β-Elimination of 9-fluorenylmethanol in aqueous solution: an E1cB mechanism

Abstract

9-Fluorenylmethanol undergoes base-catalysed β-elimination in aqueous sodium hydroxide to yield dibenzofulvene. Comparison of rate constants for elimination, k_E, and for exchange of tritium at the 9-position (β to the hydroxy-group), k_x^T, together with the observation of kinetic induction periods upon elimination of [9-¹H₁]9-fluorenylmethanol in D₂O and [9-²H₁]9-fluorenylmethanol in H₂O, indicate that a carbanion is formed in the course of reaction. A kinetic analysis of the induction period for reaction of the protio-compound in D₂O allows determination of a rate constant for its exchange, k_x^H, and an isotope effect upon elimination, k_E^H/k_E^D= 7·2. Comparison of k_E^H/k_E^D in D₂O with the solvent isotope effect upon elimination of the protio-compound in H₂O and the deuterio-compound in D₂O allows a distinction to be drawn between E2 and E1cB mechanisms of elimination. It is concluded that the observed value of 0·92 for the solvent isotope effect is too small to incorporate the primary effect of 7·2 required by the E2 mechanism, but is of the right magnitude for an E1cB mechanism.

For an E1cB mechanism, k_E^H/k_E^D corresponds to the isotope effect upon ionization of 9-fluorenylmethanol to form the carbanion, and use of the Swain–Schaad relation between deuterium and tritium isotope effects, together with the observed value of k_x^H, permits calculation of a rate constant for tritium exchange, k_x^T, which agrees well with the independently measured value. Less satisfactory agreement for the corresponding comparison in H₂O is ascribed to experimental error, and either to the presence of ‘internal return’ from a specifically solvated carbanion, or to the contribution of a minor component of E2 elimination.