Kinetic study on S<inf>N</inf>Ar 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

Abstract

Second-order rate constants (k<inf>N3-</inf>) for S<inf>N</inf>Ar reactions of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes (2a-2h) with (Formula presented.) in 80 mol % H<inf>2</inf>O/20 mol % DMSO at 25.0 ± 0.1 °C have been measured spectrophotometrically. The Brønsted-type plot is linear with β<inf>1g</inf>=-0.38. The Hammett plots correlated with (Formula presented.) constants exhibit highly scattered points. In contrast, the Yukawa-Tsuno plot results in an excellent linear correlation with <inf>ρY</inf> = 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 k<inf>N3-</inf> with the k<inf>OH-</inf> values reported previously for the corresponding reactions with OH¯ has revealed that (Formula presented.) is only 6- to 26-fold than OH¯ toward substrates 2a-2h, although the former is over 11 pK<inf>a</inf> units less basic than the latter. Solvation and polarizability effects have been suggested to be responsible for the unusual reactivity shown by (Formula presented.) and OH¯. Effects of changing nucleophile from OH¯ to (Formula presented) on reaction mechanism and reactivity are discussed in detail. © 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.