Abstract:
Disclosed is a method of separating the meta isomer of a halotoluene having the general formula ##STR1## 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:
BACKGROUND OF THE INVENTION 
     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. In particular, it relates to exposing the mixture to a brominating agent under conditions such that the meta isomer is preferentially brominated. 
     Commercial parachlorotoluene (PCT) is made by chlorinating toluene. After distilling off the unreacted toluene and most of the orthochlorotoluene (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. Pat. No. 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. 
     SUMMARY OF THE INVENTION 
     We have discovered that 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. 
     We have also found that the bromination reaction is unusually fast, which is a processing advantage. In addition, we have found that 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. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The starting substrate for the process of this invention is a mixture of isomers having the general formula ##STR2## 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 ferric 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, 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 1/2 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 HCI 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 1/2 equivalent of bromine, wait until it reacts, then add about 1/20 equivalent of chlorine. The bromination reaction is unexpectedly rapid (about 15 minutes) and can be followed by gas chromatography (GC) to determine its completion. The lower boiling unreacted 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 2  Cl 2  were added to the chlorotoluene mixture and the solution was allowed to equilibrate to the designated temperature. Then the bromine was added and the reactor was sampled at the stated time and analyzed. 
     
         __________________________________________________________________________Chloro-                  GC Analysis  toluene FeCl.sub.3 Br.sub.2 Temp. Time (Area %)Example (g) (g) (mL)             (° C.)                 (hrs)                    OCT                       MCT                          PCT Others__________________________________________________________________________Initial                  1.036                       0.516                          98.324                              0.124  1 153.5 0.1438 2.0 46 0.5 0.988 0.007 96.129 2.926  2 153.8 0.1453 1.0 46 1.0 0.989 0.082 97.529 1.400  3 153.8 0.1456 1.5 31 2.0 0.962 0.012 96.896 2.130  4 154.1 0.1484 2.0 11 3.0 0.988 0.055 97.399 1.558  5 153.9 0.1445 1.0 11 3.0 1.000 0.086 97.720 1.194  6 154.0 0.1411 0.8 0 6.0 1.526 0.091 98.080 0.303__________________________________________________________________________ 
    
     EXAMPLE 7 and 8 
     A reactor fitted with an agitator and a gas outlet was charged with a chloroluene mixture as summarized in the table below. The catalyst was added to the chlorotoluene mixture and allowed to equilibrate to 30° C., followed by 1.5 mL of bromine. The reactor was sampled at the stated time and analyzed. 
     
         ______________________________________Chloro-                    GC Analysis  toluene FeCl.sub.3 Time (Area %)Example (g)     (g)     (hrs)                        OCT  MCT  PCT   Others______________________________________Initial                      1.036                             0.516                                  98.324                                        0.124  7 154.1 0.0458 4.0 0.989 0.064 96.943 2.004  8 154.0 0.0983 3.0 0.968 0.013 96.786 2.233______________________________________ 
    
     EXAMPLE 9 and 10 
     A reactor or fitted with an agitator and a gas outlet was charged with a chloroluene 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. 
     
         __________________________________________________________________________    Chloro-   GC Analysis  toluene FeCl.sub.3 Br.sub.2 Time (Area %)Example(g) (g) (mL)           (hrs)              Toluene                  OCT MCT PCT Others__________________________________________________________________________Initial            10.913                  42.883                      0.252                          45.913                              0.039   9 155.1 0.0975 9.0 2.0 0 41.079 0.027 45.511 13.382  Initial     0.028 48.061 0.282 51.588 0.041  10 154.7 0.0927 1.5 3.0 0 46.506 0.047 51.247 2.199__________________________________________________________________________ 
    
     The above experiments show that the method of this invention is very effective in reducing the MCT content of an isomeric mixture of chlorotoluenes. 
     EXAMPLE 11 
     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 observed 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 reactor was charged 1069.8 g PCT and 0.5224 g FeCl 3 . The temperature of the mixture was adjusted to 30° C. and 31.6 g of bromine were allowed to react to completion. The resulting mixture was removed from the reactor and 863.3 g were transferred to a still pot and distilled at 100 mm Hg through a distillation column with 10 sieve plates. The results of the distillation are shown in the table below. 
     
         ______________________________________       Takeoff              GC Area %   (grams)         Rate (%) OCT     MCT  PCT  Others______________________________________First Fraction     17.7    50       1.63  0    98.28                                      0.09  Second Fraction 398.5 50 1.15 0 98.85  Third Fraction 158.8 50 0.87 0 99.13  Fourth Fraction 152.3 40 0.63 0 99.37  Fifth Fraction 45.1 33 0.25 0 99.75  Final Still Pot 62.2  0.09 0 96.61 3.3  Initial Para-   0.96 0.39 98.64 0.01  chlorotoluene______________________________________ 
    
     EXAMPLE 13 
     To a reactor was charged PCT and enough FeCl 3  to make a 500 ppm solution. Chlorine or bromine was added to the reaction mixture at 23° C. and the mixture sampled. Results illustrating the increased effectiveness of bromine are illustrated below. At the point where chlorine and bromine reduced the PCT concentration to 97.5 wt %, 0.4 wt % MCT remained in the chlorine treated sample while the assay for the bromine treated MCT was below the detection limit. 
     
         ______________________________________      GC Area %      OCT  MCT        PCT    DCT______________________________________Starting Material        1.4    0.5        98.1 0.0  Chlorine 1.4 0.4 97.5 0.7  Bromine 1.3 0.0 97.5 1.2______________________________________