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Nonlinear Brønsted and Hammett Correlations Associated with Reactions of 4-Chloro-7-nitrobenzofurazan with Anilines in Dimethyl Sulfoxide Solution

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Abstract

The kinetics and mechanism of nucleophilic aromatic substitution reactions of 4-chloro-7-nitrobenzofurazan 1 with 4-X-substituted anilines 2a–g (X = OH, OCH3, CH3, H, I, Cl, and CN) are investigated in a dimethyl sulfoxide (Me2SO) solution at 25°C. The Hammett plot of log k1 versus σ is nonlinear for all the anilines studied due to positive deviations of the electron-donating substituents. However, the corresponding Yukawa–Tsuno plot resulted in a good linear correlation with σ+r (σ+−σ). The corresponding Brønsted-type plot is also nonlinear, i.e., the slope (βnuc) changes from 1.60 to 0.56 as the basicity of anilines decreases. These results indicate a change in a mechanism from a polar SNAr process for less basic nucleophiles (X = I, Cl, and CN) to a single electron transfer for more basic nucleophiles (X = OH, OCH3, and CH3). The satisfactory log k1 versus Eo correlation obtained for the reactions of 1 with anilines 2a–d in the present system is consistent with the proposed mechanism. Interestingly, the βnuc = 1.60 value measured for 1 in Me2SO reflects one of the highest coefficients Brønsted ever observed for SNAr reactions. © 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 152–160, 2013

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