A kinetic study is reported for reactions of 2‐methoxy‐3‐X‐5‐nitrothiophenes 1a–d (X = SO2CH3, CO2CH3, CONH2, H) with piperidine in different solvents at 20°C. It is shown that the reactions take place through a SNAr mechanism with the initial nucleophilic addition step being rate limiting. The satisfactory Hammett correlations (log k1 vs. σnormalp−) obtained in the present system confirms that a 3‐X substituent exerts an effect on the 2‐position of the same type as that exerted from the 5‐position. The second‐order rate constants associated with these reactions are employed to determine the electrophilicity parameters E of the thiophenes 1a–d according to the relationship log k (20°C) = s(E + N) (Angew. Chem., Int. Ed. Engl. 1994, 33, 938–957). The E values of 1a–d are found to cover a range from −21.33 to −17.18, going from 1d, the least reactive, to 1a, the most reactive thiophene. Interestingly, a linear correlation (r2 = 0.9910) between the electrophilicity parameters E determined in this work and the Hammett's σnormalp− constants values has been observed and discussed. On the other hand, we have found that the reported rate constants of some thiophenes 1 complexation by the methoxide ion in methanol are 3.5–73.5 times higher than predicted by Mayr's approach.

Second-order rate constants for the reactions of 2-aryl-4,6-dinitrobenzotriazole 1-oxides 1a-d with some 4-X-substituted phenoxide ions 2a-d (X = OCH 3 , H, Cl, and CN) have been measured in aqueous solution at 20 • C. The pK a values for the σ -complexation processes of a series of benzotriazole 1a-d measured in water have been used to determine their electrophilicity parameters E according to the correlation E = -3.20 -0.662 pK a (F. Terrier, S. Lakhdar, T. Boubaker, and R. Goumont, J Org Chem, 2005, 70, 6242-6253). For these reactions, plots of log k versus the electrophilicity parameters E of the benzotriazoles 1a-d were linear, allowing to derive the nucleophilicity parameters N and s for phenoxide ions as defined by the Mayr equation log k 1 (20 • C) = s (E + N) (H. Mayr, M. Patz. Angew Chem, Int Ed Engl 1994, 33, 938-957). The N values are found to cover a range of nucleophilicity from 6.85 to 10.22, going from 4-cyanophenoxide 2d for the least reactive ion to 4-methoxyphenoxide 2a for the most reactive nucleophile. Good linear correlations were found between the nucleophilicity parameters N of phenoxide ions 2a-d and the pK a values of their conjugate acids (N = -3.05 + 1.25 pK a ) and the σ p constants of the substituents X (N = 9.21 -2.51σ p ). C 2014 Wiley Periodicals, Inc. Int J Chem Kinet 46: 711-717, 2014

Kinetics of the reactions of 3,5‐dinitrothiophene 1 and 3‐cyano‐5‐nitrothiophene 2 with a series of parasubstituted phenoxide anions 3a–c have been investigated in aqueous solution at 20°C. Two unsubstituted electrophilic centers (C(2) and C(4)) of the two thiophenes have been identified. The Fukui functions correctly predict the C(2) and C(4) atoms as the most electrophilic centers of these electron‐deficient thiophenes 1 and 2. Analysis of the experimental data in terms of Brønsted relationships reveals that the reaction mechanism likely involves a single‐electron transfer (SET) process. The excellent correlations upon plotting the rate constants versus the oxidation potentials Eo values is an additional evidence that reactions between thiophenes and phenoxide anions are proceeding through an initial electron transfer. It is of particular interest to note that the systems studied in this paper provide a rare example of a SET mechanism in σ‐complexation reactions. According to the free energy relationship log k = s(N + E) (Angew. Chem., Int. Ed. Engl., 1994, 33, 938–957), the electrophilicity parameters E of the C‐4 and C‐2 positions of the thiophenes have been determined and compared with the reactivities of other ambident electrophiles. On the other hand, the second‐order rate constants for the reactions of these thiophenes with the hydroxide ion has been measured in water and 50% water–50% acetonitrile and found to agree with those calculated theoretically using Mayr's equation from the E values determined in this work and from the previously published N and s parameters of OH−.

Combined experiments and modelling rationalize the large influence of solvent composition on pyrrolidine nucleophilicity.