A new method for the determination of the relative acidities of alcohols in alcoholic solutions. The nucleophilicities and competitive reactivities of alkoxides and phenoxides

Abstract

A new semiquantitative method has been developed for measuring the relative acidities of methanol, ethanol, isopropyl alcohol, and tert-butyl alcohol in mixed hydroxylic solvents. A solution of the alkoxides of two alcohols in an excess of the two alcohols is allowed to react with n-butyl bromide to form a mixture of two ethers. The composition of the ether mixture is a measure of the "competitive reactivity" of thetwo alkoxides. This can be measured directly, and in theory can be factored into two components: the relative nucleophilicity and the relative basicity of the two alkoxides. Relative nucleophilicities are determined by using solutions in which phenol is one component. Knowing the competitive reactivities and nucleophilicities, the relative acidities of methanol, ethanol, isopropyl alcohol, and tert-butyl alcohol in alcoholic media are shown to be 4.4, 1.0, 0.24, and 0.21, respectively. The relative nucleophilicities of hydroxide, methoxide, ethoxide, isopropoxide, tert-butoxide, phenoxide, and m-cresoxide are 0.08, 0.82, 1.0, 0.4, 0.04, 0.46, and 0.57 respectively.