Nucleophilic attacks on LL (Low LUMO) substrates. Part 3. Molecular stacking of 9-methylenefluorene derivatives as a source of zero-order reactions

The reactions of the three activated dibenzfulvene derivatives 9-dicyanomethylene-, 9-dinitromethylene-, and 9-nitromethylene-fluorene (FDCN, FDN, and FN respectively) with NaOH in water containing 1% dioxane were followed at 25°C. The reactions were found to be zero-order in the substrate and zero-order within a run in the hydroxide. Overall, the reaction was first-order in the hydroxide. Under these conditions zero-order kinetics are also obtained for the reaction of CN^- with FDCN. 'Normal' first-order kinetics were obtained under these conditions for the reaction of OH^- with 1,1-diphenyl-2,2-dinitroethylene as well as for FDN when the dioxane concentration was increased to 25% (v/v). It is suggested that owing to their planar structure these substrates undergo efficient stacking in the solution with only a small fraction left as a free reactive monomer at the saturation concentration. A relatively rapid equilibrium between the monomer and the aggregate maintains a constant monomer concentration in the solution. A Tyndall effect of the aggregated particles could not be observed with the naked eye. The particles also pass through a Whatman No. 1 filter paper. However, other observations such as solvent and temperature effects on the absorbance in the u.v. range strongly support the suggested explanation.