DESC:FIELD
The present disclosure relates to a process for preparing 2,4-dimethylthiophenol.
BACKGROUND
The background information herein below relates to the present disclosure but is not necessarily prior art.
2,4-Dimethylthiophenol (CAS number : 13616-82-5) is an important intermediate for the preparation of therapeutic compounds.
Conventionally, 2,4-dimethylthiophenol (I) is prepared from 1,3-xylene (CAS number: 108-38-3). The synthesis procedure involves chlorosulphonation of 1,3-xylene to obtain 2,4-dimethylbenzene-1-sulphonyl chloride (IV), which is reduced to obtain I.
However, the conventional processes are associated with drawbacks such as low yield, low purity, and need for purification steps. Further, the reduction step is difficult to control.
John F. W. Keana et al (J. Org. Chem., 1988, volume 53, issue 10, pages 2365–2367) describes chlorosulphonation of 1,3-xylene using chlorosulphonyl chloride. The reaction is carried out without the use of a fluid medium. The product obtained by the process of John F. W. Keana contains impurities, and the crude product is purified by column chromatography using organic fluid medium. Therefore, the chlorosulphonation of 1,3-xylene reported by John F. W. Keana et al has low purity and is not environmentally friendly.
US 3326981 describes the reduction of 2,4-dimethylbenzene-1-sulphonyl chloride using zinc and concentrated hydrochloric acid. The process of US 3326981 involves addition of a toluene solution of IV to a suspension of zinc in water and concentrated hydrochloric acid. The heterogeneous reaction proceeds with uncontrolled evolution of a large amount of hydrogen gas. Therefore, the process of US 3326981 is not safe.
Hiromi Uchiro et al (Tetrahedron Letters, 1999, Volume 40, Issue 16, Pages 3179-3182) describes the reduction of 2,4-dimethylbenzene-1-sulphonyl chloride using zinc and concentrated hydrochloric acid along with an additional reagent dichlorodimethylsilane. The product obtained by the process of Hiromi Uchiro et al has low purity and it needs to be purified by column chromatography using an organic fluid medium. Therefore, reduction reported by Hiromi Uchiro et al is not environmentally friendly.
There is, therefore, felt a need for a simple and environmentally friendly process that produces 2,4-dimethylthiophenol (I) in high yield with high purity.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
It is another object of the present disclosure to provide a simple and environmentally friendly process for preparation of 2,4-dimethylthiophenol (I).
It is still another object of the present disclosure to provide a process for preparation of 2,4-dimethylthiophenol (I) in high yield with high purity.
Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY
In a first aspect, the present disclosure provides a process for preparation of 2,4-dimethylthiophenol. The process comprises subjecting 1,3-xylene (II) to chlorosulphonation at a temperature in the range of 0 °C to 30 °C, to obtain a first product mixture comprising 2,4-dimethylbenzenesulphonyl chloride (IV). 2,4-dimethylbenzenesulphonyl chloride (IV) is separated from the first product mixture. In the next step, 2,4-dimethylbenzenesulphonyl chloride (IV) is reduced in the presence of a mineral acid and metallic zinc at a temperature in the range of 30 °C to 50 °C, to obtain a second product mixture comprising 2,4-dimethylthiophenol (I). 2,4-dimethylthiophenol (I) is separated from the second product mixture.
In accordance with the present disclosure, chlorosulphonation of 1,3-xylene (II) is carried out by adding chlorosulphonic acid (III) to a solution of 1,3-xylene in a first fluid medium, followed by stirring the resultant mass over a first predetermined time period.
In accordance with the present disclosure, reduction of 2,4-dimethylbenzenesulphonyl chloride (IV) is carried out by adding metallic zinc, followed by adding a mineral acid to the solution of 2,4-dimethylbenzenesulphonyl chloride (IV) in a second fluid medium under stirring while maintaning the temperature of the resultant mixture in the range of 30 °C to 50 °C.
In accordance with the present disclosure, in the step of chlorosulphonation, the molar ratio of the 1,3-xylene (II) to the chlorosulphonic acid (III) is in the range of 1:1.5 to 1:2.5.
In accordance with the present disclosure, in the step of chlorosulphonation, chlorosulphonic acid (III) is added to the solution of 1,3-xylene over a time period in the range of 3 to 9 hours.
In accordance with the present disclosure, in the step of chlorosulphonation, the chlorosulphonic acid (III) is added to the solution of 1,3-xylene at a rate in the range of 100 grams/hour to 400 grams/hour.
In accordance with the present disclosure, in the step of chlorosulphonation, the first fluid medium is selected from the group consisting of dichloromethane (MDC), chloroform, carbon tetrachloride, dichloroethane (EDC), and mixtures thereof.
In accordance with the present disclosure, the step of separating 2,4-dimethylbenzenesulphonyl chloride (IV), comprises adding the first product mixture to water maintained at a temperature in the range of 10 ?C to 20 ?C and stirring the resultant biphasic mixture, followed by allowing the biphasic mixture to separate into an upper organic phase comprising 2,4-dimethylbenzenesulphonyl chloride and first fluid medium, and a lower aqueous phase, followed by separating the organic phase from the aqueous phase, washing the organic phase with brine, and distilling out the first fluid medium from the organic phase to obtain a residue that is dried under reduced pressure in the range of 40 to 60 mm Hg, at a temperature in the range of 30 to 60 ?C to obtain 2,4-dimethylbenzenesulphonyl chloride in the form of a light yellow oil, which is used in the step of reducing.
In accordance with the present disclosure, in the step of reducing, the molar ratio of 2,4-dimethylbenzenesulphonyl chloride (IV) to zinc is in the range of 1:2 to 1:5.
In accordance with the present disclosure, in the step of reducing, the molar ratio of the 2,4-dimethylbenzenesulphonyl chloride (IV) to the mineral acid is in the range of 1:12 to 1:18.
In accordance with the present disclosure, in the step of reducing, the second fluid medium is selected from the group consisting of dichloroethane (EDC), toluene, o-xylene, m-xylene, p-xylene, monochlorobenzene, and orthodichlorobenzene.
In accordance with the present disclosure, in the step of reducing, the mineral acid is at least one selected from the group consisting of hydrochloric acid (HCl) and sulphuric acid (H2SO4).
In accordance with the present disclosure, the step of separating 2,4-dimethylthiophenol (I), comprises allowing the second product mixture to separate into a biphasic mixture comprising an upper organic phase comprising 2,4-dimethylthiophenol (I) and the second fluid medium, and a lower aqueous phase, followed by separating the organic phase from the aqueous phase, extracting the aqueous phase with fresh second fluid medium to obtain another organic phase, combining the organic phases, washing the combined organic phase with brine, and distilling out the second fluid medium from the combined organic phase to obtain a residue and drying the residue under reduced pressure at a temperature in the range of 40 ?C to 80 ?C, and then distilling the dried residue under reduced pressure of 50 mm Hg, at 125 ?C to obtain 2,4-Dimethylthiophenol.
DETAILED DESCRIPTION
Embodiments are provided so as to thoroughly and fully convey the scope of the present disclosure to the person skilled in the art. Numerous details, are set forth, relating to specific components, and methods, to provide a complete understanding of embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.
The terminology used, in the present disclosure, is only for the purpose of explaining a particular embodiment and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly suggests otherwise. The terms "comprises," "comprising," “including,” and “having,” are open ended transitional phrases and therefore specify the presence of stated features, integers, steps, operations, elements, modules, units and/or components, but do not forbid the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The particular order of steps disclosed in the method and process of the present disclosure is not to be construed as necessarily requiring their performance as described or illustrated. It is also to be understood that additional or alternative steps may be employed.
When an element is referred to as being "mounted on," “engaged to,” "connected to," or "coupled to" another element, it may be directly on, engaged, connected or coupled to the other element. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed elements.
The terms first, second, third, etc., should not be construed to limit the scope of the present disclosure as the aforementioned terms may be only used to distinguish one element, component, region, layer or section from another component, region, layer or section. Terms such as first, second, third etc., when used herein do not imply a specific sequence or order unless clearly suggested by the present disclosure.
Terms such as “inner,” “outer,” "beneath," "below," "lower," "above," "upper," and the like, may be used in the present disclosure to describe relationships between different elements as depicted from the figures.
Conventional processes for the preparation of 2,4-dimethylthiophenol (I) are often complex and provide a product with low yield and with low purity. The present disclosure envisages a simple and environmentally friendly process for the preparation of 2,4-dimethylthiophenol (I) with high yield and with high purity.

In one aspect, the present disclosure provides a process for preparing 2,4-dimethylthiophenol. The process comprises the following steps:
1,3-xylene (II) is subjected to chlorosulphonation reaction at a temperature in the range of 0 °C to 30 °C, by adding chlorosulphonic acid (III) to a solution of 1,3-xylene in a first fluid medium, followed by stirring the resultant mass over a first predetermined time period to obtain a first product mixture comprising 2,4-dimethylbenzenesulphonyl chloride (IV).
2,4-dimethylbenzenesulphonyl chloride (IV) is separated from the first product mixture to obtain 2,4-dimethylbenzenesulphonyl chloride.
In the next step, 2,4-dimethylbenzenesulphonyl chloride (IV) is reduced in the presence of a mineral acid and metallic zinc at a temperature in the range of 30 °C to 50 °C, by adding the metallic zinc, followed by adding the mineral acid to the solution of 2,4-dimethylbenzenesulphonyl chloride (IV) in a second fluid medium under stirring while maintaning the temperature of the resultant mixture in the range of 30 °C to 50 °C, to obtain a second product mixture comprising 2,4-dimethylthiophenol (I).
2,4-dimethylthiophenol (I) is separated from the second product mixture to obtain 2,4-dimethylthiophenol (I).

The process of the present disclosure is represented herein below:
Step of Chlorosulphonation of 1,3-xylene (II)

Step of Reduction of 2,4-dimethylbenzenesulphonyl chloride (IV)

In accordance with the embodiments of the present disclosure, in the step of chlorosulphonation, the molar ratio of the 1,3-xylene (II) to the chlorosulphonic acid (III) is in the range of 1:1.5 to 1:2.5.
In one embodiment of the present disclosure, in the step of chlorosulphonation, the molar ratio of the 1,3-xylene (II) to the chlorosulphonic acid (III) is 1:2.
It is observed that the optimum temperature for chlorosulphonation step is in the range of 0 ?C to 30 ?C.
Since, chlorosulphonation of II is an exothermic reaction, it very important that it proceeds in a controlled manner. Addition of reactants in a controlled manner is an effective and efficient way of controlling the temperature of the exothermic reactions.
In the step of chlorosulphonation, chlorosulphonic acid (III) is added to the solution of 1,3-xylene over a time period in the range of 3 to 9 hours.
In one embodiment of the present disclosure, chlorosulphonic acid (III) is added to the solution of 1,3-xylene over a time period of 4 hours.
In the step of chlorosulphonation, chlorosulphonic acid (III) is added to the solution of 1,3-xylene at a rate in the range of 100 grams/hour to 400 grams/hour.
In one embodiment of the present disclosure, in the step of chlorosulphonation, chlorosulphonic acid (III) is added to the solution of 1,3-xylene at a rate of 275 grams/hour.
Since, chlorosulphonation of II is an exothermic reaction, efficient stirring and an effective heat dissipation mechanism is needed in order to prevent a sudden and violent reaction.
In the process of the present disclosure, the step of chlorosulphonation is carried out in a first fluid medium selected from the group consisting of dichloromethane (MDC), chloroform, carbon tetrachloride, dichloroethane (EDC), and mixtures thereof.
In one embodiment of the present disclosure, in the step of chlorosulphonation, the first fluid medium is dichloromethane (MDC).
Stirring of the reaction mixture ensures formation of a homogeneous reaction medium by using the fluid medium. Stirring also helps in the dissipation of heat produced during the exothermic reaction, and facilitates chlorosulphonation to be carried out at a temperature in the range of 0 ?C to 30 ?C. As a result, the chlorosulphonation of II proceeds in a controlled manner.
In the step of chlorosulphonation, the first predetermined time period is in the range of 0.5 hour to 12 hours.
In one embodiment of the present disclosure, in the step of chlorosulphonation, first predetermined time period is 9.5 hours
The step of separating 2,4-dimethylbenzenesulphonyl chloride (IV), comprises adding the first product mixture to water maintained at a temperature in the range of 10 ?C to 20 ?C and stirring the resultant biphasic mixture for a second predetermined time period, followed by allowing the biphasic mixture to separate into an upper organic phase comprising 2,4-dimethylbenzenesulphonyl chloride and first fluid medium, and a lower aqueous phase, followed by separating the organic phase from the aqueous phase, washing the organic phase with brine, and distilling out the first fluid medium from the organic phase to obtain a residue that is dried under reduced pressure in the range of 40 to 60 mm Hg, at a temperature in the range of 30 to 60 ?C to obtain 2,4-dimethylbenzenesulphonyl chloride (IV) in the form of a light yellow oil, which is used in the step of reducing.
In the process of the present disclosure, chlorosulphonation proceeds with high regioselectivity and IV is obtained with a yield in the range of 70 % to 90% and purity in the range of 95 % to 99.9%.
As a result, purification of IV is not needed and it is directly taken for the reduction step. Thus, chlorosulphonation step of the present disclosure is simple, and cost effective.
It is observed that, the yield and/or purity of the IV is affected if chlorosulphonation is not controlled. Uncontrolled chlorosulphonation proceeds with lower regioselectivity and substantial amounts of isomeric impurities, 3,5-dimethylbenzenesulphonyl chloride and 2,6-dimethylbenzenesulphonyl chloride, are formed, which may need an additional purification step.
In the process of the present disclosure, in the step of reducing, the molar ratio of 2,4-dimethylbenzenesulphonyl chloride (IV) to zinc is in the range of 1:2 to 1:5.
In one embodiment of the present disclosure, in the step of reduction, the molar ratio of 2,4-dimethylbenzenesulphonyl chloride (IV) to zinc is 1:3.4.
In the process of the present disclosure, in the step of reducing, the molar ratio of the 2,4-dimethylbenzenesulphonyl chloride (IV) to the mineral acid is in the range of 1:8 to 1:18.
In one embodiment of the present disclosure, in the step of reduction, the molar ratio of the 2,4-dimethylbenzenesulphonyl chloride (IV) to the mineral acid is 1:13.2.
In another embodiment of the present disclosure, in the step of reduction, the molar ratio of the 2,4-dimethylbenzenesulphonyl chloride (IV) to the mineral acid is 1:16.8.
In the process of the present disclosure, the step of reduction is carried out in the second fluid medium selected from the group consisting of dichloroethane (EDC), toluene, o-xylene, m-xylene, p-xylene, monochlorobenzene, and orthodichlorobenzene.
In one embodiment of the present disclosure, in the step of reduction, the second fluid medium is toluene.
In another embodiment of the present disclosure, in the step of reduction, the second fluid medium is dichloroethane (EDC).
A fluid medium for the reduction step provides a homogeneous reaction mixture, and facilitates thorough mixing of reagents and results in a controlled reaction.
Reduction using zinc and mineral acid has several advantages. Firstly, zinc and mineral acid are readily available and are inexpensive. Secondly, the post-reaction work-up is simple.
In the step of reduction, the mineral acid is at least one selected from the group consisting of hydrochloric acid (HCl), and sulphuric acid (H2SO4). Other acids can also be used for reduction step.
In one embodiment of the present disclosure, in the step of reduction, the mineral acid is hydrochloric acid (HCl).
In another embodiment of the present disclosure, in the step of reduction, the mineral acid is sulphuric acid (H2SO4).
The step of reduction is carried out under controlled conditions so as to maintain reaction temperature in the range of 30 ?C to 50 ?C.
For controlling the reaction temperature in the range of 30 ?C to 50 ?C, zinc and the mineral acid are added to the solution of 2,4-dimethylbenzenesulphonyl chloride (IV) in a second fluid medium, in a controlled manner.
In accordance with the embodiments of the present disclosure, the total amount of zinc to be added to the reaction mixture can be divided into a number of lots. Similarly, the total amount of acid to be added to the reaction mixture can be divided into a number of lots. Lots of zinc and acid can be added to the reaction mixture portion by portion in a controlled manner so as to allow controlled evolution of hydrogen.
Controlling reaction conditions are amenable for performing the reduction step at a large scale. Further, the extensive cooling mechanism is not needed for this step in the process of the present disclosure.
As a result of the use of the second fluid medium and additions of zinc and acid in lots to maintain the reaction temperature in the range of 30 ?C to 50 ?C, the reduction of IV to I proceeds in a controlled manner, and provides a product in high yield and with high purity.
Thus, in the process of the present disclosure, 2,4-dimethylbenzene sulphonyl chloride is reduced to 2,4-dimethylthiophenol under controlled condition using metals and acids. The process is safe, non-hazardous, and industrially viable at commercial scale.
The step of separating 2,4-dimethylthiophenol (I), comprises allowing the second product mixture to separate into a biphasic mixture comprising an upper organic phase comprising 2,4-dimethylthiophenol (I) and the second fluid medium, and a lower aqueous phase, followed by separating the organic phase from the aqueous phase, extracting the aqueous phase with fresh second fluid medium to obtain another organic phase, combining the organic phases, washing the combined organic phase with brine, and distilling out the second fluid medium from the combined organic phase to obtain a residue and drying the residue under reduced pressure at a temperature in the range of 40 ?C to 80 ?C, and then distilling the dried residue under reduced pressure of 50 mm Hg, at 125 ?C to obtain 2,4-Dimethylthiophenol.
The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.

Experimental Details:
Experiment 1: Preparation of 2,4-dimethylbenzenesulphonyl chloride
Example 1:
A solution of m-xylene (500 g, 4.7 mol) and dichloromethane (1000 ml) was cooled to 10 ?C. Chlorosulphonic acid (1100 g, 9.4 mol) was added drop-wise to the cooled solution over 4 hours while controlling the temperature of the resultant mass at 10 ?C. The resultant mass was stirred for 1.5 hours at 10 ?C. The temperature of the resultant mass was raised to 25 ?C and was stirred for 8 hours to obtain a product mixture.
The product mixture was quenched by adding the product mixture to 3000 ml water maintained at 15 ?C and was stirred for 15 minutes. A biphasic mixture was obtained when stirring was stopped. Lower organic layer was separated and washed with brine. Dichloromethane was distilled out and the residue was dried under reduced pressure of 50 mm Hg, at 45 ?C to obtain 2,4-dimethylbenzenesulphonyl chloride in the form of a light yellow oil, which was further used directly for the next step.
[Yield = 792gm (82.5%), Purity (GC) = 99.04 %]
Example-2:
A solution of m-xylene (500 g, 4.7 mol) and dichloromethane (1000 ml) was cooled to 15 ?C. Chlorosulphonic acid (1100 g, 9.4 mol) was added drop-wise to the cooled solution over 4 hours while controlling the temperature of the resultant mass at 15 ?C. The resultant mass was stirred for 1.5 hours at 15 ?C. The temperature of the resultant mass was raised to 25 ?C and was stirred for 8 hours to obtain a product mixture.
The product mixture was quenched by adding the product mixture to 3000 ml water maintained at 15 ?C and was stirred for 15 minutes. A biphasic mixture was obtained when stirring was stopped. Lower organic layer was separated and washed with brine. Dichloromethane was distilled out and the residue was dried under reduced pressure of 50 mm Hg, at 45 ?C to obtain 2,4-dimethylbenzenesulphonyl chloride in the form of a light yellow oil.
[Yield = 776 g (80.83%), Purity (GC) = 98.65%]
Example-3:
A solution of m-xylene (500 g, 4.7 mol) and dichloromethane (1000 ml) was cooled to 0 ?C. Chlorosulphonic acid (1100 g, 9.4 mol) was added drop-wise to the cooled solution over 4 hours while controlling the temperature of the resultant mass at 0 ?C. The resultant mass was stirred for 1 hour at 0 ?C. The temperature of the resultant mass was raised to 25 ?C and was stirred for 10 hours to obtain a product mixture.
The product mixture was quenched by adding the product mixture to 3000 ml water maintained at 15 ?C and was stirred for 15 minutes. A biphasic mixture was obtained when stirring was stopped. Lower organic layer was separated and washed with brine. Dichloromethane was distilled out and the residue was dried under reduced pressure of 50 mm Hg, at 60 ?C to obtain 2,4-dimethylbenzenesulphonyl chloride in the form of a light yellow oil, which was further used directly for the next step.
[Yield = 770 g, (80.23%), Purity (GC) = 97.80%]
Example-4:
A solution of m-xylene (500 g, 4.7 mol) and dichloromethane (1000 ml) was cooled to 0 ?C. Chlorosulphonic acid (1100 g, 9.4 mol) was added drop-wise to the cooled solution over 8 hours while controlling the temperature of the resultant mass at 0 ?C. The resultant mass was stirred for 1 hour at 0 ?C. The temperature of the resultant mass was raised to 25 ?C and was stirred for 10 hours to obtain a product mixture.
The product mixture was quenched by adding the product mixture to 3000 ml water maintained at 15 ?C and was stirred for 15 minutes. A biphasic mixture was obtained when stirring was stopped. Lower organic layer was separated and washed with brine. Dichloromethane was distilled out and the residue was dried under reduced pressure of 50 mm Hg, at 65 ?C to obtain 2,4-dimethylbenzenesulphonyl chloride in the form of a light yellow oil, which was further used directly for the next step.
[Yield = 782 g, (81.45%), Purity (GC) = 98.60%]
Example-5:
A solution of m-xylene (500 g, 4.7 mol) and dichloroethane (1000 ml) was cooled to 0 ?C. Chlorosulphonic acid (1100 g, 9.4 mol) was added drop-wise to the cooled solution over 4 hours while controlling the temperature of the resultant mass at 0 ?C. The resultant mass was stirred for 1 hour at 0 ?C. The temperature of the resultant mass was raised to 25 ?C and was stirred for 8 hours to obtain a product mixture.
The product mixture was quenched by adding the product mixture to 3000 ml water maintained at 15 ?C and was stirred for 15 minutes. A biphasic mixture was obtained when stirring was stopped. Lower organic layer was separated and washed with brine. Dichloroethane was distilled out and the residue was dried under reduced pressure of 50 mm Hg, at 65 ?C to obtain 2,4-dimethylbenzenesulphonyl chloride in the form of a light yellow oil.
[Yield = 772 g, (80.41%), Purity (GC) = 97.30%]
Example-6:
A solution of m-xylene (500 g, 4.7 mol) and dichloromethane (1000 ml) was cooled to 10 ?C. Chlorosulphonic acid (1100 g, 9.4 mol) was added drop-wise to the cooled solution over 4 hours while controlling the temperature of the resultant mass at 10 ?C. The resultant mass was stirred for 1.5 hours at 10 ?C. The temperature of the resultant mass was raised to 25 ?C and was stirred for 8 hours to obtain a product mixture.
The product mixture was quenched by adding the product mixture to 3000 ml water maintained at 15 ?C and was stirred for 15 minutes. A biphasic mixture was obtained when stirring was stopped. Lower organic layer was separated and washed with brine. Dichloromethane was distilled out and the residue was dried under reduced pressure of 50 mm Hg, at 45 ?C to obtain 2,4-dimethylbenzenesulphonyl chloride in the form of a light yellow oil, which was further used directly for the next step.
[Yield = 788 g, (82.08%), Purity (GC) = 99.15 %]
Experiment 2: Preparation of 2,4-Dimethylthiophenol
Example-7:
Zinc dust (1000 g, 15.3 mol) was divided into 10 lots of 100 g each, and concentrated hydrochloric acid (5500 ml, 51.5 mol) was divided into 10 lots of 550 ml each.
2,4-Dimethylbenzenesulphonyl chloride (790 g, 3.9 mol) was added to a reaction vessel containing toluene (4000 ml) and the resultant mixture was stirred at 15 ?C for 30 minutes. The resultant mixture was then cooled to 10 ?C. First lot of 100 g zinc dust was added to the resultant mixture and it was stirred for 10 minutes. First lot of conc. HCl was slowly added to the resultant mixture in such a way that the temperature of the reaction mixture does not exceed 50 ?C and was stirred for 30 minutes. In similar way all other lots of zinc dust and conc. HCl were added while maintaining the temperature of the reaction mixture at 50 ?C.
After complete addition, the reaction mixture was stirred for 1 hour at 50 ?C. After completion of the reaction, stirring was stopped. A biphasic mixture was obtained. The upper organic layer was separated. The aqueous layer was extracted with toluene (670 ml) to obtain another organic layer. Organic layers were combined, and toluene was distilled out from the combined organic layer and the residue was dried under reduced pressure at 60 ?C. The residue was finally subjected to distillation. 2,4-Dimethylthiophenol was distilled out at 125 ?C at 50 mm Hg pressure.
[Yield = 405 g (76.5%), Purity (GC) = 99.8%]
Example-8:
Zinc dust (1000 g, 15.3 mol) was divided into 10 lots of 100 g each, and 50% dilute sulphuric acid (7000 ml, 65.7 mol) was divided into 10 lots of 700 ml each.
2,4-Dimethylbenzenesulphonyl chloride (790 g, 3.9 mol) was added to a reaction vessel containing toluene (4000 ml) and the resultant mixture was stirred at 15 ?C for 30 minutes. First lot of 100 g zinc dust was added to the resultant mixture and it was stirred for 10 minutes. First lot of diluted sulphuric acid was slowly added to the resultant mixture in such a way that the temperature of the reaction mixture does not exceed 50 ?C and was stirred for 30 minutes. In similar way all other lots of zinc dust and diluted sulphuric acid were added while maintaining the temperature of the reaction mixture at 50 ?C.
After complete addition, the reaction mixture was stirred for 1 hour at 50 ?C. After completion of the reaction, stirring was stopped. A biphasic mixture was obtained. The upper organic layer was separated. The aqueous layer was extracted with toluene (670 ml) to obtain another organic layer. Organic layers were combined, and toluene was distilled from the combined organic layer and the residue was dried under reduced pressure at 60 ?C. The residue was finally subjected to distillation. 2,4-Dimethylthiophenol was distilled out at 125 ?C at 50 mm Hg pressure.
[Yield = 368 g ,(69.5%), Purity (GC) = 99.5%]

Example-9:
Zinc dust (1000 g, 15.3 mol) was divided into 10 lots of 100 g each, and concentrated hydrochloric acid (5500 ml, 51.53 mol) was divided into 10 lots of 550 ml each.
2,4-Dimethylbenzenesulphonyl chloride (790 g, 3.6 mol) was added to a reaction vessel containing EDC (4000 ml) and the resultant mixture was stirred at 15 ?C for 30 minutes. The resultant mixture was then cooled to 10 ?C. First lot of 100 g zinc dust was added to the resultant mixture and it was stirred for 10 minutes. First lot of conc. HCl was slowly added to the resultant mixture in such a way that the temperature of the reaction mixture does not exceed 40 ?C and was stirred for 30 minutes. In similar way all other lots of zinc dust and conc. HCl were added while maintaining the temperature of the reaction mixture at 40 ?C.
After complete addition, the reaction mixture was stirred for 1 hour at 40 ?C. After completion of the reaction, stirring was stopped. A biphasic mixture was obtained. The upper organic layer was separated. The aqueous layer was extracted with EDC (670 ml) to obtain another organic layer. Organic layers were combined, and EDC was distilled out from the combined organic layer and the residue was dried under reduced pressure at 60 ?C. The residue was finally subjected to distillation. 2,4-Dimethylthiophenol was distilled out at 125 ?C at 50 mm Hg pressure.
[Yield = 376 g ,(70.5%), Purity (GC) = 99.6%]
Example-10:
Zinc dust (1000 g, 15.3 mol) was divided into 10 lots of 100 g each, and concentrated hydrochloric acid (5500 ml, 51.53 mol) was divided into 10 lots of 550 ml each.
2,4-Dimethylbenzenesulphonyl chloride (790 g, 3.9 mol) was added to a reaction vessel containing toluene (4000 ml) and the resultant mixture was stirred at 15 ?C for 30 minutes. The resultant mixture was then cooled to 10 ?C. First lot of 100 g zinc dust was added to the resultant mixture and it was stirred for 10 minutes. First lot of conc. HCl was slowly added to the resultant mixture in such a way that the temperature of the reaction mixture does not exceed 50 ?C and was stirred for 30 minutes. In similar way all other lots of zinc dust and conc. HCl were added while maintaining the temperature of the reaction mixture at 50 ?C.
After complete addition, the reaction mixture was stirred for 2 hour at 110 ?C. After completion of the reaction, the reaction mixture was cooled to 30 ?C and stirring was stopped. A biphasic mixture was obtained. The upper organic layer was separated. The aqueous layer was extracted with toluene (670 ml) to obtain another organic layer. Organic layers were combined, and toluene was distilled out from the combined organic layer and the residue was dried under reduced pressure at 60 ?C. The residue was finally subjected to distillation. 2,4-Dimethylthiophenol was distilled out at 125 ?C at 50 mm Hg pressure.
[Yield = 371g ,(69.57%), Purity (GC) = 99.20%]
Example-11:
Zinc dust (1000 g, 15.3 mol) was divided into 10 lots of 100 g each, and concentrated hydrochloric acid (5500 ml, 51.53 mol) was divided into 10 lots of 550 ml each.
2,4-Dimethylbenzenesulphonyl chloride (790 g, 3.9 mol) was added to a reaction vessel containing toluene (4000 ml) and the resultant mixture was stirred at 15 ?C for 30 minutes. The resultant mixture was then cooled to 10 ?C. First lot of 100 g zinc dust was added to the resultant mixture and it was stirred for 10 minutes. First lot of conc. HCl was slowly added to the resultant mixture in such a way that the temperature of the reaction mixture does not exceed 50 ?C and was stirred for 30 minutes. In similar way all other lots of zinc dust and conc. HCl were added while maintaining the temperature of the reaction mixture at 50 ?C.
After complete addition, the reaction mixture was stirred for 1 hour at 50 ?C. After completion of the reaction, stirring was stopped. A biphasic mixture was obtained. The upper organic layer was separated. The aqueous layer was extracted with toluene (670 ml) to obtain another organic layer. Organic layers were combined, and toluene was distilled out from the combined organic layer and the residue was dried under reduced pressure at 60 ?C. The residue was finally subjected to distillation. 2,4-Dimethylthiophenol was distilled out at 125 ?C at 50 mm Hg pressure.
[Yield = 400 g, (75%), Purity (GC) = 99.78%]
The foregoing description of the embodiments has been provided for purposes of illustration and not intended to limit the scope of the present disclosure. Individual components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.
TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of:
- a simple and environmentally friendly process for preparation of 2,4-dimethylthiophenol; and
- a process for preparation of 2,4-dimethylthiophenol with high yield and with high purity.

The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
The foregoing description of the specific embodiments so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the components and component parts of the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other changes in the preferred embodiment as well as other embodiments of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.
,CLAIMS:WE CLAIM
1. A process for preparation of 2,4-dimethylthiophenol, said process comprising the following steps:
i) subjecting 1,3-xylene (II) to chlorosulphonation at a temperature in the range of 0 °C to 30 °C, to obtain a first product mixture comprising 2,4-dimethylbenzenesulphonyl chloride (IV);
ii) separating 2,4-dimethylbenzenesulphonyl chloride (IV) from the first product mixture;
iii) reducing 2,4-dimethylbenzenesulphonyl chloride (IV) in the presence of a mineral acid and metallic zinc at a temperature in the range of 30 °C to 50 °C, to obtain a second product mixture comprising 2,4-dimethylthiophenol (I); and
iv) separating 2,4-dimethylthiophenol (I) from said second product mixture.

2. The process as claimed in claim 1, wherein chlorosulphonation of 1,3-xylene (II) is carried out by adding chlorosulphonic acid (III) to a solution of 1,3-xylene in a first fluid medium, followed by stirring the resultant mass over a first predetermined time period.

3. The process as claimed in claim 1, wherein reduction of 2,4-dimethylbenzenesulphonyl chloride (IV) is carried out by adding said metallic zinc, followed by adding said mineral acid to the solution of 2,4-dimethylbenzenesulphonyl chloride (IV) in a second fluid medium under stirring while maintaning the temperature of the resultant mixture in the range of 30 °C to 50 °C.

4. The process as claimed in claim 2, wherein the molar ratio of said 1,3-xylene (II) to said chlorosulphonic acid (III) is in the range of 1:1.5 to 1:2.5.

5. The process as claimed in claim 2, wherein said chlorosulphonic acid (III) is added to said solution of 1,3-xylene over a time period in the range of 3 to 9 hours.

6. The process as claimed in claim 2, wherein said chlorosulphonic acid (III) is added to said solution of 1,3-xylene at a rate in the range of 100 grams/hour to 400 grams/hour.

7. The process as claimed in claim 2, wherein said first fluid medium is selected from the group consisting of dichloromethane (MDC), chloroform, carbon tetrachloride, dichloroethane (EDC), and mixtures thereof.

8. The process as claimed in claim 1, wherein step (ii) comprises adding said first product mixture to water maintained at a temperature in the range of 10 ?C to 20 ?C and stirring the resultant biphasic mixture, followed by allowing the biphasic mixture to separate into an upper organic phase comprising 2,4-dimethylbenzenesulphonyl chloride and the first fluid medium, and a lower aqueous phase, followed by separating the organic phase from the aqueous phase, washing the organic phase with brine, and distilling out said first fluid medium from the organic phase to obtain a residue that is dried under reduced pressure in the range of 40 to 60 mm Hg, at a temperature in the range of 30 to 60 ?C to obtain 2,4-dimethylbenzenesulphonyl chloride in the form of a light yellow oil, which is used in the step (iii).

9. The process as claimed in claim 1, wherein in step (iii), the molar ratio of said 2,4-dimethylbenzenesulphonyl chloride (IV) to zinc is in the range of 1:2 to 1:5.

10. The process as claimed in claim 1, wherein in step (iii), the molar ratio of said 2,4-dimethylbenzenesulphonyl chloride (IV) to said mineral acid is in the range of 1:12 to 1:18.

11. The process as claimed in claim 3, wherein in step (iii), said second fluid medium is selected from the group consisting of dichloroethane (EDC), toluene, o-xylene, m-xylene, p-xylene, monochlorobenzene, and orthodichlorobenzene.

12. The process as claimed in claim 1, wherein said mineral acid is at least one selected from the group consisting of hydrochloric acid (HCl) and sulphuric acid (H2SO4).

13. The process as claimed in claim 1, wherein step (iv) comprises allowing the second product mixture to separate into a biphasic mixture comprising an upper organic phase comprising 2,4-dimethylthiophenol (I) and said second fluid medium, and a lower aqueous phase, followed by separating the organic phase from the aqueous phase, extracting said aqueous phase with fresh second fluid medium to obtain another organic phase, combining the organic phases, washing the combined organic phase with brine, and distilling out said second fluid medium from the combined organic phase to obtain a residue and drying said residue under reduced pressure at a temperature in the range of 40 ?C to 80 ?C, and then distilling the dried residue under reduced pressure of 50 mm Hg, at 125 ?C to obtain 2,4-Dimethylthiophenol.