The Williamson ether synthesis proceeds via an S N2 mechanism, in which an alkoxide ion displaces a halogen ion.
This method cannot be used with tertiary alkyl halides, because the competing elimination reaction predominates. The elimination reaction occurs because the rearward approach that is needed for an S N2 mechanism is impossible due to steric hindrance. An S N1 mechanism is likewise unfavored, because as the 3° carbon attempts to become a carbocation, the hydrogens on the adjacent carbons become acidic. Under these conditions, the alkoxide ion begins to show less nucleophilic character and, correspondingly, more basic character. This basic character leads to an acid‐base reaction, which results in the generation of an elimination product (an alkene).