Thermodynamics of the Hydroxymethylation of Melamine and Urea with Formaldehyde

The hydroxymethylation of melamine and urea with formaldehyde in an aqueous solution was studied thermodynamically. The entropy, enthalpy, and free energy of activation for the early stages of the reaction, i. e., the formation of mono-methylol derivatives, and also the changes in the entropy, enthalpy, and molecular heat, etc., were determined in connection with the functionalities. It was confirmed that no marked differences exist between the rate constant of melamine and that of urea, but that the ΔS and ΔH values of urea are considerably smaller than the respective values of melamine. Prominent differences do exist between ΔS and ΔH, the ΔS and ΔH terms of melamine amount roughly to 105 times and 105 times those of urea respectively, regardless of the functionalities. These results imply a conspicuous difference between their reaction mechanisms, a difference which is explicable in terms of the characteristics of their amino-group nitrogen. Besides, the solvent effect of dimethylsulfoxide in the hydroxymethylation of melamine was investigated; it will be discussed in connection with the solvation.