Method for chlorination of methylated aromatic compounds

Abstract: A process for chlorination of methylated aromatic compounds in a titanium tetrachloride solvent. The ring chlorination is first carried out by reacting said methylated aromatic compounds with chlorine gas under shaded conditions,and then the side chain chlorination is carried out by reacting the resulting mixture with chlorine gas under light irradiation. The process ensures continuous chlorination of aromatic rings and side chains of methylated aromatic compounds at a high yield without using any catalysts or perilous solvents such as carbon tetrachloride.

Description: The present invention relates to a method for chlorination of nuclei (aromatic rings) and side chains of methylated aromatic compounds. The reaction conditions required for chlorination of side chains and aromatic rings of methylated aromatic compounds are different. Chlorination of aromatic rings has conventionally been carried out by reacting a methylated aromatic compound with chlorine gas in the presence of a Lewis acid catalyst such as antimony chloride, ferric chloride, aluminum chloride or titanium tetrachloride (CH-A-436 237), whereas chlorination of side chains has been done by reacting a methylated aromatic compound with chlorine gas under light irradiation. These chlorination reactions are usually carried out in a carbon tetrachloride solvent, or without using a solvent. When both aromatic rings and side chains of methylated aromatic compounds are to be chlorinated, there is firstly aromatic ring chlorination in the presence of a Lewis acid catalyst and then, after removing the catalyst, side chain chlorination by subjecting the intermediate to chlorine gas under light irradiation (GB-A-582 258). For total chlorination of both aromatic rings and side chains of methylated aromatic compounds, for example, Belgian Patent No. 631170 discloses a method for manufacturing tetrachloro-1,3-bis-(dichloromethyl) benzene from m-xylene; which comprises i) ring chlorination of m-xylene by reacting the m-xylene with chlorine gas in a carbon tetrachloride solvent in which ferric chloride is contained, followed by washing with water to remove the ferric chloride catalyst from the resulting intermediate; and ii) side chain chlorination by subjecting the intermediate to chlorine gas under light irradiation. As stated above, chlorination of methylated aromatic compounds has been performed in a carbon tetrachloride solvent, which is stable under various conditions of chlorination and is easy to handle owing to its low boiling point, because the resulting products in which both the aromatic rings and side chains are chlorinated become viscous or even solid. However, development of a new chlorination method in which no carbon tetrachloride is used has been desired since carbon tetrachloride is considered to contribute to the destruction of the ozone layer. The present inventors have undertaken extensive studies on the chlorination of both aromatic rings and side chains of methylated aromatic compounds, and have found unexpectedly that advantage can be obtained by using titanium tetrachloride as the solvent. Aromatic rings and side chains can be continuously chlorinated by using this solvent and by reacting the methylated aromatic compounds with chlorine gas under shaded conditions and under light irradiation. The present invention provides a method of chlorinating aromatic rings and/or side chains of methylated aromatic compounds using a solvent which can substitute for carbon tetrachloride. The present invention permits continuous chlorination of both the aromatic rings and side chains of methylated aromatic compounds by simply changing the optical conditions for the chlorination. The method of the present invention can chlorinate methylated aromatic compounds at a high yield without using any additional catalyst. The present invention provides a method of side-chain chorinating methylated aromatic compound by reacting said methylated aromatic compound with chlorine gas in a titanium tetrachloride solvent under light irradiation. It also provides such a process wherein a reaction mixture, obtained by reacting methylated aromatic compound with chlorine gas in a titanium tetrachloride solvent under shaded conditions, is chlorinated The shaded conditions preferably involve substantial exclusion of light. According to the present invention, the methylated aromatic compounds to be chlorinated include those having methyl group(s) attached to carbon atom(s) of an aromatic nucleus, e.g. toluene, xylene, mesitylene, methylnapthalene. In one procedure, such compound is dissolved in a titanium tetrachloride solvent and aromatic ring chlorination is effected by injecting a specified amount of chlorine gas under shaded conditions. The concentration of methylated aromatic compound in the titanium tetrachloride solvent is desirably below the level where intermediate and target compounds precipitate at any point in either chlorination reaction. Nevertheless, a higher concentration is acceptable if no operational problems are encountered when precipitation occurs. Usually, it is preferable that the chlorination reaction be carried out at a methylated aromatic compound concentration of 2-20% by weight. Chlorination of aromatic rings is carried out under shaded conditions by feeding an amount of chlorine gas required for the chlorination into the solution of the methylated aromatic compound in titanium tetrachloride. The reaction temperature is for example 0-130°C, with the preferable range being 20-100°C. When the reaction temperature is lower than this range, the reaction rate is retarded; whereas when the temperature is higher than this range, undesirable reactions may take place. The rate of feeding chlorine gas and the reaction time vary depending on the target chlorinated product. These reaction conditions, therefore, can be determined appropriately so that the reaction can be completed in as short a time as possible so long as the exothermic heat can be controlled and the utilization of chlorine gas does not become extremely low. The titanium tetrachloride used as a solvent also itself works as a catalyst for ring chlorination, so that it does not necessitate any other catalysts such as ferric chloride or aluminum chloride conventionally required for chlorination of aromatic rings; at the same time, it does not disturb subsequent photo-chlorination. This is quite advantageous for the subsequent chlorination of methyl groups because no separation and removal of catalyst is required prior to the next step; since chlorination catalysts such as ferric chloride, aluminum chloride,and the like disturb the photo-chlorination reaction, they must be removed from the reactant solution by washing with water before the succeeding chlorination but such procedure can be eliminated when titanium tetrachloride solvent is adopted for use. Consequently, the reactant mixture following ring chlorination can be fed as is to the photo-chlorination step enabling the chlorination of methyl groups. These two chlorination reactions can be carried out in a single reaction vessel by simply switching the reaction conditions to light irradiation from shaded conditions. More desirably, however, ring chlorination is performed in a shaded vessel until said chlorination is accomplished to a desired level, and then the chlorinated reaction mixture is transferred to another vessel equipped with a light irradiation device to chlorinate the methyl groups to a desired level by feeding chlorine gas into the vessel under light irradiation. Conventional light irradiation conditions generated by a high-pressure mercury lamp, low-pressure mercury lamp, gallium lamp, indium lamp can be applied to this side chain chlorination, at a reaction temperature of e.g. 40-130°C, more preferably 50-100°C. When the reaction temperature is lower than this range, the reaction rate is retarded; whereas when the temperature is higher than this range, undesirable subreactions may take place. The rate of feeding chlorine gas and the reaction time vary, in the same way as in the case of the ring chlorination under shaded conditions, depending on the target chlorinated product. These reaction conditions, therefore, can be determined appropriately so that the reaction can be completed in as short a time as possible so long as the exothermic heat can be controlled and the utilization of chlorine gas does not become extremely low. After completing the reactions, the target product can be separated from the reaction mixture by distillation to remove titanium tetrachloride. The titanium tetrachloride can easily be separated and collected by distillation. When the target compound is solidified and becomes difficult to handle due to the complete removal of titanium tetrachloride, the separation of the target compound can be facilitated by solvent exchange. Specifically, another inert solvent, which has a higher boiling point than titanium tetrachloride, is added to the reaction mixture from which most of the titanium tetrachloride has been removed by distillation, and the mixture is further distilled until the remaining titanium tetrachloride is almost entirely removed. The resulting solution in which no titanium tetrachloride is contained is cooled so as to precipitate the target compound. Given as examples of these solvents are o-dichlorobenzene, decalin (decahydronaphthalene), paraffin having approximately a C 9 -C 12 content. These solvents can be used repeatedly. The target compounds thus separated are further purifiable by recrystallization or the like as required for further reactions. The chlorinated products of methylated aromatic compounds can be used as intermediates for useful final products such as dyestuffs, agricultural chemicals. For example, tetrachloro-1,2-bis (dichloromethyl) benzene, which is a chlorinated intermediate of methylated aromatic compound of which both the benzene rings and side chains are chlorinated, can easily be converted to tetrachlorphthalide, a useful compound for agricultural chemicals, by hydrolysis in the presence of sulfuric acid. As illustrated above, the use of titanium tetrachloride as the solvent for chlorination of both aromatic rings and methyl groups of methylated aromatic compounds allows the advantages of eliminating the use of chlorination catalyst requiring complicated separation following ring chlorination, and of obtaining the target products at a high yield as well. Furthermore, the ring chlorination and side chain chlorination can be performed continuously if a methylated aromatic compound is first reacted with chlorine gas under shaded conditions and then reacted under light irradiation The following description of an exemplary embodiment is given for illustration of the invention. EXAMPLE Into a four-necked 500-ml light protective flask fitted with a gas inlet tube, thermometer, reflux condenser and stirrer, 10.62 g (0.1 mol) of o-xylene and 419.5 g of titanium tetrachloride were charged. Chlorine gas was fed at a rate of 0.2 mol/hour into the mixture for 2 hours maintaining the bulk temperature at 40°C, with stirring. After the reaction temperature was raised to 60°C, chlorine gas was further fed at a rate of 0.1 mol/hour to complete the ring chlorination. Chromatographic analysis of the reaction mixture revealed that the solution consisted of 70.5 mol% of 3,4,5,6-tetrachloroxylene and 25.8 mol% of 3,4,5,6,ω-pentachloroxylene. The resulting mixture was transferred to a light reaction unit fitted with a gas inlet tube, reflux condenser and thermometer, and reacted with chlorine gas fed at a rate of 0.1 mol/hour into the mixture for 4.5 hours, while maintaining the reaction temperature at 70°C, with stirring and light irradiation generated by a 100-W high-pressure mercury lamp. Upon completion of the reaction, the resulting mixture was subjected to vacuum distillation at 37-38°C and 21 mmHg to recover 407 g of titanium tetrachloride (Recovery yield: 97%). Chromatographic analysis revealed the residual white solid (37.6 g) to be tetrachloro-1,2-bis (dichloromethyl) benzene having 95% purity.

Claims: A method for the side-chain chlorination of methylated aromatic compound by reacting said methylated aromatic compound with chlorine gas in a titanium tetrachloride solvent under light irradiation. A method according to claim 1 wherein a reaction mixture, obtained by reacting methylated aromatic compound with chlorine gas in a titanium tetrachloride solvent under shaded conditions, is chlorinated. A method according to claim 2 wherein the reaction under shaded conditions is conducted at 0 - 130°C. A method according to ally preceding claim 2 or 3 wherein the reaction mixture contains no additional catalyst. A method according to any preceding claim wherein the concentration of methylated aromatic compound in the solvent is 2 - 20 wt.%. A method according to any preceding claim wherein the reaction mixture contains no carbon tetrachloride. A method according to any preceding claim wherein the reaction mixture contains no additional solvent. A method according to any preceding claim wherein the reaction under light irradiation is conducted at 40 - 130°C.

Inventors: HOSHI HAJIME, SUNAGAWA KAZUHIKO, WATANABE TSUMORU, HOSHI, HAJIME, SUNAGAWA, KAZUHIKO, WATANABE, TSUMORU
Applicants: KUREHA CHEMICAL IND CO LTD, KUREHA CHEMICAL INDUSTRY CO., LTD.
Application year: 1992
Main classification: C07C17
IPC codes: C07C25, C07C17