Theoretical Models and Experimental Data for Reactions between Water and Protonated Alcohols: Substitution and Elimination Mechanisms
✍ Scribed by Einar Uggerud; Lihn Bache-Andreassen
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 365 KB
- Volume
- 5
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
The substitution reactions: H 2 *O R À OH 2 3R À *OH 2 H 2 O and elimination reactions: H 2 *O RÀOH 2 3(RÀH) (H 2 O ´H2 *O)H [R CH 3 , CH 3 CH 2 , (CH 3 ) 2 CH, and (CH 3 ) 3 C; (RÀH) alkene] have been studied at low pressure in a Fourier transform ion cyclotron resonance mass spectrometer, and modelled with quantum-chemical methods and microcanonical variational transition state theory. The relative rates of the substitution reactions are
this is in good agree-ment with the theoretical calculations. This is different from the situation in solution in which the trend CH 3 b CH 3 CH 2 b (CH 3 ) 2 CH b (CH 3 ) 3 C traditionally is explained by the notion that increased methyl substitution at the acarbon reduces the rate constant for S N 2 reactions owing to increased steric hin-drance. For R (CH 3 ) 2 CH and (CH 3 ) 3 C front-side nucleophilic substitution and elimination are competing with backside S N 2. Both these barriers decrease upon increased methyl substitution at the a carbon. The carbocationic character of all key transition structures and intermediates becomes more prevalent upon increased methyl substitution. Ab initio calculations with additional water molecules (to mimic water solvation) illustrate the origin of the differences in reactivity in the gas phase and solution.