REDUCING META CONTENT OF ISOMERIC MIXTURES OF HALO SUBSTITUTED TOLUENES

Abstract: Disclosed is a method of separating the meta isomer of a halotoluene having general formula (I) from a mixture with at least one other isomer, where X is Cl or Br. About 0.0001 to about 5 wt% of a Friedel-Crafts catalyst is added to the mixture and the mixture is exposed to a brominating agent which preferentially brominates the meta isomer. The mixture is then heated at a temperature above the boiling point of the other isomers but below the boiling point of the brominated meta isomer.

Description: REDUCING META CONTENT OF ISOMERIC MIXTURES OF HALO SUBSTITUTED TOLUENES This invention relates to a method of reducing the content of the meta isomer of a halo substituted toluene in a mixture with other isomers. Commercial parachlorotoluene (PCT) is made by chlorinating toluene. After distilling off the unreacted toluene and most of the oithochlorotoluene (OCT), the product is primarily PCT, but small amounts of metachlorotoluene (MCT) and OCT are also present, typically about 0.5 to about 1 wt% MCT and about 0.5 to about 1 wt% of the OCT. PCT is used as an intermediate in the preparation of pharmaceuticals, paint pigments, herbicides, and other chemicals. While the presence of small amounts of the OCT is usually innocuous, it has been found that the presence of MCT can deleteriously affect the properties of the chemicals made from PCT. Unfortunately, the boiling point of MCT is close to the boiling point of PCT and the two isomers cannot be easily separated. In U.S.-A-4,827,058, herein incorporated by reference, a chlorotoluene isomeric mixture is chlorinated in the presence of a Friedel-Crafts catalyst at a temperature of 0°C up to the boiling point of the mixture. The MCT chlorinates to dichlorotoluene to a much greater extent than does the OCT or the PCT. The PCT-OCT mixture is then separated from the higher boiling dichlorotoluenes (DCT) by distillation. According to one aspect of this invention there is provided a method of separating the meta isomer of a compound having the general formula from a mixture with at least one other isomer, where X is Cl or Br, comprising (A) adding to said mixture about 0.0001 to about 5 wt% of a Friedel- Crafts catalyst; (B) exposing said mixture to about Vi to about 10 equivalents of a brominating agent per equivalent of said meta isomer, whereby said meta isomer is preferentially brominated; and (C) heating said mixture to a temperature above the boiling point of said other isomers but below the boiling point of said brominated meta isomer. According to another aspect of this invention there is provided a method of reducing the content of metachlorotoluene in a mixture with parachlorotoluene comprising (A) adding to said mixture about 0.01 to about 1 wt% of a Friedel-Crafts catalyst; (B) heating said mixture to a temperature between 0°C and reflux; (C) adding about 2 to about 5 equivalents of Br 2 or Bid to said mixture per equivalent of said meta chlorotoluene; and (D) heating said mixture to a temperature above the boiling point of said parachlorotoluene but below the boiling point of said brominated metachlorotoluene. According to a further aspect of this invention there is provided a method of reducing the content of metachlorotoluene in a mixture with parachlorotoluene comprising (A) adding to said mixture about 0.01 to about 1 wt% of a Friedel-Crafts catalyst; (B) heating said mixture to a temperature between 0°C and reflux; and (C) adding about Vi equivalent of Br 2 to said mixture per equivalent of said meta chlorotoluene; (D) after said Br 2 has reacted, adding about '/-- equivalent of Cl 2 to said mixture per equivalent of said meta chlorotoluene; and (E) heating said mixture to a temperature above the boiling point of said parachlorotoluene but below the boiling point of said brominated metachlorotoluene. The invention also relates to a mixture from which a meta isomer has been separated by a method as described above. Meta halo substituted toluenes and can be separated from an isomeric mixture by exposing the mixture to a brominating agent under conditions such that the meta isomer is preferentially brominated. While bromine is less effective than chlorine in aromatic substitution, we have found that it is more selective for the meta isomer in this reaction than is chlorine. Thus, we are able to remove more of the meta isomer while haloginating less of the desirable para isomer than was possible using chlorine. The bromination reaction is unusually fast, which is a processing advantage. In addition, when the brominating agent is bromine, the byproduct, hydrogen bromide, is not evolved and can be converted in situ to additional brominating agent by adding chlorine. In this way, expensive bromine is not wasted. Preferred embodiments of the invention will now be described, by way of example. The starting substrate for the process of this invention is a mixture of isomers having the general formula where X is Cl or Br, but is preferably chlorine as those compounds are commercially more important. While the process of this invention will work with mixtures of isomers that contain almost any amount of the meta isomer, it is most practical for mixtures of isomers that contain about 0.01 to about 10 wt% of the meta isomer. About 0.0001 to about 5 wt% of a Friedel-Crafts catalyst is added to the isomeric mixture. Preferably, about 0.001 to about 1 wt% catalyst is used as less is less effective and more is usually unnecessary. Examples of suitable Friedel- Crafts catalysts include the chlorides of manganese, molybdenum, titanium, iron, aluminum, zinc, tin, antimony, and mixtures thereof. The preferred catalyst is feme chloride as it is inexpensive, works well, and is often the catalyst used to chlorinate toluene. It is preferable to also use about 0.001 to about 5 wt% of an optional cocatalyst. Preferably, about 0.01 to about 1 wt% of the cocatalyst is used. Examples of cocatalysts include sulfur and sulfur compounds such as diphenylsulfide, disulfur dichloride, thianthrene, thianthrene derivatives, phenoxathiin, phenoxathiin derivatives, phenothiazine, and phenothiazine derivatives, iodine, and iodine compounds. The preferred cocatalyst is thianthrene as it is often used in the chlorination of toluene. Examples of suitable brominating agents include liquid or gaseous bromine, BrCl, and sulfuryl bromide (S 2 Br 2 ), The preferred brominating agents are liquid bromine and BrCl as they are inexpensive and effective. About '/ 2 to about 10 equivalents of brominating agent can be used per equivalent of the meta isomer that is present in the mixture. It is preferable to use about 2 to about 5 equivalents of the brominating agent per equivalent of meta isomer that is present in the mixture as less may leave some meta isomer unbrominated and more may brominate some of the para isomer. Generally, proportionally less brominating agent is required at higher meta concentrations. If the starting material was prepared by halogenating toluene, unreacted toluene is preferably removed first to prevent its bromination. The brominating agent is added to the mixture of isomers, catalyst, and optional cocatalyst, which can be, for example, at a temperature of about 0°C to reflux. The preferred temperature range is between room temperature and about 50°C as at lower temperatures the reaction is slow, although the selectivity is better, while the reverse is true at higher temperatures. The brominated agent can be added before or after the mixture is heated. The bromination produces a bromochloro or dibromo substituted toluene and usually a halogenated byproduct, e.g., hydrogen bromide if Br 2 is used or HC1 if BrCl is used. We have found that when bromine is used, a substantial portion of the HBr that is formed does not evolve but remains in solution. The addition of chlorine gas to the solution results in the formation of additional bromine or BrCl in situ. Thus, to prevent the evolution and loss of expensive bromine, one can use about V2 equivalent of bromine, wait until it reacts, then add about Vi equivalent of chlorine. The bromination reaction is unexpectedly rapid (about 15 minutes) and can be followed by gas chromotography (GC) to determine its completion. The lower boiling umeacted para and ortho isomers are then distilled off, leaving behind the higher boiling brominated meta isomer. Using the method of this invention, the meta content can be reduced to less than 0.1 wt%. The following examples further illustrate this invention. Examples 1 to 6 A reactor fitted with an agitator and a gas outlet was charged with a chlorotoluene mixture as summarized in the table below. The catalyst and 0.210 mL of S C1 2 were added to the chlorotoluene mixture and the solution allowed to equilibrate to the designated temperature. Then the bromine was added and the reactor was sampled at the stated time and analyzed. Examples 7 and 8 A reactor fitted with an agitatoi and a gas outlet was chaiged with a chlorotoluene mixture as summarized in the table below The catalyst was added to the chlorotoluene mixture and allowed to equilibiate to 30°C, followed by 1 5 mL of bromine. The leactoi was sampled at the stated tune and analyzed Examples 9 and 10 A reactor fitted with an agitator and a gas outlet was charged with a chlorotoluene mixture as summarized in the table below. The catalyst was added to the mixture and allowed to equilibrate to 30°C, followed by the bromine. The reactor was sampled at the stated time and analyzed. The above experiments show that the method of this invention is veiy effective in reducing the MCT content of an isomeric mixture of chlorotoluenes. Example 1 1 A reaction calorimeter was charged with 714 g of PCT and 0.66 g of FeCl 3 . The temperature was adjusted to 30°C and 22.0 parts of liquid Br 2 were added in single dose. The reaction was monitored by the evolution of heat. Based on the obsei'ved heat effects, the reaction time to reach 95% completion was 6.5 minutes. Reaction time to 99.9% completion was 15.5 minutes. Example 12 To a leactor was charged 1069 8 g PCT and 0 5224 g FeCl 3 The tempeiature of the mixtuie was adjusted to 30°C and 31 6 g of bromine were allowed to leact to completion The lesultmg mixtuie was lemoved fiom the reactor and 863 3 g weie tiansfened to a still pot and distilled at 100 mm Hg thiough a distillation column with 10 sieve plates The iesults of the distillation are shown in the table below Example 13 To a leactoi was chaiged PCT and enough FeCl 3 to make a 500 ppm solution Chloiine oi biomine was added to the leaction mixtuie at 23 °C and the mixtuie sampled Results lllustiating the inci eased effectiveness of biomine are illustrated below. At the point where chlorine and bromine have reduced the PCT concentration to 97.5 wt%, 0.4 wt% MCT remains in the chlorine tieated sample while the assay for the bromine treated MCT is below the detection limit. In the present specification "comprise" means "includes or consists of and "comprising" means "including or consisting of. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in teims of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims: We Claim: 1. A method of separating the meta isomer of a compound having the general formula from a mixture with at least one other isomer, where X is Cl or Br, comprising (A) adding to said mixture about 0.0001 to about 5 wt% of a Friedel- Crafts catalyst; (B) exposing said mixture to about 1J to about 10 equivalents of a brominating agent per equivalent of said meta isomer, whereby said meta isomer is preferentially brominated; and (C) heating said mixture to a temperature above the boiling point of said other isomers but below the boiling point of said brominated meta isomer. 2. A method according to Claim 1 wherein X is Cl. 3. A method according to Claim 1 or 2 wherein said bromination is performed using liquid bromine. 4. A method according to Claim 3 wherein about '/-. equivalent of said bromine is used followed by about '/-> equivalent of chlorine after said bromine has reacted. 5. A method according to Claim 1 or 2 wherein said bromination is performed using BrCl. 6. A method according to any one of the preceding Claims wherein about 0.01 to about 10 wt% of said mixtui e is the meta isomer. 7. A method according to any one of the preceding Claims wherein said catalyst is ferric chloride. 8. A method according to any one of the preceding Claims wherein 0.001 to about 5 wt% of a cocatalyst is present. 9. A method according to Claim 8 wherein said cocatalyst is thianthrene. 10. A method according to any one of the preceding Claims wherein said bromination is performed at about 0°C to reflux. 11. A method of reducing the content of metachlorotoluene in a mixture with parachlorotoluene comprising (A) adding to said mixture about 0.01 to about 1 wt% of a Friedel-Crafts catalyst; (B) heating said mixture to a temperature between 0°C and reflux; (C) adding about 2 to about 5 equivalents of Br 2 or BrCl to said mixture per equivalent of said meta chlorotoluene; and (D) heating said mixture to a temperature above the boiling point of said parachlorotoluene but below the boiling point of said brominated metachlorotoluene. 12. A method according to Claim 1 1 wherein said Friedel-Crafts catalyst is ferric chloride. 13. A method according to Claim 1 1 or 12 wherein said mixture includes about 0.01 to about 1 wt% of a cocatalyst. 14. A method according to Claim 13 wherein said cocatalyst is thianthrene. 15. A method according to any one of Claims 1 1 to 14 wherein liquid bromine is used in step (C). 16. A method according to any one of Claims 1 1 to 14 wherein BrCl is used in step (C). 17. A method of reducing the content of metachlorotoluene in a mixture with parachlorotoluene comprising (A) adding to said mixture about 0.01 to about 1 wt% of a Friedel-Crafts catalyst; (B) heating said mixture to a temperature between 0°C and reflux; and (C) adding about ! -> equivalent of Br 2 to said mixture per equivalent of said meta chlorotoluene; (D) after said Br 2 has reacted, adding about !/ 2 equivalent of Cl 2 to said mixture per equivalent of said meta chlorotoluene; and (E) heating said mixture to a temperature above the boiling point of said parachlorotoluene but below the boiling point of said brominated metachlorotoluene. 18. A method according to Claim 17 wherein said Friedel-Crafts catalyst is ferric chloride. 19. A method according to Claim 17 or 18 wherein said mixture includes about 0.01 to about 1 wt% of a cocatalyst. 20. A method according to Claim 19 wherein said cocatalyst is thianthrene. 21. A mixture from which a meta isomer has been separated by a method according to any one of the preceding Claims.

Inventors: BERMEL FRANK, LESINS VIESTURS, LINDROSE MARK EDWARD, MORTH ARTHUR HENRY, RUETER WILLIAM LEO, TANG DAVID YUN-HUNG, BERMEL, FRANK, LESINS, VIESTURS, LINDROSE, MARK, EDWARD, MORTH, ARTHUR, HENRY, RUETER, WILLIAM, LEO, TANG, DAVID, YUN-HUNG
Applicants: OCCIDENTAL CHEM CO, STEPHENS DINAH, OCCIDENTAL CHEMICAL CORPORATION, STEPHENS, DINAH
Application year: 1999
Main classification: C07C17
IPC codes: C07C17, C07C25