Mechanism for basic hydrolysis of N-nitrosoguanidines in aqueous solution

Moreira, José; Leis, J. R.; Norberto, F.; García-Río, Luís; Araujo, E.; Ribeiro, Laura

http://hdl.handle.net/10400.1/3222

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Abstract(s)

A kinetic study was carried out on the hydrolysis of two N-nitrosoguanidines, 1-nitroso-1-methyl- 3-tolylsulfonylguanidine (TSGNO) and 1-nitroso-1-methyl-3-benzoylguanidine (BCGNO). We observed an absence of buffer catalysis using H2PO4 -/HPO4 2-, H3BO3/H2BO3 -, and HCO3 -/CO3 2- regulators and a complex dependency of the rate constant on the pH. We discovered the existence of three simultaneous reaction paths: spontaneous decomposition of the neutral form of the N-nitrosoguanidine, decomposition of the monoanion, and decomposition through the form of the dianion. The analysis of the kinetic data has allowed us to obtain the acidity constant for the formation of the monoanion of the N-nitrosoguanidine, with values of pKa I ) 11.5. The reaction rate for the process through the monoanion, k2, decreases as the acidity increases. The application of the principle of nonperfect synchronization shows that the basicity and reactivity do not correlate when there exists a possibility of stabilization of the negative charge by resonance. This behavior is consistent with the mechanism E1cB whereby the stabler the negative charge, the slower the elimination reaction. When dealing with the case of the elimination through the neutral form we observe that the reaction rate increases together with the capacity of stabilization of the positive charge on the nitrogen atom adjacent to the imino group. For the reaction through the dianion we used a maximum value of k3 ) 1010 s-1 to estimate the value of pKa II for the formation of the dianion of the N-nitrosoguanidine, obtaining values of pKa II < 24.

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http://hdl.handle.net/10400.1/3222

Citation

Moreira, José A.; Leis, Jose Ramon; Norberto, Fátima; García-Río, Luís; Araujo, Eduarda; Ribeiro, Lara. Mechanism for Basic Hydrolysis of N-Nitrosoguanidines in Aqueous Solution, The Journal of Organic Chemistry, 68, 11, 4330-4337, 2003.