Hydrolysis of phenylureas. Part II. Hydrolysis in acid and aqueous solutions

Abstract

The rate constants of hydrolysis of 4-methyl-, 4-methoxy-, 4-ethoxy, 4-isopropyl-, 4-n-butyl-, 4-chloro-, 4-bromo-, 4-nitro-, and 3-nitro-phenylurea have been measured in aqueous H₂SO⁴(0.5–98% w/w) at 101.0°. The rate profiles exhibit a bell shape up to intermediate acidities, followed by a localised minimum, and then an increase in rate. In acidities up to the minimum application of the standard criteria of mechanism for hydrolysis of the Oprotonated conjugate acid have proved unsatisfactory. The rate constants of hydrolysis of 4-chloro- and 4-methyl-phenylurea have also been measured in 2.5–37%(w/w) HCl and 5–60%(w/w) HClO₄ and in mixtures of LiCl–HCl at 101.0°. Results show that there is a strong dependence on water activity but that the counter anion has little effect. Rate constants of hydrolysis of 4-fluoro- and 3-methyl-phenylurea, the phenylureas listed above, and also phenylurea have been measured in water (pH 6.48), and these differ little from the ‘acid-catalysed’ rate constants. A new mechanism of hydrolysis is postulated, for which water acts as a proton transfer agent to either the unprotonated species or the minor N-protonated conjugate acid. Solvent deuterium isotope effects and Hammett ρ constants support the new mechanism. Electron donating substituents cause sulphonation in >70%(w/w) H₂SO₄ and a mechanism of hydrolysis at these higher acidities is postulated as A-1 decomposition of the O-protonated conjugate acids.