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, kE, and for exchange of tritium at the 9-position (β to the hydroxy-group), kxT, 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, kxH, and an isotope effect upon elimination, kEH/kED= 7·2. Comparison of kEH/kED 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, kEH/kED 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 kxH, permits calculation of a rate constant for tritium exchange, kxT, 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.