Gas-phase SN2 rethink

02 April 2016 - Chemical modelling

Interesting perspective by Xie & Hase in this week's Science on the topic of bimolecular nucleophilic substitution (SN2), The title "Rethinking the SN2 reaction" sounds alarming but only the gas-phase variety is in need of a reconsideration. So what is going on: the textbooks all present SN2 (X` + CH3Y -> XH3C + Y`) as a classic and clean second order reaction with a single transition state and with inversion of configuration. Add more detail and attractive ion dipole pair interactions appear resulting in two wells flanking the TS. But Xie and Hase detect more issues standing in the way of accurately modelling this reaction type. It seems more like modelling a car crash. The approach of the anion is not limited to the backside of the halide: the anion can approach head-on or it can even fly-by. There is also a complex roundabout dance and what seems like a case of retention of configuration can also be a double inversion. Of course changing one halide for another or introducing solvent molecules complicate things further. By now you are looking at an army of modellers and experimental physical chemists to get things sorted out. Main conclusion: more work needs to be done. In this way the perspective seems more directed at influential people with grant money to distribute than at fellow scientists.