4260 J. Phys. Chem. A, Vol. 103, No. 21, 1999
Belsky et al.
or the H2O solvent field (or both) would be less affected by the
steric bulk of R and more plausibly be the initial driving force
in accelerating the rate of decarboxylation. The main influence
of R remains the ease of cleaving the C-C bond.
Acknowledgment. We are grateful to the Army Research
Office (R. W. Shaw, Program Manager) for support of this work
on the University Research Initiative Grant DAAL03-92-G-
0174.
Figure 9 shows that R ) HOC(O)CH2- and NH2C(O)CH2-
decarboxylate at faster rates than expected for steric and
electronic control. This most likely results from the ability to
form the six-membered cyclic structure III, which facilitates
internal H transfer and CO2 elimination. These are the only
compounds studied that can form structure III. For example,
this structure is relatively unimportant when R ) CF3CH2-
because of the weak hydrogen bonding ability of the F atom in
a C-F bond.43
References and Notes
(1) Lewan, M. D.; Fisher, J. B. in Organic Acids in Geological
Processes; Pittman, E. D., Lewan, M. D., Eds.; Springer-Verlag: Berlin,
1994; Chapter 4.
(2) Shock, E. L. Org. Life EVol. Biosphere 1990, 20, 331.
(3) Holgate, R. H.; Meyer, J. C.; Tester, J. W. AIChE J. 1995, 41,
637; Skaates, J. M.; Briggs, B. A.; Lamparter, R. A.; Baillod, C. R. Can.
J. Chem. Eng. 1981, 59, 517.
(4) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Ind. Eng. Chem. Res.
1995, 34, 2.
(5) Shock, E. L. Am. J. Sci. 1995, 295, 496.
(6) Maiella, P. G.; Brill, T. B. J. Phys. Chem. A 1998, 102, 5886.
(7) Yu, J.; Savage, P. E. Ind. Eng. Chem. Res. 1998, 37, 2.
(8) Bell, J. L. S.; Palmer, D. A.; Barnes, H. L.; Drummond, S. E.
Geochim. Cosmochim. Acta 1994, 58, 4155.
(9) Meyer, J. C.; Marrone, P. A.; Tester, J. W. AIChE J. 1995, 41,
2108.
(10) Clark, L. W. In The Chemistry of Carboxylic Acids and Esters.
The Chemistry of Functional Groups Series; Patai, S., Ed.; Wiley: New
York, 1969; p 589.
(11) Lundegard, P. D.; Kharaka, Y. K. In Organic Acids in Geological
Processes; Pittman, E. D., Lewan, M. D., Eds.; Springer-Verlag: Berlin,
1994; Chapter 3.
(12) Shock, E. L. In Organic Acids in Geological Processes; Pittman,
E. D., Lewan, M. D., Eds.; Springer-Verlag: Berlin, 1994; Chapter 10.
(13) Bell, J. L. S.; Palmer, D. A. in Organic Acids in Geological
Processes; Pittman, E. D., Lewan, M. D., Eds.; Springer-Verlag: Berlin,
1994; Chapter 9.
(14) Fairclough, R. A. J. Chem. Soc. 1938, 1186.
(15) Hall, G. A., Jr. J. Am. Chem. Soc. 1949, 71, 2691.
(16) Maiella, P. G.; Brill, T. B. J. Phys. Chem. 1996, 100, 14352.
(17) Lee, I.; Cho, J. K.; Lee, B. S. J. Chem. Soc., Perkin Trans. 2 1998,
1319.
When R ) CF3-, Figure 9 reveals that the rate of decar-
boxylation is slower than expected from the electronic and steric
substituent effects of R alone. The electronic anomalies of the
CF3- group are well-known44-46 and frequently are interpreted
as resulting from the hyperconjugative effect IV. Such an effect
would somewhat strengthen the C-C bond relative to that
predicted by the Taft parameters and thereby retard the rate of
decarboxylation.
Conclusions
(18) Bernoulli, A. L.; Wege, W. HelV. Chim. Acta 1919, 2, 511.
(19) Hall, G. A., Jr. J. Am. Chem. Soc. 1949, 71, 2691.
(20) Fraenkel, G.; Belford, R. L.; Yankwich, P. E. J. Am. Chem. Soc.
1954, 76, 15.
(21) Kieke, M. L.; Schoppelrei, J. W.; Brill, T. B. J. Phys. Chem. 1996,
100, 7455.
(22) Schoppelrei, J. W.; Kieke, M. L.; Wang, X.; Klein, M. T.; Brill, T.
B. J. Phys. Chem. 1996, 100, 14343.
(23) Maiella, P. G.; Schoppelrei, J. W.; Brill, T. B. Appl. Spectrosc., in
press.
(24) Belsky, A. J.; Brill, T. B. J. Phys. Chem. A 1998, 102, 4509.
(25) Galat, A. J. Am. Chem. Soc. 1948, 70, 2596.
(26) Cvetanovic, R. J.; Singleton, D. L. Int. J. Chem. Kinet. 1977, 9,
481.
(27) Kortum, G.; Wogel, W.; Andrussow, K. Dissociation Constants of
Organic Acids in Aqueous Solutions; Butterworths: London, 1961.
(28) Lindsay, W. T. Proc. Int. Water Conf. Eng. Soc. W. Pa. 1980, 41,
284.
(29) Taft, R. W., Jr. Steric Effects in Organic Chemistry; Newman, M.
S., Ed.; John Wiley and Sons: New York, 1956.
(30) Unger, S. H.; Hansch, C. Prog. Phys. Org. Chem. 1976, 12, 91.
(31) MacPhee, J. A.; Panaye, A.; Dubois, J. E. Tetrahedron 1978, 34,
3553.
(32) Charton, M. J. Am. Chem. Soc. 1969, 91, 615.
(33) King, J. A. J. Am. Chem. Soc. 1947, 69, 2738.
(34) Hall, G. A., Jr. J. Am. Chem. Soc. 1980, 72, 4709.
(35) Hall, G. A., Jr.; Verhoek, F. H. J. Am. Chem. Soc. 1947, 69, 613.
(36) Yamamoto, S.; Back, R. H. Can. J. Chem. 1985, 65, 549.
(37) Davis, L. L.; Brower, K. R. J. Phys. Chem. 1996, 100, 18775.
(38) Townsend, S. H.; Klein, M. T. Fuel 1985, 64, 635.
(39) Francisco, J. S. J. Chem. Soc., Faraday Trans. 1992, 88, 3521.
(40) Ashworth, A.; Harrison, P. G. J. Chem. Soc., Faraday Trans. 1993,
89, 2409.
The stoichiometries of the decarboxylation reactions of H2O
solutions of most acetic acid derivatives, RCO2H, where R is
electron-withdrawing, are relatively straightforward at the hydro-
thermal conditions. The presence of the H2O solvent field greatly
facilitates the decarboxylation reaction. In contrast to electron-
donating groups (especially R ) CH3-) the rates depend more
on R than whether the cell is constructed of 316 SS and Ti.
The exceptions are greatest when the possibility of cell corrosion
and secondary reactions of the products of decarboxylation exist.
An explanation for the greater role of the heterogeneous reaction
component when R ) H- and CH3- compared to when R is
electron-withdrawing lies in the fact that acetic and formic acids
are more inert in H2O. On the other hand, when R is electron-
withdrawing, the acids decarboxylate more rapidly at lower
temperature and less surface assistance is needed.
Electron-withdrawing R groups accelerate the decarboxylation
reaction, and the mechanistic details appear to depend on the
nature of R. Some groups anomalously accelerate the reaction
while others anomalously retard it. Hence, a simple general
relation does not exist between the decarboxylation rate and
the electronic and steric properties of the functional group. The
anions, likewise, do not decarboxylate at rates that parallel those
of the corresponding acid. The anions of the keto derivatives
react more slowly than the acid whereas the reverse is true for
the nonketo derivatives. In almost all cases, however, the entropy
of activation for decarboxylation is more negative for the anion
than for the acid which is consistent with a greater role of H2O
in the decarboxylation of the anion. Thus, while homogeneous
and heterogeneous pathways probably compete in determining
the decarboxylation rate, electron-withdrawing R groups cause
the structure of the acid to retain a strong influence and the
homogeneous route becomes more important than the differ-
ences caused by the SS and Ti cell types.
(41) Auerbach, I.; Verhoek, F. H.; Henne, A. L. J. Am. Chem. Soc. 1950,
72, 299.
(42) Conners, K. A. Chemical Kinetics; VCH: New York, 1990; p 220.
(43) West, R.; Powell, D. L.; Whately, L. S.; Lee, M. K. T.; Schleyer,
P. von R. J. Am. Chem. Soc. 1962, 84, 3221.
(44) Roberts, J. D.; Webb, R. L.; McElhill, E. A. J. Am. Chem. Soc.
1955, 72, 408.
(45) Koppel, I. A.; Pihl, V.; Koppel, J.; Anvia, F.; Taft, R. W. J. Am.
Chem. Soc. 1994, 116, 8654.
(46) Paleta, O. Chem. Listy 1970, 64, 366.