Direct SN2 rebound survives bulky alkyl iodides
Even with extra methyl groups on the alkyl iodide, Cl− and CN− still show a direct backward SN2 rebound channel at high collision energy rather than fully statistical dynamics.
Source
Conservation of direct dynamics in sterically hindered S<sub>N</sub>2/E2 reactions
What they did
The authors crossed mass-selected Cl− or CN− with CH3I, CH3CH2I or (CH3)2CHI beams and recorded I− product velocity-map images at relative collision energies of 0.4, 1.1 and 1.9 eV, comparing angular/energy partitioning with computed SN2 and E2 barriers.
What they found
Direct backward SN2 opens at higher energy as methylation increases, yet at 1.9 eV more than half of Cl−/CN− + CH3CH2I events are direct rebound; energy into product internals stays below 60%. Direct SN2 outcompetes E2 above 1 eV.
The limits
What it doesn't show
These are isolated gas-phase ion–molecule collisions, not condensed-phase SN2 rates or stereochemical inversion yields in solution.
Key terms
- Direct rebound SN2
- Collinear backside attack that scatters the leaving-group ion backward in the center-of-mass frame.
- E2
- Bimolecular base-induced elimination competing with substitution on ethyl/isopropyl iodides.
- Velocity-map imaging
- Ion-imaging method that maps product speed and scattering angle in a single image.
- E_rel
- Relative collision energy of the ion–molecule pair in the center-of-mass frame.
- Walden inversion
- Stereochemical inversion associated with backside SN2 attack.
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The imaged leaving-group product is:
Common questions
Which product ion was imaged?
I− leaving-group ions.
At what E_rel is direct SN2 favoured over E2?
Above 1 eV.
Do methylated systems rebound at 0.4 eV?
No—Cl− and CN− with methylated alkyl iodides show no direct rebound at 0.4 eV.
What fraction of Cl− + CH3CH2I at 1.9 eV is direct rebound?
More than half of the reactive collisions.
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