Spectroscopy of hydrothermal reactions. 13: Kinetics and mechanisms of decarboxylation of acetic acid derivatives at 100--260 C under 275 bar
- Univ. of Delaware, Newark, DE (United States). Dept. of Chemistry and Biochemistry
Carboxylic acids are frequent intermediates in the degradation of organic compounds by aqueous oxidative processes, such as supercritical water oxidation and wet air oxidation. The rates and pathways of decarboxylation of acetic acid derivatives, RCO{sub 2}H, and their Na{sup +} salts, RCO{sub 2}Na, which possess electron-withdrawing groups (R = CCl{sub 3}-, CF{sub 3}-, HOC(O)CH{sub 2}-, NH{sub 2}C(O)CH{sub 2}-, CF{sub 3}CH{sub 2}-, NCCH{sub 2}-, CH{sub 3}C(O)-) were determined in H{sub 2}O at 100--260 C and a pressure of 275 bar. Simple conversion to RH + CO{sub 2} occurs in most cases, except that H{sub 2}O appears to be a required reactant for the anions. Real-time FTIR spectroscopy was used to determine the rate of formation of CO{sub 2} in flow reactors constructed of 316 stainless steel (SS) and of titanium. With a few exceptions, the rate of decarboxylation is similar within the 95% confidence interval in 316 SS and Ti and the difference is smaller than that caused by R. Therefore, while wall effects/catalysis may exist in some cases, it plays a lesser role in the relative rates than the substituent R. The acid form of the keto derivatives decarboxylates more rapidly than the anionic form, whereas the reverse is true for the nonketo derivatives. In keeping with the greater role of H{sub 2}O as a reactant, the entropy of activation for the anions is smaller or more negative than for the acids. A Taft plot of the decarboxylation rates suggests that the mechanistic details can be interpreted in terms of the various roles of R. Where R = HOC(O)CH{sub 2}- and NH{sub 2}C(O)CH{sub 2}-, decarboxylation occurs faster than expected, probably because a cyclic transition state can exist. The rate is slower than expected for R = CF{sub 3}-, perhaps because of stabilization of the acid by hyperconjugation. The mechanism of decarboxylation of acids of the remaining R groups is similar and the steric effect of R is somewhat more influential than its electron withdrawing power.
- OSTI ID:
- 352681
- Journal Information:
- Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory, Journal Name: Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory Journal Issue: 21 Vol. 103; ISSN 1089-5639; ISSN JPCAFH
- Country of Publication:
- United States
- Language:
- English
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