New Mechanism Proposed for the Base-Catalyzed Urea⁻Formaldehyde Condensation Reactions: A Theoretical Study

Base-catalyzed urea⁻formaldehyde condensation reactions were investigated by using a quantum chemistry method. It was found that monomethylolurea or N,N'-dimethylolurea can produce the methyleneurea intermediate (⁻HN⁻CO⁻N=CH₂) with the catalysis of base. The E1cb (unimolecular elimination of conjugate base) mechanism was identified for the formation of such an intermediate. The potential energy barrier was theoretically predicted to be 59.6 kJ/mol for the E1cb step, which is about half of that of previously proposed S_N2 (bimolecular nucleophilic substitution) mechanism. In the subsequentcondensation reactions, Michael addition reactions that lead to different condensed structures can occur between the methyleneurea intermediate and the anions produced from methylolureas under alkaline conditions. Based on the theoretical calculations on the kinetics and thermodynamics of the selected reactions, the competitive formations of methylene linkages, ether linkages and uron were discussed in combination with our previous experimental observations.