A kinetic investigation is made of the reactions between monomethylol urea, dimethylol urea and urea in acid aqueous solution.
Strong indications are obtained that these reactions are all of one type viz. bimolecular hydrogen ion catalyzed reactions between an amidomethylol group and an amide group, leading to the formation of methylene linkages between urea fragments:
The rate constant of this reaction appeared to depend on the type of amide group or amidomethylol group that are reacting. The activation energy is about 15 kcal/mole.
The reaction between two molecules of dimethylol urea was found to be very slow if occurring at all.
The formation of cyclic structures by trimerisation of monomeric methylene urea and the formation of dimethylene ether linkages between urea fragments may be excluded.
The reaction of urea and formaldehyde, giving monomethylolurea, is reversible in a neutral, acid or alkaline aqueous solution, the forward reaction being bimolecular and the reverse reaction monomolecular.The forward and reverse reactions are catalyzed to the same extent by hydrogen ions as well as by hydroxyl ions. The specific rates are directly proportional to the hydrogen and hydroxyl ion concentrations. There appears to be an influence of the buffer concentration on the reaction rates probably due to a general acid and general base catalysis.The equilibrium is almost independent of the pH of the solution and of the buffer concentrations.The activation energies of the forward and the reverse reaction have been determined, giving the heat of reaction. Within experimental error these activation energies may be the same for the non-catalyzed and for the catalyzed system. The reaction mechanism is discussed.
The nature and the velocities of the reactions occurring in concentrated solutions of urea and formaldehyde may be very similar to what is found in dilute systems.
A rapid initial reaction between urea and formaldehyde, as reported in literature, was not observed and is shown to be due to the analytical methods used.
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