PROCESS FOR PRODUCING 2,5-DICHLOROTOLUENE

Abstract: Disclosed herein is a process for producing 2,5-­ dichlorotoluene which is useful as a monomer of heat-resistant polymers and an intermediate raw material for medicine, agricultural chemicals and various organic synthetic substances. The process gives 2,5-­ dichlorotoluene at a high selectivity and in a high yield. It comprises the steps of chlorinating 4-t-alkyltoluene or 4-isopropyltoluene in the presence of a catalyst, thereby obtaining a 2,5-dichloro compound, and bringing the formed 2,5-dichlorocompound into contact with toluene in the presence of a catalyst, thereby subjecting the 2,5-dichloro compound to transalkylation.

Description: The present invention relates to a process for producing 2,5-dichlorotoluene which is used as a monomer of heat-resistant polymers represented by polyether sulfone, polyether ketone, polyphenylene sulfide, etc. and can be used as a raw material for various organic synthetic chemical substances, for example, medicines and agricultural chemicals. The present invention also relates to a specific process for producing 2,5-dichlorotoluene in a favorable yield, in spite of the difficulty obtaining 2,5-dichlorotoluene by ordinary chlorination of toluene. It has been known that various isomers are formed when dichlorotoluene is produced by chlorinating toluene. Namely, since a methyl group possessed by toluene and a chlorine atom which is to be introduced into toluene nucleus by chlorination are the ortho-para orientating groups and the methyl group shows a stronger ortho-para orientation, when toluene is chlorinated to obtain dichlorotoluene, both 2,4-dichlorotoluene and 2,6-dichlorotoluene are easily formed, however, 2,5-dichlorotoluene is relatively seldom to be formed. Accordingly, concerning the production of 2,5-­ dichlorotoluene, various devices have been tried. For instance, a process has been known, wherein 2-chlorotoluene is preliminarily formed from toluene, and in the next step of introducing the second chlorine atom into 2-chlorotouene, 0.5 to 0.9 molecule of chlorine is brought into reaction per molecule of 2-chlorotoluene in the presence of a metal catalyst accompanying a sulfur compound as a co-catalyst (refer to U.S. Patent No. 4,03l,l46). However, in this process, the reaction rate of 2-chlorotoluene must be cont­ rolled low, i.e. 50 to 70%, in order to suppress the formation of by-produced trichlorotoluene less than 6%, as a result, there is a problem that the low yield of 2,5-dichlorotoluene is inevitable (the yield being 30 to 42%). Next, a process of bringing 0.9 to l.l molecules of chlorine into reaction per molecule of 2-chlorotoluene in the presence of sulfur and/or a sulfur compound as a catalyst has also been known (refer to Japanese Patent Application Laid-Open (KOKAI) No. 5l-l43627/l976). However, in this process, although the yield of 2,5-dichlorotoluene is improved to 50%, there is a problem, on the other hand, that by-production of trichlorotoluene went up to 8%. Still more, among the processes using 2-chlorotoluene as a starting material, a process of bringing l.0 to l.6 molecules of chlorine into reaction per molecule of 2-­ chlorotoluene while using iodine as a catalyst has been known (refer to Japanese Patent Application Laid-Open (KOKAI) No. 57-9l934/l982). In this process, although the yield of 2,5-dichlorotoluene is as high as 53%, there is a problem that as much as l4 to 22% of trichlorotoluene is formed as a by-product. Moreover, other than these processes, a process of chlorinating 2-chlorotoluene using L-type zeolite as a catalyst has been proposed. However, the selectivity of forming 2,5-dichlorotoluene of the process is only around 56% and a by-production of trichlorotoluene is unavoidable. As has been described above, all the processes already known are not yet satisfiable for producing 2,5-­ dichlorotoluene at a high selectivity and in a high yield. The present invention has been performed in view of these situation, and as a result of the present inventors' earnest studies subjecting to offer a process for producing 2,5-dichlorotoluene at a high selectivity and in a high yield, it has been found that the subject can be attained by introducing a specified group at a para-position of toluene preliminarily, chlorinating the treated toluene under the specified conditions, and subjecting the chlorinated compound to transalkylation together with toluene. On the basis of the finding, the present inventors have attained the present invention. The object of the present invention is to provide a process for producing 2,5-dichlorotoluene which can be used as a monomer of heat-resistant polymeric substances such as polyether sulfone, polyether ketone, polyphenylene sulfide, etc. and as an intermediate raw material or various organic synthetic chemical substances including medicines and agricultural chemicals. Furthermore, the object of the present invention is to provide a special process for producing 2,5-dichlorotoluene at a high selectivity and in a high yield, in spite of the difficulty obtaining 2,5-dichlorotoluene by ordinary chlorination of toluene. The feature of the present invention lies in obtaining 2,5-dichlorotoluene by chlorinating 4-t-alkyl­ toluene or 4-isopropyltoluene at an ordinary temperature to a temperature of 80°C in the presence of a Friedel-Crafts catalyst and sulfur or a sulfur-containing compound or in the presence of iron (II) and/or (III) sulfide, thereby obtaining a 2,5-dichloro-4-t-­ alkyltoluene or 2,5-dichloro-4-isopropyltoluene and bringing the obtained 2,5-dichloro-4-t-alkyltoluene or 2,5-dichloro-4-isopropyltoluene into contact with toluene in the presence of a Friedel-Crafts catalyst in a transalkylation step. By carrying out the chlorination step with the defined process characteristics, the 2,5-dichloro-toluene derivative is formed in the substantial absence of the trichloro toluene derivative. As the starting material for the chlorination 4-t-alkyl toluene or isopropyltoluene is used. A preferred t-alkyl derivative is 4-t-butyltoluene. Namely, 4-t-alkyltoluene or 4-isopropyltoluene which is obtained by a conventional alkylation process of toluene, for instance by the reaction of toluene with t-alkyl chloride or propylene, is used as a starting material and 2,5-dichloro-4-t-alkyltoluene or 2,5-dichloro-4-isopropyl-toluene is produced by chlorinating the starting material in the presence of a Friedel-Crafts catalyst and sulfur or the sulfur-containing compound or in the presence of iron (II) and/or (III) sulfide, thereby introducing chlorine atoms to 2- and 5-positions of the starting material. The reaction for obtaining 2,5-dichloro-4-t-alkyl toluene or 2,5-dichloro-4-­ isopropyltoluene can be carried out at a room temperature to a temperature of 80°C, preferably 40 to 60°C while bringing gaseous chlorine into reaction with the starting material keeping the degree of chlorination between 2 and 2.2 (the degree of chlorination means the molar amount of chlorine reacted with one molecule of the starting material and is measured by the remaining amount of supplied chlorine in the ordinary reaction system and the theoretical amount of chlorine, or is measured by the gaschromatographic method). In these cases, when the reaction temperature is over 80°C, by-production of other dichlorotoluenes, for instance, 2,6-dichlorotoluene and 2,3-dichlorotoluene, as well as that of trichlorotoluene, etc. increase. On the other hand, when the reaction temperature is lower than room temperature, the reaction efficiency and the selectivity become poor. The selectivity herein mentioned means the rate of formation of 2,5-dichlorotoluene in all the dichloro­ toluene derivatives formed in the reaction. As the Friedel-Crafts catalyst, anhydrous ferric chloride, anhydrous aluminum chloride, anhydrous antimony (III) chloride and anhydrous antimony (V) chloride may be exemplified and such a catalyst is used together with sulfur or a sulfur-­ containing compound. As the sulfur-containing compound used herein, the compounds represented by the following formulae (I), (II) and (III) can be mentioned: R³-S-R⁴ (I); R³SR⁴ (II) and R³-SH (III) (wherein R³ and R⁴ represent respectively an alkyl group, an aryl group, a cycloalkyl group or a heterocyclic group and may be same or different from each other and n is an integer of 2 to 8), such as dodecyl sulfide, diphenyl sulfide, diphenyl disulfide, tetrahydrothiophene, thiophene, cyclohexylmercaptan, benzenethiols and mixtures of more than two of them. Furthermore, other than the case of using Friedel-­ Crafts catalyst together with the sulfur-containing compound, iron sulfide only can be used for the purpose. The amount of Friedel-Crafts catalyst used according to the present invention is 0.0l to 0.l, preferably 0.0l to 0.08 mole per one mole l of the starting material, for instance, 4-isopropyltoluene. The amount of sulfur or the sulfur-containing compound used is 0.0l to l.0 preferably 0.0l to 0.8 parts by weight of the amount of Friedel-Crafts catalyst used. Further, in the case of using iron (II) and/or (III) sulfide only, the amount thereof is 0.00l to 0.l, preferably 0.00l to 0.0l mole to one mole of the starting material. According to the present invention, after ending the chlorination reaction, the catalyst is generally removed from the reaction mixture and the reaction mixture is then subjected to distillation and preferably any unreacted starting material and monochloro derivatives of alkylated toluene are recycled to the chlorination reaction. Thereafter, 2,5-dichloro-4-t-alkyltoluene or 2,5-dichloro-­ 4-isopropyltoluene is separated from the distillation residue. The yield of the 2,5-dichlorotoluene is usually about 70% and the selectivity thereof can be more than 90%. In the next step, the obtained 2,5-dichloro-4-t-­ alkyltoluene or 2,5-dichloro-4-isopropyltoluene is brought into contact with toluene, thereby subjecting to transalkyl­ ation, disconnecting the t-alkyl group or the isopropyl group from the mother compound to obtain 2,5-dichlorotoluene. In the reaction of the transalkylation, the 2,5-­ dichlorocompound (2,5-dichloro-4-t-alkyltoluene or 2,5-dichloro-­ 4-isopropyltoluene) can be treated in an excess amount of toluene at 20 to 60°C, preferably 25 to 35°C for 4 to 5 hours in the presence of Friedel-Crafts catalyst. Toluene to be used is suitably in an amount of not smaller than 2 times, preferably l0 to 20 times by volume of the 2,5-dichloro­ compound. It should be noted that when toluene is used in an amount of less than 2 times of 2,5-dichlorocompound, the object reaction may not proceed and when using too much amount of toluene, it is uneconomical from the view point of separating the product after the reaction. Further the reaction temperature is preferably less than 60°C, since above that temperature the yield of 2,5-dichlorotoluene may be reduced and is accompanied by the by-production of colored substances. Moreover, the amount of Friedel-Crafts catalyst used in transalkylation is preferably 0.0l to 3.0, more preferably 0.0l to l.0 molecule per one molecule of the raw material, 2,5-dichlorocompound. After the transalkylation reaction is over, the catalyst may be removed from the reaction mixture and by washing with water and distilling the remaining oil layer, the object product, 2,5-dichlorotoluene is obtained, while 4-t-alkyltoluene or 4-isopropyltoluene is obtained as the by-product. The yield of 2,5-dichlorotoluene is usually not less than 90% and each of the by-products is obtained in a yield of not less than 90%. Accordingly, in the present invention, these by-­ products obtained by the transalkylation can be recycled and reused as the starting material for the chlorination reaction. As has been described, since in the present invention, a 4-t-alkyltoluene or 4-isopropyltoluene is used as the starting material and by transalkylating a 2,5-dichloro-4-t-alkyltoluene or 2,5-dichloro-4-isopropyltoluene obtained by chlorinating the starting material, 2,5-dichlorotoluene can be produced in a high yield and in the same time, the starting material can be recovered in a high yield and reused, the process of the present invention can be an extremely profitable commercial process. The present invention will be explained more concretely while referring to the non-limitative Examples as follows. EXAMPLE l: Chlorination for obtaining 2,5-dichloro-4-t-butyltoluene After adding 3.3 g (0.020 mol) of anhydrous ferric chloride (FeCl³) and l.5 g (0.047 mol) of sulfur (a simple substance), as the catalyst, to 296 g (2.0 mol) of 4-t-butyltoluene, gaseous chlorine was blown into the mixture kept at about 45°C at a rate of 40 litres per hour to carry out chlorination of 4-t-butyltoluene until the degree of chlorination reached 2.l. Then, the obtained liquid reaction mixture was washed with water to remove the catalyst and after drying the organic layer with anhydrous sodium sulfate, the dried organic layer was distilled to recover unreacted 4-t-butyl­ toluene and monochlorinated substance in the first distillate for reuse in the step of chlorination and the next distillate of boiling point of 70 to 75°C/0.5 mmHg, namely, 2,5-dichloro-­ 4-t-butyltoluene of a purity of not less than 93% was collected in a yield of 75%. Transalkylation of 2,5-dichloro-4-t-butyltoluene After adding a large excess of 2000 g of toluene and 64 g (0.48 mol) of anhydrous aluminum chloride, as a catalyst, to 284 g (l.3 mol) of 2,5-dichloro-4-t-butyltoluene obtained as stated above, the mixture was brought into reaction for 5 hours at room temperature under a vigorous agitation thereof. After washing the obtained liquid reaction mixture by water to remove the catalyst, the organic layer was distilled to obtain 2,5-dichlorotoluene and 4-t-butyltoluene respectively in a quantitative yield. The obtained 4-t-­ butyltoluene was recovered and reused in the chlorination step. EXAMPLE 2: Step of chlorination After adding 3.3 g (0.020 mol) of anhydrous ferric chloride (FeCl 3 ) and l.5 g (0.047 mol) of sulfur (a single substance), as catalysts, to 264 g (2.0 mol) of 4-isopropyltoluene, gaseous chlorine was blown into the mixture at a temperature of 45°C at a rate of 40 litres/hour to carry out chlorination until the degree of chlorination reached 2.l. After the chlorination was over, the liquid reaction mixture was washed with water to remove the catalysts and after drying the organic layer with anhydrous sodium sulfate, the dried organic layer was distilled. After recovering unreacted 4-isopropyltoluene and monochlorinated substance as the first distillate, a fraction of boiling point of 68 to 70°C/0.5 mmHg, of a purity of 97% and in a yield of 75% was obtained as 2,5-dichloro-4-isopropyltoluene. Step of transalkylation After adding 2000 g of toluene to 267 g (l.3 mol) of 2,5-dichloro-4-isopropyltoluene obtained and further adding 64 g (0.48 mol) of anhydrous aluminum chloride, as a catalyst, the mixture was kept at room temperature and the newly formed mixture was brought into reaction for 5 hours under a vigorous agitation. After washing the liquid reaction mixture with water to remove the catalyst, the organic layer was distilled to obtain l90 g of 2,5-dichlorotoluene and l68 g of 4-­ isopropyltoluene. EXAMPLE 3: In the same manner as in Example l except for carrying out chlorination while using 0.9 g of iron (II) sulfide (FeS) as a catalyst instead of using anhydrous ferric chloride and sulfur in Example l, l93 g of 2,5-dichloro­ toluene were obtained.

Claims: 1. A process for producing 2,5-dichlorotoluene, which process comprises the steps of chlorinating a 4-t-­ alkyltoluene or 4-isopropyltoluene at room temperature to a temperature of 80°C in the presence of a Friedel-Crafts catalyst and sulfur or a sulfur-containing compound or in the presence of iron sulfide to obtain a 2,5-dichloro-4-­ t-alkyltoluene or 2,5-dichloro-4-isopropyltoluene and of bringing the obtained 2,5-dichloro-4-t-alkyltoluene or 2,5-dichloro-4-isopropyltoluene into contact with toluene in the presence of a Friedel-Crafts catalyst in a transalkylation step. 2. A process according to claim 1 wherein said chlorination step is carried out at a temperature of from 40 to 60°C. 3. A process according to claim 1 or claim 2 wherein said transalkylation step is carried out at a temperature of from 20 to 60°C while using toluene in an amount of not less than 2 times by volume of 2,5-dichloro-4-t-­ alkyltoluene or 2,5-dichloro-4-isopropyltoluene. 4. A process according to any preceding claim wherein gaseous chlorine is brought into reaction with 4-t-­ alkyltoluene or 4-isopropyltoluene in said chlorination step to the extent that the degree of chlorination is to be kept at the value between 2 to 2.2. 5. A process according to any preceding claim in which the 4-t-alkyltoluene starting material is 4-t-butyltoluene. 6. A process according to any preceding claim wherein said Friedel-Crafts catalyst is selected from the group consisting of anhydrous ferric chloride, anhydrous aluminium chloride, anhydrous antimony (III) chloride, anhydrous antimony (V) chloride and mixtures thereof. 7. A process according to any preceding claim wherein said sulfur-containing compound is selected from compounds represented by the formulae (I), (II) and (III) and mixtures thereof: R³-S-R⁴ (I), R(S)R⁴ (II) and R³-SH (III) wherein R³ and R⁴ may be the same or different and each represents an alkyl group, an aryl group, a cycloalkyl group on a heterocyclic group and n is an integer of from 2 to 8. 8. A process according to any preceding claim wherein said sulfur-containing compound is selected from the group consisting of dodecyl sulfide, diphenyl sulfide, diphenyl disulfide, tetrahydrothiophene, cyclohexylmercaptan and mixtures thereof. 9. A process according to any preceding claim wherein the 4-t-alkyltoluene or 4-isopropyltoluene obtained as the by-product in the transalkylation step is used as the starting material for the chlorination step.

Inventors: HOZUMI, TOSHIO, OHTSURU, MASASHI
Applicants: KUREHA KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1987
Main classification: C07C25
IPC codes: C07C17, B01J27, C07C25, C07B61, C07C67