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By combining the results of kinetic measurements with DFT calculations we provide a clearcut evidence of the merging between the E2 and E1cb reaction mechanisms for a large series of leaving groups. Our results solve a longdebated issue in chemical reactivity with profound implications both from a fundamental and biological point of view, thus paving the way to further investigations with different substrates.© WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009

Nucleophilic substitution on meta‐ and para‐substituted methyl benzenesulphonates was studied with two chloride salts with different structures: NBu4Cl or KCl‐Kryptofix 2,2,2. Treating the results with the Acree equation shows that the reaction proceeds by two reaction paths, one involving the chloride ion and the other, slower one, involving the ion pairs. Treating the results with the Hammett equation gives consistent data, and shows that ρ is positive and nearly the same for the two reaction paths (ρ ≈ +2). The reactivity of methyl p‐nitrobenzenesulphonate was compared with that of the corresponding ethyl derivative, and it is shown that the methyl derivative reacts faster than the ethyl derivative in both paths. The results are interpreted based on the assumption that in both paths a negative charge is developed on the leaving group in the transition state, and that the activated complex is linear. Copyright © 2001 John Wiley and Sons, Ltd.

Nucleophilic substitution on meta- and para-substituted methyl benzenesulphonates was studied with two chloride salts with different structures: NBu4Cl or KCl-Kryptofix 2,2,2. Treating the results with the Acree equation shows that the reaction proceeds by two reaction paths, one involving the chloride ion and the other, slower one, involving the ion pairs. Treating the results with the Hammett equation gives consistent data, and shows that ρ is positive and nearly the same for the two reaction paths (ρ ≈ +2). The reactivity of methyl p-nitrobenzenesulphonate was compared with that of the corresponding ethyl derivative, and it is shown that the methyl derivative reacts faster than the ethyl derivative in both paths. The results are interpreted based on the assumption that in both paths a negative charge is developed on the leaving group in the transition state, and that the activated complex is linear. Copyright © 2001 John Wiley and Sons, Ltd.

β-Elimination reactions of N-[2-(4-pyridyl)ethyl]quinuclidinium and N-[2-(2-pyridyl)ethyl]quinuclidinium salts (N) in acetohydroxamate–acetohydroxamic acid buffer, H₂O, μ = 1 M KCl, 50 °C, with the formation of 4- or 2-vinylpyridine, respectively, proceed by an E1cb mechanism with carbon deprotonation occurring in the substrates protonated at the pyridine ring (NH⁺). The formation of the intermediate carbanion has a high degree of reversibility. The systems consistently present H/D exchange. NH⁺ is much more reactive than N; this can be attributed to the strong stabilization of the intermediate carbanion formed from NH⁺ due to resonance.

Nucleophilic substitution on methyl p-nitrobenzenesulfonate has been studied with a series of chloride salts with different structures and solvations: Bu4NCl, PPNCl or KCl complexed by 18-crown-6 or Kryptofix 2,2,2 as well as with a bromide salt (PPNBr) for comparison purposes. The rate constants and the activation parameters are in accordance with an SN2 mechanism. The treatment of the data, following the Acree equation, shows that the process takes place by way of two reaction paths: the first, due to the chloride ion, has the same rate for all the salts, while the second slower path, involving the ion pair, has a rate related to the dissociation constant of the salts.

Second order rate constants kEM−1s−1for the β-elimination reaction from N-[2-(p-nitrophenyl)ethyl]quinuclidinium and 2-(p-nitrophenyl)ethyl bromide induced by amines of different structure in dimethylsulfoxide at 50 °C have been measured. Application of the Brønsted equation shows a similar behaviour of the two substrates, with values of β = 0.649 and 0.584 respectively.