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.

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The conversion of four alkyl nitrile compounds, RCN, to the corresponding carboxylic acids and ammonia in fluid H2O at 150−260 °C under 275 bar is described. The reaction rate was increased by the addition of 0.3−1.0 N HCl. The kinetics and pathways of the reactions were determined in real time by IR spectroscopy with a sapphire−titanium flow cell. Ex situ IR and Raman spectral data following batch reaction aided in establishing the reaction pathways. When R = CH3−, CH3CH2−, and (CH3)2CH−, the relatively simple conversion to RCO2H and NH3 occurred and was modeled by two protonation equilibria and two forward reactions. When RHOC(O)CH2−, a more complex pH-dependent reaction took place because of the presence of two functional groups. The carboxylate group reacted at high pH, whereas the nitrile group reacted at low pH. The kinetics of these reactions were determined in real time, and the Arrhenius parameters were obtained. The activation energy for the hydrothermolysis of the nitriles qualitatively correlates with ν(CN), indicating that the electron-donating power of R is a factor in the rate of reaction. Consistent with this observation is the fact that a Taft plot is linear when the electronic effect of R is weighed more heavily than the steric contribution. This was not the case when RHOC(O)CH2−, where a different transition state is proposed.

The hydrothermolysis reaction scheme of cyanamide, NH 2 CN, and dicyandiamide, (NH 2 ) 2 CNCN, applies to the role of these compounds in chemical evolution and to their destruction in aqueous waste streams by hydrothermal methods. Real-time IR spectroscopy with an optically accessible flow cell and Raman spectroscopy with a stopped-flow cell were used at set temperatures of 130-270°C and a pressure of 275 bar to specify the details of the pathway. Rate constants and Arrhenius parameters were determined for the major steps, i.e., hydrolysis of cyanamide, conversion of cyanamide into dicyandiamide, hydrolysis of dicyandiamide, and hydrolysis of the guanylurea intermediate. Previously reported hydrolysis kinetics for guanidine and urea at hydrothermal conditions were used to complete the kinetic scheme. In addition, the apparent equilibrium constants for deprotonation of cyanamide and the protonation of dicyandiamide and its monoanion were determined. The conversion of dicyandiamide to ammeline and the hydrolysis of ammeline were observed by Raman spectroscopy at longer times than were used for the kinetic analysis. A relatively complete and consistent reaction scheme now exists for the cyanamide-dicyandiamide system at hydrothermal conditions.

Infrared spectroscopy in conjunction with an optically accessed flow reactor enable real-time concentration measurements to be made on species involved in hydrothermal reactions. The kinetics of the reaction of the aqueous cyanate ion, OCN-, at 110−160 °C and 275 bar were determined at pH = 3.94−10.5. These kinetics are useful because OCN- is frequently the precursor to the NH3 and CO2 products seen during the hydrothermolysis of organic amines. Three rate constants were needed to model the cyanate reaction. These rates were then used to model the hydrothermolysis kinetics of the progressively more complex reactions of the isoelectronic cations of semicarbazide (T = 140−170 °C) and aminoguanidine (T = 190−260 °C) in neutral and acidic solutions. Six internally consistent rate constants were obtained in order to specify the reaction network.