Kinetics of the hydrolytic deamination of N-heteroaromatic amines in aqueous alkaline solutions
Abstract
The kinetics of the hydrolytic deamination of amino-pyrimidines and -acridines with alkali has been studied.
The rate of the deamination of 2- and 4-amino-5-bromopyrimidine, and 4-aminoquinazoline in 0·05–1·0M-potassium hydroxide is first-order with respect both to the amine and the hydroxide concentration, but in 1·0–5·0M-potassium hydroxide it increases more rapidly than expected from a first-order dependence on the hydroxide concentration.
In dilute solutions the deamination of 2- and 4-aminopyrimidine, and their methyl-derivatives is too slow for convenient measurement, but in 3·0–6·0M-potassium hydroxide the rate, which is first-order with respect to the amine concentration, increases more rapidly than would be expected from the hydroxide concentration. The results seem to suggest an approximate exponential increase.
Rates of deamination of 2-aminopyrimidine in sodium hydroxide, are greater than for potassium hydroxide of the same H– values. They also show a small increase with the concentration of added sodium chloride. In solutions whose ionic strength is maintained constant by the addition of sodium chloride, the increase of the rate of the deamination with the hydroxide concentration appears to be more nearly linear.
The effect of substituents and the large negative ΔS‡ values observed in the deamination of the aminopyrimidines, are consistent with those expected from a bimolecular nucleophilic reaction.
The deamination of 9-amino-, 9-methylamino-, and 9-dimethylamino-acridine is of first-order with respect to the amine, but of zeroth-order with respect to the hydroxide concentration. The methyl groups enhance the reaction and the ΔS‡ values observed are small and positive.
pKa Values and u.v. spectra are recorded.