A kinetic study is reported for the reaction of the anionic nucleophiles OH-, CN-, and N 3 - with aryl benzoates containing substituents on the benzoyl as well as the aryloxy moiety, in 80 mol % H2O - 20 mol % dimethyl sulfoxide at 25.0 degrees C. Hammett log k vs sigma plots for these systems are consistently nonlinear. However, a possible traditional explanation in terms of a mechanism involving a tetrahedral intermediate with curvature resulting from a change in rate-determining step is considered but rejected. The proposed explanation involves ground-state stabilization through resonance interaction between the benzoyl substituent and the electrophilic carbonyl center in the two-stage mechanism. Accordingly, the data are nicely accommodated on the basis of the Yukawa-Tsuno equation, which gives linear plots for all three nuceophiles. Literature reports of the mechanism of acyl transfer processes are reconsidered in this light.

A kinetic study is reported for the reactions of 4-nitrophenyl phenyl carbonate (5) and thionocarbonate (6) with a series of alicyclic secondary amines in 80 mol% H(2)O-20 mol% DMSO at 25.0 +/- 0.1 degrees C. The plots of k(obsd) vs. amine concentration are linear for the reactions of 5. On the contrary, the plots for the corresponding reactions of 6 curve upward as a function of increasing amine concentration, indicating that the reactions proceed through two intermediates (i.e., a zwitterionic tetrahedral intermediate T(+/-) and its deprotonated form T(-)). The Brønsted-type plot for 5 the reactions of with secondary amines exhibits a downward curvature, i.e., the slope decreases from 0.98 to 0.26 as the pK(a) of the conjugate acid of amines increases, implying that the reactions proceed through T(+/-) with a change in the rate-determining step (RDS). The k(N) values are larger for the reactions of with secondary amines than for those with primary amines of similar basicity. Dissection of k(N) values for the reactions of 5 into the microscopic rate constants (i.e., k(1) and k(2)/k(-1) ratio) has revealed that k(1) is larger for the reactions with secondary amines than for those with isobasic primary amines, while the k(2)/k(-1) ratio is nearly identical. On the other hand, for reactions of 6, secondary amines exhibit larger k(1) values but smaller k(2)/k(-1) ratios than primary amines. The current study has shown that the reactivity and reaction mechanism are strongly influenced by the nature of amines (primary vs. secondary amines) and electrophilic centers (C[double bond]O vs. C[double bond]S).

Second-order rate constants have been measured spectrophotometrically for the reactions of O-2,4-dinitrophenyl thionobenzoate (1) and 2,4-dinitrophenyl benzoate (2) with a series of substituted pyridines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Brønsted-type plots obtained are nonlinear with beta(1) = 0.26, beta(2) = 1.07, and pK(a) degrees = 7.5 for the reactions of 1 and beta(1) = 0.40, beta(2) = 0.90, and pK(a) degrees = 9.5 for the reactions of 2, suggesting that the pyridinolyses of 1 and 2 proceed through a zwiterionic tetrahedral intermediate T(+/-) with a change in the rate-determining step at pK(a) degrees = 7.5 and 9.5, respectively. The thiono ester 1 is more reactive than its oxygen analogue 2 except for the reaction with the strongest basic pyridine studied (pK(a) = 11.30). The k(1) value is larger for the reactions of 1 than for those of 2 in the low pK(a) region, but the difference in the k(1) value becomes negligible with increasing the basicity of pyridines. On the other hand, 1 exhibits slightly larger k(2)/k(-1) ratio than 2 in the low pK(a) region but the difference in the k(2)/k(-1) ratio becomes more significant with increasing the basicity of pyridines. Pyridines are more reactive than alicyclic secondary amines of similar basicity toward 2 in the pK(a) above ca. 7.2 but less reactive in the pK(a) below ca. 7.2. The k(1) value is slightly larger, but the k(2)/k(-1) ratio is much smaller for the reactions of 2 with pyridines than with isobasic secondary amines in the low pK(a) region, which is responsible for the fact that the weakly basic pyridines are less reactive than isobasic secondary amines.

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Second-order rate constants (kN) have been measured spectrophotometrically for reactions of Y-substituted phenyl benzoates (1a-h) with azide ion (N3 -) in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 o C. The Brønsted-type plot for the azidolysis exhibits a downward curvature, i.e., the slope (βlg) changes from -0.97 to -0.20 as the basicity of the leaving group decreases. The pKa o (defined as the pKa at the center of the Brønsted curvature) is 4.8, which is practically identical to the pKa of the conjugate acid of N3 -ion (4.73). Hammett plots correlated with σ o and σ -constants exhibit highly scattered points for the azidolysis. On the contrary, the corresponding Yukawa-Tsuno plot results in an excellent linear correlation with ρ = 2.45 and r = 0.40, indicating that the leaving group departs in the rate-determining step. The curved Brønsted-type plot has been interpreted as a change in the rate-determining step in a stepwise mechanism. The microscopic rate constants (k1 and k2/k-1 ratio) have been calculated for the azidolysis and found to be consistent with the proposed mechanism.