2332
INDUSTRIAL A N D ENGINEERING CHEMISTRY
about 316" C. At this temperature the curve of octane uuniber vs! flame position had an inflection point a t an octane number of about 80. Consequently, a correlation for higher oct,ane fuels than this value was not obtained. Cool flames xere observed under somewhat different conditions with fuels having octane nymbers as high as 95. A survey of the literature indicated that it inlight not be possible to produce cool flames for hydrocarbons of q u c h higher octane numbers a t atmospheric pressure. ,The position of the cool flame as measured in these erperin(ents is evidently related to, but not directly proportional to, the induction period that precedes cool flame formation. \Vheneder the fuel-air mixture was introduced the cool flame appeared in, a portion of the tube downstream from its equilibrium position. Start,ing immediately the cool flame Trould slowly migrate upstream to the equilibriuni point. Thus the time required for the gases to travel to the stabilized flame front was always less than the induction Deriod would have been a t conditions uninfluenced by the presence of the flame. This behavior was possibly due to diffusion effect's and the localized heating caused b!. tmhe Cool flame reaction.
Vol. 43, No. 10
LITERATURE CITED
(1) Aivezov, B. V., and Neiman, M. B., J . Phys. Chent. ( U . S . S . R . ) ,8, 88 (1936). (2) American Petroleum Inst. Research Project 45, Eleventh Annual Report, (July 1, 1948, to .June 30, 1949) Ohio State University Research Foundation. (3) Beatty, H. A,, and Edgar, G., J . -4m.Chem. Soc., 5 6 , 107 (1934). (4) Jost, W. "Explosion and Combustion Processes in Gases," KeTzYork and London, McGraw-Hill Book Co., 1946, translated by H. 0.Croft. (5) Jost, W., "Physico-Chemical Investigations of the Combustion Process in an Engine." Technical Oil Missions Reel 52, Frames 509-549, Library of Congress, Photoduplication Service, Publication Board Project, Washington, D. C. (Dee. 16, 1937), translation FL-45, Consultants Bureau, New York (1948). ( 6 ) Lewis, B., and Yon Elbe, G. "Combustion, Flames and Explosions of Gases," London, Cambridge University Press, 1938. (7) Townend, D. T. A., and Chamberlain, E. A. C., Pror. Roy. SOC. Lon,do?s, 158,415 (1937). R E C E I V December ~~D 2, 1950. Portions of this paper were preaented before the r)irision of petroieuin chemistry at the 118th hfeeting of the AYERICAU CHE\IIC%L SOCIETY, Chicago, TI!
Some Properties of
Perfluorocarboxylic Acids d
E. A. IUUCK AKD A. R. DIESSLIN Minnesota Mining & Manufacturing Co., S t . Paul, Minn. During an investigation of the electrocheniical fluorina: drotion of organic compounds dissolved in anhydrous h gen fluoride, i t was discovered that fully fluorinated acjl fluorides Mere produced which hydrolyze readily to the corresponding perfluoro acids. Boiling points and liquid densities are presented for the following acids: perfluoroacetic acid, perfluoropropionic acid, per0tiorobutyric acid, perfluoroisobutyric acid, perfluorovaleric acid, perfluorocaproic acid, perfluoroheptanoic acid, perfluorocapr?lic acid, perfluorocapric acid, perfluoromyristic acid, perfluorocyclohexanecarboxylic acid, and perfluorocyclohexaneacetic acid. With the exception of the first three compounds listed, these substances are reported for the first time. Data are presented on vapor pressure, iiscositj, pH, and equivalent conductance of perfluoroacetic and perfluorobutyric acids. A feasible method of preparing a new series of fluorinated acids has been developed and these fluorinated acids are being made available to the chemical public. A wide variety of useful derivatives can be made from these acids. Interest in the acids and their derivatives is expected because of their unusual chemical and physical properties.
T
HE electrochemical fluorination of organic rompounds in liquid hydrogen fluoride was first accomplished by Simons ( I S , 1.4) by electrolysis of a solution of organic material in liquid hydrogen fluoride at an electrolyzing potential which is insufficient to generate free fluorine. Fluorination is accomplished in one step to obtain fully fluorinated end products Jyhich, depending upon their boiling points, either leave the cell as gases or settle t o the cell bottom and are drained out. Trifluoroacetio acid, CF,COOH, was first prepared by Swarts in 1922 (15). Its preparation since has been described by others ( 1 , 2, 6). Perfluoropropionic and perfluorobutyric acids have
recently been prepared ( 4 ) . It has been found that a great variety of fully fluorinated acid fluorides can be readily produced by the electrolysis of the corresponding hydrocarbon acids or anhydrides in hydrogen fluoride. The acid fluorides thus formed are easily hydrolyzed to the coi ponding perfluoro acids. EXPERIMENTAL PROCEDURE
An electrolytic cell was constructed from &inch standard iron pipe. Suspended from the cover were nine nickel anodes and nine iron cathodes, each l/,e X 3.5 X 6.5 inches spaced 0.125 inch apart. Figure 1 is a photograph of the cell disniantled and assembled to show the electrode pack. The cell was charged with 2000 grams of a 4% solution of acetic anhydride in anhydrous hydrogen fluoride. The electrolysis was conducted at 50 amperes, 5.2 volts direct current with an electrolyt,e t,eInperature of 20" C. a t atmospheric pressure. The gaseous products were passed through a reflux condenser maintained a t about -30" C. to reniove t,he bulk of the accompanying hydrogen fluoride and then through a dry sodium fluoride tube to remove the final traces of hydrogen fluoride. h typical reaction may be illustrated as follows: (CH3CO)ZO t l O H F 4 2 C F s C O F OF2 8H2
+
+
I n actual cell pract,ice, some decarboxylation and fragmcntation occur, producing fluorinated material with fewer carbon atoms than the starting material. Trifluoroacetyl fluoride is readily absorbed by water, hydrolyzing to trifluoroacetic acid and hydrogen fluoride. These acids are separated in various TTays-for example, by ion exchange or by the extraction of an alkali salt with a solvent ( 7 ) . The trifluoroacetic acid produced in one run was found by titration to be 99.9% pure. The electrolysis is readily operated continuously. Acetic anhydride and hydrogen fluoride are charged continuously to the cell to replace that consumed by the electrolysis and t'he reaction products are withdrawn continuously. This electrolysis has been conducted in a 2000-ampere pilot plant cell for several months
October 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
2333
all highly purified specimens. Their neutral equivalent Liquid E ~ Hydrolyzable ~ ~ agreed ~ with~ the theoretical ~ t Density, ____ Fluoride within very close limits and 20’ C. Found Theory Me./Gram 1.489 114 114 None little or no hydrolyzable fluo1 561 164 164 Kone ride ion could be detected by 1 651 214 214 Kone 1.713 262 264 0.01 the standard thorium nitrate 1 762 311 314 0.08 1 z92 360 364 titration. These values are . , . ’ 414 ... listed in Table I. The in510 614 ... a ... 714 ... frared spectra demonstrated 1.780 .. 326 ... t h e a b s e n c e of c a r b o n hydrogen bonds. Numerous 1.813 , . , 376 ,.. 1.649 219 214 ... derivatives such as esters and amides were prepared whose molecular weight agreed with the theoretical. Figure 2 compares the boiling points of the fluorinated aliphatic acids with those of the corresponding hydrocarbon acids. I t is seen that the fluorocarbon acids boil about 50” C. lower than the hydrocarbon acids with the same number of carbon atoms. As a general rule, completely fluorinated compounds boil lower than the corresponding hydrocarbon analogs. The effect of fluorination on the boiling point is more pronounced in the case of the fluorinated acids than with the fluorocarbons ( 3 ) .
TABLE I. PHYSICAL PROPERTIES OF PERFLUORO ACIDS Perfluoroacetic acid CFsCOOH CsE’sCOOH Perfluoropropionic acid CsFiCOOH Perfluorobutyric acid Perfluorovaleric acid CaFoCOOH CsFiiCOOH Perfluorocaproic acid Perfluoroheptanoic acid CsFisCOOH Perfluorocaprylic acid CiFisCOOH Perfluorocapric acid CsBiilCOOH Perfluoromyristic acid CiaFziCOOH Perfluorocyclohexanecarboxylic acid Cs1”iiCOOH Perfluorocyclohexaneacetic acid Cd‘llCF2COOH Perfluoroisobutyric acid Iso-CsF;COOH a Solids a t room temperature.
Boilitg Point, C. (760mni.) (740 mm.) (735 mm.) (749 mm.) (742 mm.) (742 mm.) (736 mm.) (740 mm.) (740 mm.)
72.4 96 120 139 167 175 189 218 270
170 (742 mm.) 184 (735 mm.) 1 1 7 . 5 (740 mm.)
I . .
,
260i
F i g u r e 1. Electrolytic Cell with no indications of accumulation of undesirable by-products in the electrolyte and with substantially no corrosion of the cell or electrodes. Higher fluorinated acids are prepared in a similar manner by the electrolysis of the corresponding hydrocarbon acid or acid anhydride. Both perfluoropropionic acid and perfluorobutyric acid can be recovered in the manner described for perfluoroacetic acid, as their acid fluorides are sufficiently low boiling to permit them to leave the cell as gases. The perfluoro acids above valeric, houever, are recovered in a slightly different manner, as their acid fluorides boil sufficiently high to allow them to remain in the cell in a liquid state. These higher perfluoro acid fluorides are insoluble in, and have a higher liquid density than, the electrolyte and they accumulate a t the cell bottom. They are withdrawn a t convenient intervals and can be converted to the perfluoro acids by hydrolyeis and removal of the fluoride ion. It is sometimes desirable, however, to convert them directly to other derivatives -for example, the amide by reacting with anhydrous ammonia or the esters by reacting with alcohols. PHYSICAL PROPERTIES
Table I lists the physical properties of some completely fluorinated acids prepared by this process. It is evident that a serien of completed fluorinated normal aliphatic acids exists. Apparently the first member of this series is perfluoroacetic acid, perfluoroformic acid (FCOOH) being either nonexistent or unstable. COF,, the perfluoro acid fluoride of formic acid ( 2 1 ), hydrolyzes to carbon dioxide and hydrogen fluoride, whereas all higher perfluoro acid fluorides hydrolyze to the corresponding perfluoro acid. The acids described in Table I and elsewhere in this paper were
6 0 1 I
I
I
I
2
3
4
I
5
I
I
6
7
I
8
I
9
L
IC
NUMBER OF CARBON ATOMS Figure 2.
Boiling Points of Fluorocarbon Hydrocarbon Acids
and
TABLE 11. VAPORPRESSURES AND VISCOSITIES OF PERFLUOROACETIC AND PERFLUOROBUTYRIC
CF~COOH T;mz., 37.0 43 1 47.3 49 5 60.8 66 7 71.1
Vapor pressure, mm. Hg 191 250 298 326 503 625 734 Viscosity, cs.
ACIDS
n-CaF7COOH (9) Vapor Temp., pressure, 0 c. mm. Hg 44 56.0 110 74.4 179 84.8 324 98.6 455 107.4 572 113.6 735 120.0 Viscosity, cs.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
2334
Figure 3 compareA the liquid densities of perfluoro aliphatic acids and their corresponding hydrocarbon analogs. It is seen that the densities of the fluorinated acids are high, ranging from 1.489 for perfluoroacetic acid to 1.792 for perfluoroheptanoic acid. I n the fluorocarbon aaid series, the densities increase regularly with the number of carbon atoms, whereas in the hydrocarbon acid series the opposite is true. This probably means that the contribution t o density of the carboxyl group is intermediate between the alkyl and fluoroalkyl groups.
Vol. 43, No. 10
In Table IV the equivalent conductance values a t infinite dilution of these two fluorinated acids and their anions are compared with the respective hydrocarbon analogs. The conductance values for the fluorinated and corresponding nonfluorinated anions are approximately the same despite their difference in molecular weight. This may indicate that the fluorinated ions are less solvated than the nonfluorinated ions. The solubility in water of the fluorinated acids was measured. At 25” C., perfluoroacetic acid, perfluoropropionic acid, perfluorobutyric acid, and perfluorocaproic acid were miscible with water in all proportions. Perfluorocaprylic acid and perfluorocapric acid are only slightly soluble in water. Their saturated fiolutions a t 25’ C. were measured to be 0.023 and 0.01 molar, respectively. Higher members of the series (perfluorobutyric acid and above) possess unusual surface active properties (12). CHEMICAL PROPERTIES
- 1
1.3 >L I
I
n - CnH 2n+, C O O H
.9 I 2 3 4 5 6 7 8 9 1 0 NUMBER OF CARBON ATOMS Figure 3. Liquid Densities of Fluorbcarbon and Hydrocarbon .4cids
I n addition t o the properties given above, further properties have been determined for perfluoroacetic and perfluorobutyric acids. The vapor pressures and viscosities of these t r o acids are presented in Table 11. From these data the heats of vaporization at the boiling point and Trouton’s ratio are calculated to be 8300 and 11,200 calories per mole, and 24.1 and 28.5, respectively. Table 111 lists the pH and equivalent conductance of aqueous solutions of perfluoroacetic acid and perfluorobutyric acid. These acids are almost completely ionized in water solution and are equivalent to mineral acids in strength.
CONDUCTANCE O F PERFLUOROTABLE 111. pH AND EQUIVALEKT ACETIC AND PERFLUOROBUTYRIC ACID:
Concentration
TABLE Iv.
’
pH a t 29‘ C. CFsCOOH n-CaFrCOOH
EQUIVALENT
Equivalent Conductance at 27’ C., hIhos/Equivalent CFgCOOH n-CaF7COOH
CONDUCTANCE AT AKD 27’ C.
INFINITE
DILUTION
*? Acid Anion
Solution
At!d
Perfluoroacetic acid Perfluorobutyric acid Acetic acid Butyric acid
400
40
389 403 393
29
43 33
The chemical properties of the fluorocarbon acids are largely unexploited. Their reactions are in some cases similar to the hydrocarbon acids; however, many differences exist (8,10). Unlike nitric and sulfuric acids, the perfluoro aliphatic acids are not oxidizing acids. The former acids will oxidize ketones, aldehydes, alcohols, olefins, etc.; the perfluoro acids will not. The perfluoro aliphatic acids can be reduced by extremely powerful reducing agents such as lithium aluminum hydride (8). The perfluoro aliphatic acids are also very difficult t o oxidize further. This is demonstrated by the fact that trifluoroacetic acid can be prepared in good yields by the oxidation of either benzotrifluoride with a chromic anhydride-sulfuric acid mixture ( 1 4 ) or CF3CCl=CCICFI with alkaline permanganate (6). The freedom of the pcrfluoro acids from oxidizing or rcducing tendencies decreases the danger of unwanted side reactions in their use. The perfluoro acids are highly thermally stable. They can be heated to 400’ C. in borosiliaate glaw without significant decomposition. LITERATURE CITED
(1) Babcock, J. H., and Kischitz, A. D., U. S. Patent 2,414,706
(1947). (2) Benning, A. F., and Park, S.D., Ibid.,2,396,076 (1946). (3) Grosse, A. V., and Cady, G. H., IXD.ENG.CHEM.,39,367 (1947). (4) Haszeldine, R. N., Nature, 166, 192 (1950). ( 5 ) Henne, A. L., et al., J . Am. Chem. Soc., 67, 918 (1945). (6) Ibid., 69, 281 (1947). (7) Ilenne, A. L., and Trott, P., Ibid,,69, 1820 (1947). (8) Husted, D. R., and Ahlbrecht, A. H., “Reactions of n-Heptafluorobutyric Acid,” Division of Industrial and Engineering Chemistry, 116th Meeting, AM. CHEY.Soc., Atlantic City, N. J., 1949. (9) Minnesota Mining & Mfg. Co., “Heptafluorobutyric Acid,” Sept. 26,1949. (10) Reid, T. 9.;and Smith, G. H., “Some Derivatives of Heptafluorobutyric Acid,” Division of Industrial and Engineering Chemistry, 116th Meeting, AM, CHEM.Soc., Atlantic City, N. J., 1949. (11) Ruff, O., and Miltschitzky. 0. Z., 2. anorg. allgem. Chem., 221, 154-8 (1934). (12) Scholberg, H. hf., “Surface Chemistry of Fluorocarbons and Their Derivatives,” Division of Industrial and Engineering Chemistry, 116th Meeting, AM. CHEM.SOC., Atlantic City, N. J., 1949. (13) Simons, J. H., U. S. Patent 2,519,983 (1950). (14) Simons, J. H., et al., J . Electrochem. Soc., 95, 2, 47-67 (1949). (15) Swarts, F., Bull. a c d . roy. belg.. 8, 343 (1922). RECEIVED October 20, 19.50. Presented before the Division of Industrial and Engineering Chemistry, Symposium o n Fluorine Chemistry, at the 116th Meeting of the AVERICANCHEXICAL SOCIETY,.4tlantic City, N. J. Contribution 22, Central Research Department, Minnesota Mining & Manufacturing Co.
