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
Study at a glance
- Design
- Other — Crossed-beam velocity-map imaging of Cl−/CN− with alkyl iodides plus computed SN2/E2 barriers
- N
- Collision energies 0.4, 1.1, and 1.9 eV across CH3I, CH3CH2I, and (CH3)2CHI
- Population
- Gas-phase ion–molecule SN2/E2 reactions
- Outcome
- Direct backward SN2 rebound persists even with steric methylation and outcompetes E2 above 1 eV
Structured fields used in claim comparison tables when every cited study has a complete layer.
Inspect source passages
Direct SN2 rebound survives bulky alkyl iodides
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
More than half of Cl− + CH3CH2I and CN− + CH3CH2I collisions at 1.9 eV follow direct rebound.
Remarkably, more than half of the reactive collisions follow the direct rebound mechanism for Cl – + CH 3 CH 2 I and CN – + CH 3 CH 2 I at 1.9 eV.
Internal excitation does not exceed 60% of the available energy.
A higher amount of energy is dissipated in form of internal excitation in the reactions Cl – + CH 3 CH 2 I and Cl – + (CH 3 ) 2 CHI, however this fraction does not exceed 60% in any case.
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.
Flashcards
0 of 11 answers reviewed
Research intelligence for this paper
See its role on concept claims, tensions it is part of, placement history, and related discoveries.
Quiz yourself
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.
More on Reaction mechanisms