Elimination and substitution compete
Halides are classified according to the number of carbon atoms attached to the carbon which bears the halogen atom. Examples are given in the table at the right.
Primary, secondary and tertiary alkyl halides that have a saturated carbon bearing a good leaving group can react by substitution with a nucleophile.
Basic nucleophiles (such as hydroxide ion and cyanide ion) may react with halides to cause elimination of HX, giving an alkene as the organic product.
The relative extents to which substitution and elimination occur for a particular halide depend on both the reaction conditions and the structure of the halide. In general, at elevated temperature for
| halide structure |  |  |  |
| carbons bonded to carbon bearing X | 1 | 2 | 3 |
| classification | primary (1°) | secondary (2°) | tertiary (3°) |
Primary, secondary and tertiary alkyl halides that have a saturated carbon bearing a good leaving group can react by substitution with a nucleophile.
For example, hydroxide ion can displace bromide ion from bromomethane:
or cyanide ion can displace bromide ion from bromomethane:
Basic nucleophiles (such as hydroxide ion and cyanide ion) may react with halides to cause elimination of HX, giving an alkene as the organic product.
The relative extents to which substitution and elimination occur for a particular halide depend on both the reaction conditions and the structure of the halide. In general, at elevated temperature for
- primary halides, substitution is much more important than elimination.
- tertiary halides, elimination is much more important than substitution.
- secondary halides, a mixture of substitution and elimination products is formed.