Abstract. Pseudo-first-order rate constants and thermodynamic activation parameters have been determined for the water-catalyzed hydrolysis of the l-acyl-l,2,4-triazoles 1-9 in water and in aqueous acetonitrile [x(H,O) = 0.801. The reaction occurs via water-catalyzed nucleophilic attack of water at the amide carbonyl group. Variation of the alkyl group in the acyl part of the substrate led to the sequence of reactivities (in water):This complex behavior strongly suggests the operation of a composite steric effect most likely involving contributions from conformational preference (SEI), change of coordination number at carbonyl carbon (SE2), steric repulsion between 0' and the alkyl group (SE3), and steric inhibition of solvation (SE4). Modelling of the relative rates in water in terms of the expanded branching equation gave satisfactory results. Two possible transition states are proposed for the neutral hydrolysis, one involving intramolecular hydrogen bonding between the second water molecule and N2 of the 1,2,4-triazole ring. The different sequence of reactivities for neutral hydrolysis in aqueous acetonitrile as well as the rates for hydroxide-ion-catalyzed hydrolysis in water (B,,2 mechanism) support the analysis of substituent effects.

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