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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 35, Number 7
BKCSDE 35(7)
July 20, 2014 

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
Tae-Ah Kang, Hyo-Jin Cho, Ik-Hwan Um*
SNAr reaction, 1-Phenoxy-2,4-dinitrobenzene, Brønsted-type plot, Hammett plot, 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 oC. The second-order rate constant (kOH−) increases as the substituent Y in the leaving group changes from an electron-donating group (EDG) to an electronwithdrawing group (EWG). The Brønsted-type plot for the reactions of 1a-1h is linear with βlg = –0.16, indicating that the reactivity of substrates 1a-1h is little affected by the leaving-group basicity. A linear Brønsted-type plot with βlg = –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 σY o constants results in a much better correlation than that correlated with σY − 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.
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