Solvents effects on aromatic nucleophilic substitutions. Part 5. Kinetics of the reactions of 1-fluoro-2,4-dinitrobenzene with piperidine in aprotic solvents

Abstract

The kinetics of the reaction between 1-fluoro-2,4-dinitrobenzene and piperidine were studied in toluene, benzene, dioxane, trichloroethane, tetrahydrofuran, chlorobenzene, ethyl acetate, chloroform, dichloromethane, acetone, acetonitrile, and nitromethane. The second-order rate coefficient is almost insensitive to the amine concentration in chloroform, acetonitrile, and nitromethane; for the other solvents this value of k₃/k₂ is greater than 10², indicating neat base catalysis. The trend in k₃/k₂ does not follow the solvent basicity order; the solvents insensitive to base catalysis are those which exhibit hydrogen-bond donor (HBD) properties. These results are interpreted as an indication that in this reaction the detachment of the nucleofuge is the rate-limiting step in most of the aprotic solvents studied; those solvents which are HBD assist the departure of fluoride, and the formation of the intermediate is the rate-determining step. This conclusion was confirmed by k_F/k_Cl determinations. A simple linear energy solvation correlation was found between the parameter E_T(30) and k_A obtained at [B] < 10^–2M. For higher amine contents increasing deviations were found.