Equilibria for the Reaction of Amines with Formaldehyde and Protons in Aqueous Solution

A RE-EXAMINATION OF THE FORMOL TITRATION

From the ¹ From the Graduate Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02154

The existence of protonated hydroxymethylamines of secondary amines, R₂(+)/NHCH₂OH, in equilibrium with hydroxy-methylamines and protonated amines, has been demonstrated by titration at equilibrium and by rapid titration of the protonated hydroxymethylamines. Analogous compounds, R₃(+)/NCH₂OH, are formed from formaldehyde and the sterically favorable tertiary amines: pyridine, N-methylimidazole, and triethylenediamine. The equilibrium constants for the formation of these cationic hydroxymethylamines are two to three orders of magnitude less favorable than for the formation of neutral hydroxymethylamines; conversely, the basicity of hydroxymethylamines is two to three orders of magnitude less than that of the parent amines. These differences are ascribed primarily to solvation and polar effects. Protonation of the dihydroxymethyl adducts of primary amines was not detected.

An analysis of the complex equilibria for the addition of protons and hydroxymethyl groups to imidazole suggests that protonation and hydroxymethylation do not occur on the same nitrogen atom.

The equilibrium constant

K_d = [R₂NCH₂NR₂]/[R₂NH]²[H₂C(OH)₂]

for the formation of the methylenediamine adduct of morpholine has been shown to be 15,500 ± 600 by two titrimetric methods.

An attempt is made to evaluate the effects on the formol titration of changes in water concentration, formaldehyde polymerization, the methanol introduced with commercial formalin, and nonspecific medium effects in concentrated formaldehyde solutions. It is concluded that these factors either have little significant effect or largely cancel each other out.

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