Sunday, August 19, 2007

Elimination reactions

Material relevant to main material blog

1. When heated with H-2SO-4, 2 degree alcohols undergo dehydration via an E1 mechanism.
The major product is the tri-substituted alkene, methylcyclohexene

2. When heated with strong bases such as NaOEt, alkyl bromides undergo E2 elimination.
The outcome of E2 reactions is dependent on the antiperiplanar arrangement of the C-H and C-LG bonds. For substituted cyclohexanes this requires that the LG be axial.
For the cis-isomer with the -Br axial, the more highly susbtituted alkene can be formed by removal of the H adjacent to the methyl group.
For the trans-isomer, when the -Br is axial the methyl group is also axial. Therefore the elimination must occur from the C3-H bond giving the anti-Zaitsev product.

The reactive conformation is an unfavourable diaxial conformer, therefore the reaction will be slower than that of the cis-isomer.

3. The lowest energy conformation of menthyl chloride has the chlorine atom in a equatorial postion.
In this position there is no antiperiplanar H , ring flip is difficult as it would require to formation of a triaxial conformer.
In contrast, in neomenthyl chloride, the lowest energy confromation has the chlorine atom axial with 2 H in the correct orientation to give the products. The major product is the more highly substituted alkene.

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