Sn2 — It’s a gas

Sn2 reactions are a lot more complicated than as taught in orgo 101 (at least in the gas phase). The classic mechanism is very easy to teach to students, it’s just an umbrella turning inside out in the wind. A current article in Science (vol. 352 pp. 32 – 33 1 April ’16) shows how complicated things can be when the reaction is carried out in the gas phase. Mechanisms illustrated include rebound stripping, frontside attack, ion-dipole complex, roundabout, hydrogen bond complex, frontside complex and double inversion.

Why study Sn2 in the gas phase? One reason is to sharpen computational and theoretical methods to be able to predict reaction rates (in gas phase reactions). I was surprised on looking up Rice-Ramsperger-Kassel-Marcus theory to find out how old it was. Back in the 60’s it was taught to us without any names attached. One assumes that before and after reaction the ion molecule complexes are trapped in potential wells. It is assumed that vibrational energies in the complex are quickly distributed to ‘equilibrium’ in the complexes so that detailed computation of rates can be carried out.

Is this of any use to the chemist actually reacting molecules in solution? Other than by sharpening computational tools, I don’t see how it can be given the present state of the art.

Gas phase kineticists are starting to try, but they’ve got a very long way to go. “Stepwise addition of solvent molecules to the bare reactant anion offers a bottom up approach to learn more about the transition of chemical reactions from the gas to liquid phase. To investigate the role of solvation in Sn2 reactions Otto et al. have performed crossed molecular beam studies of the microsolvated” Sn2 reaction (e.g. the approaching anion solvated with all of one or two waters). “The results show that “the dynamics differ dramatically from the unsolved anion.”

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