Urea synthesis from amines and cyanic acid: kinetic evidence for a zwitterionic intermediate

Abstract

The following evdence supports the conclusion that the reaction of amines with cyanic acid and the hydrolysis of urea occur through a stepwise mechanism with the formation of an intermediate zwitterion. (1) The Brønsted type plot of log₁₀k′versus ammonium ion pK_a has a slope β_nue 0·3 for basic amines and exhibits a break to a slope β_nue 0·8 for weakly basic amines; the two segments are assigned to rate-determining nucleophilic attack and protontransfer steps, respectively. (2) General acid-base catalysis is observed only with weakly basic amines, for which the proton-transfer step is rate determining. (3) A change in rate-determining step with increasing buffer concentration is observed as the uncatalysed attack step becomes rate determining.