The reaction involving the nucleophilic substitution of a halide ion from an alkyl halide by an alkoxide ion to form an ether is known as Williamson synthesis.

1. Mechanism: It follows an $S_N2$ mechanism where the alkoxide ion acts as a nucleophile attacking the alkyl halide.
2. Scope: It is used for the preparation of both symmetrical and unsymmetrical ethers. For best results, the alkyl halide should be primary, as secondary and tertiary alkyl halides favour elimination (dehydration) over substitution .
3. Differentiation: 'Williamson's continuous etherification' typically refers to the acidic dehydration of alcohols to ethers, which is a different process.