Carbocation Mechanism Resolved

Abstract

Chemists have known for 70 years that carbocation rearrangements involve slow steps and fast steps. The slow steps are crucial in that they control the degree of branching in alkanes, and understanding how that works is vital for predicting product distributions in processes such as biomass and petroleum refining. But why the key steps are slow has been a confounding mystery. Daniel J. S. Sandbeck, Daniel J. Markewich, and Allan L. L. East of the University of Regina now appear to have found the answer via a set of computer simulations (J. Org. Chem. 2016, DOI: 10.1021/acs.joc.5b02553). The team first revisited decades-old studies in which chemists proposed that the rearrangements proceed through a protonated cyclopropane intermediate. Then using hexyl ion as a model, the researchers ran simulations and uncovered 70 transition states connecting primary, secondary, and tertiary ion versions. In the past, most chemists thought the cyclopropane intermediate must be