Rate constants for cyanate hydrolysis to urea: A Raman study

Abstract

NANPING WEN and MURRAY H. BROOKER. Can. J. Chem. 72, 1099 (1994). Raman spectra of aqueous solutions of potassium cyanate have been obtained at suitable time intervals. It was found that the peaks attributed to cyanate became less intense, while the peaks attributed to urea, carbamate and carbonate, centred at 1003, 1034, and 1065 cm-', respectively, enhanced greatly with the passage of time. Further Raman spectroscopic studies revealed that the cyanate ion hydrolysed slowly and spontaneously at room temperature. Urea, carbamate, and carbonate were formed even without additional ammonium. Raman intensity measurements were used to monitor species concentrations as a function of time. The results suggested a complicated hydrolysis process: the hydrolysis of cyanate ion to form carbonate and ammonium, a rearrangement type reaction of aqueous cyanate ion with aqueous ammonium to formed urea, and an equilibrium reaction of carbonate and ammonium to form carbamate. The initial hydrolysis of cyanate with pure water was found to be first order with rate constant kl = (2.67 f 0.53) x min-' at 22°C. The reaction of cyanate with aqueous ammonium was found to be second order with rate constant k2 = (4.64 f 0.93) x rn~l-'.L.min-~. The equilibrium reaction of carbonate and ammonium to form carbamate was very fast. Urea and carbamate were formed in parallel reactions. It was not possible to convert urea to carbamate or carbamate to urea at room temperature.

Kinetics of Urea Decomposition in the Presence of Transition Metal Ions: Ni2+

“…However, Shaw and Bordeaux 24 and Frost and Pearson 28 claimed that “both mechanisms were equally supported by the kinetic evidence.” On the other hand, by comparing the pre‐exponential term for this reaction with reactions with a similar charge type, Hughes 29 argued against Frost and Pearson and favored the ionic mechanism. The ionic mechanism found further support through the work of Wen and Brooker 30 Hence, the ionic mechanism was adopted (Reaction ) for the simulations used in this study. However, the fast equilibrium established for HNCO/OCN − and NH 3 /NH 4 + alleviates the distinction between the two mechanisms.…”

Detailed modeling of the evaporation and thermal decomposition of urea‐water solution in SCR systems

“…35 Reactions R1 and R10 involve ''solid'' and ''aqueous'' urea, which behave differently toward thermal decomposition. 60 Previous studies [61][62][63] indicate that the backward step of R1 can be of significance in our conditions. However, for simplicity purposes, we choose to include only the forward step of R1.…”

Alkyl Isocyanates via Manganese-Catalyzed C–H Activation for the Preparation of Substituted Ureas

“…With a solution of Mn­(TMP)Cl in EtOAc, we obtained K d values of 0.19 mM and 0.39 mM for aqueous N 3 – and NCO – , respectively (Table and Figure S1 of the Supporting Information, SI). Interestingly, despite the affinity of isocyanate anion for Mn­(TMP)­Cl, use of NaOCN as isocyanate source resulted in low yields, presumably due to the known tendency of cyanate anions to hydrolyze in aqueous media …”