Field of the invention
The present invention provides a process for preparing fluoropropenes of the formula CF3CY-CXaHb, wherein X and Y are independently hydrogen or a halogen selected from fluorine, chlorine, and bromine; and a and b are independently integers equal to 0, 1 or 2, provided that (a+b)=2, by dehydrohalogenating halopropane of formula CF3C(YR1)C(XaHbR2) wherein R1, R2, X and Y are independently hydrogen or a halogen selected from the group consisting of fluorine, chlorine, bromine and iodine, provided one of R1, R2 is hydrogen and other is halogen.

Background of the invention
Fluoropropenes, being environmentally benign are developed to replace chlorofluorocarbons for use in the industry as refrigerants, solvents, cleaning agents, foam expansion agents, aerosol propellants, heat transfer media, dielectrics, fire extinguishing agents, sterilants and power cycle working fluids.
Most of the processes known in literature for the preparation of fluoropropene involve dehydrohalogenation either using alkali or a catalyst.
U.S. Patent No. 7,230,146 provides a process for producing fluoropropenes by dehydrohalogenating the corresponding fluoropropane by either thermal dehydrohalogenation or using concentrated solution of sodium hydroxide. Thermal dehydrohalogenation and the concentrated solution of alkali hydroxide results in the dehydrohalogenation of the product resulting in the formation of impurities, thereby demanding multiple distillation and purification.
U.S. Patent No. 8,084,653 discloses a process for preparation of fluoropropenes via the step of catalytic dehydrohalogenation at high temperature, preferably above 150ºC. The use of catalyst increase the cost of running the process at commercial scale.
Thus there remains a need to develop an economic, selective, safe and robust process for preparation of fluoropropenes using mild reagents.
Object of the invention
The present invention provides a simple, cost effective and alternative process for preparing fluoropropenes by dehydrohalogenating corresponding halopropane using metal acetate.

Summary of the invention
The first aspect of present invention provides a process for preparation of fluoropropenes of formula I
CF3CY-CXaHb,
Formula I
wherein X and Y are independently hydrogen or a halogen selected from fluorine, chlorine, and bromine; and a and b are independently integers equal to 0, 1 or 2, provided that (a+b)=2,
comprising the step of dehydrohalogenating a halopropane of formula II,
CF3C(YR1)C(XaHbR2)
Formula II
wherein R1, R2, X and Y are independently hydrogen or a halogen selected from the group consisting of fluorine, chlorine, bromine and iodine, provided one of R1, R2 is hydrogen and another is halogen, using metal acetate.

Detailed description of the invention
As used herein, “dehydrohalogenation” refers to a chemical reaction that involve elimination of hydrogen halide from the substrate.
As used herein, the term about refers to 10% variation in both sides from specified value.
As used herein, “metal acetate” refers to alkali metal acetate selected from a group consisting of potassium acetate, sodium acetate and lithium acetate or a mixture thereof.
The dehydrohalogenation process is carried out at a molar ratio of metal acetate relative to the amount of compound of formula II from about 1:1 to about 20:1, preferably from about 1:1 to about 15:1; and more preferably from about 1:1 to about 10:1.
In an embodiment of the present invention, the strength of metal acetate is from about 2 wt % to about 100 wt %, more preferably from about 5 wt % to about 90 wt % and most preferably from about 10 wt % to about 80 wt %. The reaction is preferably conducted at a temperature of about 80°C to about 120°C, more preferably from about 90°C to about 120°C and most preferably from about 95°C to about 105°C. The reaction pressure is not critical. The reaction can be conducted at atmospheric pressure, super-atmospheric pressure. Preferably, the reaction is conducted at atmospheric or super-atmospheric pressure.
As used herein, “purity” refers to the analysis performed on gas chromatography or GC purity.
The unreacted halopropane can be recycled back to the reaction vessel to provide a continuous process. Alternatively, fresh halopropane may be supplied to the reaction mixture in order to run the process continuously.
In an embodiment of the present invention, the dehydrohalogenation is carried out in a polar aprotic solvent selected from the group consisting of nitriles, amides, sulfoxides, phosphates, and combinations of two or more of these.
Polar aprotic solvent is selected from acetonitrile, dimethylacetamide, dimethylformamide, dimethylsulfoxide, hexamethyl phosphoramide or the like.
In an embodiment, the present invention provides a process for preparation of 1,1,1,2-tetrafluoropropene, comprising the step of dehydrohalogenating either 1,1,1,2-tetrafluoro-3-chloropropane or 1,1,1,2-tetrafluoro-3-bromopropane using metal acetate in a polar aprotic solvent.
In an embodiment, the present invention provides a process for preparation of 1,1,1,2-tetrafluoropropene comprising the step of dehydrohalogenating 1,1,1,2,3-pentafluoropropane using metal acetate in a polar aprotic solvent.
The dehydrohalogenation reaction is completed in time period selected from 5 to 15 hours, preferably from 8 to10 hours.
In a particular embodiment, dehyrohalogenation of compound of formula II is carried out using alkali metal acetate such as potassium acetate or sodium acetate at 80 to 120°C.
In an embodiment, the compound of formula I is obtained with a purity greater than 97%. In another embodiment, the compound of formula I is obtained with a purity greater than 97%. In an embodiment, the compound of formula I is obtained with a yield greater than 72%. In a particular embodiment, the compound of formula I is obtained with a yield greater than 73% and purity greater than 97%.
The compound of Formula I is isolated by any method known in the art, for example, chemical separation, extraction and distillation or a mixture thereof.
Preferably, the compound of formula 1 obtained, has purity greater than 97%.
The compound of Formula II used a raw material in present invention can be prepared by various methods known in the art or can be obtained commercially.
The preferred method for analysis for present invention is gas chromatography. The halopropanes used in the process of the present invention are either prepared following the methods known in the literature or can be procured from the commercial sources. Completion of the reaction was monitored by gas chromatography technique. The reagents used in the above process are obtained commercially.
Unless stated to the contrary, any of the words “comprising”, “comprises” and includes mean “including without limitation” and shall not be constructed to limit any general statement that it follows to the specific or similar items or matters immediately following it.
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.
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 Preparation of 1,1,1,2-tetrafluoropropene using potassium acetate and N,N-dimethylformamide
3-chloro-1,1,1,2-tetrafluoropropane (75g), potassium acetate (100g) and dimethylformamide (100ml) were charged in an autoclave. The reaction mixture was heated to 100°C with constant stirring. After 8 hours, gaseous product was vented and collected at -50°C. The obtained 1,1,1,2-tetrafluoropropene gas was analyzed by GC.
Yield: 73%, Purity by GC: 97%
Example: 2 Preparation of 1,1,1,2-tetrafluoropropene using sodium acetate
3-chloro-1,1,1,2-tetrafluoropropane (75g), sodium acetate (100g) and N, N-dimethylformamide (100ml) were charged in an autoclave. The reaction mixture was heated to 100°C with constant stirring. After 8 hours, gaseous product was vented and collected at -50°C. The obtained 1,1,1,2-tetrafluoropropene gas was analyzed by GC.
Yield: 75%, Purity by GC: 98%
Example: 3 Preparation of 1,1,1,2-tetrafluoropropene using potassium acetate and dimethylsulfoxide
3-chloro-1,1,1,2-tetrafluoropropane (75g), potassium acetate (100g) and dimethylsulfoxide (100ml) were charged in an autoclave. The reaction mixture was heated to 100°C with constant stirring. After 8 hours, gaseous product was vented and collected at -50°C. The obtained 1,1,1,2-tetrafluoropropene gas was analyzed by GC.
Yield: 74%
Purity by GC: 97.8%

WE CLAIM:
1. A process for preparation of fluoropropenes of formula I
CF3CY-CXaHb,
Formula I
wherein X and Y are independently hydrogen or a halogen selected from fluorine, chlorine, and bromine; and a and b are independently integers equal to 0, 1 or 2, provided that (a+b)=2,
comprising the step of dehydrohalogenating a halopropane of formula II,
CF3C(YR1)C(XaHbR2)
Formula II
wherein R1, R2, X and Y are independently hydrogen or a halogen selected from the group consisting of fluorine, chlorine, bromine and iodine, provided one of R1, R2 is hydrogen and another is halogen, using metal acetate.

2. The process as claimed in claim 1, wherein metal acetate is selected from a group consisting of potassium acetate, sodium acetate and lithium acetate or a mixture thereof.

3. The process as claimed in claim 1, wherein the dehydrohalogenation is carried out in a polar aprotic solvent selected from a group consisting of acetonitrile, N, N-dimethylacetamide, N,N-dimethylformamide, phosphoramide and dimethylsulfoxide or a mixture thereof.

4. The process as claimed in claim 1, wherein dehydrohalogenation is carried out at a temperature ranging from 80°C to 120°C.

5. The process as claimed in claim 1, wherein the compound of formula 1 is 1,1,1,2-tetrafluoropropene and compound of formula II is 3-chloro-1,1,1,2-tetrafluoropropane, 1,1,1,2,3-pentafluoropropane, or 3-bromo-1,1,1,2-tetrafluoropropane.
6. The process as claimed in claim 1, wherein, the preparation of fluoropropene of formula I from halopropane of formula II is a continuous process.

7. The process as claimed in claim 1, wherein the compound of formula I is obtained with a purity of 97 to 100%.

8. The process as claimed in claim 1, wherein the compound of formula I is obtained with a yield of 72 to 85%.