Reactions in strongly basic solutions. Part I. Kinetics and mechanism of the alkylation and benzylation of the 9-cyanofluorenyl anion

The rate coefficients for the reaction in ethanol of a number of alkyl and benzyl halides with the 9-cyanofluorenyl anion have been measured. The high reactivity and low selectivity of the carbanion, considered to be due to its high polarisability, are compared with those for other nucleophiles. Increased rates are observed for benzyl halides with both electron-releasing and electron-withdrawing substituents. This is attributed to changes in the relative amounts of bond-making and bond-breaking in the transition state. The nucleophilic reactivity parameter, n°MeI, in methanol has been evaluated. Unusually, added dimethyl sulphoxide decreases the rate of reaction. This is taken to indicate that the carbanion is not significantly solvated in ethanol. These observations, together with salt effects, are discussed in terms of a simple mechanism involving a bimolecular rate-determining step.