A theoretical study on the water-mediated asynchronous addition between urea and formaldehyde

The reaction between urea and formaldehyde in water solution was theoretically investigated by using B3LYP and MP2 methods. It was found that the addition of the nitrogen atom in urea to the carbonyl group in formaldehyde precedes the proton transfer and the proton migration from water to the carbonyl group occurs before the proton abstraction from the nitrogen. With one or two water molecules involved in the TS, the activation energy barrier is lowered compared to the TS of the mechanism with no water participation. The energy change along the reaction coordinate clearly shows that a zwitterionic-like intermediate does not exist on the PES. The reaction between urea and formaldehyde occurs in a concerted mechanism but with asynchronous characters. This is different from the stepwise mechanism recently found for the amination reactions of formaldehyde.