Orientation Effects in Elimination Reactions

With halides having unsymmetrical R groups, such as 2-chloro-2-methylbutane, it is possible to form two or more different alkenes, the proportion depending on the relative rates at which the different β hydrogens are removed. Most E2 eliminations of alkyl halides with common bases, such as HO^⊖, C2H5O^⊖, and NH2^⊖, tend to give mixtures of alkenes with a preference for the most stable alkene, which usually is the one with the fewest hydrogens or most alkyl groups attached to the carbons of the double bond. Thus

However, the precise distribution of alkenes formed is found to vary enough with the nature of the leaving group, or the base used, so either product will predominated with some combination of reagents or conditions. For example, a change in the base alone can be decisive: