METHOD OF MAKING 1,2,4-TRICHLOROBENZENE

Abstract: Disclosed is a method of making 1,2,4-trichlorobenzene. Dichlorobenzene containing less than an isomeric amount of meta-dichlorobenzene is isomerized to produce an isomeric mixture of dichlorobenzenes substantially rich in meta-dichlorobenzene. The isomeric mixture of dichlorobenzenes is chlorinated with gaseous chlorine to produce 1,2,4-trichlorobenzene and unreacted dichlorobenzenes. The mixture of 1,2,4-trichlorobenzene and unreacted dichlorobenzenes is separated by distillation and the unreacted dichlorobenzenes are recycled to the isomerizer.

Description: METHOD OF MAKING 1.2■4-TRTCHLOROBENZENE Cross-Reference to Related Application This application is related to application Serial No. _ _, filed by R. Krishnamurti, titled, "Method of Making Chlorobenzenes." (Case 6720) Background of the Invention This invention relates to a method of making 1,2,4- trichlorobenzene from ortho and para-dichlorobenzenes. In particular, it relates to isomerizing ortho and para- dichlorobenzenes, chlorinating to produce 1,2,4- trichlorobenzene, and recycling unreacted dichlorobenzene (DCB) . The compound 1,2,4-trichlorobenzene (1,2,4-TCB) is a commercially important raw material which is used in making herbicides and fungicides. It is typically obtained as a by¬ product during the chlorination of benzene. However, it is very difficult to obtain 1,2,4-TCB in a high yield and purity because large amounts of the dichlorobenzenes must be recycled in order to minimize the formation of undesirable tetra and higher chlorinated products. This recycling requires a large and expensive reactor, and a substantial amount of 1,2,3- trichlorobenzene (1,2,3-TCB) is also produced, which must be separated from the 1,2,4-TCB. Summary of the Invention I have discovered a method of making reasonably pure 1,2,4-TCB at high yield from dichlorobenzene that is substantially the ortho or para isomers. In my invention, such a dichlorobenzene mixture is isomerized, which produces a mixture of dichlorobenzenes of which at least half is m-DCB. In electrophilic chlorination, m-DCB is expected to be considerably more reactive than o-DCB and p-DCB. The cooperative mesomeric effect of the two chlorines enhances attack of chlorine at the 4-position. When this mixture is chlorinated, the meta isomer preferentially reacts to produce 1,2,4-TCB and the unreacted ortho and para-dichlorobenzenes are recycled to the isomerizer. Currently manufacturers who produce the industrially more valuable p-DCB inevitably produce a large amount of o-DCB which has a more limited market. This trend is expected to lower the price of o-DCB substantially. The method of this invention can take this inexpensive ortho-dichlorobenzene and convert it into a valuable product, 1,2,4-TCB. Another advantage of the process is the relative ease of product separation. The unreacted 1,2-DCB and 1,4-DCB can be easily distilled away from the higher boiling 1,2,4-TCB/1,2,3- TCB mixture. Additionally, the recovered DCB mixture is not wasted since it can be sent back to the isomerizer to produce more 1,3-DCB-containing DCB mixture. The relatively high throughput of product in the process of this invention will also decrease the capital required for large-scale production. Brief Description of the Drawing Figure 1 is a block diagram illustrating the process of this invention. Figure 2 is a graph giving the relative concentrations of various components in the chlorinator with the passage of time. It is further explained in the examples. Description of the Preferred Embodiments In Figure 1, dichlorobenzene in line 1 is isomerized in isomerizer 2 to produce an isomeric mixture of ortho, meta, and para-dichlorobenzenes. This isomeric mixture is sent through line 3 to chlorinator 4 where chlorine from line 5 is added, converting the meta-dichlorobenzene into 1,2,4-TCB. The mixture of 1,2,4-TCB and unreacted ortho and para- dichlorobenzene is sent through line 6 to distillation column 7, which separates the mixture into ortho and meta dichlorobenzenes, which are recycled through line 8 to isomerizer 2, and the product 1,2,4-TCB in line 9. The feedstock for the process of this invention consists of dichlorobenzene, typically a mixture of ortho, meta, and para-dichlorobenzene (i.e., 1,2-dichlorobenzene, 1,3- dichlorobenzene, and 1,4-dichlorobenzene, respectively.) The chlorination of benzene using a Lewis acid catalyst (and possibly also a sulfur-containing cocatalyst) produces a dichlorobenzene feedstock which is typically about 25 to 35 wt% ortho, 0.5 to 2 wt% meta, and 65 to 75 wt% para- dichlorobenzene. If the feed is from benzene chlorination then, of course, distillation to remove benzene, monochlorobenzene, and trichloro and higher chlorinated benzene is performed to obtain the mixture of dichlorobenzenes. A feedstock of ortho-dichlorobenzene is particularly preferred because it could be potentially inexpensive. While the particular ratios of ortho, meta, and para-dichlorobenzenes produced by the isomerizer will depend upon isomerization conditions and catalyst used, a typical conposition of the product isomeric mixture is about 10 wt% ortho, about 55 wt% meta, and about 35 wt% para- dichlorobenzene. In the first step of the process of this invention, the dichlorobenzene is isomerized. Isomerization of dichlorobenzene is a known process and is described, for example, in U.S. Patent Nos. 2,666,085, 2,727,075, 2,819,321, and 2,920,109, herein incorporated by reference. Typical isomerization conditions are at about 200 to about 250°C for a few hours at about 135 to about 415 kPa (about 20 to about 60 psi) in the presence of Lewis catalyst such as aluminum chloride, antimony chloride, or iron chloride; aluminum chloride is the preferred catalyst because it is a stronger Lewis acid. In the next step of the process of this invention, the isomeric mixture of ortho, meta, and para-dichlorobenzene is treated with chlorine gas. The chlorine gas is bubbled into the mixture of dichlorobenzenes in an amount that is preferably at least about stoichiometric with the amount of meta-dichlorobenzene that is present. In order to increase the yield of 1,2,4-TCB, it is preferable to use an amount of chlorine that is up to 10 mole % in excess of the amount of meta-dichlorobenzene present in the feedstock. Primarily, it is the meta-dichlorobenzene that is chlorinated and when meta- dichlorobenzene is chlorinated it produces predominantly 1,2,4-TCB. Some ortho-dichlorobenzene is also chlorinated, which produces a mixture of 1,2,4 and 1,2,3-trichlorobenzene, and some para-dichlorobenzene is chlorinated, which produces only 1,2,4-TCB. Chlorination is a rapid reaction and proceeds easily at temperatures of 0 to 100°C. The chlorination can be monitored using a gas chromatograph and, to reduce the formation of undesirable by- products, the chlorination can be terminated when the presence of 1,3-dichlorobenzene falls to less than 5 wt%. A catalyst is required for chlorination and usually a Lewis acid catalyst, such as aluminum chloride, antimony chloride, or ferric chloride, is used; preferably, ferric chloride is used as it is inexpensive. The amount of catalyst can vary from about 10 micromoles per mole of the dichlorobenzene feed mixture up to about 1 millimole per mole of the dichlorobenzene feed mixture. A cocatalyst is not required for chlorination but it can be used if desired. Cocatalysts are typically sulfur- containing compounds such as elemental sulfur, aryl sulfides, and thianthrenes. Elemental sulfur is preferred as it is inexpensive and readily available. The amount of cocatalyst used is typically about 10 micromoles to about 1 millimole per mole of the dichlorobenzene feed mixture. Next, the 1,2,4-TCB is separated from the unreacted dichlorobenzene. This can be accomplished most advantageously by distillation. The lower-boiling unreacted dichlorobenzenes (primarily ortho and para) are distilled off and recycled to the isomerizer and the 1,2,4-TCB is collected. The yield of 1,2,4-TCB is about equal to the amount of meta-dichlorobenzene that is present. Other components in the product include 1,2,3-TCB and dichlorobenzenes. Purification of the 1,2,4-TCB can be accomplished, if desired, by techniques well known in the art, such as distillation and fractional crystallization. The following examples further illustrate this invention. EXAMPLE 1 The apparatus consisted of a 3-necked flask containing a magnetic stir bar and fitted with a disentrainer, a Friedrich condenser, a twin adapter for a thermometer and a polytetrafluorethylene (Teflon) tube for chlorine delivery, and an adapter containing a Teflon-backed septum for sample withdrawal. The top of the condenser was connected in series with an ice-cooled empty trap (to condense any benzene and product vapors that may not condense) , a stirred water trap, and a stirred 10% aqueous KOH trap. Chlorine flow was regulated to the desired level using a needle valve and flow rate was measured using a transducer. The apparatus was purged with nitrogen for 15 min, and the calculated quantities of the Lewis acid catalyst, sulfur containing co-catalyst (whenever necessary) , and the appropriate chlorobenzene were introduced into the reactor under a gentle nitrogen sweep. In the case of chlorinating the o/m/p-dichlorobenzene mixture, calculated amounts of each isomer were introduced into the reactor. Cold water to the condenser was turned on, the reactor flask was immersed in an oil bath pre-heated to the desired reaction temperature, nitrogen flow was stopped, and chlorine was bubbled through the solution at an appropriate rate. Reaction progress was followed by withdrawing an aliquot by syringe and quickly diluting it with dichloromethane. This solution was analyzed by gas chromatography. The results were used to generate chlorination curves which relate yield of products as a function of wt% chlorine passed though the reactor. RUN NO. l The charge consisted of chlorobenzene (47.1 g, 0.418 mole), FeCl 3 (0.068 g, 4.19 x 10 "4 mole), and sulfur powder (0.0067 g, 2.09 x 10 "4 mol) . Chlorine was bubbled through the solution at the rate of 0.19 mole/min at a reaction temperature of 80°C. RUN NC 2 The charge consisted of 1,2-DCB (74.5 g, 0.507 mole), FeCl 3 (0.041 g, 2.53 x 10 "4 mole), and sulfur powder (0.0081 g, 2.53 x 10 "4 mol). Chlorine was bubbled through the solution at the rate of 0.15 mole/min at a reaction temperature of 55°C. RUN NO. 3 The charge consisted of chlorobenzene (47.1 g, 0.32 mole) FeCl 3 (0.056 g, 3.47 x 10 "4 mol) . Chlorine was bubbled through the solution at the rate of 0.15 mole/min. RUN NO. 4 The charge consisted of 1,2-DCB (24.7 g, 0.168 mole), 1,3-DCB (49.3 g, 0.335 mole), 1,4-DCB (24.7 g, 0.168 mole), FeCl 3 (0.0545 g, 3.36 x 10 "4 mole), and sulfur powder (0.0108 g, 3.38 x 10 "4 mol). Chlorine was bubbled through the solution at the rate of 0.148 mole/min. The following table gives the feeds used, the mole ratios of catalysts and cocatalysts, the temperature, and the composition of the reaction mixture at the end of the indicated reaction time. ♦Comparative Run 1. Does not include amounts of the other chlorinated benzenes formed. The above table shows that chlorination of 1,3-DCB is considerably faster than that of 1,2-DCB and 1,4-DCB. If one takes a mixture of the o/m/p isomers, essentially only the m- isomer reacts. In fact, the highest yield of 1,2,4-TCB that one can obtain from the 1/2/1 DCB mixture iε around 55%. The yield of 1,2,4-TCB is roughly equal to the quantity of 1,3-DCB present in the DCB mixture. The chlorination curve for Run No. 4 is shown in Figure 2, where the ordinate is GC Area % and the abscissa is wt % Cl 2 . Curve 1 is 1,2-DCB, Curve 2 is 1,3-DCB, Curve 3 iε 1,4- DCB, Curve 4 is 1,2,3-TCB, Curve 5 is 1,2,4-TCB, Curve 6 is 1,2,3,4-tetrachlorobenzene, and Curve 7 is 1,2,4,5- tetrachlorobenzene. Figure 2 shows that the concentration of 1,3-DCB declines more rapidly than o- and p-DCB due to its higher reactivity as it is converted into 1,2,4-TCB. Very little 1,2,3-TCB and still less tetrachlorobenzene was produced.

Claims: WE CLAIM : 1. A method of making 1,2,4-trichlorobenzene comprising (A) producing an isomeric mixture of dichlorobenzenes by isomerizing a liquid which comprises dichlorobenzene containing less than the isomeric amount of meta- dichlorobenzene; (B) chlorinating said isoueric mixture to produce a mixture which comprises 1,2,4-trichlorobenzene and unreacted diclorobenzenes; (C) separating said 1,2,4-trichlorobenzene from said unreacted dichlorobenzenes; and (D) recycling said unreacted dichlorobenzenes to step (A) . 2. A method according to Claim 1 wherein said liquid comprises ortho-dichlorobenzene. 3. A method according to Claim 1 wherein said liquid comprises about 25 to 35 wt% ortho-dichlorobenzene, about 65 to 75 wt% para-dichlorobenzene, and about 0.5 to 2 wt% meta-dichlorobenzene. 4. A method according to Claim 1 wherein said liquid is produced by chlorinating benzene using a Lewis catalyst. 5. A method according to Claim 1 wherein said isomeric mixture is produced by heating said liquid at about 200 to about 250°C in the presence of a Lewis acid catalyst. 6. A method according to Claim 5 wherein said Lewis acid catalyst is aluminum trichloride. 7. A method according to Claim 1 wherein said chlorinating is performed at about 0 to about 100°C in the presence of a Lewis acid catalyst. 8. A method according to Claim 7 wherein said Lewis acid catalyst is ferric chloride. 9. A method according to Claim 1 wherein said chlorinating is monitored using a gas chromatagraph and said chlorinating is reduced when the presence of 1,3- dichlorobenzene in said isomeric mixture falls below 5 wt%. 10. A method according to Claim 1 wherein said chlorinating is performed in the presence of a sulfur-containing cocatalyst. 11. A method according to Claim 10 wherein said sulfur- containing cocatalyst is elemental sulfur. 12. A method of making 1,2,4-trichlorobenzene comprising (A) isomerizing a liquid which comprises ortho- dichlorobenzene to produce an isomeric mixture of ortho-dichlorobenzene, meta-dichlorobenzene, and para-dichlorobenzene; (B) chlorinating said isomeric mixture by bubbling gaseous chlorine through said isomeric mixture in the presence of a Lewis acid catalyst to produce a mixture which comprises 1,2,4-trichlorobenzene and unreacted dichlorobenzenes; (C) distilling said mixture of 1,2,4-trichlorobenzene and unreacted dichlorobenzenes to separate said 1,2,4-trichlorobenzene from said unreacted dichlorobenzenes; and (D) recycling said unreacted dichlorobenzenes to step (A). 13. A method according to Claim 12 wherein said isomerizing is performed at about 200 to about 250°C under pressure of about 135 to about 415 kPa in the presence of a Lewis acid catalyst. 14. A method according to Claim 13 wherein said Lewis acid catalyst is aluminum trichloride. 15. A method according to Claim 12 wherein said chlorinating is performed at a temperature of about 0 to about 100°C. 16. A method according to Claim 15 wherein the amount of chlorine gas is about 0 to 10 mole % in excess of the amount that is stoichiometric with the amount of meta- dichlorobenzene that is present. 17. A method according to Claim 12 wherein the amount of catalyst used in said chlorinating is about 10 micromoles per mole of meta-dichlorobenzene up to about 1 millimole per mole of meta-dichlorobenzene. 18. A method of making 1,2,4-trichlorobenzene comprising (A) chlorinating benzene to produce a mixture containing dichlorobenzenes; (B) separating said dichlorobenzenes from said mixture; (C) isomerizing said dichlorobenzenes to produce an isomeric mixture of dichlorobenzenes; (D) chlorinating said isomeric mixture of dichlorobenzenes to produce 1,2,4-trichlorobenzene and unreacted dichlorobenzenes; (E) distilling said mixture of 1,2,4-trichlorobenzene and unreacted dichlorobenzenes to separate said 1,2,4-trichlorobenzene from said unreacted dichlorobenzenes; and (F) recycling said unreacted dichlorobenzenes to step (C). 19. A method according to Claim 18 wherein said isomeric mixture is produced by heating said liquid at about 200 to about 250°C in the presence of a Lewis acid catalyst. 20. A method according to Claim 18 wherein said chlorinating is monitored using a gas chromatagraphy and said chlorinating is terminated when the presence of 1,3- dichlorobenzene in said isomeric mixture falls below 5 wt%.

Inventors: KRISHNAMURTI RAMESH, KRISHNAMURTI, RAMESH
Applicants: OCCIDENTAL CHEM CO, OCCIDENTAL CHEMICAL CORPORATION
Application year: 1997
Main classification: C07C25
IPC codes: C07B61, C07C17, C07C25