Proton-transfer reactions. 3. Differences in the protonation of localized

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point where it becomes as good or better a leaving group than protonated imidazole.22 2. The values of kCNfrom Table I represent the facility with which the protonated amide acetals undergo C-N cleavage. A Brernsted plot of log kCNvs. pKa of the parent imidazole (slope -1 .O) indicates that the least basic imidazoles undergo C-N cleavage most rapidly. 3. Bicyclic amide acetal 2 a t pHs >5 adheres to the general mechanism of hydrolysis common to the acyclic examples la-c with the exception that the ring-opening process is a reversible one such that the rate of hydrolysis of 2 is (1-2) X 102-foldslower than that of la-c. The reversibility can be demonstrated by the observation that added nucleophiles such as N3- or HzNOH increase the rate of disappearance of 2 by trapping the intermediate open ion which prevents its reclosure to 2-H+. The additional sigmoidal section to the pH/log koW profile of 2 that is observed at pHs