J. RADELL,B. W. BRODMAN, A. HIRSHFELD, AXD E. D. BERGMANN
928
Acidity and Autocatalysis of Esterification of Acetylenic and Fluoro Acids
by Jack Radell,’ B. W. Brodman, Pitman-Dunn Institute for Research, Frankfotd Arsenal, Philadelphia, Pennsylvania
19137:
Amiram Hirshfeld, and E. D. Bergmann The Hebrew University of Jerusalem, Jerusalem, Israel
(Received September 26, 1964)
The acetylenedicarboxylic acid and perfluoro acids, unlike propiolic acid, were found to be autocatalytic in esterification reactions with unfluorinated alcohols. Only the diester could be isolated from the diacid. A determination of pK. values showed the acidity to decrease in the following order: perfluoro acids > acetylenedicarboxylic acid > propiolic acid. The acidity of the acetylenedicarboxylic acid was compared with that of fumaric, maleic, and succinic acids. The enhanced acidity of the acetylenedicarboxylic acid was discussed. A simplified, self-consistent method was employed to determine the effect of added salt on the pK, and the thermodynamic pK,. The measured and thermodynamic pK. values were found to be insignificantly different in each case studied.
Introduction In order to obtain compounds containing acetylenic ester groups several prototype compounds were prepared. To determine the feasibility of obtaining esters of acetylenic acids without added acid catalysts, the pK. values of propiolic, acetylenedicarboxylic, and some perfluoro acids were measured. The latter acids were included since several cases are known2 where these acids are autocatalytic in esterification reactions with nonfluorinated alcohols. For comparison, some perfluoro acids were autocatalytically esterified. The pK. of acids were measured in order to determine which are sufficiently strong to autocatalyze their own esterification. Discussion Titration curves for the acetylenedicarboxylic acid and the hydrated samples of n-C7F&02H, n-C3F7C02HJ and HC=CC02H are listed in Table I. All the curves had a single strong inflection (Figure 1) including acetylenedicarboxylic acid for the titration of both carboxyl groups. Two pK, values were obtained by Dondon3 and Ashton and Partington4 for the dihydrate of scetyleriedicarboxylic acid using an approximately 50% aqueous ethanol solution (pK,, = 2.2; pK,, = 4.0). Chartons reporting on the work of Ashton and Partington4gave a single pk’, value. The Journal of Physical Chemistry
Our measurements which resulted in a single pK, value for acetylenedicarboxylic acid differed primarily in that we started with anhydrous material as noted by the neutralizat,ion equivalent in Table I. Dondon3 did note that a single pK, value was obtained for the diacid when 20% ethanol in water was used as a solvent. If we compare the pKnIand pK,, values of succinic, fumaric, and maleic acids with the single value we obtained for acetylenedicarboxylic acid, we note the values in Table 11. The succinic acid has the highest pK, of those listed (1) Holder of a U. S. Secretary of the Army Research and Study Fellowship a t The Hebrew University of Jerusalem, 1962-1963. (2) (a) M. Hudlicky, “Chemistry of Organic Fluorine Compounds,” The Macmillan Co., New York, N. Y., 1962, p. 197; (b) J. Radell and J . W. Connolly, Chem. Eng. Data, 6 , 282 (1961); (c) E. E. Burgoyne and F. E. Condon, J . A m . Chem. SOC.,72, 3276 (1950). (3) M. L. Dondon, J . chim. phya., 54, 304 (1957). (4) H. W. Ashton and J. R. Partington, Trans. Faraday Soc., 30, 598 (1934). (5) M. Charton, J . Org. Chem., 26, 735 (1961), erroneously reported “It should be rioted that Ashton and Partington have reported values of 1.85 X lo-* for the first and for the second ionization constant of acetylenedicarboxylic acid.” In Ashton and Partington’s paper4 there are two sets of K , values measured with different titrant and sample concentrations giving no indication of the solvent presumed to be largely or completely aqueous. The respective values they4 reported for the molality of acetylenedicarboxylic acid, normality of titrant, Kal X lo-*, and K,, X 10-6 are 0.0101. 0.0314, 1.82, 4.20, and 0.00202, 0.00720, 1.88, 3.85.
ESTERIFICATION OF ACETYLENIC AND FLUOROACIDS
1
12 10
ii i i
8
r 6 a
2 HCZCC02H 3 O-C3F7C02H 4 P-C7FSCO2H
i -----7-------...i .. .:.
