The reactivity of phenyl isocyanate in aqueous solution

Abstract

The rate of hydrolysis of 1-phenylcarbamoylimidazole (3) in water at 30° is markedly depressed by the addition of imidazole in the pH region 6–10. This is rationalised in terms of the formation of a small equilibrium concentration of phenyl isocyanate in solution. Phenyl isocyanate, besides reacting with imidazole to regenerate (3), can also be competitively trapped by H₂O, HO^–, and other nucleophiles; the relative rates of reaction of HO^– and H₂O with phenyl isocyanate are 7 × 10⁵: 1. Phenyl isocyanate also shows a low sensitivity to the nature of primary amine nucleophiles with a Brønsted β value of 0.30. Secondary and α-effect amines lie on the same correlation line as primary amines, consistent with a transition state in which there is little bond formation. In all cases, urea formation shows simple second-order kinetics (first order each in amine and phenyl isocyanate) and no catalyiss by general acids or bases (or by products formed) is obseved; this contrasts with the results of previous studies which have been carried out in non-aqueous solution.