Mechanistic studies in strongly basic media. Part VIII. Kinetic studies on the displacement of halide from benzyl halides by alkali-metal salts of 9-substituted fluorenes in t-butyl alcohol solution

Abstract

The kinetics of the displacement of halide from benzyl fluoride, chloride, and bromide by alkali-metal 9-cyano-, 9-methoxycarbonyl-, and 9-phenylsulphonylfluorenides have been studied in t-butyl alcohol solution in which the salts have been shown to exist largely in the form of ion pairs. The effect of variation in the reactant concentrations and of the addition of a common cation salt, sodium tetraphenylborate, on the kinetic form and reaction rate have been examined. The reaction of sodium 9-cyano- and 9-phenylsulphonyl-fluorenides with the three halides show essentially clean second-order kinetics with rate constants little affected by dilution of the carbanion salt or addition of the common ion. Reactions involving 9-methoxycarbonylfluorenide, however, display an increase in the second-order velocity constant as the reactions progress, and sodium tetraphenylborate retards the reaction; both observations indicate that the free carbanion is substantially more reactive than the associated form(s). Since sodium 9-cyanofluorenide is more dissociated than the 9-methoxycarbonyl salt, the absence of these kinetic effects implies that the free ion and ion pair are of equal reactivity in this system.