The acid-catalyzed rearrangement of deuterium-labeled 2,3-dimethylbutan-2-ol; the concentration of the intermediate cation

Abstract

The rate of rearrangement of hexadeuterated 2,3-dimethylbutan-2-ol 1A in sulfuric acid solution in D₂O was measured by means of ²H NMR. Kinetic data revealed that after the acid-induced formation of the carbocation intermediate, it rearranges through two parallel paths: (a) by way of a 1,2-hydride shift, and (b) by way of elimination of the methine proton and readdition of water. The elimination/addition reaction is approximately seven times slower than the rearrangement via the hydride shift, which is the major reaction pathway. In an experiment carried out with 2,3-dimethylbutan-2-ol-2-[¹³C] as a substrate in H₂¹⁸O, the ratio of the rearrangement rate and the rate of the exchange of the hydroxy oxygen was found to be k_–1/k₂ = 9.5. Consequently, the highest point on the energy surface in the overall rearrangement reaction presented in Scheme 1 is the hydrido-bridged 2,3-dimethylbutyl cation. The K_R⁺ = [ROH][H⁺]/[R⁺] = 2.2 × 10¹⁵ was obtained. The concentration of the carbocation 2 obtained under experimental conditions (20 vol% H₂SO₄ in D₂O at 40 °C) was calculated to be [R⁺] = 6 × 10^–15 M.