GB2213478A - Chlorination and bromination of aromatic or heterocyclic compounds - Google Patents

Abstract

A process for the chlorination or bromination of an aromatic or heterocyclic compound comprises reacting the aromatic or heterocyclic compound with chlorine or bromine in the presence of a gallium catalyst. The effective conversion of the halogen is high and in the case of an alkyl aromatic compound the extent of nuclear halogenation is high and side chain halogenation is low.

Description

PREPARATION OF HALOGENATED ORGANIC COMPOUNDS This invention relates to the halogenation of aromatic and heterocyclic hydrocarbons, especially alkyl aromatic hydrocarbons.

British Patent Specification 1436657 discloses a process for brominating aromatic compounds containing two or more non-condensed benzene nuclei in the liquid phase in which the starting material is reacted with elemental bromine in the presence of a halogen carrier.

The preferred halogen carrier is iron powder, but aluminium chloride, aluminium bromide, antimony pentachloride or iron and iodine (in combination) may also be used.

USP 3,192,272 discloses a process for the preparation of mixed isomeric ring-tribrominated ethyltoluenes or diethyl benzenes wherein the alkyl groups are predominantly in the meta position. The bromination is carried out under conditions favouring ring substitution, that is at temperatures below about 50"C and in the presence of an effective amount of a ring bromination catalyst. About 1-3% by weight of ferric chloride based on the hydrocarbon is stated to be suitable.

We have now discovered that a gallium catalyst is particularly effective for the halogenation of aromatic and heterocyclic compounds.

Thus according to the present invention there is provided a process for the chlorination or bromination, preferably the latter, of an aromatic or heterocyclic compound which process comprises reacting the aromatic or heterocyclic compound with chlorine or bromine in the presence of a gallium catalyst under chlorination or bromination conditions.

Suitable aromatic compounds include benzene, alkyl benzenes, and benzenes with various substituents (e.g. nitro, cyano, formyl, carboxy, acetyl, halo, amino, acetamino, hydroxy, alkoxy, thio, sulphonate, halomethyl, carboxamide, carboxylester and acetoxy).

Other suitable aromatic compounds include compounds containing a plurality of condensed or non-condensed aromatic nuclei such as diphenyls and diphenyl ethers.

Suitable heterocyclic compounds include pyridine and suitable aromatic heterocyclic compounds include benzofurans and indole.

The preferred aromatic compounds are mono-alkyl substituted benzenes wherein the alkyl group contains 2 or 3 carbon atoms, i.e.

ethyl benzene or isopropyl benzene.

The gallium catalyst may be gallium metal or a trivalent gallium salt, e.g. gallium trichloride or gallium tribromide.

The catalyst may be used as the free metal or salt, optionally in the case of the salt, deposited on a support such as a metallic oxide, e.g. silica or alumina, a molecular sieve, or polymer beads.

The activity of the catalyst may be enhanced by the addition of a promoter such as iodine, water, an alcohol or a carboxylic acid.

It is a feature of the gallium catalyst that the effective conversion of the halogen is high and in the case of an alkyl substituted aromatic compound the extent of nuclear halogenation is high and side chain halogenation is low, as opposed to conventional Friedel Crafts catalysts such as iron chloride or aluminium chloride, which at an equivalent mole proportion give rise to relatively high side chain halogenation and less efficient halogen utilisation.

The quantity of chlorine or bromine employed depends on the desired extent of halogenation of the benzene nucleus. If- a high density liquid product is desired then 2-4 equivalents of the halogenating agent should be employed, preferably 3-4.

The amount of catalyst employed is preferably in amount 0.1 to 2% by weight of the aromatic or heterocyclic compound, expressed as percentage by weight gallium.

If desired, the reaction may be effected in the presence of an inert diluent. Suitable diluents include halogenated lower alkanes, e.g. carbon tetrachloride and dibromomethane.

Pressure is not a significant parameter and the reaction is conveniently carried out at atmospheric pressure.

If light is not inherently excluded from the reaction vessel by virtue of its design, then measures should be taken to ensure that the reaction takes place in the dark.

The reaction is preferably carried out at a temperature in the range 15"C to 50etc.

In the case of halogenated alkyl aromatic compounds, maintaining the product at elevated temperature, i.e. greater than 500C, can promote transalkylation reactions, thus enabling the products and hence the properties of a chlorinated or brominated mixture to be adjusted if desired.

If desired, the level of transalkylation products can be minimised by using additives such as nitromethane. The amount of nitromethane employed is preferably in amount 0.1X to 10% by weight of the aromatic compound, expressed as percentage by weight of nitromethane.

The product itself, which in most cases will be a mixture of compounds, depends, of course, on the initial compound(s) and the extent to which it or they are halogenated. Increasing the extent of halogenation, particularly bromination, increases the density of the product, but also the likelihood that the product will be a solid.

For many purposes, the most useful products are liquids with the highest density which is consistent with the product remaining in the liquid phase down to temperatures of, say, -10 C. Such products will in general be stable up to elevated temperatures of, say, 200"C. They are non-flammable, non-corrosive liquids having high boiling points, high densities and low viscosities. They are suitable for use as functional fluids such as hydraulic fluids and insulating oils and flame retardant additives.

In addition, providing certain criteria are met, the bromination products of ethylbenzene and cumene may be suitable for use as well bore fluids.

The invention is illustrated with reference to the following Examples 1-8 of which Examples 2, 3, 5, 6 and 8 are comparative examples not in accordance with the invention.

ExamPles The following procedure was employed.

0.1 mole ethyl benzene, 1.4 m mole catalyst (except for Example 7 where the concentration was reduced to 0.84 m mole) and 10 ml carbon tetrachloride were added to a three-neck flask in the dark.

3-4 equivalents of bromine (see Table) were added dropwise at a rate of 1 ml per 10 minutes at a temperature in the range 15 to 20"C.

After addition of the bromine, the reaction mixture was raised to and held at 45 C for 1 hour. It was then cooled back to room temperature and another 40 ml carbon tetrachloride added. The resulting mixture was neutralised with sodium metabisulphite and sodium hydroxide and washed three times with water. The organic phase was then dried over magnesium sulphate, filtered and the carbon tetrachloride removed by vacuum distillation at 800C/0.1 mm Hg for 1 hour. The products were analysed by 13C NMR and the following results obtained.

Table Bromination of Ethylbenzene

Catalyst Example 1.4 x Br2 S.G. -10 C Br2 (mmole) (moles) stability utilisation 1 GaBr3 0.3 2.10 L 100 2 AlBr3 0.3 1.29 L 55 3 FeBr3 0.3 1.87 P 75 4 GaBr3 0.35 2.11 L 95 5 AlBr3 0.35 1.44 L 55 6 FeBr3 0.35 1.76 L 67 7 Ga (0.84 mmoles) 0.3 2.05 L 100 8 FeBr3 0.4 2.10 P 70 L = liquid product at this temperature P = precipitated solids at this temperature The product of Example 1 was

Br Br 1 3r ( Br B 4 9 by t + 3Z other B Br Br Br Br 57% 20Z 20% The product of Example 3 contained in addition to the above a significant amount of

A comparison of the Examples shows (i) greater utilisation of bromine by gallium than by iron or aluminium, (ii) the effectiveness of gallium at lower catalyst loadings, and (iii)even although an iron catalyst can produce products of high density, approaching the density of the gallium produced compounds, these products are less stable than their gallium equivalents since they precipitate at -100C and in addition require the use of more bromine, cf Examples 1 and 8.

Claims (11)

Claims:

1. A process for the chlorination or bromination of an aromatic or heterocyclic compound which process comprises reacting the aromatic or heterocyclic compound with chlorine or bromine in the presense of a gallium catalyst under chlorination or bromination conditions.

2. A process according to claim 1 wherein the aromatic compound is a substituted or unsubstituted benzene or alkyl benzene.

3. A process according to claim 2 wherein the aromatic compound is ethyl benzene or isopropyl benzene.

4. A process according to any of the preceding claims wherein the catalyst is gallium trichloride or gallium tribromide.

5. A process according to any of the preceding claims wherein 2-4 equivalents of chlorine or bromine are employed per equivalent of aromatic or heterocyclic compound.

6. A process according to any of the preceding claims wherein the catalyst is employed in amount 0.1 to 2% by weight of the aromatic or heterocyclic compound, expressed as percentage by weight gallium.

7. A process according to any of the preceding claims wherein the reaction is carried out in the presence of an inert diluent.

8. A process according to claim 7 wherein the inert diluent is a halogenated lower alkane.

9. A process according to claim 8 wherein the inert diluent is carbon tetrachloride.

10. A process according to any of the preceding claims wherein the process is carried out at a temperature in the range 15 to 50"C.

11. A process according to claim 1 as hereinbefore described with reference to Examples 1, 4 and 7.