Field of the invention
The present invention relates to an improved process for preparation of trifluoromethyl benzenes.

Background of the invention
Trifluoromethyl benzenes are very useful intermediates in the preparation of 2,6-bis(trifluoromethyl)benzoic acid which has been used in selective separation of brominated aromatic compounds using the polymer-based separation media by the fragment imprinting technique. Trifluoromethyl benzenes are also useful intermediates in the preparation of dyes.
The U.S. Patent No. 3,742,074 discloses a process for preparation of
1,3-bis(trifluoromethyl)benzene by reacting 1, 3-bis(trichloromethyl)benzene with hydrogen fluoride in the presence of a solution of hexamethylenetetramine /glacial acetic acid (0.3:1). The reaction is performed at 40 atmosphere gage pressure in an autoclave at 125°C in 48 hours. Liberated hydrogen chloride gas is allowed to escape. The crude product i.e., 1,3-bis(trifluoromethyl)benzene is then purified by steam distillation.
The above process suffers several disadvantages, for example, use of catalyst makes the process expensive and high reaction temperature results in several undesired side products, which needs extra purification steps.
The US Patent No. 3,966,832 discloses a process for preparation of benzotrifluoride by charging 200 litre of liquid hydrogen fluoride in an autoclave and then feeding 500 kg of benzotrichloride and 220 kg of hydrofluoric acid at 120°C. The hydrogen chloride gas formed in the reaction is removed. The product is recovered by distillation.
The above process also suffers several disadvantages, for example, high reaction temperature makes the process unsafe at industrial scale and also results in several undesired side products which are difficult to remove.
Thus there is a need to develop a cost effective, simple and safe process for the formation of trifluoromethyl substituted benzene derivatives.

Object of the invention
The present invention provides a simple, cost effective and industrially safe process for preparation of trifluoromethyl substituted benzene derivatives.

Summary of the invention
The present invention provides a process for preparation of a compound of formula 1,

comprising the steps of:
a) reacting a compound of formula 2),

with hydrogen fluoride at a temperature below 50°C to obtain a reaction mixture;
b) adding aqueous basic solution and organic solvent to the reaction mixture of step a);
c) isolating the compound of formula 1), from step b).

Detailed description of the invention
In an embodiment, the present invention provides a process for preparation of a compound of formula 1a,

comprising the steps of:
a) reacting a compound of formula 2a),

with hydrogen fluoride at a temperature below 50°C to obtain a reaction mixture;
b) adding aqueous basic solution and organic solvent to the reaction mixture of step a);
c) isolating the compound of formula 1a), from step b).

The compound of formula-2 may be prepared by any method known in the prior art or may be obtained commercially.
The fluorination reaction of the step a) is carried out at a temperature below 50°C, preferably below 40°C, more preferably in the range of 25°C to 40°C within a time period of 24 hours.
The step a) is carried out at a pressure below 30 kg/cm2, more preferably below 25 kg/cm2.

The aqueous basic solution used in the step b) consists of water and inorganic base selected from the group consisting of alkali hydroxides, alkali earth hydroxides, alkali carbonates, alkali earth carbonates, alkali hydrogen carbonates, alkali earth hydrogen carbonates and ammonium hydroxides or mixture(s) thereof.
The preferred inorganic base used in the step b) is selected from the group consisting of sodium carbonate, potassium carbonate, lithium carbonate, lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide, lithium hydrogen carbonate, sodium hydrogen carbonate and potassium hydrogen carbonate or mixture thereof.
The organic solvent used in the step b) is selected from a group consisting of dichloromethane, di-isopropyl ether, chloroform, ethyl acetate, carbon tetrachloride, tetrahydrofuran, methyl tert- butyl ether, 1,2-dichloroethane,
butyl acetate, C5-C12 aliphatic and aromatic hydrocarbons, preferably heptane, hexane, pentane, xylene, toluene, benzene, fluoro benzene, cyclohexane, and chlorobenzene or the mixture thereof.
In a preferred embodiment, 1,3-bis(trichloromethyl) benzene was charged in an autoclave. The system was then cooled to 0°C. The hydrogen fluoride gas was passed into the autoclave and the reaction mixture was brought up to 40°C and maintained at 25-40°C and pressure up to 25 kg/cm2 for 24 hours.
The progress of the reaction was monitored by gas chromatography. The excess of hydrogen fluoride gas was vented out after the reaction completion and the crude reaction mass was neutralized with aqueous sodium hydroxide solution and the product was extracted using dichloromethane as a solvent. The title compound i.e., 1,3-bis(trifluoromethyl)benzene was then obtained using distillation at 130 to 140°C.
The compound of formula-1 is isolated by using techniques known in the art for example distillation, evaporation, column chromatography and layer separation or combination thereof.
The compound of formula-1 so obtained by the present invention has a purity greater than 95 %, more preferably greater than 98 %, most preferably greater than 99% by gas chromatography.

While the present invention has been described in terms of its specific embodiments, certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present invention.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.

EXAMPLE
Preparation of 1, 3-bis(trifluoromethyl)benzene
1,3-bis(trichloromethyl) benzene (200g) was charged in an autoclave. The system was then cooled to 0°C. The hydrogen fluoride gas (307g) was passed into the autoclave and the reaction mixture was heated to 40°C and maintained at 25-40°C and pressure up to 25 kg/cm2 for 24 hours. The reaction progress was monitored by gas chromatography. The excess of hydrogen fluoride gas was vented out after the reaction completion and the crude reaction mass was unloaded. The unloaded mass was diluted with 1 litre dichloromethane (DCM) and it was washed with diluted (1%) NaOH solution and layers were separated. The organic layer was taken for simple distillation to remove DCM followed by title compound i.e., 1,3-bis(trifluoromethyl)benzene was obtained using distillation at 110 to 116°C without using distillation column.
Yield: 75%
Purity: 99.10% (by gas chromatography).

WE CLAIM:
1. A process for preparation of a compound of formula 1,

comprising the steps of:
a) reacting a compound of formula 2,

with hydrogen fluoride at a temperature below 50°C to obtain a reaction mixture;
b) adding aqueous basic solution and organic solvent to the reaction mixture of step a);
c) isolating the compound of formula 1, from step b).
2. The process as claimed in claim 1, wherein, the step a) is carried out at a temperature below 50°C, preferably below 40°C, most preferably in the range of 25°C to 40°C.
3. The process as claimed in claim 1, wherein, the step a) is carried out at a pressure below 30 kg/cm2.
4. The process as claimed in claim 1, wherein, the aqueous basic solution consists of water and inorganic base selected from the group consisting of alkali hydroxides, alkali earth hydroxides, alkali carbonates, alkali earth carbonates, alkali hydrogen carbonates, alkali earth hydrogen carbonates and ammonium hydroxides or a mixture thereof.
5. The process as claimed in claim 4, wherein, the preferred inorganic base is selected from a group consisting of sodium carbonate, potassium carbonate, lithium carbonate, lithium hydroxide, sodium hydroxide, potassium hydroxide, ammonium hydroxide, lithium hydrogen carbonate, sodium hydrogen carbonate and potassium hydrogen carbonate or a mixture thereof.
6. The process as claimed in claim 1, wherein, the organic solvent used is selected from a group consisting of dichloromethane, di-isopropyl ether, chloroform, ethyl acetate, carbon tetrachloride, tetrahydrofuran, methyl tert- butyl ether, 1,2-dichloroethane, butyl acetate, C5-C12 aliphatic and aromatic hydrocarbons, preferably heptane, hexane, pentane, xylene, toluene, benzene, fluoro benzene, cyclohexane, and chlorobenzene or a mixture thereof.