The Williamson synthesis of ethers proceeds by which mechanism?

1. A${\mathrm{S}}_{\mathrm{N}}1$ ionisation of the alkyl halide followed by attack of an alcohol
2. Bfree radical substitution
3. C${\mathrm{S}}_{\mathrm{N}}2$ attack of an alkoxide ion on an alkyl halide
4. Delectrophilic addition of an alcohol to an alkene