A METHOD FOR PREPARING 4-CHLORO-2-METHYLPHENOXYALKANOIC ACIDS

Abstract: 4-chloro-2-methylphenoxyalkanoic acids useful as herbicides, e.g. 4-chloro-2-methylphenoxyacetic acid (MCPA), 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP) and 4-(4-chloro-2-methylphenoxy)butyric acid (MCPB), are prepared by reacting the corresponding 2-methylphenoxyalkanoic acid with a water-compatible chlorinating agent in an aqueous medium in the presence of a catalyst which is a chemical compound comprising two carbon atoms bonded together with a single bond or via a methylene or methine group, one of the carbon atoms being substituted with an electropositively functioning group, the other carbon atom being the carbon atom in an electronegatively functioning group. In particular, the catalyst is compound of the general formula (I) X - CR1R2 - (CHRx)n - CR3R4 - Z, wherein R1, R2, R3 and R4 each independently is hydrogen or C1-4 alkyl; or R1 and R2 together are oxo, and R3 and R4 each independently is hydrogen or C1-4 alkyl; n is an integer 0 or 1; and Rx is hydrogen; or, when n is 1, Rx and R4 may together designate a carbon-carbon bond; X, when R1 and R2 together are oxo, is C1-6 alkoxy, hydrogen, C1-6 alkyl or NR5R6, wherein R5 and R6 each independently is hydrogen or C1-6 alkyl; or X, when R1 and R2 are not together oxo, is NR'5R'6 wherein R'5 each is C1-4 alkyl; and Z is NR7R8, wherein R7 and R8 each independently is C1-4 alkyl. The catalyst ensures a high ratio between 4-chlorinated and 6-chlorinated reaction product of at least 15, preferably at least 25, and up to 50-150 and even higher. The method has a high environmental acceptability.

Description: A METHOD FOR PREPARING 4-CHL0R0-2-METHYLPHEN0XYALKAN0IC ACIDS The present invention relates to a method for preparing 4- chloro-2-methylphenoxyalkanoic acids which are widely used as hormone-type herbicides for controlling the growth of dicotyledonous weeds in cereals and grass lands. Particu¬ larly interesting 4-chloro-2-methylphenoxyalkanoic acids used as herbicides are: 4-chloro-2-methylphenoxyacetic acid (MCPA) , 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP), and 4-(4-chloro-2-methylphenoxy)butyric acid (MCPB). Among these, MCPP may exist as pure enantiomers thereof or as mixtures of these such as the racemic mixture. Traditionally, 4-chloro-2-methylphenoxyalkanoic acids of formula II (CH 2 ) n COOH 0 CH. (ID § Cl ■" wherein n is an integer 1, 2 or 3, have been prepared according to the following method: 2-Methylphenol (o-cresol) is chlorinated by means of sul- furylchloride (SO2CI2) at a temperature of 30-40°C to yield a mixture of chlorocresols containing approximately 93% w/w 4-chloro-o-cresol and 6% w/w 6-chloro-o-cresol. The mixture is distilled in order to obtain a satisfactory purity of the desired product. The purity of the distilled product is 97-98% w/w 4-chloro-o-cresol. A by-product of the chlorina- tion step is sulfur dioxide (S0 2 ) which is recycled by reaction with chlorine (Cl 2 ) to form S0 2 C1 2 . The 4-chloro-o-cresol is reacted either with a chloroalka- noic acid (for the preparation of MCPA or MCPP) or with a lactone (for the preparation of MCPB) in a strongly alka¬ line medium at reflux temperature. This is a condensation type reaction. After the condensation, the reaction mixture is extracted with e.g. an organic solvent at neutral pH whereby unreacted 4-chloro-o-cresol is removed from the mixture. The aqueous phase containing the desired material is acidified with mineral acid (HC1 or H 2 S0 4 ) whereby the phenoxyalkanoic acid is precipitated. The resulting phenoxyalkanoic acid may be formulated di¬ rectly into a herbicidal composition without further sepa¬ ration or the phenoxyalkanoic acid can be crystallized as granules or flakes. In the above process, the yield of 4-chloro-2-methylphen- oxyalkanoic acid based on 2-methylphenol (o-cresol) is only approximately 88%. The losses are mainly due to chlorina- tion at undesired positions in the molecule, e.g. chlorina- tion at the 6-position or at both the 4- and 6-position, and also losses in the distillation step contribute con¬ siderably to lower the total yield. Another disadvantage of the traditional method for prepar¬ ing the 4-chloro-2-methylphenoxyalkanoic acids is the very unpleasant odour caused by the presence of chlorinated cresols at high temperatures. It is well-known that this unpleasant odour is very penetrating, thus leading to malodorous environments in the production area and neigh¬ bouring areas. Finally, the necessary equipment for the traditional pro- cess, i.e. the distillation columns and kettles and the S0 2 C1 2 regeneration system, is very expensive. The above-mentioned disadvantages can be avoided by prepa¬ ring 4-chloro-methylphenoxyalkanoic acids by a synthetic route comprising a condensation step prior to the chlorina¬ tion step and thereby avoiding the presence of chlorinated cresols. It is known that condensation of 2-methylphenol (o-cresol) can be carried out without any problems and, furthermore, that the yield of this condensation step is high. The condensation of 2-methylphenol should be followed by a chlorination of the 2-methylphenoxyalkanoic acid which is the product of the condensation step. This chlorination step is termed an after-chlorination reaction. It is known that this after-chlorination reaction can be carried out by using various chlorination methods. US 3,920,757 discloses the chlorination of 2-methylphenoxy- acetic acid (MPA) dissolved in glacial acetic acid (i.e. an organic solvent) by using S0 2 C1 2 and a catalyst. The pro¬ duct contained 90.8% w/w MCPA and the ratio of 4-chlorina- ted to 6-chlorinated product, i.e. MCPA to 6-chloro-2- methylphenoxyacetic acid, is 15.6. PL 100642 and PL 115188 disclose the chlorination of MPA with Cl 2 in aqueous organic solvents, namely tetrachloro- ethane or chlorobenzene. The reported yields are as high as 95% and the purities are as high as 93%. However, the chlorination of MPA is carried out at high temperatures and comprises chlorination in two steps with several separa¬ tions of the reaction mixture and, consequently, the chlo¬ rination process described is not an improvement in rela¬ tion to the traditional method of preparing the present phenoxy acids. EP 55 357 discloses the chlorination of MPA as a suspension in water with Cl 2 at 60°C. The reported yield is 92.1% and the purity is 96%. The Applicant of the present application 4 has carried out experiments according to the chlorination process described in the European patent (Example 5) where¬ by a product was obtained with the following composition: 14.4% w/w 2-methylphenoxyacetic acid 0.7% w/w 6-chloro-2-methylphenoxyacetic acid 79.6% w/w 4-chloro-2-methylphenoxyacetic acid (desired product) 3.9% w/w 4,6-dichloro-2-methylphenoxyacetic acid These results indicate that the conversion of starting material (2-methylphenoxyacetic acid) to the desired prod¬ uct MCPA proceeds with a selectivity which does not seem completely satisfactory: the resulting product contains 3.9% w/w of a dichlorinated product. Furthermore, the total yield based on chlorine is rather low, since 17.2% (mol/- mol) MPA remained unchlorinated and 3.3% (mol/mol) MPA was dichlorinated. CS 226909 discloses the chlorination of 2-methylphenoxy- acetic acid with NaOCl at such pH which results in the conversion of NaOCl into HCIO in the reaction mixture; in this process, the HCIO is actually the chlorinating agent. The product of the chlorination process is reported to contain: 0.99% w/w 2-methylphenoxyacetic acid 3.23% w/w 6-chloro-2-methylphenoxyacetic acid 32.91% w/w 4-chloro-2-methylphenoxyacetic acid (desired product) 0.24% w/w 4,6-dichloro-2-methylphenoxyacetic acid The total yield of MCPA is reported to be 82.8% w/w. As above, neither the yield of MCPA nor the relative amounts of the various resulting compounds in the product mixture can be considered completely satisfactory. The ratio of 4- chlorinated (MCPA) to 6-chlorinated phenoxy acid is 10.2. The object of the present invention is to provide a simple and easy method for preparing 4-chloro-2-methylphenoxyalka- noic acids by chlorination of the corresponding 2-methyl- phenoxyalkanoic acid to obtain high yields of the desired product, with a high selectivity for chlorination in the 4- position, which process can be carried out in simple and cheap equipment and with improved relations to the environ¬ ment, using reagents and additives which are acceptable from an environmental point of view and from the point of view of labour safety and acceptability. According to the invention, this object is obtained by reacting the corresponding 2-methyl-phenoxyalkanoic acid with a water-compatible chlorinating agent in an aqueous medium in the presence of a catalyst which is a chemical compound comprising two carbon atoms bonded together with a single bond or through a methylene (-CH 2 -) or methine (-CH=) group, one of the carbon atoms being substituted with an electropositively functioning group, the other carbon atom being the carbon atom in an electronegatively functioning group. In the present specification and claims, the term "an electropositively functioning group" designates a group which is capable of attracting an electronegative part of the chlorinating agent (such as the oxygen atom of hypo- chlorous acid or one of the chlorine atoms when the chlorinating agent is Cl 2 ) . Correspondingly, the term "an electronegatively functioning group" designates a group which is capable of attracting an electropositive part of the chlorinating agent (such as the chlorine atom of hypo- chlorous acid or one of the chlorine atoms when the chlorinating agent is Cl 2 ) . The electropositivity of the electropositive function and the electronegativity of the electronegative function should, of course, be of a sufficient strength so that the desired catalytic effect is above a certain level with respect to selective 4-chlorination. Thus, a suitable catalyst may be defined as a compound of the kind identified above which is capable of ensuring, in the chlorination in question, a ratio between 4-chlorinated and 6-chlorinated reaction product of at least 15, preferably at least 25. Alternatively, expressed by reference to a test related to a "standard synthesis", a suitable catalyst may be defined as a compound of the kind defined above which is capable of ensuring a ratio between 4-chloro-2-methylphenoxyacetic acid and 6-chloro-2- methylphenox acetic acid of at least 15, preferably at least 25, when used as the catalyst in the production of 4- chloro-2-methylphenoxyacetic acid by chlorination of 2- methylphenoxyacetic acid under the conditions as defined in Example 8 herein. Examples of catalysts corresponding to the above definition are compounds in which the electropositively functioning group is dialkylsubstituted amino, and the electronegatively functioning group is selected from carbonyl, thiocarbonyl and selenocarbonyl. Interesting catalysts are compounds wherein the elec¬ tronegatively functioning group is carbonyl or thiocar¬ bonyl, and the carbon atom of the carbonyl or thiocarbonyl group additionally carries hydrogen or a group attached through a nitrogen, carbon, oxygen, or sulfur atom, such as an amido, hydrazido, alkoxy, or thioether group. Effective and interesting catalysts are compounds in which the electronegatively functioning group is carbonyl, and the carbon atom of the carbonyl group additionally carries a group R-O- in which R is an aliphatic group, such as an alkoxy group, or additionally carries an optionally dialiphatic-substituted amido group, such as a dialkyla ido group. As more specific examples of suitable kinds of compounds useful as the catalyst may be mentioned compounds of the general formula I X - CR λ R 2 - (CHR χ ) n - CR3R 4 - Z (I) wherein R- , R 2 , R 3 and R4 each independently is hydrogen or C ] __ 4 alkyl; or R- j _ and R 2 together are oxo, and R 3 and R 4 each independently is hydrogen or C -_- - alkyl; n is an integer 0 or 1; and R x is hydrogen; or, when n is 1, R χ and R4 may together designate a carbon- carbon bond; X, when Rι and R 2 together are oxo, is C- ) __ 6 alkoxy, hydrogen, C*-__ 6 alkyl or NR5R 5 , wherein R5 and R- each independently is hydrogen or C- j ^g alkyl; or X, when R -_ and R 2 are not together oxo, is NR' 5 R' 6 wherein R' 5 and R' 6 each is C 1 _ 4 alkyl; and Z is NR 7 R 3 wherein R 7 and R 8 each independently is C- ] __4 alkyl. FR 1,470,160 discloses a method for preparing 2-methyl-4- chlorophenoxycarboxylic acids, in which a 2-methyl- phenoxycarboxylic acid dissolved in an organic solvent or in suspension is reacted with sulfuryl chloride at 40-150°C in the presence of a catalyst. The catalyst may be an a ine. The amines disclosed in FR 1,470,160 do not correspond to the definition given above for the catalysts to be used according to the invention. Compounds of formula I, wherein n is 0, are preferred as the catalysts for use in the method of the invention. It is preferred that at the most one of R 5 and R 6 (when present) is hydrogen. In particular, it is preferred that each of R 5 and R 6 is C-*^ alkyl. It is also preferred that each of R 7 and R 8 is C-*__ 2 alkyl. It is further preferred that each of R' 5 and R'g is C- j __ 2 alkyl. Preferred compounds of formula I are compounds wherein R*-_ and R 2 together are oxo, and X is NRsRg, wherein R 6 and R 5 each preferably is C^«4 alkyl; these compounds are amides of aminosubstituted alkanoic acids. Examples of preferred compounds of formula I are: N,N-dimethyl-2-aminopropionic acid N' ,N'-dimethylamide, N,N-dimethyl-3-aminopropionic acid N' ,N'-dimethylamide, N,N-dimethyl-2-aminobutyric acid-N' ,N'-dimethylamide, methyl N,N-dimethylaminoacetate, N,N-dimethylaminoacetic acid N' ,N'-dimethylamide, N,N-diethylaminoacetic acid N' ,N'-diethylamide, N,N,N' ,N'-tetramethyl-l,2-diaminoethane, ethyl N,N-dimethylaminoacetate, and N,N-dimethylaminoacetic acid amide. The compounds to be used as catalysts in the method of the invention are either known compounds or can be prepared analogously with known compounds. Compounds containing an ester function, e.g. compounds of formula I wherein X is ~ χ- - alkoxy, and R and R 2 together are oxo, may be prepared by esterification in a well-known manner of the corresponding acid, e.g. in a suitable solvent and in the presence of an acid catalyst such as sulfuric or p-toluenesulfonic acid. Compounds which are amides of aminosubstituted alkanoic acids as indicated above may be prepared by separate amida- tion and amination of the corresponding acid comprising a leaving group in the position of the amine function. How- ever, if the substituents on the a ine and amide nitrogens, respectively, are identical, the amination and amidation reactions may be conducted simultaneously by treating the leaving group-substituted acid (or an activated derivative thereof such as the acid chloride) with 2 equivalents of the appropriate amine. Compounds of the formula I in which X is NRsRg, and R-- , R 2 , R 3 and R 4 are hydrogen or C 1 _ 4 alkyl, i.e. compounds of the diamine-type, may be prepared by well-known methods such as amination by nucleophilic substitution of the corresponding alkane having leaving groups instead of the amino functions. The amount of catalyst to be used in the method according to the invention should preferably be in the range, of 0.2-5% w/w (based on the 2-(2-methylphenoxy)alkanoic acid). Usually an amount of 0.5-1% w/w catalyst will be sufficient to obtain satisfactory results in terms of yield and ratio of desired product to undesired by-product. The chlorination of 2-methyl-phenoxyalkanoic acid in the presence of a catalyst as defined above ensures high conversion of the starting material and, most importantly, a very high ratio of desired product (4-chlorinated phenoxyalkanoic acids) to by-product (6-chlorinated and 4,6-dichlorinated phenoxyalkanoic acids, the amount of the dichlorinated by-product being insignificant) . Experiments have verified that the ratio of 4-chlorinated phenoxy acid to 6-chlorinated phenoxyalkanoic acid is considerably increased; when the chlorination with the water-compatible chlorinating agent is carried out in the aqueous medium in the absence of the catalyst, the ratio is approximately 8, whereas it is possible to achieve ratios of 50-150 or even higher when the chlorination of the 2-methyl-phenoxyalkanoic acid is carried out in the presence of the catalyst. Furthermore, the yield based on the chlorinating agent (ex¬ pressed as chlorine equivalents) is good, and also the yield based on 2-methylphenol (o-cresol) is considerably • improved in comparison with existing methods due to the excellent chlorination according to the method of the invention. In the present context, the term "water-compatible chlori¬ nating agent" denotes a chlorinating agent which in the presence of water does not hydrolyze to a compound incap- able of chlorinating 2-methyl-phenoxyalkanoic acids. The water-compatible chlorinating agent as used in the method according to the invention is preferably selected from elementary chlorine and HCIO, normally formed in situ from a salt thereof, in particular an alkali metal salt, such as the sodium salt or the potassium salt, and a mineral acid. HCIO is preferred, but it is contemplated that elementary chlorine will also be useful in the process of the invention. The method of the invention is carried out in an aqueous medium. In the present specification and claims, the term "aqueous medium" designates water and mixtures of water and one or several organic solvents which do not to any sig¬ nificant extent react with the chlorinating agent, the mixtures being mixtures containing a sufficient proportion of water to secure that the 4-chloro-2-methylphenoxyalkan- oic acid salt formed does not precipitate, in other words, remains in solution in the aqueous medium. The organic solvent or solvents which may be present in the mixture will normally be solvents which are miscible with water, but mixtures of water and organic solvents which are not miscible with water are within the definition herein of the aqueous medium, provided such mixtures do not give rise to precipitation of the salt of the desired acid such as explained above. Examples of mixtures of water and water- miscible or water-immiscible solvents are, e.g., a mixture of 180 ml of water and 25 ml of acetone (water-miscible) and a mixture of 180 ml of water and 25 ml of xylene (wa- ter-immiscible) . Both of these examples have been found to be useful and to perform excellently in the method of the invention, such as appears from Example 12. Examples of water-miscible solvents which are contemplated to be useful as constituents of the aqueous medium are methanol, ethanol, isopropanol, butanol, ethyleneglycol, propyleneglycol, polyethyleneglycol, acetone, ethyl acetate, tetrahydrofuran and dimethylformamide. Examples of water-immiscible solvents which are contemplated to be useful as constituents of the aqueous medium are petroleum ether, cyclohexane and xylene, diethylether, methylene chloride, chloroform, carbontetrachloride and 1,1,1- trichloroethane. Most preferably, the method according to the invention is carried out in water. The method of the invention should preferably be carried out at a pH below 10, preferably at a pH in the range of 0-9, more preferably in the range of 3-9, most preferably in the range of 5-9. As indicated above, hypochlorous acid (HCIO) is a preferred chlorinating reagent; an advantage of HCIO is that it results primarily in monochlorination of the starting material. When Cl is dissolved in water, it enters an equilibrium with HCIO. The equilibrium pH is 4.3. When pH is increased, a salt of HCIO with an acid dissociation constant of 7.4 is formed. 12 Accordingly, it may be preferred to maintain a pH higher than 6.3, at which pH practically no Cl 2 is present. On the other hand, not all HCIO present should be in the form of a salt thereof. Thus, pH should preferably be maintained below 9.4, at and above which pH practically no HCIO is present. HCIO undergoes disproportionation into oxygen and hydro¬ chloric acid on prolonged standing and under influence of light and heat: 2 HCIO - 0 2 + 2 HC1 It has been found that the above reaction is of practically no importance at a pH of 7 or slightly above 7 as compared to the desired chlorination reaction: At a pH of 9 or slightly below 9, the disproportionation reaction becomes more important as compared to the chlori¬ nation reaction which results in a lower yield of phenoxy¬ alkanoic acid based on HCIO which in turn results in the requirement for an excess of HCIO in the reaction mixture. Consequently, it is particularly preferred that the reac¬ tion is carried out at a pH in the range of 7-9 to ensure a satisfactory yield based on HCIO, especially in the range of 7-8.5. Preferably, the method according to the invention is car¬ ried out at a temperature between 0°C and 50°C, more preferably between 10°C and 30°C. It has been found that the temperature has a significant influence on the ratio of the 4-chlorinated to the 6-chlorinated phenoxyalkanoic acids. Experiments have shown that when chlorinating 2-(2- methylphenoxy)propionic acid to obtain MCPP, the above- mentioned ratio is approximately 180 when the method according to the invention is carried out at approximately 10°C, and that the ratio is approximately 100 when the method according to the invention is carried out at approximately 30°C (see Example 7). Furthermore, the amount of starting material which is not converted (i.e. chlorinated) is approximately 3-4% higher when the method according to the invention is carried out at 30°C than when carried out at 10°C. It is believed that this is due to the disproportionation of HCIO (see Example 7). According to an interesting embodiment of the invention, the reaction is started at a temperature of 0 α C or below, and the reaction temperature is allowed to increase, the average temperature being in the range between 10°C and 20°C over the course of the reaction. The background for the preference of this embodiment is as follows: During the chlorination process, energy in the form of heat is released. It has been found that the heat of reaction in the chlorination is about 50 kcal/mol reaction. Thus, the temperature in a mixture with 0.5 mol of phenoxyacid per litre (approx. 9% w/w) increases by approx. 25°C during the chlorination process. This means that if the starting temperature in a batch process is about 20°C, and no external cooling is applied, the end temperature will be about 45°C, which is somewhat higher than preferred. One way to control the temperature is, of course, to apply external cooling with a brine in a cooling mantle on the reaction vessel or tank. However, in accordance with what is stated above, an elegant embodiment is to use direct cooling by adding ice to the chlorination reaction mixture. The ice can be added either before the start of the chlorination process, thereby lowering the starting temperature to approx. -5°C and the " end temperature to approx. 30°C, the average temperature then being approx. 12-15°C which is a very preferred average temperature. The variation in temperature during the process in this embodiment will give rise to a variation in selectivity of the chlorination, the selectivity being very high at the lower temperature at the beginning and somewhat lower at the higher temperature at the end; however, the average selectivity will be very satisfactory. Another embodiment of direct cooling is to continuously add ice to the chlorination reaction mixture, thereby keeping the temperature at the desired level. Besides its function as a cooling measure, the addition of ice during the reaction serves the purpose of keeping the concentration of the phenoxyacid in the final chlorination reaction mixture at a suitable level. Use of hypochlorous acid as chlorinating agent offers an excellent opportunity of controlling the process. An aque¬ ous solution containing minor amounts of hypochlorite has a redox-potential significantly different from pure water. The redox-potential can be measured by means of a mV-meter using a platinum and a calomel or mercury sulphate electro¬ de. Due to the influence of the catalyst, which in addition to the selective effects raises the reaction rate, the chlori¬ nation is very fast and, consequently, the hypochlorite is used in the reaction as fast as it is added. When all of the 2-methylphenoxyalkanoic acid is reacted, there is no consumption of hypochlorite and, consequently, the redox- potential changes significantly. Using the principle of end-point titration, the addition of hypochlorite can be stopped automatically. The advantages of this control are obvious: 1) Only minor or no over-chlorination takes place. 2) An initial analysis of the solution of 2 methylphenoxy- alkanoic acid is unnecessary. 3) An analysis of the hypochlorite solution is unnecessary. 4) A control analysis of the chlorinated mixture is un¬ necessary. This embodiment is illustrated in Example 13. The method of the invention may be carried out batchwise or continuously, the continuous operation being possible because the reaction rate is so fast due to the catalyst that only a very short retention time in the reaction vessel is necessary. As an example of a continuous operation of the method, solutions of 2-methylphenoxyalkanoic acid and hypochlorite can be added continuously to the reaction vessel in stoich- iometric amounts, the amounts being determined on the basis of analysis of the solutions, in such concentrations that the chlorination ends with a solution of product in a concentration not leading to precipitation. pH is measured and regulated in the reaction vessel, and the addition of hypoclorite can be controlled by measuring the redox poten¬ tial. The solution leaving the reaction vessel may be treated batchwise or continuously to obtain the final product. This embodiment is illustrated in Example 15. It will be understood from the above that the method of the invention may be used as the chlorinating step when prepar¬ ing 4-chloro-2-methyl-phenoxyalkanoic acids of formula II from 2-methylphenol (o-cresol) by e.g. a) condensing o-cresol with the corresponding chloro- alkanoic acid (for the preparation of MCPA or MCPP) or with the corresponding lactone (for the preparation of MCPB) in a strongly alkaline medium at reflux temperature. The reac¬ tion mixture is water vapour-distilled or extracted with e.g. an organic solvent whereby unreacted o-cresol is removed from the mixture. The resulting mixture is cooled to the chlorination temperature, i.e. at least below 50°C, and if necessary diluted with water to a suitable content of 2-methyl-phenoxyalkanoic acid; b) adding a suitable amount of a catalyst as defined above, such as a compound of formula I, to the reaction mixture followed by thorough agitation of the mixture while maintaining the average temperature thereof at 0-50°C and pH below 10. Thereafter, a chlorinating agent, e.g. elementary chlorine, HCIO, normally formed in situ by adding an aqueous solution of an alkali metal salt of HCIO, is added over a period of up to several hours, monitored by measuring the redox potential. The reaction mixture is allowed to stand for about 15 minutes, followed by addition of Na 2 Sθ 3 to remove the excess of the chlorina¬ ting agent. The resulting mixture contains approximately 8- 20% w/w 4-chloro-2-methyl-phenoxyalkanoic acid; c) separating the 4-chloro-2-methyl-phenoxyalkanoic acid from the reaction mixture in any suitable manner depending on the desired form of the resulting product. A preferred method of separation is to heat the reaction mixture to about 60-90°C and add an equimolar amount of a mineral acid, whereby the acid prepared is liberated as a heavy oil which easily separates from the water. After decanting the water, the oil is washed once with an equal volume of water and either crystallized as flakes or formu- lated directly as a salt. In one preferred embodiment of the invention, the corre¬ sponding 2-methylphenoxyalkanoic acid is subjected to chlorination in water as the reaction medium and with an alkali metal salt of HC10 added thereto to provide the chlorinating agent. The pH of the reaction mixture is maintained at about 8.5 whereby the added alkali metal salt of HCIO is partly converted into HCIO which then is the chemical compound which is believed to be the actual reac- tant in the chlorination reaction process. More alkali metal salt of HOC1 is converted into HCIO when HCIO is removed in the chlorination process, thus maintaining the equilibrium between HOC1 salt and HCIO. The temperature is maintained at about 20°C. The reaction mixture subjected to the method of the inven¬ tion, i.e. the resulting reaction mixture from step a) in the above-mentioned process for preparing 4-chloro-2-meth- ylphenoxyalkanoic acids from 2-methylphenol (o-cresol) , may contain 2-methylphenol (o-cresol) which may remain in minor amounts in the mixture after steam-distillation or extrac¬ tion. If the chlorinating agent is HCIO, optionally added as an alkali metal salt thereof, this 2-methylphenol will be oxidized to water-soluble aliphatic acids during the chlorination process step since HCIO acts as oxidizing agent. This final oxidation is well-known in waste-water treatment and it may be considered advantageous that no chlorinated o-cresols will be present in the waste-water from the preparation of 4-chloro-2-methylphenoxyalkanoic acids according to the method of the invention. An excess of HCIO in the chlorination step may be required, depending on the content of 2-methylphenol. In a very interesting embodiment of the method of the invention, the chlorination is performed by means of HCIO formed in situ from an alkali metal salt thereof and hydro- chloric acid, and the waste water from the reaction con¬ taining alkali metal chloride formed in the chlorination is subjected to electrolysis to form alkali metal hydroxide and chlorine, some of the chlorine being reacted with some of the alkali metal hydroxide and recycled as hypochlorite, the rest being burned with hydrogen to form hydrochloric acid, which is then recycled for use in the chlorination. When the starting 2-methyl-2-phenoxyalkanoic acid is prepa¬ red from o-cresol and chloroacetic acid in the presence of an alkali metal hydroxide such as described above, the alkali metal hydroxide formed in the electrolysis may be recycled for use in the preparation of the 2-(2-methyl- phenoxy)alkanoic acid. In this manner, the consumption of alkali metal hydroxide and mineral acid in the method of the invention may be minimized. This is of interest because these are the rea- gents the consumption of which is otherwise increased in the method of the invention compared with the traditional methods (which, as explained above, have the disadvantage that they consume other, more complex, reagents) . The electrolysis may be performed in any suitable electro- lysis plant. In order to obtain the best economy in the electrolysis, the concentration of chloride in the waste water should be as high as possible. A high content of chloride results when the concentration of phenoxyalkanoic acid is high. A particularly high concentration of phenoxyalkanoic acid is obtained when the salt of the phenoxyalkanoic acid formed is the potassium salt, which is considerably more soluble than the sodium salt. This means that the alkali metal salt of HCIO should preferably be the potassium salt. The fact that potassium hydroxide is more expensive than sodium hydroxide is of little importance in this connection, as, in principle, all alkali metal can be recycled in this embodiment of the invention. The embodiment in which potassium hydroxide is used iε illustrated in Example 14. It should be noted that the conversion of 2-methylphenol may not be fully completed when the chlorination step has been completed. The 2-methylphenol itself may have disap¬ peared but some of the degradation products will remain in the reaction mixture and thus lower the purity of the technical acid. This is especially the case in the prepara¬ tion of MCPP. Subsequent treatment with small amounts of NaClO at a pH above 9.5 (chlorination of the phenoxy acid does not take place in this pH range) ensures the removal of such impurities, and the colour of the mixture and of the resulting acid is lightened, cf. Example 4 and 5 below. Also, this subsequent treatment does not result in any loss of product. The following examples are illustrative of the method of the invention. In these examples, all amounts are expressed in terms of % w/w (weight/weight) unless otherwise indicated. The following abbreviations are used: 2MPA 2-methylphenoxyacetic acid 2MPP 2-(2-methylphenoxy)propionic acid 2MPB 4-(2-methylphenoxy)butyric acid 6C2MPA 6-chloro-2-methylphenoxyacetic acid 6C2MPP 2-(6-chloro-2-methylphenoxy)propionic acid 6C2MPB 4-(6-chloro-2-methylphenoxy)butyric acid MCPA 4-chloro-2-methylphenoxyacetic acid MCPP 2-(4-chloro-2-methylphenoxy)propionic acid MCPB 4-(4-chloro-2-methylphenoxy)butyric acid 4, 6DC2MPA 4, 6-dichloro-2-methylphenoxyacetic acid 4 , 6DC2MPP 2-(4, 6-dichloro-2-methylphenoxy)propionic acid 4, 6DC2MPB 4-(4,6-dichloro-2-methylphenoxy)butyric acid EXAMPLE 1 Preparation of MCPA using N,N-dimethyl-2-aminopropionic acid N' .N'-dimethylamide as catalyst 935 g of a solution in water containing 166.06 g (1.0 mol, 17.76% w/w) of 2MPA and 2.52 g (0.27% w/w) o-cresol was diluted further with 300 ml of water. 2 g of N,N-dimethyl- 2-aminopropionic acid N' ,N'-dimethylamide was added. The reaction mixture was agitated thoroughly by means of a propeller stirrer, and the temperature of the mixture was maintained at 20°C (water bath) and pH was maintained at 8.5 by addition of a 37% w/w HCl solution. 601.8 g of an aqueous solution of NaOCl containing 80.04 g (1.13 mol, 13.3% w/w) of elementary chlorine was added to the reaction mixture over a ptriod of approximately 5 hours, whereby a sodium salt of MCPA precipitated at the end of the addition. The reaction mixture was allowed to stand for 15 minutes, followed by addition of sufficient Na 2 S0 3 (sodium sulphite) to remove the excess of NaOCl. The content of NaOCl (delivering the oxidizing agent) was monitored by means of potassium iodide/starch paper. HPLC analysis of the resulting reaction mixture showed the following content of reaction products and starting materi¬ al: % w/w The yield of MCPA: > 98%. The ratio of MCPA to 6C2MPA is 9.57/0.051 = 188. The reaction mixture was slowly poured into 100 ml of HC1 37% w/w (1.22 mol) at 50°C to precipitate MCPA in the acid form. The precipitated acid was removed by filtration and washed with 3 x 400 ml of water followed by melting the acid at 87°C whereby water was released. The melted acid was further heated to 130°C to release and remove the remaining water and poured into a tray to obtain crystal¬ line material resembling industrial flakes. An analysis of the flakes showed the following content: % w/w MCPA/6C2MPA 633 2100 ml of mother liquor containing 0.899 g (0.4%) of MCPA was obtained. The total amount of washing water was 1130 ml with a content of 1.56 g (0.8%) of MCPA. EXAMPLE 2 Illustration of the effect of the presence of a catalyst in the chlorination 0.1 mol of 2MPA was chlorinated as described in Example 1 with and without the presence of a catalyst, respectively. As in Example 1, the catalyst was N,N-dimethyl-2-amino- propionic acid N' ,N'-dimethylamide which was added in an amount of 0.2 g. Analysis of the two reaction mixtures appears from Table I: TABLE 1 With catalyst Without catalyst % w/w % w/w 1.32 0.680 5.40 0.000 MCPA/6C2MPA 178 8.0 From the results it is obvious that considerable amounts of unchlorinated product (2MPA) are present in the reaction mixture without catalyst, and the effect of the presence of the catalyst is clearly demonstrated. EXAMPLE 3 Preparation of a commercial solution of 2MPA potassium salt 0.5 mol of 2MPA was chlorinated and precipitated as de¬ scribed in Example 1. After filtration and washing with 2 x 100 ml of water, the filter cake was dissolved in 88 g (30% w/w) solution of KOH, and water was added to the solution in such an amount that the total volume of the resulting solution was 240 ml (275.2 g) . An analysis of the solution gave the following results: *) A typical concentration in commercial products. The yield of MCPA was 96.4%. 750 ml of mother liquor with a content of 0.098 g (0.1% w/w) of MCPA was obtained. The total amount of washing water was 200 ml with a content of 1.33 g (1.3%) of MCPA. The washing water was recirculated to the next batch and used for dilution of the initial 2MPA solution. This resul¬ ted in an increase in the total yield to a total of 97.7% (96.4% + 1.3%) . EXAMPLE 4 Preparation of MCPP using N,N-dimethyl-2-aminopropionic acid N -N'-dimethylamide as catalyst and subsequent treat¬ ment to remove impurities To 311.0 g of a solution in water containing 71.97 g (0.40 mol, 23.14% w/w) of 2MPP and 0.53 g (0.17% w/w) of o-cresol was added 0.8 g N,N-dimethyl-2-aminopropionic acid N' ,N'-dimethylamide. Temperature of the mixture was main¬ tained at 20°C and pH was maintained at 8.5 as described in Example 1. 240.1 g of a solution of NaOCl in water containing 16.25 g (0.43 mol, 12.8% w/w) of elementary chlorine were added to the reaction mixture over a period of approximately 3 hours. The reaction mixture was allowed to stand for 15 minutes, followed by addition of sufficient Na 2 S0 3 (sodium sulphite) to remove the excess of NaOCl as described in Example l. In order to establish whether the chlorination process was terminated, a sample of the reaction mixture was analyzed. The following result was obtained: w/w The reaction mixture was divided into two parts. The first part was subjected to treatment with 10 ml of NaOCl at pH > 9.5 whereas the second part was untreated. The acid present in the treated and the untreated part, respectively, was precipitated by pouring the corresponding part of the reaction mixture slowly into 20 ml HC1 37% w/w at 25°C. Each of the resulting reaction mixtures were subjected to filtration on a Buchner funnel and washed with 2 x 50 ml of water and the precipitated acid was dried overnight in a vacuum oven. The resulting fine granules were analyzed with the following result: TABLE 2 NaOCl-treated Untreated % w/w % w/w 0.29 0.40 93.88 0.00 MCPP/6C2MPP 313 235 EXAMPLE 5 Treatment of MCPA to remove impurities 2MPA was chlorinated as described in Example l, and the resulting reaction mixture was divided into two parts. The first part was treated with NaOCl as described in Example 4. The NaOCl-treated and the untreated part were precipitated, filtered, washed and dried as described in Example 4. Analysis of the resulting fine granules showed the follow¬ ing results: 26 TABLE 3 NaOCl-treated Untreated % w/w % w/w 0.08 0.33 96.25 0.04 0 MCPA/6C2MPA 359 292 The ratio of MCPA to 6C2MPA in the fine granules subjected to NaOCl-treatment was 97.02/0.27 = 359, whereas the cor- 5 responding ratio for the untreated fine granules was 96.25/0.33 = 292. EXAMPLE 6 Preparation of MCPB using N,N-dimethyl-2-aminopropionic acid N' .N'-dimethylamide as catalyst -0 19.4 g of 2MPB (0.100 mol) was dissolved in 100 ml of water. 0.3 g N,N-dimethyl-2-aminopropionic acid N',N'- dimethylamide was added and the reaction mixture was stir¬ red. The temperature of the mixture was maintained at 20°C and pH was maintained at 8.5 as described in Example l. 5 75 g of a solution of NaOCl in water containing 7.5 g (0.106 mol, 10.0% w/w) of elementary chlorine were added to the reaction mixture over a period of approximately 45 minutes. The reaction mixture was allowed to stand for 15 minutes, followed by addition of sufficient Na 2 S0 3 (sodium 0 sulphite) to remove the excess of NaOCl as described in Example 1. Analysis of the resulting reaction mixture showed the following: w/w EXAMPLE 7 Illustration of the effect of temperature and pH on the chlorination 16.6 g of 2MPP (0.10 mol) were dissolved in 150 ml of water. The solution was subjected to chlorination at dif- ferent temperatures and different pH values by the addition of 52.9 g of a solution of NaOCl in water containing 7.46 g (0.105 mol, 14.1% w/w) of elementary chlorine # The method of chlorination was as described in Example 1. The temperatures were maintained at 10°C, 20°C and 30°C, respectively, and the pH was maintained at 7.0, 8.0 and 9.0, respectively. Each of the resulting reaction mixtures were analyzed to determine the percentage of unreacted 2MPP (on a molar basis, i.e. mol/mol) as well as the ratio of MCPP to 6C2MPP, respectively. For each combination of temperature and pH, the chlorina¬ tion of 2MPP was carried out twice. The following table shows the results : TABLE 4 % unreacted 2MPP (mol/mol) 10°C 20°C 30°C pH = 7.0 pH = 8.0 pH = 9.0 Average 1.84 3.19 5.15 MCPP/6C2MPP ratio 10°C 20°C 30°C pH = 7.0 pH = 8.0 pH = 9.0 Average 187.4 132.3 99.1 A statistical analysis (2 sided variance analysis) was performed in order to determine the influence of tempera¬ ture and pH on the chlorination reaction. The results in terms of percent of unreacted 2MPP show that the parameters do not interact and, furthermore, that pH has no influence on the degree of conversion of 2MPP. How- ever, the temperature has a significant effect, the conver¬ sion being higher at lower temperatures. The results in terms of the MCPP/62 ' CMPP ratio are that according to the statistical analysis, the parameters temperature and pH do not interact and, furthermore, pH has no influence on the ratio obtained. However, the tempera¬ ture has a significant influence, the ratio being signifi¬ cantly higher at lower temperatures. EXAMPLE 8 Illustration of the effect of various catalysts in the chlorination The effect of various catalysts on the chlorination of 2MPA to MCPA is illustrated below. The various catalysts were prepared from a chloroalkanoic acid and an amine or from an amino acid and an alkanol or are commercially available, cf. Example 9 and 10 below. The following catalysts were tested: A = N,N-dimethylaminoacetic acid N' ,N'-dimethylamide B = N,N-dimethyl-2-aminopropionic acid N' ,N'-dimethylamide C = N,N-diethylaminoacetic acid N' ,N'-diethylamide D = N,N-dimethyl-3-aminopropionic acid N' ,N'-dimethylamide E = methyl N,N-dimethylaminoacetate F = N,N,N' ,N'-tetramethyl-l,2-diaminoethane G = N,N-dimethyl-2-aminobutyric acid N' ,N'-dimethylamide H = ethyl N,N-dimethylaminoacetate I = N,N-dimethylaminoacetic acid amide Each of the catalysts mentioned were used in a conversion reaction of 2MPA as described in Example 1, and the reac- tion mixtures were subjected to analysis without further treatment. All experiments were carried out twice. The exact manner in which the experiments were performed (the conditions referred to in claim 1 herein) was as follows: 0.15 mol of 2MPA (128.4 g of a mixture with 19.4% 2MPA) was diluted to 175 ml. The compound to be tested for cata¬ lytic effect was added (1.5% mol/mol relative to 2MPA or 13 mmol per litre). pH was kept at 8.5 by addition of HC1 and the temperature was kept at 20°C by means of a water bath. 0.16 mol of active chlorine (a solution of 98 g NaOCl with 11.7 w/w% active chlorine) was added within 1 hour followed by stirring for 15 minutes. Excess of NaOCl was removed with NaS 2 S0 3 . Table 5 shows the results in terms of the calculated ratios of MCPA product to 6C2MPA, i.e. the ratio of 4- to 6-chlor- inated product as a function of the catalyst (the various catalysts are represented by their starting materials) . When no catalyst is added, the ratio between 4- and 6- chlorinated product is 8.0, which means that a ratio of 8.0 or very close thereto indicates no catalytic effect. TABLE 5 Catalyst MCPA/6C2MPA ratio 172 168 B 199 203 40. 43. D 19.4 18.6 74.2 79.7 63.7 50.6 135 130 H 101 100 116 117 As can be seen from the table above, N,N-dimethylamino- acetic acid N' ,N'-dimethylamide (= A), N,N-dimethyl-2- aminopropionic acid N' ,N'-dimethylamide (= B) and N,N,N' ,N'-tetramethy1-1,2-diaminoethane (= F) have the highest impact on the MCPA/6C2MPA ratio. Using exactly the same procedure as described above, a number of compounds which to some extent resemble the catalytically active compounds used according to the inven¬ tion, but which do not fulfill the above definition accord¬ ing to which one of the carbon atoms should be substituted with an electropositively functioning group, and the other 32 carbon atom should be the carbon atom in an electronega¬ tively functioning group, were tested. The compounds were the following: 1 = N,N,N' ,N'-tetramethyloxalamide 2 - ethyleneglycoldimethylether 3 = methyl methoxyacetate 4 = dimethyloxalate 5 = N,N-dimethyl-2-methoxyethylamine 6 = N,N-dimethyl-methoxyacetic acid amide 7 = methyl N,N-dimethyl-oxamate 8 = N,N-dieth l-2-aminopropionic acid N' ,N'-diethylamide 9 = 2-aminopropionic acid amide 10 = N-methyl-2-aminopropionic acid N'-methylamide 11 = N,N-dimethyl-4-aminobutyric acid N' ,N'-dimethyl mide 12 = N,N,N',N'-tetramethylurea 13 = acetic acid N,N-dimethylamide 14 = N,N-dimethylaminoacetic acid 15 = ethyleneglycol 16 = N,N-diethylethanolamine 17 = N,N-dimethylethanolamine 18 = N,N,N' ,N'-tetramethyldiaminomethane The results appear from Table 6. TABLE 6 Compound MCPA/6C2MPA ratio 8.4 8.3 8.3 8.4 8.3 8.4 10 11 12 13 14 15 16 17 18 As will be seen from the above results, the only compounds which have some appreciable effect on the reaction are compounds 5, 10 and 17, although thiε effect is far below the lower limit which defined above for interesting cata- lyεtε. Thiε small effect is in good accordance with the definition of the catalyst herein, as the oxygen atom in compounds 5 and 17 does have an electronegative function, although of a rather weak electronegativity, and the monosubstituted amino group of compound 10 does have an electropositive function, but, in the initial stage of the chlorination, loses most of its electropositivity because the hydrogen atom of the amino group reacts with the chlorine of the chlorinating agent. EXAMPLE 9 Preparation of N,N-dimethyl-2-aminopropionic acid N',N'- dimethylamide To 2 moles of dimethyla ine (188 ml of 48% w/vol dimethyl- amine) were added 10 ml of water which resulted in a 10 N solution. 0.5 mol of 2-chloropropionic acid chloride (63.5 g) was added dropwise thereto while the reaction mixture was cooled in an ice bath to maintain the tempera- ture at 25°C for 1.5 hours. Stirring was continued for 15 minutes. 1.05 mol of NaOH pellets dissolved in 125 ml of water (8 N solution) was added to the mixture to remove any hydrochloride present in such a way that pH did not exceed 11. Each time the pH had reached 11, the addition of NaOH solution was stopped and the reaction mixture was subjected to evaporation on a rotary'evaporator (20 mm Hg; 30°C) and the procedure was repeated until the reaction mixture did not require further addition of base, i.e. the amine had disappeared. The mixture was evaporated to dryness on the rotary evaporator (up to 50°C) and the product was dried overnight in a vacuum oven at 50°C. The residue waε ex¬ tracted with 3 x 50 ml of dichloromethane and evaporated. The yield waε 80%. The title compound waε identified by IR analysis which showed a peak at 1640-1660 cm" 1 corresponding to a di- εubεtituted amide and a εignificant peak at 2900-3000 cm" 1 which characterizes the amino group of an amino acid. Boiling point (12 mm Hg) : 85°C N-Analysiε: 18.8% (theor. 19.4%) The following catalytic compounds were prepared analogously to the above by using the corresponding starting materials: N,N-dimethyl-3-aminopropionic acid N' ,N'-dimethylamide, N,N-dimethylaminoacetic acid N' ,N'-dimethylamide, and N,N-diethylaminoacetic acid N' ,N'-diethylamide. Each of these compounds were identified by IR analysiε which εhowed the above-mentioned major peakε. The following catalyεt uεed in the above Exampleε is com¬ mercially available: N,N,N' ,N'-tetramethyl-l,2-diaminoethane (available from Fluka AG Che ische Fabrik, Buchε, Switzerland) EXAMPLE 10 Preparation of methyl N.N-dimethylaminoacetate 0.05 mol of N,N-dimethylglycine and 0.1 mol thionyl¬ chloride were combined in a reaction veεsel equipped with a reflux condenser and heated to 60°C for 2 hours. The resul¬ ting mixture waε evaporated on a rotary evaporator to remove the excess of thionylchloride. 40 ml of anhydrous ethanol were added and the mixture was heated to reflux temperature for 2 hours. The resulting mixture was evapo¬ rated to remove the excess of methanol. The product was subjected to IR analysis which showed a characteristic peak at 1750 cm" 1 (the ester) and a peak at 2950 cm -1 (charac¬ terizing the amine functions) . 36 EXAMPLE 11 Preparation of MCPP using Cl 2 as chlorinating agent and N.N-dimethyl-2-aminopropionic acid N' , '-dimethylamide as catalyst 0.2 mol of 2MPP and 2 g of N,N-dimethyl-2-aminopropionic acid N' ,N'-dimethyl amide (approximately 50% purity) were treated aε deεcribed in Example 1 (pH = 8.5, temperature = 20°C). 0.24 mol of Cl 2 waε bubbled through the reaction mixture while the pH was maintained at 8.5 by addition of 4 N NaOH solution. Analysis of the resulting reaction mixture showed the following results: % w/w MCPP/6C2MPP 26 EXAMPLE 12 Illustration of the effect of uεing an agueouε medium con¬ taining an organic water-miεcible or water-immiεcible solvent in the chlorination process. 0.15 mol of 2MPA (143.9 g of a solution containing 17.3% 2MPA) and 0.3 g N,N-dimethyl-aminoacetic acid N',N'-di- ethylamide was diluted with 60 g of water. 25 ml of organic solvent waε added (acetone and xylene, respective- iy ) . 0.16 mol of active chlorine (100.6 g NaOCl solution with 11.4% active chlorine) waε added within 1 hour. pH waε kept at 8.5 by addition of HCl, and the temperature waε kept at 20°C by meanε of a water bath. When xylene was used, the phaεes were separated before analysis. Result, % w/w: EXAMPLE 13 Preparation of MCPA with cooling of the reaction mixture and control of the chlorination by measuring the redox potential 0.15 mol of 2MPA (124.5 g of a solution containing 20.0% of 2MPA) and 0.3 g N,N-dimethylaminoacetic acid N',N'-di- methyl amide were mixed with 75 g of ice. The redox potential was measured by means of a mV-meter uεing platinum and mercury sulphate electrodes. Start temperature = -5.5°C Start redox potential * = 320 V pH was kept at 8.5 by addition of HCl. Addition of NaOCl solution (11.7% active chlorine) was performed by means of a pump controlled by the pH-meter. The end point was -750 mV, and the delay time was approximately 50 sec . which means that the pump was adjusted to εtart again if the redox potential roεe above -750 mV within 50 sec. During the addition, which took about 56 min. , the redox-potential was stable at around -470 V until the end of the chlorination at which time the redox potential fell to -750 mV, causing the pump to stop. The pump started again a few times before the redox-poten¬ tial was stable below -750 mV. End temperature = approximately 30.0°C Average temperature = approximately 15°C Consumption of NaOCl = 99.6 g = 0.164 mol of active chlorine Result, % w/w: MCPA/ 2MPA 2C6MPA MCPA 2,4-DC6MPA 2C6MPA 0.018 0.050 8.204 0.000 166 EXAMPLE 14 Preparation of MCPA using potassium εaltε of 2MPA and HOCl To a mixture of 0.20 mol of 2MPA (83.0 g of a εolution containing 40% 2MPA aε potaεsium salt) and 0.22 g N,N- dimethyl-aminoacetic acid N' ,N'-dimethyl amide was added approx. 0.21 mol active chlorine (119 g of a solution of KOC1 containing 12.5% active chlorine). pH was kept at 8.5 by means of addition of HCl, and the temperature was kept at 20°C by means of a water bath. Addition of K0C1 was controlled by measuring the redox potential as described in Example 13. Result, % w/w: MCPA/ 2MPA 2C6MPA MCPA 2,4-DC6MPA 2C6MPA 0.006 0.085 16.008 0.057 190 The above result clearly demonstrateε that the chlorination takes place as selectively as with sodium salt and that a higher concentration of end product can be obtained using potassium salt than when using sodium salt. EXAMPLE 15 Preparation of MCPA in a continuous chlorination process Into a reactor with a bottom-valve, stirrer and external cooling were led streams of 2MPA (1.00 mol 2MPA per liter and 0.014 mol o-cresol) and NaOCl (1.09 mol active chlo- rine per liter), each at a rate of 9.5 ml/min. The catalyst, N,N-dimethyl-aminoacetic acid N' ,N'-dimethyl amide, was dissolved in the 2MPA solution. The reaction mixture left the reactor through a tube in the bottom- valve placed with the opening of the tube in level with the surface of the reaction mixture in the reactor. The reac¬ tion volume was 680 ml, resulting in a retention time of approx. 40 min. pH was measured in the reactor and controlled by addition of HCl. When the system had stabilized, a sample is taken from the mixture leaving the reactor. Result, % w/w: MCPA/ 2MPA 2C6MPA MCPA 2,4-DC6MPA 2C6MPA 0.089 0.060 7.874 0.000 132

Claims: CLAIMS 1. A method for preparing a 4-chloro-2-methyl-phenoxyalka- noic acid, comprising reacting the corresponding 2-methyl- phenoxyalkanoic acid with a water-compatible chlorinating agent in an aqueous medium in the presence of a catalyst which is a chemical compound comprising two carbon atoms bonded together with a single and or via a methylene or methine group, one of the carbon atoms being subεtituted with an electropositively functioning group, the other carbon atom being the carbon atom in an electronegatively functioning group; and which catalyst is capable of ensuring a ratio between 4-chlorinated and 6-chlorinated reaction product of at least 15, preferably at least 25; or which catalyst is capable of ensuring a ratio between 4- chloro-2-methylphenoxyacetic acid and 6-chloro-2- methylphenoxyacetic acid of at least 15, preferably at least 25, when used as the catalyst in the production of 4- chloro-2-methylphenoxyacetic acid by chlorination of 2- methylphenoxyacetic acid under the conditions as defined in Example 8 herein. 2. A method according to claim 1 in which the electro¬ positively functioning group is dialkylεubstituted amino, and the electronegatively functioning group is selected • from carbonyl, thiocarbonyl and selenocarbonyl. 3. A method according to claim 2, wherein the electronega¬ tively functioning group is carbonyl or thiocarbonyl, and the carbon atom of the carbonyl or thiocarbonyl group additionally carries hydrogen or a group attached via a nitrogen, carbon, oxygen, or sulfur atom. 4. A method according to claim 3, wherein the electronega¬ tively functioning group is carbonyl, and the carbon atom of the carbonyl group additionally carries a group R-O- in which R is an aliphatic group, or carries an optionally dialiphatic-substituted amido gro p. 5. A method according to claim 1, in which the catalyεt iε a compound of the general formula I X - CR- ] _R 2 - (CHR χ ) n - CR 3 R4 - Z (I) wherein R l , R 2 , R 3 and R 4 each independently is hydrogen or C-L_ 4 alkyl; or R-^ and R 2 together are oxo, and R 3 and R 4 each independently is hydrogen or C- ] __4 alkyl; n is an integer 0 or 1; and R x is hydrogen; or, when n iε l, R x and R4 may together deεignate a carbon- carbon bond; X, when R-*_ and R 2 together are oxo, is C 1 _ 6 alkoxy, hydrogen, C- ] __g alkyl or NR 5 R 6 , wherein R 5 and R5 each independently is hydrogen or C-^.g alkyl; or X, when R-^ and R 2 are not together oxo, is NR' 5 R'g wherein R' 5 and R' 6 each is C 1 _ 4 alkyl; and Z is NR7R3 wherein R7 and R 8 each independently is C^ - alkyl. 6. A method according to claim 5, wherein n is 0. 7. A method according to claim 5 or 6, wherein each of R 7 and R 8 is C* j __ 2 alkyl. 8. A method according to any of claims 5-7, wherein each of R' 5 and R'g is C;L_ 2 alkyl. 9. A method according to any of claimε 5-8, wherein R 5 and Rg each independently iε C- j __ 4 alkyl. 9. A method according to claim 5, wherein the catalyst iε N,N-dimethyl-2-aminopropionic acid N' ,N'-dimethylamide, N,N-dimethyl-3-aminopropionic acid N' ,N'-dimethylamide, N,N-dimethyl-2-aminobutyric acid-N' , '-dimethylamide, methyl N,N-dimethylaminoacetate, 42 N,N-dimethylaminoacetic acid N' ,N'-dimethylamide, N,N-diethylaminoacetic acid N' ,N'-diethylamide, N,N,N' ,N'-tetramethy1-1,2-diaminoethane, ethyl N,N-dimethylaminoacetate, or ' N,N-dimethylaminoacetic acid amide. 10. A method according to any of the preceding claims, wherein the 4-chloro-2-methyl-phenoxyalkanoic acid is 4-chloro-2-methylphenoxyacetic acid (MCPA) , 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP) or 4-(4-chloro-2-methylphenoxy)butyric acid (MCPB). 11. A method according to claim 10, wherein the 2-(4- chloro-2-methyl-phenoxy)alkanoic acid is the optically active R-(+)-2-(4-chloro-2-methylphenoxy)propionic acid (MCPP) . 12. A method according to any of the preceding claims, wherein the aqueous reaction medium is water. 13. A method according to any of the preceding claims, wherein the water-compatible chlorinating agent is selected from elementary chlorine and HCIO. 14. A method according to claim 13, wherein the chlorinat¬ ing agent is HCIO formed in εitu in the reaction mixture from a εalt thereof and a mineral acid. 15. A method according to any of the preceding claimε, wherein the reaction iε carried out at a pH below 10, preferably in the range of 5-9, eεpecially in the range of 7-9. 16. A method according to any of the preceding claims, wherein the reaction iε carried out at an average tempera¬ ture between 0°C and 50°C, preferably between 10°C and 30°C. 17. A method according to claim 16, wherein the reaction iε εtarted at a temperature of 0°C or below, and the reaction temperature iε allowed to increaεe, the average temperature being in the range between 10°C and 30°C over the courεe of the reaction. 18. A method according to any of the preceding clai ε, wherein the chlorinating agent iε HCIO (optionally formed in situ from a salt thereof) which is added continuously or intermittently during the course of the reaction, the redox potential of the reaction mixture being monitored, and the addition of the chlorinating agent being stopped when the redox potential of the mixture has reached a stable value numerically higher than the εtarting value. 19. A method according to any of the preceding claimε, wherein the chlorination iε performed by meanε of HCIO formed in situ from an alkali metal salt thereof and hydrochloric acid, and the waste water from the reaction containing alkali metal chloride formed in the chlorination is subjected to electrolysis to form alkali metal hydroxide and chlorine, some of the chlorine being reacted with the alkali metal hydroxide and recycled as hypochlorite, the reεt being burned with hydrogen to form hydrochloric acid, which is then recycled for use in the chlorination. 20. A method according to claim 19, wherein the alkali metal salt of HCIO is the potassium salt.

Inventors: HOLM-JENSEN BJARNE, OXBOEL ARNE, HOLM-JENSEN, BJARNE, OXBOEL, ARNE
Applicants: KVK AGRO AS, KVK AGRO A/S
Application year: 1991
Main classification: C07C59
IPC codes: C07C51