(N/A) The Williamson synthesis involves the reaction of an alkyl halide with a strong nucleophile,the alkoxide ion.
Alkoxide ions are strong bases as well as strong nucleophiles.
When a tertiary alkyl halide is used as a substrate,the alkoxide ion acts primarily as a base rather than a nucleophile.
This leads to an elimination reaction (specifically $E2$ mechanism) instead of a substitution reaction.
Consequently,the major product formed is an alkene rather than the desired ether.
For example,the reaction between sodium tert-butoxide and tert-butyl bromide yields $2$-methylpropene as the major product instead of di-tert-butyl ether.