydrogen are reacted in the presence of a rhodium compound and a base, the reaction proceeds at lower temperatures and pressures than in the conventional method using a cobalt carbonyl catalyst, and at a sufficient rate. We have found that selectivity to the target product can be obtained, but further detailed research on this method revealed that a solvent that is insoluble or poorly soluble in water was used as the reaction solvent, and the reaction was carried out at a pH value of 25°C. The present invention has been completed by discovering that this reaction can proceed more efficiently and can be continued for a long time without deterioration of catalytic activity by extraction with an aqueous medium in which reached.

That is, in the present invention, when producing 2-chloropropionaldehyde by reacting vinyl chloride, carbon monoxide, and hydrogen in the presence of a rhodium compound, a base, and a solvent, a solvent that is insoluble or poorly soluble in water is used as a solvent. 2-chloropropionaldehyde, and the reaction is carried out under extraction with an aqueous medium having a pH value in the range of 0.5 to 7 at 25°C.

The bases mentioned herein generally mean Lewis bases containing Group B elements of the Periodic Table, such as nitrogen, phosphorus, or arsenic. In the method of the present invention, a trivalent organic phosphorus compound or an oxide of a trivalent organic phosphorus compound is particularly preferred as the base.

The trivalent organic phosphorus compound or the oxide of the trivalent organic phosphorus compound preferably used in the method of the present invention is exemplified as follows. That is, as a trivalent organic phosphorus compound,
General 2 P (R'R"R' ) C;: Here, P represents a phosphorus atom, and R1, R2, and R2 are the same or different alkyl, aryl, cycloalkyl, alkoxy, aryloxy, or cycloalkoxy groups, respectively. Examples include trivalent organic phosphorus compounds represented by: trimethylphosphine, triethylphosphine,
Phosphines such as tripropylphosphine, tributylphosphine, trioctylphosphine, triphenylphosphine, tricyclohexylphosphine, triberdylphosphine, trimethylphosphite, triethylphosphite, tripropylphosphite, tributylphosphite, trioctylphosphine , triphenyl phosphite, tricyclohexyl phosphite,
Examples include phosphites such as tribenzyl phosphite.

In addition, as a special type of phosphine, the above general 2P
In addition to those represented by (R'R2R' ), diphosphines such as bisdiphenylphosphinomethane and bisdiphenylphosphinoethane, phosphines bonded to crosslinked polystyrene, and the like are also preferably used.

In addition, as the oxide of the trivalent organic phosphorus compound, alkylphosphine oxide such as triethylphosphine oxide, tributylphosphine oxide, and trioctylphosphine oxide, arylphosphine oxide such as triphenylphosphine oxide and tritolylphosphine oxide, or an alkyl group and an aryl Examples thereof include alkylarylphosphine oxides having both groups. In addition, triethyl phosphite oxide, tributyl phosphite oxide,
Alkyl or aryl phosphite oxides such as triphenyl phosphite oxide, alkylaryl phosphite oxides having both an alkyl group and an aryl group, etc. can also be used. Furthermore,
Polydentate phosphine oxides such as bis-1,2-diphenylphosphinomethane dioxide and the like can also be used.

Rhodium compounds used in the method of the present invention include rhodium oxides, mineral acid salts, organic acid salts, and rhodium complex compounds. Examples of these are rhodium oxide, rhodium chloride, rhodium bromide, rhodium iodide, rhodium nitrate, rhodium sulfate, rhodium acetate, rhodium triacetylacetonate, rhodium dicarbonylacetylacetonate, dodecacarbonyl tetrarhodium, hexadecacarbonyl. Examples include hexalodium, and in addition to these, complex compounds formed with rhodium and other bases are also preferably used. The base may be a base preferably used in the method of the present invention, but other bases may also be used. Examples of these include, for example, hydridocarbonyltristriphenylphosphine rhodium (Rh)I(Co)(PPh+)il], nitrosyltristriphenylphosphine rhodium (Rh(No)
) (PPh, l) 3 ), η-cyclopentadienylbistriphenylphosphine rhodium (Rh(C5H
s) (PPhs)z ], chlorotris(triphenylphosphine) rhodium (RhCI (PPhz)
s), etc.

In the method of the present invention, the rhodium compound is in the range of 0.0001 to 1000 milligram atoms, preferably 0.001 to 100 milligram atoms, as rhodium atoms per liter of water-insoluble or sparingly soluble solvent in the reaction system. used in amounts equivalent to Further, the base used in the method of the present invention is preferably 0.1 to 500 mol, preferably 0.5 to 1 mol, per 1 gram atom of rhodium.
It is used in the range of 00 mol.

The method of the present invention is carried out in the presence of a solvent that is insoluble or poorly soluble in water. The water-insoluble or sparingly soluble solvent mentioned herein means a solvent that has a solubility in the aqueous phase of 5% by volume or less, particularly preferably 0.5% by volume or less under the reaction conditions. Among such solvents, those that do not adversely affect the reaction are preferably used. Hydrocarbons are particularly preferred as such solvents. in particular,
Saturated hydrocarbons such as hexane, heptane, octane, nonane, and decane, aromatic hydrocarbons such as benzene, toluene, and xylene, etc. are preferably used, and ligroin, kerosene, etc., which are industrially obtained as a mixture of hydrocarbons, Light oil, diesel oil, etc. are also included in these examples.

In addition, various halogenated hydrocarbons such as dichloromethane, 0-dichlorobenzene, p-chlorotoluene, vinyl chloride, etc. are also preferable, and in particular, vinyl chloride is also one of the raw materials for this reaction, so it is suitable for simplifying the process. It is a particularly preferred solvent. In addition, ethers such as dipropyl ether and dibutyl ether, diisobutyl ketone,
Examples of preferred solvents include ketones such as holon, esters such as butyl butyrate and butyl benzoate, and carbonate esters such as diethyl carbonate.

In the method of the invention, the reaction is carried out at 25°C with p)l
It is carried out under extraction with an aqueous medium with values in the range 0.5-7. Under extraction with an aqueous medium having a pit value in the range of 0.5 to 7 at 25°C means that during the reaction, 2-
This means that the reaction is carried out while extracting reaction products such as chloropropionaldehyde. but,
In some cases, the method of the present invention also includes a method in which this operation is performed immediately after the reaction and the water-insoluble or sparingly soluble solvent containing the extracted catalyst component is subjected to the reaction again. By adopting such a method, the reaction results are improved and the catalyst can be continuously reused.

The pH value at 25°C used in the method of the present invention is 0.
．． An aqueous medium having a p! value in the range of 5 to 7 is defined as a p! (Means a solution adjusted so that the value is in the range of 0.5 to 7. Preferred examples of such solutions include aqueous solutions of various acids, aqueous solutions of salts made of these acids and bases, or buffer solutions. As such acids, acids having a pKa in the range of 0.5 to 7 are preferred, and carboxylic acids are particularly preferred. Examples of these carboxylic acids include formic acid, acetic acid, and propionic acid. , butyric acid, isobutyric acid, heptanoic acid, acrylic acid,
Aliphatic saturated or unsaturated mono- or polycarboxylic acids such as methacrylic acid, crotonic acid, oxalic acid, malonic acid, methylmalonic acid, succinic acid, adipic acid, maleic acid, fumaric acid, 1,2,3-propanetricarboxylic acid and monovalent or polyvalent aromatic carboxylic acids such as benzoic acid, toluic acid, phthalic acid, isophthalic acid, trimellitic acid, pyromellitic acid, and mellitic acid. Further, carboxylic acids in which a substituent such as a halogen, an amino group, or a hydroxyl group is attached to the alkyl group or aryl group of these carboxylic acids are also preferable, and examples of these include monofluoroacetic acid, difluoroacetic acid, trifluoroacetic acid, monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, dibromoacetic acid,
2-chloropropionic acid, 3-chloropropionic acid, 2
．． Halogen-substituted aliphatic carboxylic acids such as 2-dichloropropionic acid, halogen-substituted aromatic carboxylic acids such as 0-chlorobenzoic acid, m-chlorobenzoic acid, p-chlorobenzoic acid, and 0-fluorobenzoic acid, glycine, and sarcosine. , amino acids such as alanine, β-alanine, 4-aminobutyric acid, valine, serine, aspartic acid, glutamic acid, glycolic acid, lactic acid, 2-hydroxybutyric acid, glyceric acid,
Examples include hydroxycarboxylic acids such as malic acid, tartaric acid, citric acid, p-hydroxybenzoic acid, salicylic acid, and 2,4-dihydroxybenzoic acid.

In addition, phenylacetic acid, pyruvic acid, anisic acid, 0-
Preferred examples include carboxylic acids with substituents other than those mentioned above, such as nitrobenzoic acid and cinnamic acid. In the method of the present invention, among these carboxylic acids, halogen-substituted aliphatic carboxylic acids and -valent or polyvalent aromatic carboxylic acids are particularly preferably used. In the method of the present invention, acids other than carboxylic acids include phosphoric acid, phosphorous acid, and various phosphorus oxyacids, such as phosphonic acids, phosphonic acids 7, phosphinic acids, and phosphinic acids. used. Examples of these are:
Methylphosphonic acid, ethylphosphonic acid, phenylphosphonic acid, phenylphosphonic acid, dimethylphosphinic acid,
Examples include diethylphosphinic acid, diphenylphosphinic acid, diphenylphosphinic acid, and the like. Furthermore, other preferred acids include boric acid, carbonic acid, and the like.

In the method of the present invention, aqueous solutions of salts consisting of these acids and bases are also preferably used. Preferred examples of the base include alkali metal or alkaline earth metal oxides or hydroxides, specifically magnesium oxide, calcium oxide, strontium oxide, lithium hydroxide, sodium hydroxide, potassium hydroxide. , cesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide, and the like. Guanidines are also preferred bases, and specific examples include various substituted guanidines in addition to guanidine, such as methylguanidine and 1,1-dimethylguanidine.

In addition, various ammonium hydroxides, particularly quaternary ammonium hydroxide, are also mentioned as preferred bases, and specific examples include tetramethylammonium hydroxide, tetraethylammonium hydroxide, and choline. Salts of these acids and bases can be obtained by mixing them, for example, by mixing carbonates or bicarbonates of acids and bases, or by mixing ammonium salts of bases and acids. A method of obtaining the same result as obtained by mixing an acid and a base is also included within the scope of the present invention.

The buffer solution used in the method of the present invention refers to a solution that has a buffering capacity against pH fluctuation factors, and is usually prepared by dissolving an appropriate combination of acids, bases, and various salts in a solvent. . For these buffers, it is necessary to select the types of acids and bases that make up the buffer, as well as their concentrations and mixing ratios, depending on the desired pH value. A buffer solution is prepared.

Examples of these include hydrochloric acid-potassium chloride buffer (typical pH range: 1-2.2), potassium hydrogen phthalate-potassium
Hydrochloric acid buffer (typical pH range: 2.2-4.0), potassium hydrogen phthalate-sodium hydroxide buffer (typical pH range: 4.1-5.9L potassium dihydrogen phosphate-hydroxide) Sodium buffer (typical pH range: 5.8-8
), and in addition, various buffer solutions with different types of acids, bases, or salts constituting these buffer solutions are also mentioned. Particularly preferred examples of these include an aqueous solution containing both the above-mentioned acid and a salt consisting of a nucleic acid and a base. In particular, when using polyhydric acids such as phthalic acid and pyromellitic acid, it is possible to form part of the acid group in the form of a salt with a base and the rest in the form of a free acid, or to form a salt of an acid or a nucleic acid with a base. Preferred examples include lithium hydrogen phthalate, sodium hydrogen phthalate, potassium hydrogen phthalate, and guanidine hydrogen phthalate. In addition to the above-mentioned polyhydric carboxylic acids and phosphoric acids, examples of acids that can be used in this method include phosphoric acid and phosphorous acid. Further, although it is preferable to use an aqueous buffer in the method of the present invention, many aromatic polycarboxylic acids have low solubility in water. However, by employing the method described above, many aromatic polyhydric carboxylic acids have good solubility in water, making it possible to form a buffer solution that is highly suitable for the method of the present invention.

Examples of this include phthalic acid and pyromellitic acid, as well as isophthalic acid and mellitic acid. In addition, terephthalic acid, which was not included in the examples of preferred acids mentioned earlier, can also be used using this method. is included as an example of a preferred acid. pH of these buffers
The value usually varies depending on the measurement temperature, but in the method of the present invention, it is conveniently represented by the pH value at 25°C.

In the method of the present invention, among these buffers, p
An aqueous buffer solution having an H value in the range of 0.5 to 7 is preferably used.

Under extraction with such an aqueous medium, the rhodium compound exhibits high activity in the synthesis of 2-chloropropionaldehyde from vinyl chloride and synthesis gas in the presence of the above-mentioned base. Furthermore, since the reaction product 2-chloropropionaldehyde is extracted into an aqueous medium immediately after being produced in the reactor, it is easily separated from the organic layer containing raw materials vinyl chloride, rhodium, and a base. Furthermore, in this case, the aqueous medium containing 2-chloropropionaldehyde separated from the raw material vinyl chloride and the catalyst is easily separated from the 2-chloropropionaldehyde by distillation operation under normal pressure or slightly reduced pressure. be able to. On the other hand, the aqueous medium thus recovered can be reused by removing a portion of it and removing foreign ions, which are reaction byproducts, or by replacing it with a newly prepared aqueous medium. The method is highly preferred as an industrial method for producing 2-chloropropionaldehyde.

The concentration of these aqueous media is also important in the method of the invention. When a low-concentration aqueous medium is used, reaction results similar to those of a high-concentration aqueous medium can be obtained at the initial stage, but as the reaction progresses, the pH value of the aqueous medium usually decreases, which is preferable (p)! If the value is outside the range, the catalyst activity will be impaired. For this reason, it is generally preferable for the concentration of the aqueous medium to be high, but it is not preferable for handling to make the concentration too high.

Usually, the concentration of the acid and salt constituting the aqueous medium is preferably in the range of 0.001 mol to 10 mol, particularly preferably in the range of 0.01 mol to 1 mol, per liter of the aqueous medium. Further, the amount of the aqueous medium used may be small if the aqueous medium has a high concentration, and conversely, it is preferable to use a large amount if the concentration is low. The concentration and amount of these aqueous media are usually determined by the ptt of the aqueous media at the reactor outlet.
It is particularly preferable to select a value in the range of 0.5 to 7. Usually, the amount of the aqueous medium is 0.000. It is preferably in the range of 1 to 10 liters.

In the method of the present invention, it is preferable to carry out the reaction in a continuous reaction apparatus, but in this case, extraction is carried out within the reactor or in an extractor provided separately from the reactor.

When extraction is performed in a reactor, a solvent containing a catalyst component is placed in the reactor, and raw materials vinyl chloride, carbon monoxide, and hydrogen are continuously supplied to the reactor.

A fixed amount of the solvent containing the catalyst component can be kept in the reactor and the reaction can be continued without supplying new catalyst. It is also possible to continuously supply a solvent containing a catalyst component to a reactor, and to continuously withdraw from the reactor a proportionate amount of the solvent.

While the reaction is being carried out in this manner, an aqueous medium having a pH value in the range of 0.5 to 7 at 25°C is continuously supplied from above the reactor, and reaction products such as 2-chloropropionaldehyde are transferred to the aqueous phase. extracted and taken out of the reaction system.

In addition, when extraction is performed using an extractor installed separately from the reactor, a solvent containing the catalyst component, vinyl chloride, carbon monoxide, and hydrogen are continuously supplied to the reactor, and the reaction liquid coming out of the reactor is Lead to the reaction liquid supply port provided at the bottom of the extraction device.

From the top of the extraction device, the pH value at 25°C is 0.5~
An aqueous medium in the range of 7 is continuously fed to extract the reaction products such as 2-chloropropionaldehyde into the aqueous phase, while the reaction liquid is recycled to the reactor for reuse.

The method of the present invention is usually carried out at a reaction temperature of 20 to 150° C. and a reaction pressure of 1 to 200 kg/cat gauge, preferably 5 to 150 kg/c+fl gauge. The reaction temperature is preferably low from the viewpoint of thermal stability of the 2-chloropropionaldehyde produced, and for this reason, 20 to 1
00°C is a particularly preferred temperature range. In addition, the mixing molar ratio of carbon monoxide and hydrogen as raw materials is 11, usually 10 to 0.
1, preferably 4 to 0.2.

Carbon monoxide and hydrogen can be used as long as they are mixed gases containing both components in the above composition ratio (water gas, water gas and gas inert to the reaction such as methane or nitrogen, or carbon dioxide or moisture). The other raw material, vinyl chloride, is used in the form of a gas, liquid, or solution dissolved in the solvent used in the reaction.

(Example) Hereinafter, the method of the present invention will be explained in more detail with reference to Examples.

Example 1 Reactor 1 with a pressure resistance of 100 Kg/cJ gauge (SUS 316
Made in L, inner diameter 25 +nm, height 350cm, actual volume approx. 17cm
0c+flte, inner diameter 25 mm at the bottom, height 150 m
0.5 mmol of hydridocarbonyltristriphenylphosphine rhodium, 5 mmol of triphenylphosphine, and 50 d of toluene as a reaction solvent were charged into a static separation tank 2 (equipped with a static separation tank 2) at a reaction temperature of 45°C.
Reaction pressure cuff 5Kg/c++! Under gauge conditions,
8.7 g/hour of vinyl chloride and about 241 g/hour of a mixed gas of carbon monoxide and hydrogen at a molar ratio of 1:2 were continuously supplied from inlet pipes 4 and 5 provided at the bottom of the reactor, respectively. At the same time, 1 liter per liter from the aqueous medium storage tank 3.
An aqueous solution containing 0 g of potassium hydrogen phthalate and having a p)I value of 3.9 at 25° C. was supplied at a rate of 150 g/hour from a liquid inlet pipe 6 provided at the top of the reactor. A liquid outlet pipe 7 is installed below the stationary separation tank 2 provided at the bottom of the reactor, and the liquid outlet pipe 7 is used to collect reaction products so that the liquid level in the reactor is kept constant. An aqueous phase consisting of an aqueous solution of potassium hydrogen phthalate containing 2-chloropropionaldehyde is continuously taken out of the reactor, while the gas inside the reactor 1 is discharged from a gas vent pipe 8 installed at the top of the reactor. Gas containing unreacted vinyl chloride and unreacted carbon monoxide and hydrogen was continuously withdrawn so that the pressure remained constant. In addition to potassium hydrogen phthalate, the aqueous phase contains 6.8
The presence of g/h of 2-chloropropionaldehyde and small amounts of chloride ions, propionate ions and rhodium at a concentration of 0.1 ppm was confirmed.

However, potassium hydrogen phthalate was analyzed by dry weighing, 2-chloropropionaldehyde and propionate ions were analyzed by GC, chlorine ions were analyzed by silver nitrate titration, and rhodium was analyzed by atomic absorption spectrometry.

The reaction was continued in this way for 6 hours, but
No substantial change in catalyst performance was observed even 6 hours after the start of the reaction.

In addition, from the aqueous phase, a pressure of 50 mm of mercury column and a can temperature of 6
It was confirmed that the reaction product, 2-chloropropionaldehyde (approximately 10% water content), was isolated by applying it to a glass batch distillation apparatus operated at 0°C.

Example 2 The reaction was carried out in the same manner as in Example 1, except that instead of potassium hydrogen phthalate, an aqueous solution containing 13 g of dichloroacetic acid per liter and having a pH value of 1.1 at 25° C. was used. It was confirmed that 2-chloropropionaldehyde was produced in the aqueous phase from the liquid extraction tube 7 at a rate of 6.4 g/hour over a period of 6 hours after the start of the reaction.

However, 2-chloropropionaldehyde was analyzed by CC method.

Example 3 5% instead of potassium hydrogen phthalate in Example 1
Add a 10% caustic soda aqueous solution to an aqueous sodium hydrogen phthalate solution to obtain a +1) value of 6.0 at 25°C.
The reaction was carried out in the same manner except that an aqueous buffer solution of Over 6 hours after the start of the reaction, 2-chloropropionaldehyde was added to the aqueous phase from the liquid take-out tube 7 at a rate of 6.2% per hour.
It was confirmed that it was produced at a rate of 1.5 g.

However, 2-chloropropionaldehyde was analyzed by GC method.

(Effects of the Invention) According to the method of the present invention, 2-chloropropionaldehyde can be produced using vinyl chloride, carbon monoxide, and hydrogen in higher yields at lower temperatures and lower pressures than conventional methods. In particular, the method of the present invention allows continuous use of the catalyst with higher catalytic activity than previously possible.

[Brief explanation of the drawing]

FIG. 1 is a detailed process diagram illustrating the present invention. In the figure,
1 is a reactor, 2 is a static separation tank, and 3 is a p) at 25°C.
Indicates an aqueous medium storage tank with an I value in the range 0.5-7. Patent applicant: Mitsui Toatsu Chemical Co., Ltd. Figure 1

Claims (8)

[Claims] (1) In producing 2-chloropropionaldehyde by reacting vinyl chloride, carbon monoxide, and hydrogen in the presence of a rhodium compound, a base, and a solvent, a solvent that is insoluble or poorly soluble in water is used as a solvent, and the reaction is performed. 25
A method for producing 2-chloropropionaldehyde, which is carried out under extraction with an aqueous medium having a pH value in the range of 0.5 to 7 at °C. (2) The method according to claim 1, wherein the base is a trivalent organic phosphorus compound or an oxide of a trivalent organic phosphorus compound. (3) The method according to any one of claims 1 to 2, wherein the water-insoluble or sparingly soluble solvent is a hydrocarbon. (4) The method according to any one of claims 1 to 2, wherein the water-insoluble or sparingly soluble solvent is vinyl chloride. (5) A patent claim in which the aqueous medium having a pH value in the range of 0.5 to 7 at 25°C is an aqueous solution of a carboxylic acid or an aqueous solution of a carboxylic acid salt having a pH value in the range of 0.5 to 7 at 25°C. The method according to items 1 to 4 of the range. (6) The method according to claim 5, wherein the carboxylic acid is a halogen-substituted aliphatic carboxylic acid. (7) The method according to claim 5, wherein the carboxylic acid is a monovalent or polyvalent aromatic carboxylic acid. (8) Claims 1 to 7, wherein the aqueous medium having a pH value in the range of 0.5 to 7 at 25°C is an aqueous buffer solution having a pH value in the range of 0.5 to 7 at 25°C. 4
The method described in section.