Many organic reactions, including industrially significant reactions in biomass processing and petroleum refining, involve chemical intermediates known as carbocations, which feature positively charged carbon atoms. Carbocations often quickly rearrange before proceeding with a reaction and some rearrangements are slower than others. The slower pace of chain-branching rearrangements has allowed industrial control of the degree of branching of alkanes, an important factor in the quality of the final product, such as the octane rating of a fuel. 
As important as the outcome may be, just why this complex molecular interplay slows down at a key juncture is a question that has eluded researchers for decades. “What people really didn’t understand was how it rearranged — the actual steps in the rearrangement,” says University of Regina theoretical chemistry professor Allan East. He began to investigate these steps more formally about six years ago, eventually unravelling the details of carbocation dynamics.

With the help of three students, East performed 80 simulations of hexyl ion behaviour, optimized 70 transition states for the various elementary steps and generated a complete map of hexyl ion rearrangement. A key focus point was the structure of protonated cyclopropanes (PCP+), which have traditionally been regarded as crucial intermediates in these reactions. Earlier work had concluded that these unstable forms feature protonation on the corner or edge of a ring structure but this research instead pointed to a meso or hybrid state. It turns out that particular meso-PCP+ structures involving primary carbocations are what slow down the chain-branching steps. 

The findings were published earlier this year in the Journal of Organic Chemistry, including rearrangement step classifications and a new table listing the energy barrier heights for various steps. East acknowledges that the patience to run and watch many simulations proved vital to learning the mechanism, which is why the underpinnings of the process had remained unanswered for so long. But he adds that the resulting insights come with some tempting potential. “Now that we know this reaction, we can work towards designing new catalysts that might be able to go in there and help it go faster with less energy cost, while maintaining product control,” East says.