One of the best-known chemical reaction mechanisms – the S_N2 substitution – sometimes proceeds in a completely different way than expected, as quantum chemical models by a team from Leiden and Amsterdam show.

You learn this chemical reaction mechanism as early as upper secondary school these days: the bimolecular nucleophilic substitution reaction (SN2), in which the attacking particle (nucleophile) attacks the substrate (electrophile) from the back and the leaving group leaves the substrate at almost the same time (see image below). However, in their experiments eight years ago, Wouter Remmerswaal, Jeroen Codée, Thomas Hansen and colleagues observed reaction products that cannot be explained via this mechanism. They have now finally solved part of the puzzle, they show in Chemistry - A European Journal.