Reaction by substitution and elimination
Substitution at a saturated carbon by a nucleophile is possible if the carbon bears a good leaving group.
Basic nucleophiles (such as those above) may react with halides to cause elimination of HX giving the alkene as product.
The relative extents to which substitution and elimination occur for a particular halide depend on the reaction conditions and the structure of the halide.
The halide ions Cl– and Br– are good leaving groups as these ions are not basic, due to being the conjugate of the strong acids HCl and HBr. Nucleophiles that react by substitution at saturated carbon have an atom with non-bonding electron pairs, one of which becomes bonding in the reaction product.
| alkoxide anions |  + CH3Br → CH3OCH3(ether) + Br– |
| alkynide anions | HC≡C:– + CH3Br → HC≡CCH3(alkyne) + Br– |
| cyanide ion | :N≡C:– + CH3Br → :N≡CCH3(nitrile) + Br– |
Basic nucleophiles (such as those above) may react with halides to cause elimination of HX giving the alkene as product.
B:– + H–CH2–CH2–Br → B–H + CH2=CH2 + Br–
The relative extents to which substitution and elimination occur for a particular halide depend on 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