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

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, 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.

Nonlinear Taft Polar Free Energy Relationship: Reactions of N‐Substituted Benzyl Amines with Benzyl Bromide in Methanol

“…This shows that the reactions are highly sensitive to the basicity of the amines used irrespective of the steric hindrances of the substituents on the amine moiety. Similar observations were made by Goumont et al and Um et al in the reactions of anilines with 4‐chloro‐7‐nitrobenzofuran and pyridinolysis of O ‐4‐nitrophenyl benzoates and thionobenzoates. The p K a values of the amines in water with R = H (9.34), methyl (9.58), and ethyl (9.68) substituents are from .…”

“…A plot of log k versus pK a values of amines again showed a linear correlation ( Fig. 4) with two different straight lines with β values of 3.68 and −9.74 for electron-withdrawing substituents and [15,20] in the reactions of anilines with 4-chloro-7-nitrobenzofuran and pyridinolysis of O-4-nitrophenyl benzoates and thionobenzoates. The pK a values of the amines in water with R = H (9.34), methyl (9.58), and ethyl (9.68) substituents are from [28].…”

“…Such kinds of studies using physical properties such as melting points and dipole moments of benzoic acids were even reported from our laboratory [10]. But to the authors' knowledge, the present article presents the first of its kind to observe a nonlinear Taft correlation, though there are several reports on nonlinear Hammett correlations in the literature [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27].…”

The ambident electrophilic behavior of 5‐nitro‐3‐X‐thiophenes in σ‐complexation processes

“…Careful examination of the literature has shown that Brönsted‐type plots with slopes β nuc > 1 are rarely observed in σ‐complexation reactions. However, β nuc values close to or greater than 1 have been reported in the literature for nucleophilic aromatic substitutions reactions, for example, β nuc = 1.24 for reaction of N1‐methyl‐4‐nitro‐2,1,3 benzothiadiazolium tetrafluoroborate with a series of 4‐X‐substituted anilines (X = OH, OMe, Me, and H) in acetonitrile at 20°C, and β nuc = 1.60 for reactions of 4‐chloro‐7‐nitrobenzofurazan with 4‐X‐substituted anilines (X = OH, OMe, Me, H) in dimethyl sulfoxide solution at 25°C . In this regard, it is interesting to note that unusual high Brønsted β nuc value of 1.22 has also been reported by El‐Bardan in the reaction of phenoxide anions with 2‐chloro‐5‐nitropyridine in methanol …”

Kinetics of the reaction of phenyl picrates with phenoxide ions in water. Concerted or stepwise?

“…That this value is higher than those, 0.5-0.7, normally associated [2] with nucleophilic substitutions may beattributed to the strongly electron-withdrawing effect of the trinitrobenzene ring. Values of β close to or higher than one have been attributed to the intervention of an SET pathway [21,22] for substitution but there is no evidence that this applies here. Phenols were the purest available commercial specimens.…”