Monday, November 19, 2007

Stereoselectivity in E1

There have been several questions about the stereoselectivity of an E1 elimination mechanism. Two arguments can be made. A kinetic argument compares the relative energies of the carbocation intermediates leading to each product. The carbocation intermediate on the left is more stable than the carbocation intermediate on the right due to reduced steric strain; therefore the trans alkene is favored. Because this reaction is reversible a thermodynamic argument can also be made and is actually more correct; although the outcome is the same in this instance. Thus, the trans alkene product is favored because it is more stable (lower in energy) than the cis alkene. Trans is more stable than cis for the same reason already mentioned, less steric strain. Caution: thermodynamic arguments (comparing relative stabilities of products) can only be made when reactions are reversible (equilibrium). This reaction is reversible and explains why distillation was necessary to remove the alkene products and shift the equilibrium toward products (LeChatleir principle).

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