Ion pairs in solvolysis reactions. Kinetic deuterium isotope effects for deprotonation of carbocation intermediates in aqueous solvent

Abstract

AbstractSolvolysis of 2‐X‐2‐phenylpropane (1‐X) in 25 vol% acetonitrile in water at 25°C produces 2‐hydroxy‐2‐phenylpropane (1‐OH) and 2‐phenylpropene (3). The carbocationic intermediate discriminates between different nucleophiles; azide anion, acetate anion, and methanol are more efficient nucleophiles than water, k/k = 42 kOAc/k = 3, and kMeOH/k = 2·9 (ratio of second‐order rate constants). The fraction of the elimination product 3 increases with increasing basicity of the leaving group X as well as by addition of general bases. The Brønsted parameter for this catalysis is small, β = 0·13, with substituted acetate anions. The kinetic deuterium isotope effect for the dehydronation of the intermediate has been measured (assuming the reaction from intermediate to alcohol is insensitive to isotopic substitution) employing the hexadeuterated substrate d6‐1‐X as k/k = 3·5 ± 0·2 for the chloride 1‐Cl with acetate anion, and, without added base, 3·1 ± 0·2 for the acetate 1‐OAc, and 3·1 ± 0·2 for the p‐nitrobnzoate 1‐PNB, respectively, and ∼5 for the protonated methyl ether 1‐OMeH+. The variation in isotope effect with change in leaving group is discussed in terms of elimination from contact ion pairs and ‘free’ carbocation. The overall kinetic isotope effect for the solvolysis was found to be k/k = 1·31 (1‐OMeH+), 1·38 (1‐OAc), 1·40 (1‐PNB), and 5·7 (1‐OH). These isotope effects consist of the isotope effect k/k for the formation of the substitution product 1‐OH and k/k for production of the olefin 3. It is concluded that the latter isotope effect is enlarged owing to a branched mechanism in which the deprotonation of the carbocationic intermediate competes with formation of the substitution product. As large an isotope effect as k/k ∼6·5 has been measured for 1‐OMeH+.