GB2135310A - Preparation of halogenated phenols and salts thereof - Google Patents

Abstract

A halogenated phenol having at least three halogen atoms in the molecule, is obtained by halogenating a compound selected from phenol, monohalophenols and dihalophenols, and distilling the resulting reaction mixture in the presence of a base which contains a primary, secondary or tertiary nitrogen atom to give the desired halogenated phenol as distillate. The halogenated phenol produced as distillate may be reacted with an alkali metal hydroxide to give the corresponding salt. The invention enables the yield of salt of the desired halogenated phenol to be substantially increased as compared with its yield in prior art methods. The starting phenolic compounds may be selected from o-chlorophenol, p-chlorophenol, 2,4,- dichlorophenol, 2,6-dichlorophenol and phenol, and the resulting halogenated phenol may be 2,4,6- trichlorophenol. By using hydrazine as the base, a virtually colourless solution of the sodium salt of 2,4,6- trichlorophenol can be obtained.

Description

SPECIFICATION Preparation of halogenated phenols and salts thereof The present invention relates to the preparation of halogenated phenols containing at least three halogen atoms in the molecule and alkali metal salts thereof and in particular but not exclusively relates to the preparation of 2,4,6-trichlomphenol (2,4,6-T.C.P.) and the sodium salt thereof (Na-2,4,6,-T.C.P.).

It is known to produce 2,4,6-T.C.P. by chlorination of either a mixture of o-chlorophenol, pchlorophenol and phenol, or phenol alone, using gaseous chlorine as chlorinating agent. The starting material (i.e. said mixture of phenols or phenol alone) is first chlorinated until it is substantially wholly di-chlorophenol. At this point in the process, it is usual practice to add a catalyst to the reaction mixture for the purpose of accelerating chlorination. Typical catalysts include AICI3 and FeCI3. When the reaction of di-chlorophenol to form 2,4,6-T.C.P. is substantially completed, the reaction mixture is air-blown to remove hydrogen chloride as gas and then charged to a still and distilled. The resulting distillate is distilled in aqueous sodium hydroxide to produce a solution containing 2,4,6-T.C.P. as its sodium salt, up to the concentration of a saturated solution.

In preparing 2,4,6-T.C.P., by the known process described above, large losses in theoretical yield can occur prior to and during distillation. These losses can be in excess of 25%.

The reasons for this are complex and are related to: (a) the addition of the catalyst to accelerate the final stages of chlorination concurrently promotes the formation of phenoxy phenols by side reactions, and (b) the crude chlorinated mixture in the still charge undergoes thermally initiated reactions during distillation causing a further loss as intractable residues.

The phenoxy phenols and products of the thermally initiated reactions are left as residues in the still.

In order to reduce the amount of residue, and thereby increase the yield of 2,4,6-T.C.P., we have observed that by omitting the addition of the catalyst, this reduces the amount of phenoxy phenol in the residue left in the still. However, after distillation, when the distillate is added to the sodium hydroxide solution, the 2,4,6-T.C.P. reacts to form an instantaneous precipitate of chlorinated polyphenylene oxides.

It has now been discovered that the yield of Na-2,4,6-T.C.P. can be substantially increased by adding a base, which contains a primary, secondary or tertiary nitrogen atom, to the reaction mixture when it is distilled. It is envisaged that this discovery will be of applicability, more generally, to the preparation of halogenated phenols containing at least three halogen atoms in the molecule.

Accordingly the present invention provides a method of preparing a halogenated phenol having at least three halogen atoms in the molecule, comprising: halogenating one or more phenolic compounds selected from phenol, monohalophenols and dihalophenols, and distilling the resulting reaction mixture in the presence of a base which contains a primary, secondary or tertiary nitrogen atom to give the desired halogenated phenol as distillate.

The halogenation may be chlorination and such chlorination may be carried out using gaseous chlorine as chlorination agent.

It is understood that no catalyst need be added to the phenols or phenol being halongenated during halogenation.

The base may be an amine, such as an organic primary, secondary or tertiary amine, or hydrazine or a substituted hydrazine. Such substituted hydrazines include hydrazine wherein one or more of the hydrogen atoms in the molecule are replaced by one or more substituents such as alkyl, aralkyl and aryl groups, the alkyl substituents of alkyl groups of the aralkyl substituents preferably containing 1 to 6 carbon atoms.

The base may be supplied to the reaction mixture as free base or as a salt thereof.

Preferably the base is hydrazine and preferably the hydrazine is present in the reaction mixture being distilled in an amount of 0.1 to 2.0% by weight, based on the amount of reaction mixture to be distilled.

The amount and nature of the base selected is dependent on whether a mixture of phenols or phenol itself is used as the starting material and, if a mixture of phenols is used as the starting material on the composition of that mixture.

The halogenated phenol produced as distillate may be reacted with an alkali metal hydroxide, such as sodium hydroxide, to give the corresponding salt.

When the salt of the halogenated phenol is to be prepared, the amount and nature of the base selected for use in the distillation depends also on the colour specification required for the solution of the salt which is produced.

Especially, but not exclusively, when the halogenation is chlorination and the alkali metal hydroxide is sodium hydroxide, the base present in the reaction mixture being distilled both keeps the distillation residue at an acceptably low level and prevents precipitation of halogenated polyphenylene oxides when the resulting halogenated phenol is reacted with the alkali metal hydroxide.

By using hydrazine as the base, a solution of an alkali metal salt of the hydrogenated phenol may be obtained virtually colourless.

In one particular aspect, the present invention provides a method of preparing 2,4,6-T.C.P.

comprising: chlorinating one or more phenolic compounds selected from o-chlorophenol, p-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol and phenol using gaseous chlorine or other chlorinating agent; and distilling the resulting reaction mixture in the presence of a base, which contains a primary, secondary or tertiary nitrogen atom, to give 2,4,6-T.C.P. as distillate. The 2,4,6-T.C.P.

may then be reacted with sodium hydroxide in solution to give Na-2,4,6-T.C.P.

Example 1 below is a comparison example and Examples 2 to 1 2 below are to illustrate the invention.

Examples I to ii In each example a melt of phenol or a mixture of partially chlorinated phenols was reacted with chlorine gas, which was fed into the phenols of mixture of phenols through a dispenser, at 80"C until the specific gravity, measured at 70"C, of the reaction mixture was 1.508-1.510.

Hydrogen chloride generated in the reaction was then removed from the resulting melt (i.e.

reaction mixture) by blowing air through the melt for approximately 3 hours until the blownthrough air was substantially free of hydrogen chloride as determined by appropriate tests.

A base was them added (except in Example 1) and the mixture was charged to a still and Engler distilled until the temperature at the base of the still reached 1 40 C.

The distillate, consisting of 2,4,6-T.C.P., was then added to an aqueous solution of sodium hydroxide to form Na-2,4,6-T.C.P.

Results of Examples 1 to 11 are given in the Table below.

TABLE Amount of Prec Distillation ipitation in NaOH Colour or Conc. (%) Starting Residue (aq.) as % wt. of Na-2,4,6-TCP Example Base of base Material (%) 2,4,6-TSP added Soln.

1. None 0 25 40 Black 2. Hydrazine 0.4 2,4-D.C.P. 6.2 0 Colourless 3. Hydrazine 0.4 P.C.P. 4.0 0 Pale Straw 4. Hydrazine 1.0 O.C.P./P.C.P. 7.1 0 Straw 5. Penyl hydrazine 2.5 10.3 0 Straw 6. n-butylamine 1.7 16.5 0 Straw 7. Pyridine 1.9 11.5 0 Straw 8A. Ethanolamine 1.0 9.4 0 Amber 8B. Ethanolamine 1.4 9.6 0 Pale Straw 9. 1,2-diamino-ethane 1.4 13.1 0 Pale Straw 10. Diphenylamine 1.4 5.9 slight turbidity Red Brown observed 11. Cyclohexylamine 1.4 12.8 0.2 Pale yellow 2,4-D.C.P.=2,4-dichlorophenol P.C.P. =p-chlorophenol D.C.P. =o-chlorophenol The percentage concentration of base and percentage of distillation residue are weight percentages based on the reaction mixture which is distilled.

Example 12 A sample of tribromophenol was prepared by a standard bromination technique. The analysis of the reaction mixture was:2,6-dibromophenol 0.2% 2,4-dibromophenol 0.2% 2,4,6-tribromophenol 99.6% The reaction mixture was halved and distilled as follows: (a) distilled without the addition of any additives, (b) distilled from 0.75% hydrazine (1.171% of a 64% hydrazine hydrate solution).

The distillations were carried out under identical conditions and the figures for each distillation were: A B % distillate 89.3 85.6 % residue 9.7 13.2 % loss 1.0 1.2 A portion of each distillate was then converted to its sodium salt solution by dissolving 679.

of the distillateina solution of 8. lug. sodium hydroxide dissolved in 639. water.

lDistillateA produced a heavy dark-coloured precipitate when added to the sodium hydroxide solution.

Distillation B, when added to the sodium hydroxide solution, gave a pale-coloured solution which was free from any precipitated material.

Claims (11)

1. A method of preparing a halogenated phenol having at least three halogen atoms in the molecule, comprising: halogenating one or more phenolic compounds selected from phenol, monohalophenols and dihalophenols, and distilling the resulting reaction mixture in the presence of a base, which contains a primary, secondary or tertiary nitrogen atom, to give the desired halogenated phenol as distillate.

2. A method according to claim 1, wherein the halogenation is chlorination.

3. A method according to claim 2, wherein the chlorination is carried out using gaseous chlorine as chlorinating agent.

4. A method according to any preceding claim, wherein the halogenation is carried out in the absence of catalyst.

5. A method according to any preceding claim, wherein the base is an organic primary, secondary or tertiary amine.

6. A method according to any of claims 1 to 4, wherein the base is hydrazine or a substituted hydrazine.

7. A method according to claim 6, wherein the base is hydrazine and is present in the reaction mixture being distilled in an amount of 0.1 to 2.0% by weight based on the amount of reaction mixture to be distilled.

8 A method of preparing 2,4,6-trichlorophenol comprising: chlorinating one or more compounds selected from phenol, o-chlorophenol, p-chlorophenol, 2,4-dichlorophenol and 2, 6-dichlorophenol; and distilling the resulting reaction mixture in the presence of a base, which contains a primary, secondary or tertiary nitrogen atom, to give 2,3,6-trichlomphenol as distillate.

9. A method according to claim 1, substantially as described herein in any of Examples 2 to 12.

1 0. A method of preparing an alkali metal salt of a halogenated phenol having at least three halogen atoms in the molecule, comprising reacting a halogenated phenol produced as distillate by a method according to any preceding claim with an alkali metal hydroxide.

11. A method according to claim 10, wherein the alkali metal hydroxide is sodium hydroxide.

1 2. A method according to claim 10, substantially as described herein in any of the examples.