During an elimination reaction, a bond forms by the removal of two atoms or groups from the original molecule. In most instances, the bond that forms is a π bond. Elimination reactions compete with substitution reactions when alkyl halides react with a nucleophile.
The elimination of hydrogen halide (a halogen acid) from an alkyl halide requires a strong base such as the alkoxide ion, RO −. Weaker bases such as the OH − ion give poor yields of elimination product.
If an alkyl halide contains more than two carbons in its chain, and the carbon atoms adjacent to the carbon atom bonded to the halogen each have hydrogen atoms bonded to them, two products will form. The major product is predicted by Zaitsev's Rule, which states that the more highly branched alkene will be the major product. For example, in the dehydrohalogenation reaction between 2‐chlorobutane and sodium methoxide, the major product is 2‐butene.