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

The rate of rearrangement of hexadeuterated 2,3-dimethylbutan-2-ol 1A in sulfuric acid solution in D2O was measured by means of H-2 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-[C-13] as a substrate in (H2O)-O-18, the ratio of the rearrangement rate and the rate of the exchange of the hydroxy oxygen was found to be k(-1)/k(2) = 9.5. Consequently, the highest point on the energy surface in the overall rearrangement reaction presented in Scheme 1 is the hydride-bridged 2,3-dimethylbutyl cation. The KR+ = [ROH][H+]/[R+] = 2.2 x 10(15) was obtained. The concentration of the carbocation 2 obtained under experimental conditions (20 vol% H2SO4 in D2O at 40 degrees C) was calculated to be [R+] = 6 x 10(-15) M.