The first step of the Williamson Ether Synthesis is the reaction of a metal with an alcohol to form an alkoxide ion plus hydrogen gas.
Alkoxide ion is a powerful nucleophile, and reacts well in an Sn2
reaction.
The alkyl halide must be primary so that the backside attack is not sterically hindered. When it is not primary, elimination usually results
Information provided by: http://www.chemhelper.com