Spectroscopy of Hydrothermal Reactions 13. Kinetics and Mechanisms of Decarboxylation of Acetic Acid Derivatives at 100−260 °C under 275 bar

The rates and pathways of decarboxylation of acetic acid derivatives, RCO2H, and their Na+ salts, RCO2Na, which possess electron-withdrawing groups (R = CCl3−, CF3−, HOC(O)CH2−, NH2C(O)CH2−, CF3CH2−, NCCH2−, CH3C(O)−) were determined in H2O at 100−260 °C and a pressure of 275 bar. Simple conversion to RH + CO2 occurs in most cases, except that H2O appears to be a required reactant for the anions. Real-time FTIR spectroscopy was used to determine the rate of formation of CO2 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 H2O 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)CH2− and NH2C(O)CH2−, decarboxylation occurs faster than expected, probably because a cyclic transition state can exist. The rate is slower than expected for R = CF3−, 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.