The proton exchange reaction between indene and the indenyl anion

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Abstract
The proton exchange reaction between the indenyl carbanion and its parent compound indene has been studied by NMR as a function of temperature. The rate of this bimolecular reaction is very low and has been found to be strongly dependent on the polarity of the solvent. In solvents like dimethoxyethane (ϵ = 7·2) and diglyme the reaction becomes manifest in the NMR spectrum only at elevated temperatures (T > 150°C). In hexamethylphosphortriamide (ϵ = 30) the rate is much greater and line broadening may be observable at room temperature. The reaction in this solvent is characterised by a frequency factor f = 7 × 107 1 mol−1 s−1, an activation enthalpy ΔH ≠ = 9·5 kcal mol−1 and an entropy of activation ΔS≠ = −23 e.u. The low reaction rate and its solvent dependence are briefly discussed.