Question

Why do elimination reactions compete with substitution reactions when alkyl halides react with a nucleophile?

Answer 1

Not always!

Explanation:

Elimination reactions compete with substitution reactions because both reaction mechanisms favour the same conditions: Alkyl halide and a nucleophile.

Under some conditions, the elimination reactions cannot compete with substitution reactions because they cannot occur and vice versa.

Example on No Elimination reaction:

The acetate `CH_3COO^-` is a strong nucleophile and a weak base, therefore, the substitution `S_(N1)` will occur at a very fast rate and consume all the alkyl halide before any elimination product is formed.

Example on No substitution reaction:

The tert-butoxide `(CH_3)_3CO^-` is a strong bulky base and a weak nucleophile. The elimination will occur at a very fast rate and consume the alkyl halide before any substitution product is formed.