Kinetic Study on SNAr Reaction of 1-Y-Substituted-phenoxy-2,4-dinitrobenzenes with Hydroxide Ion: Effect of Substituent Y on Reactivity and Reaction Mechanism

A kinetic study is reported for the SNAr reaction of 1-Y-substituted-phenoxy-2,4-dinitrobenzenes (1a-1h) with OH- in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The second-order rate constant (k(OH)-) increases as the substituent Y in the leaving group changes from an electron-donating group (EDG) to an electron-withdrawing group (EWG). The Bronsted-type plot for the reactions of 1a-1h is linear with beta(lg) = -0.16, indicating that the reactivity of substrates 1a-1h is little affected by the leaving-group basicity. A linear Bronsted-type plot with beta(lg) = -0.3 +/- 0.1 is typical for reactions reported previously to proceed through a stepwise mechanism in which formation of a Meisenheimer complex is the rate-determining step (RDS). The Hammett plot correlated with ay constants results in a much better correlation than that correlated with cry constants, implyng that no negative charge is developing on the O atom of the leaving group (or expulsion of the leaving group is not advanced at all in the TS). This excludes a possibility that the SNAr reaction of 1a-1h with OH- proceeds through a concerted mechanism or via a stepwise pathway with expulsion of the leaving group being the RDS. Thus, the current reactions have been concluded to proceed through a stepwise mechanism in which expulsion of the leaving group occurs rapidly after the RDS.