PYROLYSES OF THE SALTS OF PERFLUORO CARBOXYLIC ACIDS
Sept. 20, 1953
to be favorable circumstances may reflect the requirement t h a t the transition state in the formation of the salt be solvated to reduce its energy t o an accessible value. The solvation weakens the electrostatic interactions which provide chemical inertia to the formation of the salt. And though the reverse reaction, the decomposition of the salt, derives a driving-force from the electrostatic interactions under discussion, this is diminished because the ions must be solvated to remain in solution, a
[CONTRIBUTION No. 64
FROM THE
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condition which applies to the transition state though to a lesser extent. I n this way the electrostatic effect is so weakened as t o leave the nonelectrostatic factors dominant. Achowledgment.-This work was supported in part by grant from the Research Corporation, for which we wish to express our sincere appreciation, CHICAGO, ILLINOIS
CENTRAL RESEARCH DEPARTMENT OF MINNESOTA MININGAND MANUFACTURING COMPANY ]
Pyrolyses of the Salts of the Perfluoro Carboxylic Acids1 BY J. D. LAZERTE,L. J. HALS,T. S. REIDAND G. H. SMITH RECEIVEDJANUARY 14, 1953 The thermal decomposition of a number of salts of the straight chain peduoro acids has been investigated. From the sodium salts, terminally unsaturated perfluoro olefins were prepared in yields ranging from 65 to 100%. The reaction is represented by the equation C , F ~ , + I C F ~ C F ~ C O ~N ~ + CnFtn+lCF=CF2 COZ NaF. Salts of other metals occurring in groups I, I1 and I11 of the periodic table, gave varying yields of olefins. Silver peduorobutyrate and silver perfluorocaprylate decomposed to give CaFlr and ClaF30, respectiyely. Tetrafluoroethylene was formed when a mixture of CF3COzNa and NaOH was heated. A series of fluorocarbon hydrides, CnFzn+lHwas prepared by heating salts of the perfluoro acids in ethylene glycol.
+
+
While the literature contains references to the The yields of perfluoro olefin resulting from the decomposition of salts of trifluoroacetic acid, no pyrolytic reaction varied widely with the cation data had been reported on the pyrolyses of salts of present in the salt. The sodium salts consistently the longer chain perfluoro acids until recently.2 gave the highest yields of terminally unsaturated Experimentation with other salts and other reaction olefins. The thermal decomposition of the sodium conditions warrant further discussion of these pyro- salts of the straight chain acids began a t about 230' lytic reactions a t this time. with the rate of decomposition increasing rapidly The formation of an olefin by the decomposition as the temperature was raised. At 250-260" a of a salt was first reported by Pagenstecher3 who rapid, yet controllable, evolution of perfluoro found that when an aqueous solution of sodium olefin and C02 was obtained. The products from a-methyl-P-iodobutyrate was warmed, butene-2 many of the reactions were pure enough after was formed. Wislicenus4 observed that a hot treatment with aqueous base to make fractionation aqueous solution of sodium 2,3-dichlorobutyrate unnecessary. The yields, some of the physical decomposed to give 1-chloropropene. Later Boe- properties, and the analytical data obtained for the seken6 found that sodium perchloropropionate terminally unsaturated perfluoro olefins prepared underwent a similar type of reaction a t 25' to give from the sodium salts are shown in Table I. Identetrachloroethylene. tification of these unsaturates was based on molecWhile the salts of the straight chain perfluor0 ular data, oxidation to perfluoro acids,' elemental acids do not decompose under such mild conditions analyses and infrared absorption spectra of the to give peduoro olefins, i t has been observed in this compounds. Each member of the series having Laboratory that when certain of the dry salts three or more carbon atoms had an infrared absorpcontaining three or more carbon atoms were heated, tion peak a t 1795 cm.-I, which is characteristic of the following reaction takes place the double bond in CnF2n+1CF=CF2.7p8 The products resulting from the decomposition R/CF2CFoCO*M +R/CF=CF2 COS MF of other salts of perfluoro acids are shown in Table By means of this reaction, excellent yields of per- 11. fluoro olefins were obtained. Since many of the Pyrolyses of potassium salts gave good yields of perfluoro acids are now available from the electro- perfluoro olefins. Potassium perfluorobutyrate dechemical fluorination6 of the parent hydrocarbon acids, a simple synthesis of these olefinic compounds composed t o give a 98% yield of perfluoropropene. Potassium perfluorovalerate, not shown in the is now available. table, gave both perfluorobutene-1 and perfluoro(1) Presented before the Section on Industrial and Engineering butene-2 in a ratio of about 1:4. To form the Chemistry a t the 122nd Meeting of the American Chemical Society, internally unsaturated olefin, a shift of fluorine in Atlantic City, 1952. (2) L. J. Hals, T. S. Reid and G. H. Smith, THIS JOURNAL, 73,4054 the molecule had to occur. With the perfluoro(1951). butyric acid salts of the alkaline earth metals i t was (3) A. Pagenstecher, Ann., 196, 115 (1879).
+
+
A n n , '248, 297 (1888). (5) J. Boeseken, Rec. I Y Q W .chim., 16, 841 (1927). (6) E. A. Kauck and A. R. Diesslin, I n d . En#. Chrm., 48, 2332 (i961). (4) J. Wislicenus,
(7) T. J. Brice, J. D. LaZerte, L. J. Hals and W. H. Pearlson, THISJOURNAL, 711, 2898 (1953). (8) Infrared curves on the olefins and hydrides have been filed with Dr. E. G Crsltr ef the National Butonu of Stnndards,
J. D. LAZERTE,L. J. HALS,T.
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Vol. 7 5
S.REIDAND G. H. SMITH
TABLE I TERMINALLY UNSATURATED PERFLUORO OLEFINS
CF-CFa CFaCF=CFs C2FsCF=CFz CIF~CF=CFZ CsFliCF=CFz C7Fi6CF=CFz * At 730-745 mm.
PYROLYSES OF
% 90 97 91
90 86 65
B.p., 0c.a
-74
-29
$1 29 81 123
+D
...
... ...
1.2571 1.2782 1.2868
Mol. weight Calcd. Found
100 150 200 250 350 450
Calcd.
-
.4nalyses Carbon Fluoriw Found Calcd Found
7 -
Yield,
Olefin
I -
99 150 201 250 350
...
...
... ...
...
...
24.0 24.0 24.0
...
24.0
24.2 24.1 24.1 24.0
76.0 76.0 76.0 76.0
75.9 75.9 75.9 75.5
...
...
the peduoro acids has interesting possibilities in TABLE 11 PERFLUOROBUTYRIC ACID the synthesis of fluorocarbons.
THE SALTS OF Dec. Yield temp., Ctk, OC. %
Wide differences in the thermal stabilities of some of the salts were noted. To obtain an accurSalt ate measure of these differences, small samples were C ~ F ~ C O Z L ~240-250 20 weighed on an analytical balance, then heated for 30 minutes a t 20-25' intervals until almost comCaF7COzK 215-235 98 plete decomposition had resulted. The tempera(GF7COz)zMg 275-300 < 5 ture a t which 20% decomposition was obtained, (C3F7CO&Ca 275-300
