GB2155009A - Compositions suitable for use as electrophilic halogenating agents and their use in the electrophilic halogenation of organic substrates - Google Patents

Abstract

A compostions suitable for use as an electrophilic halogenating agent comprises as a first component one or more organic halogen-containing compounds or inorganic hypochlorites and as a second component an inorganic solid. A preferred inorganic solid is a crystalline or partially crystalline zeolite, for example faujasites of the X and Y types, ZSM5, large pore mordenite and zeolite L.

Description

SPECIFICATION Compositions suitable for use as electrophilic halogenating agents and their use in the electrophilic halogenation of organic substrates The present invention relates to compositions suitable for use as electrophilic halogenating agents and their use in the electrophilic halogenation of organic substrates.

Halogenated hydrocarbons are valuable industrial products having a variety of uses. For example, chlorotoluenes are used as intermediates in the pesticide, pharmaceutical, peroxide, dye and other industries and toluene chlorination isomer mixtures are used widely as solvents.

Chlorobenzenes, particularly p-dichlorobenzene, are also used widely as chemical intermediates in the production of dyes and pesticides and to a lesser extent as solvents.

Hydrocarbon chlorinations have been carried out with a wide variety of chlorinating agents, catalysts and reaction conditions too numerous to mention in detail. Thus, monochlorotoluene mixtures are obtained on an industrial scale using elemental chlorine as the chlorinating agent and Lewis acids such as ferric chloride and the chlorides of aluminium, tin, titanium and zirconium as catalysts. Other chlorinating agents which have been employed in the chlorination of toluene include Lewis acid halides and hydrogen chloride. Monochlorobenzene is produced either by direct benzene chlorination in the presence of iron turnings or by benzene oxychlorination. A problem commonly associated with the use of hydrogen chloride or chlorine as the chlorinating agent is the rapid rate of corrosion of process plant.

Elemental chlorine and bromine have previously been reported (J. van Dijke, J.J. van Daalen and G.B. Paerels, Recl. Trav. Chim. Pays-Bas, 1974, 93, 72; T. Huizinga, J.J.F. Scholten, Th.M. Wortel and H. van Bekkum, Tetrahedron Lett., 1980, 21. 3809) to halogenate aromatic substrates in the presence of zeolites of the X-type, but only those with calcium and ferric counter ions were used. Selectivities obtained showed little improvement over halogenation in the absence of the zeolite and of course these systems are likely to suffer from the general disadvantages associated with the use of free halogens as hereinbefore mentioned.

tert-Butyl hypochlorite on silica gel has been reported to bring about chlorination of alkenes (W. Sata, N. Ikeda, H. Yamamoto, Chem.Letts, 1982, 141) but the role of the silica gel in this process was not established. Free halogens and N-halogensuccinimides have also been used in conjunction with silicic acid for halogenation of aromatics, but yields were low and conditions harsh (C. Yaroslavsky, Tetrahedron Lett, 1974, 3395).

Finally, electrophilic chlorination by sulphuryl chloride in the presence of silica gel has been reported by Masaru Hojo and Ryoichi Masuda in Synthetic Communications, 5(3), 169-171 (1975). Sulphuryl chloride suffers from the disadvantage that it is sensitive to moisture.

We have now found novel electrophilic halogenating agents which are effective for the halogenation of organic substrates under very mild conditions.

Accordingly, the present invention provides a composition suitable for use as an electrophilic halogenating agent comprising as a first component one or more organic halogen-containing compounds or inorganic hypochlorites and as a second component an inorganic solid.

Because the halogenating agents are solids they can, in contrast to many of the prior art halogenating agents, be estimated by standard techniques and metered out accurately by weight. They can also be used in batch or flow modes, on a column or in a fluidised bed or be separated from excess reagents and by-products by filtration.

Although the organic halogen-containing compound(s) or inorganic hypochlorite(s) comprising the first component may suitably be physically admixed with the inorganic solid, they may also be supported on the inorganic solid.

As regards the first component, although the organic halogencontaining compound embraces organic iodine-containing compounds, these are very much less preferred than organic chlorineand bromine-containing compounds. The most reactive organic halogen-containing compounds are the chlorine-containing compounds.

Suitable organic halogen-containing compounds which may be employed include tert-butyl hypochlorite, N,N-dichlorourethane, dichloramine-T, N, N-dichlorodiethylphosphoramidate and Nchloro-succinimide, and their bromo-analogues. Suitable inorganic hypochlorites which may be employed include calcium hypochlorite and sodium hypochlorite.

Inorganic solids which may be employed incude silica, alumina, silica/alumina, titania, zirconia and both natural and synthetic aluminosilicates. It is particularly preferred to employ as the inorganic solid, a crystalline or partially crystalline zeolite which may be natural or synthetic.

Zeolite-containing compositions are not only mild halogenating agents, but also for aromatic hydrocarbon halogenation the isomer distributions can be strongly influenced by the zeolite in the direction of increasing the relative proportions of para isomer in the product. Generally, in the chlorination of toluene, using an inorganic solid such as silica or an amorphous aluminosilicate, the isomer distribution of chlorotoluene is ca 65% ortho, 35% para, which is within the range (ca. 30-60% para) normally associated with catalysed chlorinations of toluene with chlorine.Using the partially crystalline zeolite, proton-exchanged faujasite X, however, as the inorganic solid in combination with tert-butyl hypochlorite, the chlorination of toluene at 25"C in carbon tetrachloride can give a 100% yield of monochlorotoluenes in the ratio 75% para: 25% ortho-isomers. A wide range of crystalline zeolites may be employed. Examples of suitable zeolites include faujasites of the X and Y types, ZSM5, large pore mordenite and zeolite L. As is well known in the zeolite art, the qrginal cations associated with natural zeolites and the as prepared forms of synthetic zeolites may be replaced in whole or in part by other cations using conventional cation-exchange techniques.The preferred form of the zeolite for use in the compositions of the present invention is the hydrogen-exchanged form, though zeolites exchanged with alkali metal, alkaline earth metal, for example calcium, transition metal or lanthanide element cations may also be employed. The hydrogen form of the zeolite may be obtained from the original form of the zeolite by, for example, treatment with an acidic ion exchange resin, or hot ammonium chloride solution followed by calcination at elevated temperature. Such methods are well known to those skilled in the art.

A particularly suitable composition comprises an organic halogen-containing compound supported on a crystalline zeolite.

The organic halogen-containing compound or inorganic hypochlorite may be deposited on the inorganic solid by any of the techniques conventionally employed for such purpose. It will usualy be found convenient to impregnate the inorganic solid with a solution of the halogen containing compound or inorganic hypochlorite. The ratio of the first component to the second component in the composition may vary over a wide range.

The properties of the resulting compositions as electrophilic halogenating agents are quite different from those of the individual components. It is an advantage of the invention that many of the compositions active as electrophilic halogenating agents can be stored in the form of a dry powder without substantial loss of activity. For example, dichloramine-T supported on silica can be stored for at least one month at ambient temperature without any substantial loss in chlorination activity. Many of the compositions falling within the scope of the invention will also tolerate the presence of water.

As hereinbefore mentioned, the compositions of the present invention can be used to effect high-yield halogenation of organic substrates.

In another aspect, therefore, the present invention provides a process for the electrophilic halogenation of an organic substrate which process comprises reacting the organic substrate with a composition as hereinbefore described.

The composition may be added in the form of a preformed physical admixture or in the supported form or the individual components of the composition may be added separately.

Organic substrates which may be halogenated by the process of the present invention include aromatics, alkenes, alkynes and carbonyl compounds. The process is particularly applicable to the chlorination or bromination of aromatic compounds, which may be substituted for example by alkyl or halogen groups. Suitable aromatic compounds include anisole, benzene, toluene, I xylenes, biphenyl, chlorobenzene and bromobenzene.

The process may be carried out in the presence or absence of a solvent, preferably in the presence of a solvent. Suitable solvents include linear, branched and cyclic paraffins or their halogenated derivatives, alkyl ethers, cyclic ethers and alcohols. Examples of suitable solvents include pentane, diethyl ether, dichloromethane, carbon tetrachloride, methanol, acetonitrile or mixtures thereof. The nature of the solvent employed can profoundly influence the isomer distribution in aromatics halogenations involving a crystalline zeolite. For example, replacement of carbon tetrachloride as solvent by diethyl ether in the aforedescribed chlorination of toluene by tert-butyl hypochlorite/proton-exchanged faujasite X can result in an isomer distribution of monochlorotoluenes of greater than 90% para, less than 10% ortho.

In addition to the nature of the solvent, if any, the isomer distribution in aromatics halogenation can also be influenced by reaction conditions such as the temperature, the pore structure and silicon to aluminium ratio of the zeolite. In our experience optimum reaction rates and optimum para-selectively can be achieved using zeolites of the faujasite type having a silicon to aluminium ratio in the range of about 1 to 1.5 in a form wherein a substantial proportion of the counter ions are protons.

The process may suitably be operated at ambient temperature, though lower and higher temperatures may be employed if so desired. Atmospheric, subatmospheric or elevated pressures may be employed. The rate of the reaction will depend on the amount of the inorganic solid used (for reaction using dichloramine-T supported on silica the maximum rate is achieved at a dichloramine-T: silica ratio of about 1:4), on the physical characteristics of the inorganic solid (for example, grain size and moisture content) and on the chemical nature of the particular composition employed. Because of the variations in reactivity of the different compositions falling within the scope of the invention, a considerable degree of selectivity can be achieved by appropriate choice of composition.

The invention will now be further illustrated by reference to the following Examples. Example 1 Silica (3.76g, 60-120 BSS mesh), carbon tetrachloride (10 ml), toluene (0.005 mol) and tert-butyl hypochlorite (0.005 mol) were gently stirred together at 25"C for 30 mins. The reaction mixture was filtered and the solid was washed with a little extra carbon tetrachloride to give a filtrate containing orthochlorntoluene (65%) and para-chlorotoluene (35%), as estimated by gas chromatography.

Example 2 The procedure of Example 1 was repeated except that N,N-dichlorourethane was used instead of tert-butyl hypochlorite and the reaction period was increased to 1 hour. Monochlorotoluenes were obtained in 100% yield in the proportion ortho-chlorotoluene (65%) and para-chlorotoluene (35%) as estimated by gas chromatography.

Example 3 The procedure of Example 1 was repeated except that dichloramine-T was used instead of tertbutyl hypochlorite and the reaction period was increased to 2 hours. Monochlorotoluenes were obtained in 100% yield in the proportion orthochlorntoluene (65%) and para-chlorotoluene (35%) as estimated by gas chromatography.

Example 4 The procedure of Example 1 was repeated except that calcium hypochlorite (0.01 mol) plus a trace of acetic acid was used instead of tert-butyl hypochlorite and the reaction period was increased to 6 hours. Monochlorotulenes were obtained in 80% yield.

Example 5 A proton-exchanged zeolite was prepared by heating sodium zeolite 1 3X (1.5g) with 1 M ammonium chloride solution (15ml) for 1 h at 100"C. The solid was filtered, washed several times with distilled water, and dried in a furnace at 500"C. After cooling, to this material was added carbon tetrachloride (10ml), toluene (0.0025 mol) and tert-butyl hypochlorite (0.0025 mol), and the mixture was stirred gently at 25"C for 2h. The reaction mixture was filtered and the solid was washed with a little extra carbon tetrachloride to give a filtrate containing ortho chlorotoluene (26%,yield) and para-chlorotoluene (74%), as estimated by gas chromatography.

Example 6 The procedure of Example 5 was repeated except that ca. 5% of the sodium ions in the initial zeolite were replaced by calcium by treating the zeolite with 0.014 M CaCI2 solution (15ml) for 1 h at 100"C prior to treatment with ammonium chloride. The yield of monochlorotoluenes obtained after 1 2h at 25"C was 75% and the isomer distribution was 35% ortho, 65% para.

The remainder of the starting material was unreacted and could be recovered.

Example 7 The procedure of Example 5 was repeated except that the solvent used was diethyl ether instead of carbon tetrachloride. The yield of monochlorotoluenes was 65%, and the isomer distribution was less than 10% ortho, greater than 90% para. The remainder of the toluene was unreacted, but could be reacted by subsequent addition of a further aliquot of tert-butyl hypochlorite. In this way the yield could be increased to 98% without significant change in isomer distribution.

Example 8 The procedure of Example 5 was repeated except that the substrate was anisole; monochloroanisoles were produced in 100% yield, with an isomer distribution of 18% ortho-, 82% para.

Example 9 To proton-exchanged zeolite 1 3X (1.5 g) prepared as in Example 5, was added acetonitrile (10 ml), chlorobenzene (0.3643 g) and tert-butyl hypochlorite (0.5355 g), and the mixture was gently stirred at 40"C for 7 days. The reaction mixture was filtered and the solid was washed with extra acetonitrile (2.5 ml) to give a filtrate containing paradichlorobenzene (87.3%), ortho dichlorobenzene (2.7%) and unreacted chlorobenzene (10%), as estimated by gas chromatography.

Example 10 To a proton-exchanged zeolite 1 3X (20.4 g) prepared as in Example 5, was added acetonitrile (136 ml), chlorobenzene (3.8 g) and tert-butyl hypochlorite (4.1 g), and the mixture was gently stirred at 40"C for 1 6 days. The reaction mixture was filtered and the solid was washed with extra acetonitrile (150 ml) to give a filtrate containing paradichlornbenzene (97%), ortho dichlorobenzene (3%), as estimated by gas chromatography. The isolated yield of dichlorobenzenes was 92%.

Claims (16)

1. A composition suitable for use as an electrophilic halogenating agent comprising as a first component one or more organic halogen-containing compounds or inorganic hypochlorites and as a second component an inorganic solid.

2. A composition according to claim 1 wherein the first component is supported on the second component.

3. A composition according to either claim 1 or claim 2 wherein the inorganic halogencontaining compound is a chlorine-containing compound.

4. A composition according to either claim 1 or claim 2 wherein the first component is an organic halogen-containing compound which is either tert-butyl hypochlorite, N,N-dichlorourethane, dichloramine-T, N,N-dichlorodiethylphosphoramidate, N-chlorosuccinimide, or their bromo-analogues.

5. A composition according to either claim 1 or claim 2 wherein the first component is an inorganic hypochlorite which is either calcium hypochlorite or sodium hypochlorite.

6. A composition according to any one of the preceding claims wherein the inorganic solid is either silica, alumina, silica/alumina, titania, zirconia or an aluminosilicate.

7. A composition according to any one of claims 1 to 5 wherein the inorganic solid is crystalline or partially crystalline zeolite.

8. A composition according to claim 7 wherein the crystalline zeolite is either a faujasite of the X or Y type, a ZSM-5, a large pore mordenite or zeolite L.

9. A composition according to claim 8 wherein the zeolite is wholly or partially in the hydrogen form.

1 0. A composition substantially as herein before described with reference to Examples 1 to 6, 9 and 10.

11. A process for the electrophilic halogenation of an organic substrate which process comprises reacting the organic substrate with a composition as claimed in any one of the preceding claims.

1 2. A process according to claim 11 wherein the organic substrate in an aromatic compound.

1 3. A process according to either claim 11 or claim 1 2 wherein the process is carried out in the presence of a solvent.

14. A process according to any one of claims 11 to 1 3 wherein the organic substrate is an aromatic compound and the composition contains a zeolite of the faujasite type having a silicon to aluminium ratio in the range of about 1 to

1.5 in a form wherein a substantial proportion of the counter ions are protons.

1 5. A process according to claim 11 substantially as herein before described with reference to Examples 1 to 10.

16. Electrophilically halogenated organic substrates whenever obtained by a process as claimed in any one of claims 11 to 14.