Steric Hindrance in E2 Elimination of 2-Bromo-2,3-Dimethylbutane by Alkoxide Bases

The E2 elimination of 2-bromo-2,3-dimethylbutane yields different product mixtures, depending on the size of the base. With methoxide the more highly substituted (Zaitsev Rule) double bond product, 2,3-dimethyl-2-butene, is produced in 80%, whereas the less substituted double bond in 2,3-dimethyl-1-butene accounts for 20%. With tert-butoxide the product ratio nearly reverses to 25% : 75%. The following models illustrate the influence of steric hindrance on this reaction. If all three models are not displayed, it may be necessary to reload the page by clicking the reload button.
In all cases an anti-transition state is assumed, with the base approaching the alkyl halide from below. The α and β carbon atoms are colored deep blue and are slightly distorted to reflect rehybridization. The β C-H and the C-Br bonds have been lengthened to 1.25 and 2.1 Angstroms respectively to indicate partial breaking, and the alkoxide oxygen β-H distance is set at 1.8 Angstroms with a thin green line representing the incipient bond. These transition state models are initially shown in the ball & stick form, but should be viewed in the space-filling mode to appreciate the steric hindrance effect.

2,3-Dimethyl-2-Butene Transition State

2,3-Dimethyl-1-Butene Transition State

Methoxide
Base

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tert-Butoxide
Base

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