Kinetic Study on SNAr Reactions of 1‐(Y‐Substituted‐phenoxy)‐2,4‐dinitrobenzenes with Azide Ion: Effect of Changing Nucleophile from Hydroxide to Azide Ion on Reaction Mechanism and Reactivity

Second-order rate constants (kN3−) for SNAr reactions of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes (2a–2h) with in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 °C have been measured spectrophotometrically. The Bronsted-type plot is linear with βlg = −0.38. The Hammett plots correlated with and constants exhibit highly scattered points. In contrast, the Yukawa–Tsuno plot results in an excellent linear correlation with ρY = 1.02 and r = 0.51, indicating that a negative charge develops partially on the O atom of the leaving Y-substituted-phenoxy moiety in the transition state. Accordingly, the reactions have been concluded to proceed through a stepwise mechanism, in which expulsion of the leaving group occurs in the rate-determining step. Comparison of kN3− with the kOH− values reported previously for the corresponding reactions with OH− has revealed that is only 6- to 26-fold less reactive than OH− toward substrates 2a–2h, although the former is over 11 pKa units less basic than the latter. Solvation and polarizability effects have been suggested to be responsible for the unusual reactivity shown by and OH−. Effects of changing nucleophile from OH− to on reaction mechanism and reactivity are discussed in detail.