Field of the invention
The present invention provides a process for preparation of fluoro substituted benzenes.
Background of the invention
Fluorobenzenes, such as 1,2,3,5-tetrafluorobenzene is versatile and valuable intermediate for preparing other highly fluorinated aromatic compounds. One or more of the fluorine atoms in the molecule may be replaced by functional groups such as amino, hydroxyl, sulphydryl, alkoxy, etc. by nucleophilic attack using methods known to the art.
The GB Pat. Pub. No. 996498A discloses a process for the preparation of 1,2,3,5-tetrafluorobenzene, wherein 1,3-dibromotetrafluorobenzene is reduced to a mixture of 1,2,3,5-tetrafluorobenzene and l-chloro-2,3,4,6-tetrafluorobenzene by treatment with hydrogen over palladium on active carbon (Pd/C). Then, the products from the five runs are combined and distilled through a column to give 1 to 9 fractions and residue. One of the fraction is re-distillation to give 1,2,3,5-tetrafluorobenzene. The above said patent is silent about the yield and purity of 1,2,3,5-tetrafluorobenzene. Further, it is tedious to separate the desired product with outstanding yield from large fractions of compounds. Journal of the American Chemical Society, 1951, 73 (1), pp 152-153 discloses a process for the preparation of 1,2,3,5-tetrafluorobenzene by deamination of 3-nitro-4-amino-l,2,5-trifluorobenzene to 3-nitro-l,2,5-trifluorobenzene, followed by iron reduction and a subsequent diazotization-Schiemann transformation give 40% yield of 1,2,3,5-tetrafluorobenzene. All the above methods give 1,2,3,5-tetrafluorobenzene in about 50% yield. Since it is an important intermediate in polyfluoroaromatic chemistry, so simple synthesis for this compound with a high yield and purity is desirable. There is an urgent need in the art to develop a commercially viable process for preparation of tetrafluorobenzene.

Object of the invention
The main object of the present invention is to provide simple, commercially viable and industrially applicable processes for preparation of fluoro substituted benzenes. The process discloses selective preparation of fluoro substituted benzenes especially from respective nitrile compounds.
Summary of the invention
A first aspect of present invention provides a process for preparation of a compound of formula 1, comprising the steps of:
i) hydrolysing a compound of formula 4 to obtain a compound of
formula 3;
Formula 4 wherein X= fluoro and n=l-4, Y= chloro, bromo and p=l-4, provided
n+p=5
Formula 3 wherein X= fluoro and n=l-4, Y= chloro, bromo and p=l-4, provided
n+p=5
ii) decarboxylating the compound of formula 3 to obtain a compound of formula 2: and

Formula 2
wherein X= fluoro and n=l-4, Y= chloro, bromo and p=l-4,
provided n+p=5
iii) dehalogenating the compound of formula 2 to obtain compound of formula 1.
Formula 2 wherein X= fluoro
A second aspect of present invention provides a process for selective dehalogenation of a compound of formula 2 to obtain compound of formula 1.
Formula 2
wherein X= fluoro and n=l-4, Y= chloro, bromo and p=l-4,
provided n+p=5

Formula 1 wherein X= fluoro
Detailed description of the invention
In one embodiment, present invention provides a process for preparation of 1,2,3,5-tetrafluorobenzene comprising the step of: i) hydrolysing 3-chloro-2,4,5,6-tetrafluorobenzonitrile to obtain 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid;
ii) decarboxylating 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid to obtain 1-chloro-2,3,4,6-tetrafluorobenzene and
iii) dechlorinating l-chloro-2,3,4,6-tetrafluorobenzene to obtain 1,2,3,5-tetrafluorobenzene.
In another embodiment, present invention provides a process for preparation of l-chloro-2,3,4,6-tetrafluorobenzene comprising the step of: i) hydrolysing 3-chloro-2,4,5,6-tetrafluorobenzonitrile to obtain 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid and;
iii) decarboxylating 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid to obtain 1-chloro-2,3,4,6-tetrafluorobenzene.
In one embodiment, present invention provides a process for preparation of 1,2,3,5-tetrafluorobenzene comprising the step of: i) decarboxylating 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid to obtain 1-chloro-2,3,4,6-tetrafluorobenzene and
ii) dechlorinating l-chloro-2,3,4,6-tetrafluorobenzene to obtain 1,2,3,5-tetrafluorobenzene.
As used herein, fluorinating agent can be selected from metal fluorides of Group 1A &2A and C1-C4 alkyl quaternary ammonium fluorides. The metal fluoride can be selected from potassium fluoride (KF),

Cesium fluoride (CsF), sodium fluoride (NaF) and alkyl quaternary ammonium fluoride can be selected from tributylammonium fluoride, triethylammonium fluroride, butyldiethylammonium fluoride and their mixture thereof.
The step of fluorination is carried out at a temperature of about 150-
300°C in presence of polar aprotic organic solvents or non-polar organic
solvents. The polar aprotic organic solvent can be selected from sulfolane,
dimethylsulfoxide (DMSO), dimethylforamide (DMF),
tetramethylenesulfolane, benzonitrile, l,3-dimethyl-2-imidazolidinone and the like or mixture thereof.
The non-polar aprotic solvent can be selected from toluene, hexane, cyclohexane or the like.
In a particular embodiment of present invention, pentachlorobenzonitrile is fluorinated to 3,5- using potassium fluoride in sulfolane at a temperature of 200-220°C.
The step of hydrolysis as used herein is carried out in presence of mineral acids or metal hydroxides at 100-200°C in presence of water.
The hydrolysis reaction is carried out at a temperature in the range of 120-160°C or preferably in the range of 130-160°C or more preferably in the range of 140-150°C.
The mineral acid can be selected from sulphuric acid, hydrochloric acid, phosphoric acid or the like.
The metal hydroxides can be selected from sodium hydroxide, potassium hydroxide, lithium hydroxide, cesium hydroxide, calcium hydroxide and the like or mixture thereof.
In a particular embodiment of the present invention, 3-chloro-2,4,5,6-tetrafluorobenzonitrile is hydrolysed to 3-chloro-2,4,5,6-tetrafluorobenzoic acid using aqueous sulphuric acid at a temperature of about 100-150°C.
The step of decarboxylation is carried out using a base in presence of a catalyst at a temperature of about 150-250°C in a solvent.

The solvent can be selected from a group consisting of aprotic solvents and polar protic solvents.
The polar protic solvent can be selected from water, methanol, ethanol, propanol, isopropanol, and diethylene glycol.
The aprotic solvent can be selected from a group consisting of
sulfolane, dimethylformamide, diethylformamide, dimethylacetamide,
dimethylacetamide, N-methylpyrrolidone, N-vinylpyrrolidone,
hexamethylphosphoramide, acetonitrile, dimethylsulfoxide, propylene carbonate, l,3-Dimethyl-3,4,5,6-tetrahydro-2-pyrimidinone, toluene, dichloromethane, carbon tetrachloride, 1,2-dichloroethane, 1,1,2,2-tetrachloroethane, hexane, heptane, cyclohexane, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, diethyl ether, diisopropyl ether, tert-butyl methyl ether, tetrahydrofuran, dioxane, acetonitrile or mixtures thereof.
The base used in the decarboxylation can be selected from a group consisting of an inorganic or an organic base.
The inorganic base used in the present invention can be selected from the group consisting of alkali metal hydroxides, alkali earth hydroxides, metal carbonates and metal oxides.
The preferred inorganic base can be selected from the group consisting of lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium carbonate, potassium carbonate, sodium carbonate and ammonium carbonate or mixture thereof. The organic base can be selected from a group consisting of ammonia, Cl-C4 alkyl amine, alkanolamine, and mixture thereof.
The C1-C4 amine can be selected from tri-N-butylamine, tri-N-ethylamine, butyldiethyl-N-amine or the like.
The alkanolamine can be selected from diethanolamine, 2-amino-2-hydroxymethyl-1,3-propanediol or the like.
The catalyst can be selected from the group consisting of the catalyst can be selected from metal oxides or metal sulphates.

The metal oxides can be selected from a group consisting of calcium oxides (CaO), copper oxide (CuO), zinc oxide (ZnO).
The metal sulphates can be selected from the group consisting of sodium sulphate, potassium sulphate, ammonium sulphate, calcium sulphate and hydrates thereof or the like.
The step of decarboxylation is carried out at a temperature of about 150-250°C, preferably at 150°C.
The decarboxylation of 3-chloro-2,4,5,6-tetrafluorobenzoic acid is carried out using calcium hydroxide in presece of calcium sulphate in diethyleneglycol at a temperature in the range of 110-160°C or preferably in the range of 110-150°C or more preferably in the range of 120-140°C.
The step of dechlorination is carried out in presence of hydrogen source, a base and catalyst at 110-200°C in presence of polar solvent.
The hydrogen source can be selected from hydrogen gas, ammonium hydroxide, ammonium formate or the like.
The base for chlorination can be selected from a group consisting of organic base, organic alkoxide, metal hydroxides and the mixture thereof. In preferred embodiment, present invention provides a selective dechlorination of l,3-dichloro-2,4,6-trifluorobenzene to obtain 1,3,5-trifluorobenzene.
The organic bases can be selected from ammonia, ammonium hydroxide, ammonium acetate, trimethylamine, 2,2-bipyridine.
The metal hydroxides can be selected from sodium hydroxide, potassium hydroxide, calcium hydroxide or the like.
The catalyst used in the step of dechlorination can be selected from carbon based catalyst, metal, metal chlorides and the like or mixture thereof.
The metal can be selected from Zinc, copper, iron, palladium, platinum and metal chlorides can be selected nickel chloride (NiCb), Zinc chloride (ZnCb), copper chloride (CuCb), cuprous chloride (CuCl), silver chloride (AgCl), iron chloride(FeCb), palladium chloride (PdCb) and carbon based catalyst can be selected from Pd/C and the like or mixture thereof.

The step of dechlorination is carried out at a temperature of about 100-200°C, preferably at 140°C.
In particular embodiment l-chloro-2,3,4,6-tetraflurobenzene is dechlorinated with hydrogen in the presence of palladium on carbon and dipotassium hydrogen phosphate to give 1,2,3,5-tetrafluorobenzene.
The dechlorination is carried out in in the presence of nitrogen at a temperature in the range of 120-160°C or preferably in the range of 130-150°C.
The palladium on carbon is recovered after completion of the reaction and 1,2,3,5-tetrafluorobenzene is isolated having yield not less than 90% and purity greater than 98% by Gas chromatography.
In one embodiment, present invention provides 3-chloro-2,4,5,6-tetrafluorobenzoic acid, prepared by the process of the present invention, have yield of about 95% and purity greater than 98% by HPLC (High-performance liquid chromatography).
In one embodiment, 3-Chloro-2,4,5,6-tetrafluorobenzoic acid, prepared by the process of the present invention, have yield not less than 95% and purity greater than 98% by HPLC (High-performance liquid chromatography).
The present invention also provides a method for preparation of pentafluorobenzonitrile.
In one embodiment, present invention provides a process for preparation of 1,2,3,5-tetrafluorobenzene comprising the step of: i) reacting pentachlorobenzonitrile with fluorinating agent to obtain 3 -chloro-2,4,5,6-tetrafluorobenzonitrile;
ii) hydrolysing 3-chloro-2,4,5,6-tetrafluorobenzonitrile to obtain 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid;
iii) decarboxylating 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid to obtain 1-chloro-2,3,4,6-tetrafluorobenzene and
iv) dechlorinating l-chloro-2,3,4,6-tetrafluorobenzene to obtain 1,2,3,5-tetrafluorobenzene.

In another embodiment, present invention provides a process for preparation of l-chloro-2,3,4,6-tetrafluorobenzene comprising the steps of: i) reacting pentachlorobenzonitrile with fluorinating agent to obtain 3-chloro-2,4,5,6-tetrafluorobenzonitrile;
ii) hydrolysing 3-chloro-2,4,5,6-tetrafluorobenzonitrile to obtain 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid;
iii) decarboxylating 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid to obtain 1-chloro-2,3,4,6-tetrafluorobenzene.
In one embodiment, the present invention provides a process for preparation of 3-chloro-2,4,5,6-tetrafluorobenzoic acid comprises: i) reacting pentachlorobenzonitrile with fluorinating agent to obtain 3 -chloro-2,4,5,6-tetrafluorobenzonitrile and
ii) hydrolysing 3-chloro-2,4,5,6-tetrafluorobenzonitrile to obtain 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid.
Pentachlorobenzonitrile may be prepared either by using methods known in the art or it may be obtained from market.
The completion of the reaction may be monitored by any one of chromatographic techniques such as thin layer chromatography (TLC), high pressure liquid chromatography (HPLC), ultra-pressure liquid chromatography (UPLC), Gas chromatography (GC), liquid chromatography (LC) and alike.
Isolation and purification of the products after completion of the reaction is accomplished by well recognized procedures. The routine method consists quenching, agitating, filtering, removing the solvent and distillation.
Unless stated to the contrary, any of the words "comprising" and "comprises" mean "including without limitation" and shall not be construed to limit any general statement that it follows to the specific or similar items or matters immediately following it.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.

EXAMPLES
Example 1: Process for the preparation of 3-chloro-2,4,5,6-tetrafluorobenzonitrile
Sulfolane (2400 g), pentachlorobenzonitrile (600 g) and potassium fluoride (600 g) were added sequentially in reactor. The reaction mixture was heated gradually to 200-220°C for 9-10 hours. After completion of the reaction, reaction mass was cooled to room temperature and filtered. 3-chloro-2,4,5,6-tetrafluorobenzonitrile was recovered by fractional distillation under reduced pressure. Yield: 40% Purity: >98% (GC)
EXAMPLE 2: Process for the preparation of 3-chloro-2,4,5,6-tetrafluoro benzoic acid
An aqueous sulphuric acid (2280g, 72%) and 3-chloro-2,4,5,6-tetrafluoro benzonitrile (600g) were added sequentially in reactor to obtain a reaction mixture. The reaction mixture was heated to 150°C for 6 to 8 hours. After completion of the reaction, the reaction mixture was cooled to 70-80°C and water, (2400g) was added. After addition of water, reaction mixture was cooled to room temperature and filtered to obtain a crude solid. Crude solid was slurry washed with fresh water (1200g). The solid was filtered and dried to obtain 3-chloro-2,4,5,6-tetrafluorobenzoic acid. Yield: 96% Purity: >98% (HPLC)
EXAMPLE 3: Process for the preparation of l-chloro-2,3,4,6-tetrafluoro benzene
Diethylene glycol (610g), calcium hydroxide (41g) and calcium sulphate hemihydrate (69g) were added sequentially in a reactor to obtain a reaction

mixture. The reaction mixture was heated from 120-140°C. A solution of 3-chloro-2,4,5,6-tetrafluorobenzoic acid in diethylene glycol (1830g) was added to the reaction mixture. After complete addition of 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid, the resulting reaction mixture was stirred for 1-2 hours. l-chloro-2,3,4,6-tetrafluorobenzene was recovered by boil off. Yield: 90% Purity: >98% (GC)
EXAMPLE 4: Process for the preparation of 1,2,3,5-tetrafluorobenzene
l-chloro-2,3,4,6-tetrafluorobenzene (250g), water (1100 g), dipotassium hydrogen phosphate (355 g), and Pd/C (2.5g) were added in a reactor. Reactor was flushed first with nitrogen and then with hydrogen. The reaction mixture was heated to 140°C. Hydrogen was charged in reactor at 140°C continuously at 15 kg/cm2. Progress of the reaction was monitored by GC. After completion of the reaction, the reaction mixture was cooled to 10-15°C and excess hydrogen pressure was released. 1, 2, 3, 5-Tetrafluorobenzene was recovered from the reaction mass. The bottom mass containing Pd/C was filtered, washed with fresh water and recycled. Yield: 92% Purity: >98% (GC).
Example 5: Process for the preparation of 3,5-dichloro-2,4,6-trifluorobenzonitrile
Pentachlorobenzonitrile (600 g), dimethylformamide (2390g) were added in the reactor. Potassium fluoride (550g) was added sequentially in reactor. The reaction mixture was heated gradually to up to 220°C for 9-10 hours. After completion of the reaction, reaction mass was cooled to room temperature and filtered to obtain 3,5-dichloro-2,4,6-tetrafluorobenzonitrile. Product was recovered by fractional distillation under reduced pressure. Yield: 60% Purity: >98% (GC)

EXAMPLE 6: Process for the preparation of l,3-dichloro-2,4,6-trifluoro benzene
A mixture of calcium hydroxide and sodium carbonate (40g), diethylene glycol (610g), and calcium sulphate hemihydrate (66.6g) were added sequentially in a reactor to obtain a reaction mixture. The reaction mixture was heated from 120-140°C. A solution of 3,5-dichloro-2,4,6-trifluorobenzoic acid in diethylene glycol (1830g) was added to the reaction mixture. After complete addition of 3,5-dichloro-2, 4, 6-tetrafluorobenzoic acid, the resulting reaction mixture was stirred for 1-2 hours and 1,3-chloro-2,4,6-trifluoro benzene was recovered by boil off. Yield: 95% Purity: >98% (GC)
EXAMPLE 7: Process for the preparation of 1,2,3,5-tetrafluorobenzene
Water (lOOOg) and l-chloro-2,3,4,6-tetrafluorobenzene (250g) were added in a reactor, followed by addition of dipotassium hydrogen phosphate (340 g), and Pd/C (2.5g) were added in a reactor. Reactor was flushed first with nitrogen and then with hydrogen. The reaction mixture was heated to 140°C. Hydrogen was charged in reactor at 140°C continuously at 15 kg/cm2. Progress of the reaction was monitored by GC. After completion of the reaction, the reaction mixture was cooled to 10-15°C and excess hydrogen pressure was released. 1, 2, 3, 5-Tetrafluorobenzene was recovered from the reaction mass. The bottom mass containing Pd/C was filtered, washed with fresh water and recycled. Yield: 96% Purity: >98% (GC).
EXAMPLE 8: Process for the preparation of 3-chloro-2,4,5,6-tetrafluoro benzoic acid
An aqueous sulphuric acid (2280g, 75%) and 3-chloro-2,4,5,6-tetrafluoro benzonitrile (600g) were added sequentially in reactor to obtain a reaction mixture. The reaction mixture was heated to 150-160°C for 8 hours. After

completion of the reaction, the reaction mixture was cooled to 70-80°C and water, (2400g) was added. After addition of water, reaction mixture was cooled to room temperature and filtered to obtain a crude solid. Crude solid was slurry washed with fresh water (1200g). The solid was filtered and dried to obtain 3-chloro-2,4,5,6-tetrafluorobenzoic acid.
Yield: 97%
Purity: >98% (HPLC)
EXAMPLE 9: Process for the preparation of 3-chloro-2,4,5,6-tetrafluoro benzoic acid
An aqueous sulphuric acid (2010g, 50%) and 3-chloro-2,4,5,6-tetrafluoro benzonitrile (500g) were added sequentially in reactor to obtain a reaction mixture. The reaction mixture was heated to 150-160°C for 8 hours. After completion of the reaction, the reaction mixture was cooled to 70-80°C and water, (2400g) was added. After addition of water, reaction mixture was cooled to room temperature and filtered to obtain a crude solid. Crude solid was slurry washed with fresh water (1200g). The solid was filtered and dried to obtain 3-chloro-2,4,5,6-tetrafluorobenzoic acid. Yield: 93% Purity: >98% (HPLC)
EXAMPLE 10: Process for the preparation of l-chloro-2,3,4,6-tetrafluoro benzene
Isopropanol (500g), mixture of calcium hydroxide and cesium hydroxide (40g) and potassium sulphate and calcium sulfate hemihydrate (65g) were added sequentially in a reactor to obtain a reaction mixture. The reaction mixture was heated from 120-140°C. A solution of 3-chloro-2,4,5,6-tetrafluorobenzoic acid in diethylene glycol (1830g) was added to the reaction mixture. After complete addition of 3-chloro-2, 4, 5, 6-tetrafluorobenzoic acid, the resulting reaction mixture was stirred for 1-2 hours and l-chloro-2,3,4,6-tetrafluorobenzene was recovered by boil off.

Yield: 95%
Purity: >98% (GC)
Example 11: Process for the preparation of 3-chloro-2,4,5,6-
tetrafluorobenzonitrile
Pentachlorobenzonitrile (300 g), N-methylpyrrolidone (1210g) were added in the reactor. Potassium fluoride (310g) was added sequentially in reactor. The reaction mixture was heated gradually to 205-215°C up to 10 hours. As reaction completed, reaction mass was cooled to room temperature and filtered to collect 3-chloro-2,4,5,6-tetrafluorobenzonitrile. Product was recovered by fractional distillation under reduced pressure. Yield: 72% Purity: >97% (GC)
EXAMPLE 12: Process for the preparation of 1,3,5-tetrafluorobenzene
Water (550) and l,3-dichloro-2,4,6-tetrafluorobenzene (100g) were added in a reactor, followed by addition of dipotassium hydrogen phosphate (130 g), and Pd/C (0.52g) were added in a reactor. Reactor was flushed first with nitrogen and then with hydrogen. The reaction mixture was heated to 140°C. Hydrogen was charged in reactor at 140°C continuously at 15 kg/cm2. Progress of the reaction was monitored by GC. After completion of the reaction, the reaction mixture was cooled to 10-15°C and excess hydrogen pressure was released. 1, 3, 5-Trifluorobenzene was recovered from the reaction mass. The bottom mass containing Pd/C was filtered, washed with fresh water and recycled. Yield: 97% Purity: >99% (GC).
EXAMPLE 13: Process for the preparation of 1,3,5-tetrafluorobenzene
Water (250) and l-bromo-2,4,6-tetrafluorobenzene (50g) were added in a reactor, followed by addition of dipotassium hydrogen phosphate (70 g), and Pd/C (0.3g) were added in a reactor. Reactor was flushed first with nitrogen

and then with hydrogen. The reaction mixture was heated to 140°C. Hydrogen was charged in reactor at 140°C continuously at 15 kg/cm2. Progress of the reaction was monitored by GC. After completion of the reaction, the reaction mixture was cooled to 10-15°C and excess hydrogen pressure was released. 1, 3, 5-Trifluorobenzene was recovered from the reaction mass. The bottom mass containing Pd/C was filtered, washed with fresh water and recycled. Yield: 97% Purity: >99% (GC).
Embodiments of the invention are not mutually exclusive, but may be implemented in various combinations. The described embodiments of the invention and the disclosed examples are given for the purpose of illustration rather than limitation of the invention as set forth in the appended claims.

WE CLAIM:
1. A process for preparation of a compound of formula 1,
Formula 1
wherein X= fluoro and n=l-4 and p=l-4, provided n+p=5
comprising the steps of:
i) hydrolysing a compound of formula 4 to obtain a compound of
formula 3;
Formula 4 wherein X= fluoro and n=l-4, Y= chloro, bromo and p=l-4, provided n+p=5
Formula 3 wherein X= fluoro and n=l-4, Y= chloro, bromo and p=l-4, provided n+p=5 ii) decarboxylating the compound of formula 3 to obtain a compound of formula 2: and

Formula 2 wherein X= fluoro and n=l-4, Y= chloro, bromo and p=l-4, provided n+p=5 iii) dehalogenating the compound of formula 2 to obtain compound of formula 1.
2. The process as claimed in claim 1, wherein hydrolysis is carried out using mineral acid selected from a group consisting of sulphuric acid, hydrochloric acid, phosphoric acid.
3. The process as claimed in claim 1, wherein hydrolysis is carried out using base selected form a group consisting of sodium hydroxide, potassium hydroxide, lithium hydroxide, cesium hydroxide, calcium hydroxide and mixture therof
4. The process as claimed in claim 1, wherein decarboxylation is carried out using organic base selected from a group consisting of ammonia, C1-C4 alkylamine, alkanolamine and mixture thereof.
5. The process as claimed in claim 1, wherein decarboxylation is carried out using inorganic base can be selected from the group consisting of lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium carbonate, potassium carbonate, sodium carbonate and ammonium carbonate or mixture thereof.
6. The process as claimed in claim 1, wherein dechlorination is carried out using a catalyst selected from a group consisting of metal such as Zinc,

copper, iron, palladium, platinum and metal chlorides such as nickel chloride (NiCb), Zinc chloride (ZnCb), copper chloride (CuCb), cuprous chloride (CuCl), silver chloride (AgCl), iron chloride(FeCl3), palladium chloride (PdCb) and carbon based catalyst such as from Pd/C and the like or mixture thereof.
7. The process as claimed in claim 1, wherein dechlorination is carried out using a hydrogen source selected from a group consisting of hydrogen gas, ammonium hydroxide, ammonium formate or mixture therof
8. The process as claimed in claim 1, wherein dechlorination may be carried out using a base selected from a group consisting of ammonia, ammonium hydroxide, ammonium acetate, trimethylamine and 2,2'-bipyridine.
9. The process as claimed in claim 1, wherein dechlorination is carried out in presence of dipotassium hydrogen phosphate.
10. A second aspect of present invention provides a process for selective dehalogenation of a compound of formula 2 to obtain compound of formula 1.
Formula 2 wherein X= fluoro and n=l-4, Y= chloro, bromo and p=l-4, provided n+p=5

Formula 1 wherein X= fluoro