GB2165241A - Preparing vinylidene fluoride - Google Patents

Abstract

A method for the preparation of vinylidene fluoride comprises treating 1,2-dichloro-1,1-difluoroethane with an alkali metal amalgam in an active hydrogen containing liquid medium.

Description

SPECIFICATION Chemical process This invention relates to a chemical process and more particularly to a method for the preparation of vinylidene fluoride.

Vinylidene fluoride (1,1-difluoroethylene) is a valuable monomer for the preparation of homopolymers and copolymers which have good thermal stability and exhibit a high level of resistance to chemical attack. It is usually manufactured by the thermal dehydrochlorination of 1 -chloro-1, 1-difluoroethane, Other methods that have been proposed include the dehydrofluorination of 1,1,1-trifluoroethane and the copyrolysis of C compounds capable of generating :CF2 and :CH2 radicals.

It has also been proposed to make vinylidene fluoride by the dechlorination of 1 ,2-dichloro-1 ,1 difluoroethane. Thus, United States Patent 2734090 reports a 43% conversion of 1,2-dichloro-1,1- difluoroethane to vinylidene fluoride when the former is passed with hydrogen over nickel wire at 490-500"C. In the method of United States Patent 2774798, 1 ,2-dichloro-1 ,1 -difl uoroethane is treated in a sealed tube at 50"C with zinc in the presence of water containing a polyglycol ether stearate.

It has now been found that vinylidene fluoride may be produced in excellent yield from 1,2-dichloro-1,1difluoroethane by the action of an alkali metal amaigam under very mild conditions, for example at normal ambient temperatures or thereabouts.

Accordingly, the invention provides a method for the preparation of vinylidene fluoride which comprises treating 1,2-dichloro-1,1-difiucroethanewith an alkali metal amalgam in an active hydrogen containing liquid medium.

The alkali metal amalgam used may be, for example, sodium amalgam which may be prepared in conventional manner by the electrolysis of an aqueous solution of sodium hydroxide our a sodium salt using a mercury cathode or by dissolving sodium in mercury. Suitable amalgams can contain from about 0.3% to about 1% by weight of sodium.

The active hydrogen containing reaction medium is or includes a compound having one or more labile hydrogens. Suitable reaction media include alcohols, for example methanol, ethanol and propanols, and aqueous alcohols. Water may be employed as the reaction medium but the yield of vinylidene fluoride is much lower than when using an active hydrogen containing material that is miscible with the 1,2-dichloro-1,1-difiuornethane. When water is used as the active hydrogen containing reaction medium, the yield of vinylidene fluoride may be increased by using the water in conjunction with an aprotic solvent for the dichlorodifluoroethane, for example acetonitrile or dimethyl formamide, andlor a surface active agent.

The reaction is conveniently performed at temperatures in the range from -35OC to 80us, preferably in the range from 0 to 30"C. Since the reaction is exothermic, it will usually be necessary to cool the reaction mixture in order to maintain these temperatures. The pressure under which the reaction is performed is not important and, consequently, atmospheric pressure conditions will usually be employed.

The vinylidene fluoride may be separated from unchanged starting material and from the reaction by products by virtue of its lower boiling point using conventional distillation techniques.

The invention is illustrated but not limited by the following Examples.

EXAMPLE 1 A multi-necked 250 ml round-bottomed flask was fitted with an efficient magnetic stirrer bar, a compensating dropping funnel, a means of introducing liquids through a rubber septum and a Drikold-acetone condenser at -78CC. The condenser in turn was attached to two isolatable glass collection tubes in series; the first tube was kept in a petroleum ether (bp 40-60 C)-liquid nitrogen slush bath at -135 C and the second tube was immersed in liquid nitrogen at -196"C. Into the round-bottomed flask were placed methanol (20 mls, 0.4916 moles) and distilled water (8.9 mls, 0.4925 moles).The whole apparatus was purged with a slow stream of nitrogen. 1,2-Dichloro-l,l -difluoroethane (1.0 ml, 0.01049 moles) was injected into the flask through the rubber septum. The homogeneous aqueous alcohol and chlorofluorocarbon mixture was stirred efficiently and freshly prepared sodium amalgam (100.0g; 0.5W/w, 0.021744 mole Na) was added from the dropping funnel over 12 minutes. Following the addition the mixture was left stirring under a slow purge of nitrogen for 4 hours. The Drikoldacetone mixture was removed from the condenser and replaced by lukewarm water at 30 - 35"C. Warm water (ca 45"C, 150 mls) was injected into the flask through the septum to expel traces of products and unreacted dichlorodifluoroethane.

After 1 hour, the contents of the two cold traps were transferred by evaporation into an evacuated volumetrically calibrated 5 litre flask (5300 mls) which was then let down to atmospheric pressure with nitrogen. The contents of the flask were quantitatively analysed for residual starting material and products by gas chromatography. The identity of the products was confirmed by gas chromatographic mass spectrometry and in the cases of vinylidene fluoride, 1,2-dichloro-1,1-difluoroethane and 1-chloro-1,1difluoroethane, by comparison of retention times with authentic samples. The analytical results are reported in the Table. At the conclusion of the experiment, dilute hydrochloric acid was added to the reaction flask.

Gas was evolved from the surface of the mercury indicating, qualitatively at least, that not all of the sodium present in the amalgam had been reacted.

EXAMPLE2 The experimental apparatus, quantities and procedure were similar to those described in the previous example except that the solvent mixture comprised methanol (6.2 mls, 0.2465 moles) and distilled water (13.3 mls, 0.7389 moles) and the sodium amalgam (100.0g; 0.5WIw%, 0.02174 moles Na) was added over 5 minutes. At the conclusion of the experiment, the products and unreacted starting material were collected and quantitatively analysed. The data are recorded in the Table. When dilute hydrochloric acid was added to the reaction flask, gas was evolved from the surface of the mercury indicating qualitatively at least that not all the sodium present in the amalgam had reacted.

EXAMPLE 3 The experimental apparatus, quantities and procedure were similar to those described in Example 1 except that the solvent mixture comprised methanol (18.6 mIs, 0.4572 moles) and distilled water (4.4 mis, 0.2465 moles). At the conclusion of the experiment, the products and unreacted starting material were collected and quantitatively analysed. The data are recorded in the Table. An attempt was made to determine the approximate quantity of unreacted amalgam by addition of hydrochloric acid and titration with sodium hydroxide. Thus N-hydrochloric acid (25.0 mls) was added to the aqueous-methanol residues in the reaction flask; effervescence indicated that excess amalgam remained.When the reaction had ceased, a few drops of indicator (methyl red) were added and the acidic solution was titrated against N-sodium hydroxide solution (20.7 mls were required for ncutralisation). From this operation, and a similar titration of the amalgam that had remained stuck to the glass walls of the addition funnel and knowing the original quantity of sodium originally added to the mercury, it was possible to calculate an approximate figure for the utilisation of sodium in the reaction; this figure is included in the Table.

EXAMPLE 4 The experimental apparatus, quantities and procedure were similar to those described in Example 1 except that the solvent was anhydrous methanol alone (40.0 mls, 0.9833 moles) and the sodium amalgam (100.0g; 0.5g, 0.02174 moles Na) was added over 8 minutes. After ca 30 minutes of the stirring period, sodium chloride was seen to be deposited. At the conclusion of the reaction the products and unreacted starting material were collected and analysed quantitively. The data are reported in the Table.

EXAMPLES The experimental apparatus, quantities and procedure were similar to those described in Example 1 except that the solvent was distilled water (17.7 mls, 0.98833 moles) and the sodium amalgam (100.0g; 0.5g, 0.5W w% Na) was added over 10 minutes to the well stirred two-phase water-chlorofluorocarbon mixture. At the conclusion of the reaction, the products and unreacted starting material were collected and analysed quantitatively. In addition, the sodium utilisation was determined by the acidification and titration procedure outlined in Example 3. The data are included in the Table.

EXAMPLE 6 This experiment was a repeat of Example 3 except that the volume of the aqueous methanol solvent was halved and the amalgam was added over 7 minutes. The data are reported in the Table.

EXAMPLE 7 The experimental apparatus, quantities and procedure were identical to those described in Example 3 except that the reaction vessel was placed in a thermostatted oil bath at 60t3"C. At the conclusion of the reaction the products and starting material were collected and analysed quantitatively. In addition the sodium utilisation was determined by the acidification and titration procedure. The data are included in the Table.

EXAMPLE8 The experimental apparatus, quantities and procedure were identical to those described in Example 5 except that the reactior flask was placed in a thermostatted oil bath at 60 t2C. At the conclusion of the reaction, the products and starting material were collected and analysed quantitatively. In addition, the sodium utilisation was determined by the acidification and titration procedure. The data are included in the Table.

EXAMPLE 9 The experimental apparatus, quantities and procedure were identical to those described in Example 3 except that the reaction flask was placed in an ammonium chloride-ice slush bath at - 1 0. At the conclusion of the reaction, the products and starting material were collected and analysed quantitatively. In addition, the sodium utilisation was determined by the acidification and titration procedure. The data are included in the Table.

ANALYTICAL PROCEDURE Unreacted starting material and product fluorochlorocarbons were quantitatively analysed by the procedure outlined below. The condensed materials collected in a petroleum ether-liquid nitrogen slush bath and a liquid nitrogen bath were evaporated into an evacuated ( < 0.5mm Hg) 5-litre 3-necked flask fitted with a tap and a rubber septur7. The flask had been volumetrically calibrated. Following evaporation and transfer of the cold trapped pr zducts, the flask was made up to atmospheric pressure with nitrogen.Three samples (2.0 mls) of the contents of the flask were taken in turn in a gas-tight syringe and injected onto a glass column (13' X 1/4") packed consecutively with approximately equal volumes of Porapak O and Porapak Tat 160 C. The identities of the components were confirmed by gas chromatographic mass spectrometry and comparison of retention times with authentic samples. Quantitative assessments of vinylidene fluoride and 1,2-dichloro-1,1-difluoroethane contents were made by previously calibrating the column with various gas concentrations. In the case of vinylidene fluoride, non linearity of the FID towards the fluoro olefin was compensated for by the use of a polynomial expression calibration curve.For quantitive assessment of the minor components 1-chloro-2,2-difluoroethane, 1-chloro-1 1-difluoroethane and the C2HXFyCI olefins it was assumed that the chloro-difluc roethanes had similar flame characteristics to 1,2-dichloro-1,1 -difluoroethane and that the C2H,FyCl olefins h 3d similar flame characteristics to vinylidene fluoride.

TABLE Example Solvent Composition Temp Initial CClF2CH2Cl NaHg Convn Product yields Product selectivities (Molar Ratios) ( C) CClF2OH2Cl CONVN (%) (m Moles) (%) CONON A B C D A B C D (mMoles) 1 MeOH : H2O (50:50) 20 10.49 83.7 81.8 0.00 0.00 0.00 0.00 95.2 1.1 0.0 0.1 2 MeOH : H2O (25:75) 20 10.49 68.2 65.9 6.77 0.29 0.05 0.05 94.6 4.0 0.6 0.7 3 MeOH : H2O (65:35) 20 10.49 87.2 82.9 8.59 0.51 0.02 0.02 93.9 5.6 0.2 0.3 4 MeOH (100) 20 10.49 72.1 69.8 7.30 0.26 0 0 96.5 3.5 0 0 5 H2O (100) 20 10.49 17.2 22.1 1.64 0.15 0.01 0 91.1 8.1 0.8 0 6 MeOH : H2O (65:35) 20 10.49 78.1 75.5 7.80 0.35 0.02 0.03 95.1 4.2 0.3 0.4 7 MeOH : H2O (65.35) 60 10.49 83.9 97.9 7.82 0.93 0.03 0.03 88.7 10.5 0.3 0.3 8 H2O (100) 60 10.49 19.2 33.3 1.77 0.22 0.22 0 88.2 10.9 0.9 0 9 MeOH : H2O (65:35) -10 10.49 48.6 51.6 4.91 0.18 0.02 0 96.3 3.4 0.3 0 The volume of solvents in Example 6 was half that in Example 3.

PRODUCT KEY A = CH2 = CF2 B = CH2 x FyCl, an inseparable mixture of CFCl = CH2 and/or CFH = CClH with CF2 - CClH C = CClF2CH3 D = CHF2CH2Cl

Claims (6)

1. A method for the preparation of vinylidene fluoride which comprises treating 1,2-dichloro-1 ,1 - difluoroethane with an alkali metal amalgam in an active hydrogen containing liquid medium.

2. A method according to Claim 1 wherein the alkali metal amalgam is sodium amalgam.

3. A method according to Claim 1 or Claim 2 wherein the active hydrogen containing liquid medium is an alcohol.

4. A method according to Claim 3 wherein the active hydrogen containing liquid mediurn is an aqueous alcohol.

5. A method according to any one of the preceding claims wherein the dichlorodifluoroethane is treated with the amalgam at a temperature in the range from -35' to 80"C.

6. A method according to Claim 5 wherein the treatment is effected at a temperature in the range from 0 to 30"C.