Kinetics and Mechanism of Azidolysis of Y-Substituted Phenyl Benzoates
Bulletin of the Korean Chemical Society (Print)
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 Brønstedtype 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
... to the pKa of the conjugate acid of N3ion (4.73). Hammett plots correlated with σ o and σconstants 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.