Process for the preparation of halogenotetrafluoropropionic acid

The halogenotetrafluoropropionic acid 3-iodotetrafluoropropionic acid of the formula ICF.sub.2 CF.sub.2 COOH, it is prepared by reacting iodine, tetrafluoroethylene and ethylene in a one-pot reaction under specific conditions, reacting the resulting compound ICF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 I with a base in order to eliminate hydrogen iodide and oxidizing the compound ICF.sub.2 CF.sub.2 --CH.dbd.CH.sub.2 thus obtained to give 3-iodotetrafluoropropionic acid. The halogenotetrafluoropropionic acid is an advantageous starting compound for the preparation of valuable unsaturated compounds containing functional groups.

The invention relates to a new 3-halogenotetrafluoropropionic acid. It also 
relates to processes for the preparation of this propionic acid and to its 
use. 
3-Halogenotetrafluoropropionic acids, namely 3-bromotetrafluoropropionic 
and 3-chlorotetrafluoropropionic acid, and also pentafluoropropionic acid 
are already known (cf. Chemical Abstracts, volume 70, 1969, Abstract No. 
77,334 q). 
They are prepared by a thermal addition reaction, elimination of hydrogen 
halide acid from the addition compound and subsequent oxidation to give 
the corresponding propionic acid; the equations below, in which X 
represents bromine, are intended to illustrate this: 
##STR1## 
Since the bromine atom and, in particular, the chlorine and fluorine atom 
are relatively firmly attached and are therefore more or less slow to 
react, the known 3-halogenotetrafluoropropionic acids are largely 
unsuitable as starting materials for the preparation of compounds 
containing an advantageous functional group instead of the halogen atom. 
3-Iodotetrafluoropropionic acid, which would not exhibit the disadvantages 
mentioned, is not yet known. 
A process for the preparation of the compounds ICF.sub.2 CF.sub.2 CH.sub.2 
CH.sub.2 I and ICH.sub.2 CH.sub.2 CF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 I by 
a thermal addition reaction between 1,2-diiodotetrafluoroethane (ICF.sub.2 
CF.sub.2 I) and ethylene is disclosed in U.S. Pat. No. 3,016,407. In this 
reaction ICF.sub.2 CF.sub.2 I and CH.sub.2 =CH.sub.2 are heated under 
pressure at a temperature of 180.degree. to 220.degree. C., and the 
desired compound is isolated from the resulting reaction product. If the 
process is carried out discontinuously, it is recommended that ICF.sub.2 
CF.sub.2 I should first be prepared by reacting I.sub.2 and C.sub.2 
F.sub.4 at a temperature of about 150.degree. C., and the reaction of 
ICF.sub.2 CF.sub.2 I with ethylene should be carried out subsequently in a 
second reaction stage. It is emphasised expressly in the U.S. patent 
mentioned that the preparation of ICF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 I 
in a single-stage process, that is to say by simultaneous reaction of 
iodine, tetrafluoroethylene and ethylene, is not possible, because 
considerable amounts of unwanted 1,2-diiodoethane (ICH.sub.2 CH.sub.2 I) 
would then be obtained. 
It has now been found, surprisingly, that the compound ICF.sub.2 CF.sub.2 
CH.sub.2 CH.sub.2 I can be prepared in good yields in a one-pot process 
from iodine, tetrafluoroethylene and ethylene without the formation in 
appreciable amounts of the compound ICH.sub.2 CH.sub.2 I. It has also been 
found that the compound ICF.sub.2 CF.sub.2 --CH.dbd.CH.sub.2 can be 
obtained from the butane compound mentioned by eliminating hydriodic acid, 
and that the corresponding propionic acid can be obtained from the 
compound ICF.sub.2 CF.sub.2 --CH.dbd.CH.sub.2 by oxidation.

The invention therefore relates to 3-iodotetrafluoropropionic acid: 
ICF.sub.2 CF.sub.2 COOH. The process, according to the invention, for the 
preparation of this acid comprises reacting iodine, tetrafluoroethylene 
and ethylene in a molar ratio of 1:(1.2-2): (1-1.2) at a temperature of 
170.degree. to 200.degree. C. in a one-pot reaction, and isolating from 
the resulting reaction product the compound 
1,4-diiodo-1,1,2,2-tetrafluorobutane (ICF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 
I), reacting this butane compound with a base in order to eliminate 
hydrogen iodide (HI), and isolating the compound 
1-iodo-1,1,2,2-tetrafluoro-3-butene (ICF.sub.2 CF.sub.2 --CH.dbd.CH.sub.2) 
from this reaction product, reacting this butene compound with an 
oxidizing agent in order to convert it into the corresponding propionic 
acid, and isolating 3-iodo-2,2,3,3-tetrafluoropropionic acid (ICF.sub.2 
CF.sub.2 COOH) from this reaction product. The process according to the 
invention is in accord with the equations (1) to (3) below: 
##STR2## 
The reaction shown in equation (1) is possible and also results in a high 
yield of the desired compound if the conditions according to the 
invention, indicated above, are maintained. An excess of 
tetrafluoroethylene greater than the 1.2 to 2 mol indicated favors the 
formation of ICF.sub.2 CF.sub.2 I and ICH.sub.2 CH.sub.2 (CF.sub.2 
CF.sub.2).sub.n I, while an excess of ethylene greater than the 1.2 mol 
indicated favors the formation of ICH.sub.2 CH.sub.2 I and ICH.sub.2 
CH.sub.2 CF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 I (n=an integer&gt;1). The 
reaction time required at the reaction temperature of 170.degree. to 
200.degree. C. indicated is about 5 to 20 hours, depending on the 
temperature chosen, i.e. the reaction time for virtually complete 
conversion is about 5 hours if the reaction temperature is 200.degree. C. 
and is about 20 hours if the reaction temperature is 170.degree. C. 
Clearly, a pressure corresponding to the compounds present will prevail 
during the reaction according to the invention. At the start of the 
reaction it is about 40 to 70 bar and it decreases as the reaction 
proceeds. The desired compound is isolated from the reaction product 
obtained. This isolation is preferably effected by fractional distillation 
of the liquid reaction product. If the reaction conditions according to 
the invention are maintained, not only is a high yield of the desired 
compound (ICF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 I) obtained, but also only 
very few by-products, such as diiodoethane (ICH.sub.2 CH.sub.2 I), or none 
at all, are formed. 
In the reaction according to equation (2), hydrogen iodide is eliminated by 
means of a base. The nature of the base is not critical. Examples of 
suitable bases are alkali metal hydroxides, such as potassium hydroxide 
and sodium hydroxide, alkoxides, such as sodium methylate and potassium 
methylate, or tertiary amines. In a preferred embodiment of equation (2), 
the compound ICF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 I is reacted with at 
least a stoichiometric amount of potassium hydroxide or sodium hydroxide 
in the form of an approximately 10 to 60% strength by weight aqueous 
solution at a temperature of 20.degree. to 100.degree. C., preferably 
50.degree. to 90.degree. C. (the reaction can be followed by means of the 
amount of potassium iodide or sodium iodide, respectively, formed), and 
the desired compound (ICF.sub.2 CF.sub.2 --CH.dbd.CH.sub.2) is isolated 
from the reaction product by phase separation or fractional distillation. 
It is advantageous to employ an excess (in comparison with the 
stoichiometric amount) of potassium hydroxide or sodium hydroxide, 
specifically an excess of about 0.5 to 2 mol. It has been found that the 
reaction described, with alkali metal hydroxide solution, proceeds 
particularly advantageously and is accelerated if phase transfer 
catalysts, for example in the form of tetraalkylammonium salts, are 
present. Examples of suitable tetraalkylammonium salts are 
tetrabutylammonium bromide, tricetylmethylammonium chloride, 
dioctyldimethylammonium chloride and dodecyltrimethylammonium bisulfate. 
The amount of phase transfer catalyst is about 0.1 to 5 mol %, preferably 
0.5 to 2 mol %, relative to the amount of compound ICF.sub.2 CF.sub.2 
CH.sub.2 CH.sub.2 I employed. In the reaction described, with alkali metal 
hydroxide, it is preferable to add the phase transfer catalyst to the 
alkali metal hydroxide solution and to meter in the compound ICF.sub.2 
CF.sub.2 CH.sub.2 CH.sub.2 I to the initially taken alkali metal hydroxide 
solution containing the phase transfer catalyst, at the temperature 
indicated. 
The oxidation according to equation (3) is carried out by means of an 
oxidizing agent. The nature of the oxidizing agent is not critical. 
Examples of suitable oxidizing agents are potassium permanganate, chromium 
trioxide, chromic sulfuric acid, osmium tetroxide, ozone and peroxoacetic 
acid. In a preferred embodiment of the equation (3), the compound 
ICF.sub.2 CF.sub.2 --CH.dbd.CH.sub.2 is reacted with at least a 
stoichiometric amount of potassium permanganate in the form of an 
approximately 10 to 40% strength aqueous solution or suspension at a 
temperature of 5.degree. to 50.degree. C., preferably 10.degree. to 
25.degree. C. (the reaction can be followed by means of consumption of 
oxidizing agent), and the desired 3-iodotetrafluoropropionic acid is 
isolated from the reaction product by fractional distillation after 
removing the manganese dioxide formed and acidification. It is preferable 
to employ a slight excess (in comparison with the stoichiometric amount) 
of potassium permanganate, specifically an excess of about 0.01 to 0.1 
mol. The equation below is intended to illustrate the oxidation described 
with potassium permanganate: 
EQU 3 ICF.sub.2 CF.sub.2 --CH.dbd.CH.sub.2 +10 KMnO.sub.4 .fwdarw.3 ICF.sub.2 
CF.sub.2 --CO.sub.2 K +10 MnO.sub.2 +3 K.sub.2 CO.sub.3 +KOH+4 H.sub.2 O 
It has been found that the oxidation described, using an aqueous solution 
of potassium permanganate, affords particularly high yields of the desired 
propionic acid if phase transfer catalysts, for example in the form of 
quaternary ammonium or phosphonium salts, are present. The 
tetraalkylammonium salts indicated above and the corresponding phosphonium 
salts constitute suitable phase transfer catalysts in this case too. The 
amount of phase transfer catalyst is about 0.5 to 5 mol %, preferably 1 to 
3 mol %, relative to the amount of compound ICF.sub.2 CF.sub.2 
--CH.dbd.CH.sub.2 employed. In the reaction indicated above it is 
preferable to employ a procedure in which the aqueous potassium 
permanganate solution containing the added phase transfer catalyst is 
initially taken, and the compound ICF.sub.2 CF.sub.2 --CH.dbd.CH.sub.2 is 
metered in at the temperature indicated. The reaction rate can be 
controlled conveniently by means of the rate of metering at which the 
olefin is fed to the stirred mixture of potassium permanganate, water and 
phase transfer catalyst. The heat of reaction can readily be controlled by 
cooling. At the end of the reaction, the 3-iodotetrafluoropropionic acid 
is present in the form of its potassium salt, together with manganese 
dioxide. The potassium salt can be separated off from the manganese 
dioxide (MnO.sub.2) by hot filtration, and the free propionic acid can be 
isolated by evaporating the filtrate to dryness, adding sulfuric acid, 
preferably concentrated sulfuric acid, and distillation. 
3-Iodo-tetrafluoropropionic acid (ICF.sub.2 CF.sub.2 -COOH) is a colorless 
liquid which slowly turns pink in the light and crystallizes at room 
temperature. It has a boiling point of 78.degree. C. at 13 mbar. 
3-Iodotetrafluoropropionic acid is a valuable starting compound for the 
preparation of other .OMEGA.-iodo compounds into which functional groups 
can be introduced at the terminal iodine atom by known processes. Thus, 
the new 3-halogenotetrafluoropropionic acid is an advantageous starting 
compound for the preparation of unsaturated comonomers containing 
functional groups for the production of electrolysis membranes. Thus it is 
possible, for example, to obtain the compound FSO.sub.2 --(CF.sub.2).sub.3 
--O--CF.dbd.CF.sub.2 from ICF.sub.2 CF.sub.2 COOH by means of known 
methods. This compound, and its advantageous use for the preparation of 
polymers for the production of electrolysis membranes, is known. 
The invention will now be illustrated in greater detail by means of an 
example. The nuclear magnetic resonance data indicated in the example 
relate to deuterochloroform solutions (Standard: tetramethylsilane for 
.sup.1 H-NMR and trifluorofacetic acid for .sup.19 F-NMR. Signals having a 
positive sign are at a lower field, compared with the standard resonance). 
Preparation of 1,4-diiodo-1,1,2,2-tetrafluorobutane 
A 250 ml shaking autoclave is charged with 76.2 g (0.3 mol) of iodine, 
cooled to -76.degree. C. and freed from oxygen by being evacuated and 
filled with argon three times. After a further evacuation, 50 g (0.5 mol) 
of tetrafluoroethylene and 9.0 g (0.32 mol) of ethylene are condensed 
successively into the autoclave. The autoclave is then heated to 
180.degree. C. in the course of 5 hours and is shaken at this temperature 
for 15 hours. In the course of this, the pressure falls from 58 bar to 14 
bar. After cooling to room temperature and releasing the pressure, 121.3 g 
of a liquid product are obtained which, according to nuclear magnetic 
resonance analysis (.sup.19 F-NMR and .sup.1 H-NMR) has the following 
composition (in mol %): 
0.9% of ICH.sub.2 CH.sub.2 I 
6.5% of ICF.sub.2 CF.sub.2 I 
3.0% of ICF.sub.2 CF.sub.2 H 
80.1% of ICF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 I, 
4.3% of I(CF.sub.2 CF.sub.2).sub.2 CH.sub.2 CH.sub.2 I, 
5.2% of ICH.sub.2 CH.sub.2 CF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 I. 
Fractional distillation in vacuo of this mixture gives 88.5 g of ICF.sub.2 
CF.sub.2 CH.sub.2 CH.sub.2 I (liquid of boiling point 72.degree./15 mbar; 
turns violet in light). The yield is 77.3%, relative to the iodine 
employed. Nuclear magnetic resonance data: 
.sup.19 F-NMR: 18.8 ppm (CF.sub.2 I), -29.3 ppm (CF.sub.2). 
.sup.1 H-NMR: 3.24 ppm (CH.sub.2 I, triplet), 2.74 ppm (CH.sub.2, 
multiplet). 
Preparation of 1-iodo-1,1,2,2-tetrafluoro-3-butene 
200 g (5.0 mol) of NaOH, 1 liter of demineralized water and 7.5 g (0.025 
mol) of [(C.sub.8 H.sub.17).sub.2 N(CH.sub.3).sub.2 ].sup.+ Cl.sup.- as 
phase transfer catalyst are initially placed in a 2.5 liter glass flask 
equipped with a dropping funnel, a stirrer, a thermometer and an upright 
20 cm packed column. The mixture is heated to 90.degree. C., and 819 g 
(2.14 mol) of ICF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 I are added dropwise 
slowly, with stirring, to the heated mixture, the latter being kept at the 
boil. The take-off reflux ratio at the head of the column is adjusted to 
1:1 and a mixture of fluorine compounds and water is collected 
continuously in an ice-cooled receiver until no further organic 
constituents pass over (bottom temperature at the conclusion 105.degree. 
C.; reaction time: 2.5 hours). The distillate is extracted by shaking with 
water, and the organic phase is dried with sodium sulfate. 451 g of crude 
product are obtained, and fractional distillation of this gives 335 g 
(1.319 mol) of ICF.sub.2 CF.sub.2 --CH.dbd.CH.sub.2 (boiling point 
86.degree. to 88.degree. C./980 mbar) and 105 g (0.275 mol) of starting 
compound. The yield is 70.7%, relative to ICF.sub.2 CF.sub.2 CH.sub.2 
CH.sub.2 I which has reacted. Nuclear magnetic resonance data: 
.sup.19 F-NMR: 18.2 ppm (CF.sub.2 I), -30.0 ppm (CF.sub.2). 
.sup.1 H-NMR: 5.78-6.09 ppm (CH.dbd.CH.sub.2, multiplet). 
Preparation of 3-iodo-tetrafluoropropionic acid 
383.9 g (2.43 mol) of potassium permanganate and 1 liter of demineralized 
water are placed in a 2.5 liter glass flask equipped with dropping funnel, 
stirrer and thermometer, 5.0 g (0.016 mol) of [(C.sub.8 H.sub.17).sub.2 
N(CH.sub.3).sub.2 ].sup.+ Cl.sup.- are added, with stirring, as phase 
transfer catalyst, and the mixture is stirred for 10 minutes. 187 g 
(0.7365 mol) of ICF.sub.2 CF.sub.2 --CH.dbd.CH.sub.2 are then added 
dropwise in the course of 3 hours, with vigorous stirring and external 
cooling, the internal temperature being kept between 15.degree. and 
20.degree. C. The mixture is then stirred for a further 4 hours at room 
temperature and is heated to 70.degree. C., and the manganese dioxide 
formed is filtered off by means of a heatable pressure filter. The 
manganese dioxide is stirred with 1 liter of boiling water and is again 
filtered off. The combined filtrates are concentrated to dryness on a 
rotary evaporator under reduced pressure. The residue is transferred to a 
distillation apparatus, and 500 ml of concentrated sulfuric acid are added 
cautiously (initial foaming caused by evolution of CO.sub.2). Vacuum 
distillation gives 290.6 g (72.5% yield) of pure 
3-iodotetrafluoropropionic acid (boiling point 78.degree. C./13 mbar): a 
colorless liquid which slowly turns pink in light and crystallizes at room 
temperature. 
Elemental analysis: 
C: 13.1%, H: 0.5%, F: 27.7%, I: 46.8% 
(calculated: C: 13.25%, H: 0.37%, F: 27.95%, I: 46.66%). 
Nuclear magnetic resonance data: 
.sup.19 F-NMR: 18.0 ppm (CF.sub.2 I), -32.9 ppm (CF.sub.2) 
.sup.1 H-NMR: 12.5 ppm (CO.sub.2 H).