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

The kinetics and mechanism of nucleophilic aromatic substitution reactions of 4‐chloro‐7‐nitrobenzofurazan 1 with 4‐X‐substituted anilines 2a–g (X = OH, OCH 3 , CH 3 , H, I, Cl, and CN) are investigated in a dimethyl sulfoxide (Me 2 SO) solution at 25°C. The Hammett plot of log k 1 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 ( β nuc ) changes from 1.60 to 0.56 as the basicity of anilines decreases. These results indicate a change in a mechanism from a polar S N Ar process for less basic nucleophiles (X = I, Cl, and CN) to a single electron transfer for more basic nucleophiles (X = OH, OCH 3 , and CH 3 ). The satisfactory log k 1 versus E o correlation obtained for the reactions of 1 with anilines 2a–d in the present system is consistent with the proposed mechanism. Interestingly, the β nuc = 1.60 value measured for 1 in Me 2 SO reflects one of the highest coefficients Brønsted ever observed for S N Ar reactions. © 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 152–160, 2013