FIELD

The present disclosure relates to a process for the synthesis of carbazole and its derivatives.

BACKGROUND

The background information hereinbelow relates to the present disclosure but is not necessarily prior art.

Carbazole is one of the constituents of coal tar, wherein the coal tar contains around 1.5% carbazole. The carbazole is represented by the following structural formula:

Carbazole and its derivatives (I) are widely used in the synthesis of dyes, pigments, insecticides, electrical industry, high heat-resistant polymers, concrete plasticizers, and the like. In particular, the carbazole derivative such as N-ethyl carbazole is used in the synthesis of Pigment Violet-23 (PV-23), which is widely used in the printing ink, plastic, and paint industry.

Conventionally, there are various methods known for the preparation of carbazole and its derivatives. The conventional methods involve the use of metal such as palladium, requires a dehydrating agent, and requires high temperatures with evolution of hydrogen gas in the process. Further, the conventional methods involve production of a large amount of inorganic salts that require additional purification steps which makes the process uneconomical. Moreover, the product obtained by using the conventional methods has a comparatively low yield and low purity.

Therefore, there is felt a need to provide a process for the synthesis of carbazole and its derivatives that mitigates the aforestated drawbacks. 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.

Another object of the present disclosure is to provide a process for the synthesis of carbazole and its derivatives.

Another object of the present disclosure is to provide a process for the synthesis of carbazole and its derivatives that is free of palladium.

Another object of the present disclosure is to provide a process for the synthesis of carbazole and its derivatives that is free from metal catalyst which avoids the evolution of hydrogen.

Yet another object of the present disclosure is to provide a process for the synthesis of carbazole and its derivatives that provides a better yield and purity of the final product.

Still another object of the present disclosure is to provide a simple, safe, and cost-effective process for the synthesis of carbazole and its derivatives.

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

The present disclosure relates to a process for the synthesis of a carbazole compound represented by formula (I), wherein,

X and Y are independently selected from H, halogen, amino, and Ci to C20 straight or branched chain alkyl; and

R is H, Ci - C20 straight or branched chain alkyl, aromatic, or aliphatic substituents. The process comprises dehydrogenating at least one tetrahydrocarbazole by using a nitro compound optionally in a fluid medium at a predetermined temperature for a predetermined time period to obtain a product mixture comprising a crude carbazole compound. The carbazole compound is separated from the product mixture to obtain a pure carbazole compound of formula (I).

DETAILED DESCRIPTION

Embodiments, of the present disclosure, will now be described herein. 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.

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. Carbazole and its derivatives (I) are used in the synthesis of dyes, pigments, insecticides, electrical industry, high heat-resistant polymers, and concrete plasticizers. Carbazole is also one of the constituents of coal tar, which contains around 1.5% carbazole.

Several methods for the preparation of carbazole and its derivatives are reported in the art. The conventional methods involve the use of palladium, require a dehydrating agent and high temperatures with the evolution of hydrogen gas. Further, the conventional method of preparation involves a large amount of production of inorganic salts that required additional purification steps. Moreover, the product obtained by using the conventional methods has a comparatively low yield and low purity.

The process of the present disclosure provides a simple, environment friendly, and economical process for the synthesis of carbazole and its derivative that results in improved yields and higher purity of the final product (carbazole and its derivative).

In an aspect of the present disclosure, there is provided a process for the synthesis of carbazole and its derivatives. Particularly, the present disclosure provides a process for the preparation of carbazole and N-ethyl carbazole.

The schematic representation of the process is provided below wherein,

X & Y are independently selected from H or halogen, amino; Ci to C20 straight or branched chain alkyl substituents; and

R is selected from the group consisting of H, Ci to C20 straight or branched chain alkyl substituents, aromatic substituents, aliphatic substituents, and combinations thereof.

The process is described in detail.

In a first step, tetrahydrocarbazole is dehydrogenated by using a nitro compound optionally in a fluid medium at a predetermined temperature for a predetermined time period to obtain a product mixture comprising a crude carbazole compound.

In accordance with one embodiment, the tetrahydrocarbazole compound is selected from

1,2,3,4-tetrahydrocarbazole and 9-alkyl-tetrahydrocarbazole. In an exemplary embodiment of the present disclosure, the tetrahydrocarbazole compound is 1,2,3,4-tetrahydrocarbazole. In another exemplary embodiment of the present disclosure, the tetrahydrocarbazole compound is 9-alkyl-tetrahydrocarbazole.

In an embodiment, the nitro compound is at least one selected from alkyl nitroethane, aryl nitroethane, nitrobutane, nitro anthracene, nitro naphthalene, and nitrobenzene. In an exemplary embodiment, the nitro compound is nitrobenzene. In another exemplary embodiment, the nitro compound is nitro naphthalene.

In an embodiment, the fluid medium is at least one selected from neat tetrahydrocarbazole, toluene, monochlorobenzene, dichlorobenzene, trichlorobenzene, and xylene. In an exemplary embodiment, the fluid medium is toluene.

In an embodiment, the neat tetrahydrocarbazole works both as a reactant and as a fluid medium in the synthesis of carbazole compounds. The neat tetrahydrocarbazole is a viscous liquid at 30°C.

In an embodiment, the predetermined temperature is in the range of 120°C to 250°C. In an exemplary embodiment of the present disclosure, the temperature is 200°C to 230°C.

In an embodiment, the predetermined time period is in the range of 8 hours to 22 hours. In an exemplary embodiment, the time period is 12 hrs. In another exemplary embodiment, the time period is 15 hrs. In yet another exemplary embodiment, the time period is 10 hrs. In still another exemplary embodiment, the time period is 18 hrs.

In an embodiment, a weight ratio of the tetrahydrocarbazole to the nitro compound is in the range of 1:0.3 to 1:2. In an exemplary embodiment of the present disclosure, the weight ratio of tetrahydrocarbazole to the nitro compound is 1.5. In another exemplary embodiment, the weight ratio of tetrahydrocarbazole to the nitro compound is 0.75. In yet another exemplary embodiment, the weight ratio of tetrahydrocarbazole to the nitro compound is 0.45. In still another exemplary embodiment, the weight ratio of tetrahydrocarbazole to the nitro compound is 1.05.

In a final step, the crude carbazole compound is separated from the product mixture to obtain a pure carbazole compound formula (I).

In an embodiment of the present disclosure, the conversion of tetrahydrocarbazole compound to carbazole; and the conversion of tetrahydrocarbazole to N-alkyl carbazole is greater than 98%. In an embodiment of the present disclosure, the carbazole compound of formula (I) is selected from carbazole and N-ethyl carbazole.

The reaction is carried out for a time period such that desirable conversion of 1 ,2,3,4- tetrahydrocarbazole to carbazole and the conversion of tetraalkylhydrocarbazole to N- alkylcarbazole is greater than 95%. The conversion of the tetrahydrocarbazole in accordance with the process of the present disclosure is >98%.

In an embodiment of the present disclosure, the so obtained carbazole or N- alkylcarbazole compound has a purity of 99.9% and a yield of 95%.

In an embodiment of the present disclosure, the carbazole and its derivatives have a purity greater than 98%.

In an embodiment of the present disclosure, a process for the synthesis of a carbazole compound represented by formula (II),

II wherein the process comprising the following steps: a) dehydrogenating 1,2,3,4-tetrahydrocarbazole compound by using a nitronaphthalene in toluene at a temperature in the range of 200°C to 230°C for 18 hours to obtain a product mixture comprising a crude carbazole compound; and b) separating the carbazole compound from the product mixture to obtain a pure carbazole compound of formula (II).

In an embodiment of the present disclosure, a process for the synthesis of ethyl carbazole compound represented by formula (III), wherein the process comprising the following steps: a) dehydrogenating 9-ethyl-l,2,3,4-tetrahydrocarbazole compound by using a nitrobenzene in toluene at a temperature in the range of 200°C to 230°C for 15 hours to obtain a product mixture comprising a crude ethyl carbazole compound; and b) separating the ethyl carbazole compound from the product mixture to obtain a pure ethyl carbazole compound of formula (III).

Therefore, the process of the present disclosure provides a higher yield of the product with greater purity while also being cost-efficient, environment friendly, safe, and economical.

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.

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 can be extrapolated to industrial scale.

EXPERIMENTAL DETAILS

EXAMPLE 1:

199 g of 9-ethyl-l,2,3,4-tetrahydrocarbazole (98.6% purity) was charged into a reactor followed by the addition of 70 ml of toluene to obtain a mixture. The mixture was heated to 140 °C to distill out toluene to obtain a reaction mixture. 250 ml (300 gm) of nitrobenzene was added to the reaction mixture and raised the temperature to 200°C to 230°C such that nitrobenzene, aniline, and water was distilled at a rate of 15 ml/hr to obtain a product mixture. The reaction was monitored by GC and terminated when starting compound reaches

< 1 %. The total reaction time was 12 hrs. The product mixture was taken for further distillation to get the main cut at 145°C/10 mm vacuum to obtain ethyl carbazole. The purity of the product was > 98% and the yield was 95% (185 gm).

EXAMPLE 2:

199 g of 9-ethyl-l,2,3,4-tetrahydrocarbazole (98.6% purity) was charged into a reactor followed by the addition of 70 ml of toluene to obtain a mixture. The mixture was heated to 140 °C to distill out toluene to obtain a reaction mixture. 125 ml (150 gm) of nitrobenzene was added to the reaction mixture and raised the temperature to 200 °C to 230 °C such that nitrobenzene, aniline, and water was distilled at a rate of 15 ml/hr to obtain a product mixture. The reaction was monitored by GC and terminated when starting compound reaches

< 1 %. The total reaction time was 15 hours. The product mixture was taken for further distillation to get the main cut at 145°C/10 mm vacuum to obtain ethyl carbazole. The purity of the product was > 98% and the yield was 85% (170 gm).

EXAMPLE 3:

199 g of distilled 9-ethyl-l,2,3,4-tetrahydrocarbazole (>98% purity) was charged into a reactor followed by the addition of 75 ml (90 gm) of nitrobenzene and heated to 200°C to 230°C such that nitrobenzene, aniline, and water was distilled at a rate of 5 ml/hr to obtain a product mixture. The reaction was monitored by GC and terminated when starting compound reaches < 5 %. The total reaction time was 10 hours. The product mixture was treated with 2 gm of activated carbon followed by crystallization by using a mixture of toluene and hexane to obtain N-ethyl carbazole. The purity of the product was 99.9% and the yield was 75% as crop 1.

EXAMPLE 4:

171 g of 1,2,3,4-tetrahydrocarbazole (>98% purity) was charged into a reactor followed by the addition of 150 ml (180 gm) of nitro naphthalene and heated to 200°C to 230°C such that amino naphthalene and water was distilled at a rate of 7 ml/hr to obtain a product mixture. The reaction was monitored by GC and terminated when starting compound reaches < 5 %. The total reaction time was 18 hours. The product mixture was treated with 2 gm of activated carbon followed by crystallization by using a mixture of toluene and hexane to obtain carbazole. The purity of the product was 98 % and the yield was 70% as crop 1.

Table 1- Summary of examples It is evident from the above table that in examples 3 and 4, the carbazole compounds were synthesized in the absence of toluene (fluid medium). The tetrahydrocarbazole itself acts as a reactant and a fluid medium as well, providing enhanced purity of carbazole compounds.

TECHNICAL ADVANCEMENT

The present disclosure described hereinabove has several technical advantages including, but not limited to, the realization of a process for the synthesis of carbazole and its derivatives that;

• avoids metals (palladium);

• no evolution of hydrogen gas;

• is a simple, economical, and environment friendly process; and • provides carbazole and its derivatives in comparatively high purity and high yield.

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 invention to achieve one or more of the desired objects or results. While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Variations or modifications to the formulation of this invention, within the scope of the invention, may occur to those skilled in the art upon reviewing the disclosure herein. Such variations or modifications are well within the spirit of this invention.

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 given for various physical parameters, dimensions, and quantities are only approximate values and it is envisaged that the values higher than the numerical value assigned to the physical parameters, dimensions and quantities fall within the scope of the invention unless there is a statement in the specification to the contrary. While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment 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.