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Rates of protolysis of aqueous urea have been measured by steady-state and spin-echo nmr methods, with good agreement between the two techniques. Below about 2 M urea, the lifetime of a proton on urea is independent of urea concentration. The rate constants for acid-, base-, and water-catalyzed protolysis are (9.0 ± 1) X 10® M-1 sec-1, (2.4 ± 1)10® M~l sec-1, and about 1.5 ± 0.5 sec-1, respectively. Protonation occurs at urea nitrogen rather than oxygen. Above about 2 or 3 M urea, formation of aggregated species of urea causes the lifetime of a urea proton to depend on urea concentration.

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