The Compensation in ΔH+ and ‡S+ Accompanying the Conversion of Lower Order Nucleophilic Displacement Reactions to Higher Order Catalytic Processes. The Temperature Dependence of the Hydrazinolysis and Imidazole-Catalyzed Hydrolysis of Substituted Phenyl Acetates

Thomas C. Bruice, Stephen J. Benkovic

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Abstract

The hydrazinolysis and imidazole-catalyzed hydrolysis of a series of substituted phenyl acetates were found to be dependent on the concentration of the conjugate acid and base species of the nitrogen bases in the following manner: -d(ester)/dt = [kn (H2NNH2) + kgb(H2NNH2)2 + kga(H2NNH2)(H2XNH3⊕)](ester) and -d-(ester)/dt = (kn(C3H4N2) + kgb(C3H4N2)2](ester), respectively. The terms kn are second-order rate constants for nucleophilic displacement at the ester bond, and the kgb and kga terms are third-order rate constants for general-base and general-acid assisted nucleophilic displacement reactions at the ester bond. The kn - ρ value for hydrazine is considerably greater than that for ammonia, so that the α-effect is greatest with the best leaving group. Also, the α-effect is of greater importance in kgb and in kga than in kn- The ρ-values for the kgb and terms are very much smaller than those for the kn terms, as previously found for general base-catalyzed ammonolysis reactions. The consistent finding that the p-values for the third-order, generally assisted reactions are always much less than the p-values for the simple unassisted bimolecular reactions has the result that the assisted reactions are always of much greater importance for nucleophilic displacement reactions on esters with poorer leaving groups. The kgb term for hydrazine, as previously found for ammonia, exhibits no deuterium solvent kinetic isotope effect; however, the kgb term for imidazole does. Unlike the kgb, term, the kga term for hydrazine does exhibit a deuterium solvent isotope effect. The ΔH + for the kn term does not appear to be simply related to the nature of the nucleophile nor the substituent on the phenyl ester. With change in the electronic nature of substituent groups on the ester, the TΔS± term may follow a consistent pattern of decrease, of increase, or a random variation. The electronic effects on TΔS+ may be considered to be secondary perturbations of that value determined by the nucleophile and the kinetic order of the reaction. By averaging the TΔS+ values for the series of esters for a particular term (i.e., kn, kgb, or kga) with a particular nucleophile, the electronic effects of substituent groups may be compensated for. The kinetic order of the reaction when multiplied by 5 ± 1 kcal. mole-1 provides in each case (eleven reactions) the value of TΔS+av. This result is discussed on the basis of the arbitrary division of TΔS+ into a component (TΔAS+t.r.) determined by the number of species which must be. brought to a position of close approach in order to enter the transition state and a component (TΔS +e) which relates to the kinetic energy terms in taking the reactants from a position of close approach to the transition state. The expected lowering of ΔH+ and TΔS+ accompanying the conversion of a lower order displacement reaction to a higher order catalytic process has been examined. The values of ΔH+ and TΔS+ for kn, kgb, and kga for the hydrazinolysis reactions are found to behave in a compensatory manner so that ΔF+ becomes only slightly more negative for the catalytic reactions.

Original languageEnglish (US)
Pages (from-to)418-426
Number of pages9
JournalJournal of the American Chemical Society
Volume86
Issue number3
DOIs
StatePublished - Feb 1 1964

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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