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Pictures of the Day CH320M/CH328M Fall 2012

Sometimes alkyl halides must undergo bond rotation before they are able to react by the E2 reaction mechanism. Shown above is an alkyl halide in a conformation that DOES NOT ALLOW an E2 process because there is no hydrogen anti-periplanar to the halogen atom. Note there are other hydrogen atoms in the molecule that could potentially react to some small extent, but we are ignoring these because we are focusing on the reaction that gives the predominant Zaitsev (most highly substituted) alkene product.  
By rotating the central C-C bond, a configuration can be found in which the H and Br are anti-periplanar with respect to each other. This will be the conformation that leads to reaction, meaning that the only product observed will be the Z alkene as shown. Thus, being able to identify the reactive conformation (H and Br are anti-periplanar allows you to predict correctly the alkene product (E vs. Z).
Cyclohexyl Halides - Halide Must Be Axial
An important consequence of the required anti-periplanar geometry for an E2 reaction is that only cyclohexyl derivatives with the halogen atom axial can take part in the reaction. The reason, as shown above left, is that when the halogen atom is equatorial, only the carbon atoms circled are anti-periplanar with respect to the halogen atom. There are no hydrogen atoms that can be removed according to the E2 mechanism! The anti-periplanar atoms are the two circled carbon atoms. On the other hand, when the ring flips and the halogen atom is axial, either of the two circled axial hydrogen atoms can take part in an E2 reaction.