GB2044246A - Process for producing 4-bromo- 2,5-dichlorophenol - Google Patents

Abstract

A process for the production of 4- bromo-2,5-dichlorophenol comprises the bromination of 2,5-dichlorophenol in the presence of a compound of the formula <IMAGE> in which R1 represents a C1-8 alkyl, or the phenyl or benzyl group, R2 and R3 independently of one another represent a C1-8 alkyl, R4 represents hydrogen, or C1-8 alkyl, and X represents chlorine, bromine, or iodine or the hydrogen sulfate anion. The new process substantially avoids the undesired formation of the 2,6- isomer and the 4-bromo-2,5- dichlorophenol is obtained in excellent purity.

Description

SPECIFICATION Process for producing 4 - bromo - 2, 5 - dichlorophenol The present invention relates to a process for producing 4 - bromo - 2, 5 - dichlorophenol by bromination of 2, 5 - dichlorophenol.

4 - Bromo - 2, 5 - dichlorophenol is a valuable intermediate for producing insecticidal and acaricidal active substances. To be mentioned among these is in particular the 0, 0 - dimethyl - 0 - (4 bromo - 2, 5 - dichlorophenyl) - thiophosphoric acid ester, the production and use of which are described in the U.S. Patent Specification No.3.227.610.

It is known that 4 - bromo - 2 - chlorophenol can be produced by reacting 2 - chlorophenol with bromine at room temperature in carbon tetrachloride. The desired 4 - bromo - 2 - chlorophenol is obtained in this manner in a yield of 87% of theory (see Ch.

Raiford et. al.,J.Amer, Chem.55,2125-2131 (1933)).

Furthermore, it is known from the U.S. Patent Specification No.3.449.443 that on reaction of 2 chlorophenol with bromine chloride in carbon tetrachloride at 0 C there is formed, besides 74% of theory of 2 - bromo - 6 - chlorophenol, only 22% of theory of 4 - bromo - 2 - chlorophenol; and at 23 to 26 C-there is formed, besides 61.7% of theory of 2 - bromo - 6 chlorophenol, only 26.3% of theory of 4 - bromo - 2 chlorophenol.

From the German Offenlegungsschrift No.

2.144.259 is also known the procedure of reacting phenols in the melt, in the presence of brominating catalysts such as iron, aluminium, iron halides, aluminium halides and iodine or mixtures of these substances, with a mixture of bromine and chlorine.

This process yields however exclusively highly brominated compounds.

The processes mentioned above are lacking above all in that they are insufficiently selective with regard to the formation of the desired 4 - bromo - 2, 5 dichlorophenol. This leads on the one hand to losses in yield, and on the other hand to the necessity of separating the isomers which are always concomitantly formed, especially the 2, 6 - isomer, since these impairthe effectiveness of the final products; and this separating operation constitutes an undesirable additional expenditure.

It is therefore the aim of the present invention to provide a process by which 2, 5 - dichlorophenol can be brominated in the 4-position more selectively than it can be by the processes known hitherto.

It has been found that 4 - bromo - 2, 5 dichlorophenol can be produced with excellent selectivity by brominating 2, 5 - dichlorophenol in the presence of a compound of the formula I

in which R1 represents an alkyl group having 1 to 8 carbon atoms, or the phenyl or benzyl group, R2 and R3 independently of one another represent an alkyl group having 1 to 8 carbon atoms, R4 represents hydrogen, or an alkyl group having 1 to 4 carbon atoms, and X represents chlorine, bromine, iodine or the hydrogen sulfate anion.

The bromination of 2, 5 - dichlorophenol by the process according to the invention can be performed either in the presence or in the absence of an inert solvent. Suitable solvents are for example halogenated hydrocarbons, such as methylene chloride, chloroform, carbon tetrachloride, 1,2 - dich loroethane, 1, 1 - dichloroethane, 1, 1, 1 -trich- loroethane, chlorobenzene, o - dichlorobenzene or Frigen - 113 (trichlorotrifluoroethane). Furthermore, hydrocarbons such as hexane, benzene, toluene and xylenes are likewise suitable. With the absence of a solvent, the process according to the invention is performed in the melt.

The process according to the invention is performed at temperatures within the range of 0 to 75"C.

In the case where the reaction is performed in a solvent, the preferred temperature range is between 0 and 20"C; and in the melt the reaction is performed in each case preferably just above the melting point of the reaction mixture. This means in practice that the bromination in the melt is commenced at the melting point of the starting material and the temperature is successively raised in the course of bromination. When 4 - bromo - 2,5 - dichlorophenol is produced by melt bromination, the reaction is performed for example initially at 58"C, and the temperature during the course of bromination is gradually raised, to an extent corresponding to the rising melting point of the reaction mixture, to 70 to 75"C.

The compounds of the formula I are added according to the invention in amounts of 1 to 10 percent by weight, preferably 3 to 6 per cent by weight, relative to the employed amount of 2,5 - dichlorophenol.

Suitable compounds of the formula I are hydrochlorides, hydrobromides, hydroiodides and hydrogen sulfates of tertiary amines, such as trimethylamine hydrochloride, trimethylamine hydrobromide, triethylamine hydrochloride, triethylamine hydrobromide, triethylamine hydroiodide, tributylamine hydrochloride, tributylamine hydrobromide, tributylamine hydrogen sulfate or trioctylamine hydrochloride, and also quaternary ammonium salts, such as trimethylethylammonium bromide, trimethylethylammonium chloride, trimethylethylammonium iodide, trimethylphenylammonium chloride, triethylphenylammonium chloride, trimethylbenzylammonium chloride, trimethylbenzylammonium bromide and trimethylbenzylammonium hydrogen sulfate.

As brominating agents it is possible to use according to the invention elementary bromine, mixtures of chlorine and bromine, bromine chloride, N - bromine compounds such as N - bromosuccinimide, or hypobromites. Elementary bromine is preferred amongst these brominating agents. The brominating agent is used in stoichiometric amounts, or in a slight excess of up to 0.02 mole of bromine per mole of 2,5 - dichlorophenol. It is also possible however to use the brominating agent in an amount slightly less than the equivalent amount, and after bromination to remove the unreacted 2, 5 - dichlorophenol by distillation.

It is possible by the process according to the invention to convert 2, 5 - dichlorophenol, with excellent selectivity, into 4 - bromo - 2, 5 - dichlorophenol.

2, 5 - Dichlorophenol is converted practically quantitatively into bromodichlorophenol. The resulting reaction products contain when the process according to the invention is performed in an inert solvent about 1 percent by weight of undesired 2,6 - isomer, and when performed in the melt about 2 per cent of undesired 2,6 - isomer. The 4 - bromo - 2,5 - dichlorophenol produced according to the invention is therefore suitable, without further purification, as intermediate for producing insecticidal phosphoric acid esters, particularly 0, 0 - dimethyl - 0 - (2, 5 dichloro - 4 - bromophenyl) - thiophosphate.Compared with the processes known hitherto, the process according to the invention has the advantage that on the one hand the yield 4 - bromo - 2,5 dichlorophenol is increased, and on the other hand that expensive purifying operations on the intermediates produced are rendered unnecessary.

The process according to the invention is further illustrated by the following Example.

Example: 81,5 g (0.5 mole) of 2, 5 - dichlorophenol is dissolved in 120 g of chlorobenzene. After addition of 4.1 g (0.03 mole) of triethylamine hydrochloride the mixture is cooled to 5 C. Then 80.0 g (0.5 mole) of bromine is added during 3 hours. During the first hour of bromine addition the temperature of the reaction mixture is kept at 5 to 8"C. At the end of the bromine addition the temperature is allowed to raise to about 15"C. During the additon of bromine an evolution of hydrogen bromide gas can be observed.

Afterthe additon of bromine the reaction mixture is stirred at 15 to 20"C for 1 hour. Then the solvent is distilled off in vacuo at 70"Cll 5 to 20 torr.

126.0 g of product is obtained containing 94% by weight of 4 - bromo - 2, 5 - dichlorophenol (118.4 g) which corresponds to a yield of 98% of the theoretical amount calculated on 2, 5 - dichlorophenol. The product contains 0.5 to 1% by weight of 6 - bromo - 2, 5 - dichlorophenol.

The same result is obtained when the triethylamine hydrochloride is replaced with 5.0 g (0.03 mole) of ethyl -trimethyl ammonium bromide.

When the bromination of 2,5 - dichlorophenol is carried out under the same conditions but in the absence of a compound of formula I, such as triethylamine hydrochloride and ethyl trimethylammonium bromide, there are obtained 124.0 g of product containing 88% by weight (109.1 g) of4 - bromo - 2, 5 - dichlorophenol which corres pongs to a yield of 90 /O of the theoretical amount calculated on 2, 5 - dichlorophenol. The product contains 5 to 6% by weight of 6 - bromo - 2,5 - dich lorophenol as by-product.

Claims (13)

1. A process for producing 4 - bromo - 2, 5 - dich lorophenol by bromination of 2, 5 - dichlorophenol, which process comprises brominating 2, 5 - dich lorophenol in the presence of a compound of the formula I

in which R1 represents an alkyl group having 1 to 8 carbon atoms, orthe phenyl orbenzyl group, R2 and R3 independently of one another represent an alkyl group having 1 to 8 carbon atoms, R4 represents hydrogen, or an alkyl group having 1 to 4 carbon atoms, and X represents chlorine, bromine, iodine or the hydrogen sulfate anion.

2. A process according to Claim 1, wherein the bromination of 2, 5 - dichlorophenol is performed in the presence of an inert solvent.

3. A process according to Claim 1, wherein the bromination of 2,5 - dichlorophenol is performed in the presence of methylene chloride, chloroform, carbon tetrachloride, 1, 2 - dichloroethane, 1, 1 - dichloroethane, 1,1,1 -trichloroethane, chlorobenzene or o - dichlorobenzene, as solvent.

4. A process according to Claim 1, wherein the bromination of 2, 5 - dichlorophenol is performed in the melt, in the absence of a solvent.

5. A process according to Claim 1, wherein the bromination of 2,5 - dichlorophenoi is performed in the temperature range of 0 to 75"C.

6. A process according to Claim 1, wherein the bromination of 2, 5 - dichlorophenol is performed in the presence of an inert solvent in the temperature range of between 0 and 20"C.

7. A process according to Claim 1, wherein the bromination of 2,5 - dichlorophenol is performed in the melt in each case just above the melting point of the reaction mixture.

8. A process according to Claim 1, wherein there is used 1 to 10 per cent by weight of a compound of the formula I, relative to the employed 2, 5 - dichlorophenol

9. A process according to Claim 1, wherein there is used 3 to 6 per cent by weight of a compound of the formula I, relative to the employed 2, 5 - dichlorophenol.

10. A process according to Claim 1, wherein there are used as brominating agents: elementary bromine, mixtures of chlorine and bromine, bromine chloride, N - bromosuccinimide or hypobromites.

11. A process according to Claim 1, wherein the brominating agent used is elementary bromine.

12. A process according to Claim 1 substantially as described in the foregoing Example.

13. 4 - Bromo - 2,5 - dichlorophenol when produced by the process of any of the preceding Claims.