| Abstract
| - Second-order rate constants, kOHN, M-1 s-1, for the β-elimination reactions of HF with 2-(2-fluoroethyl)pyridine (2), 3-(2-fluoroethyl)pyridine (3), and 4-(2-fluoroethyl)pyridine (4) in OH-/H2O,at 50 °C and μ = 1 M KCl, are= 0.646 × 10-4 M-1 s-1,= 2.97 × 10-6 M-1 s-1, and=5.28 × 10-4 M-1 s-1, respectively. When compared with the second-order rate constants for thesame processes with the nitrogen-methylated substrates 1-methyl-2-(2-fluoroethyl)pyridinium iodide(5), 1-methyl-3-(2-fluoroethyl)pyridinium iodide (6), and 1-methyl-4-(2-fluoroethyl)pyridinium iodide(7), the methyl-activating factor (MethylAF) can be calculated from the ratio kOHNCH3/, and avalue of 8.7 × 105 is obtained with substrates 5/2, a value of 1.6 × 103 with 6/3, and a value of 2.1× 104 with 7/4. The high values of MethylAF are in agreement with an irreversible E1cb mechanism(ANDE* + DN) for substrates 5 and 7 and with the high stability of the intermediate carbanionrelated to its enamine-type structure. In acetohydroxamate/acetohydroxamic acid buffers (pH 8.45−9.42) and acetate/acetic acid buffers (pH 4.13−5.13), the β-elimination reactions of HF, withsubstrates 2 and 4, occur at NH+, the substrates protonated at the nitrogen atom of the pyridinering, even when the [NH+] is much lower than the [N], the unprotonated substrate, due to thehigh proton-activating factor (PAF) value observed: 3.6 × 105 for 2 and 6.5 × 104 for 4 withacetohydroxamate base. These high PAF values are indicative of an irreversible E1cb mechanismrather than a concerted E2 (ANDEDN) mechanism. Finally, the rate constant for carbanion formationfrom NH+ with 2 is kBNH+ = 0.35 M-1 s-1, which is lower than when chlorine is the leaving group(= 1.05 M-1 s-1; Alunni, S.; Busti, A. J. Chem. Soc., Perkin Trans. 22001, 778). This is directexperimental evidence that some lengthening of the carbon−leaving group bond can occur in theintermediate carbanion. This is a point of interest for interpreting a heavy-atom isotope effect.
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