Spectroscopy of hydrothermal reactions - 13. Kinetics and mechanisms of decarboxylation of acetic acid derivatives at 100-260 degrees 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 de termined in H2O at 100-260 degrees C and a pressure of 275 bar. Simple conv ersion to RH + CO2 occurs in most cases, except that H2O appears to be a re quired reactant for the anions. Real-time FTIR spectroscopy was used to det ermine the rate of formation of CO2 in flow reactors constructed of 316 sta inless steel (SS) and of titanium. With a few exceptions, the rate of decar boxylation is similar within the 95% confidence interval in 316 SS and Ti a nd the difference is smaller than that caused by R. Therefore, while wall e ffects/catalysis may exist in some cases, it plays a lesser role in the rel ative rates than the substituent R. The acid form of the keto derivatives d ecarboxylates more rapidly than the anionic form, whereas the reverse is tr ue 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 neg ative than for the acids. A Taft plot of the decarboxylation rates suggests that the mechanistic details can be interpreted in terms of the various ro les of R. Where R = HOC(O)CH2- and NH2C(O)CH2-, decarboxylation occurs fast er than expected, probably because a cyclic transition state can exist. The rate is slower than expected for R = CF3-, perhaps because of stabilizatio n of the acid by hyperconjugation. The mechanism of decarboxylation of acid s of the remaining R groups is similar and the steric effect of R is somewh at more influential than its electron withdrawing power.