The overall kinetic solvent isotope effects on the acid catalyzed hydrolysis of a series of 2-aryl-2-diazocarboxylic esters ArCN2COOCH3, and one 2-aryl-2-diazocarboxamide C6H5CH2CON (CH,), vary inversely with the reactivity of the substrate, between limits of 3.14 and 1.46. A linear Hammett plot for the hydrolysis rates of the a-diazocarboxylic esters indicates that there is no mechanistic change for the hydronium-ion-catalyzed reaction. The relation between hydrolysis rate and buffer acid concentration deviates from linearity for high values of the latter. It is shown on the basis of the solvent isotope effects for the non-linear region that this deviation does not stem from a mechanistic change caused by the buffer base component. The specific salt effects on the general acid-catalyzed reaction are discussed.
Introduction.
-The values of solvent kinetic isotope effects are often employed for the elucidation of hydrolysis mechanisms: a value of kH2,/kD2,z 1 normally implies a rate-determining protonation (A SE2-mechanism), whereas kH20/kD20 < 0.5 is consistent with a protonation equilibrium preceding a rate-determining substitution (A 2-or A 1-mechanism).
Application of this criterion to hydrolyses of aliphatic diazo compounds has led to mechanistic distinction according to the degree of substitution of the diazo C-atom. Deactivated primary diazoalkanes, e.g. diazoketones, are hydrolyzed by the A2-mechanism, whereas introduction of an alkyl or aryl substituent entails an ASE2-mechanism [lI3). Variation in the solvent isotope effect on hydrolysis of a series of diazo compounds, has been interpreted in terms of differences in transition state geometries for the proton transfer from the hydronium ion to the diazo substrate. In particular the A SE2-hydrolyses of a series of aryl diazoalkanes I)