A computational exploration of the mechanisms for the acid-catalytic urea-formaldehyde reaction: New insight into the old topic

摘要

Some initial acid-catalytic reactions involved in the synthesis of the urea-formaldehyde resin were theoretically investigated at B3LYP and MP2 levels with solvent effects included. The results suggest that the addition between urea and formaldehyde in neutral condition undergoes with a concerted mechanism represented by a four-member ring transition state. For this reaction, a notable barrier (above 130 kJ/mol) was identified at all theoretical levels. The reactions between urea and different protonated forms of formaldehyde in acid solution were investigated. The reaction between protonated methanediol with urea can produce the methylol urea cation via an S _N2 transition state with a lower barrier of 54.8 kJ/mol. With the mediation of a water molecule, the intra-molecular proton transfer produces the stable methylol carbonium (NH _2CONHCH _2 ~+), which plays an important role in the following formation of methylene and methylene ether linkages.