Mechanisms in the reaction between olefins and alcohols catalyzed by ion exchange resins

Abstract

An experimental study was carried out on the addition of alcohols to tertiary olefins catalyzed by a macroporous sulfonic acid resin. Initial rates of the reaction between methanol or n-butanol with isobutene show different mechanisms depending on the ratio of the reactants. At stoichiometric or higher alcohol/isobutene ratios, the initial rates show a zero order on the alcohol and a first order on the olefin; in this case the experimental data agree with an ionic mechanism wherein the protonation of the olefin by the solvated proton is the rate determining step. In such conditions the influence of the alcohol is displayed only by control of the protonetlon power of the catalyst, and the higher reactivity of n-butanol vs. methanol should reflect the acidity order of the corresponding solvated proton. On reducing the alcohol/olefin ratio, a negative order on the methanol and an increase in the rates are observed, until a maximum in the rate is reached; a further reduction leads to a zero order on the olefin and a first order on the alcohol. The mechanism agrees in this case with a concerted reaction on the associated network of sulfonic groups. The lower reactivity found for the n-butanol can be attributed to the difficulty of the polymeric structure adapting to the higher steric hindrance of n-butanol.