Williamson synthesis: RONa + R'X -> R-O-R' + NaX (SN2 mechanism). Most important ether preparation. Critical rule: R'X must be primary (or methyl). If R'X is secondary, some elimination competes. If R'X is 3°, elimination dominates completely — no ether formed. Strategy for mixed ethers: Always make the more hindered group the alkoxide and the less hindered group the halide. Example: to make (CH3)3C-O-CH3, use (CH3)3CONa + CH3I (not CH3ONa + (CH3)3CBr which gives elimination). Dehydration method: 2ROH + H2SO4 (413 K) -> R-O-R + H2O. Only works for symmetric ethers from 1° alcohols. Mechanism: SN2 of one alcohol molecule on the protonated form of another.