Kinetic analysis reveals a stepwise mechanism with a zwitterionic intermediate in urea synthesis, indicating that the rate-determining step shifts with amine basicity and buffer level.
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 pKa has a slope βnue 0·3 for basic amines and exhibits a break to a slope βnue 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.
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Williams et al. (1974) studied this question.
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