β-elimination reactions to occur the carbon atom adjacent to the carbon bearing the halide (that is, the β -carbon) must bear a hydrogen atom.
During the β-elimination reaction, a hydrogen atom from the β-carbon and the halogen atom from the carbon bearing halide are eliminated, resulting in the formation of a double bond between the adjacent carbon atoms. This process is facilitated by a strong base, which takes the hydrogen atom, leaving a negative charge on the β-carbon and enabling the halogen to leave as a halide ion.
If the β-carbon does not bear a hydrogen atom, the β-elimination reaction cannot occur. This is because there is no hydrogen atom available to be taken by the strong base, and thus the elimination cannot proceed. In such cases, alternative reactions, such as nucleophilic substitution, may occur instead.