Nucleophilic activity is prototypical Lewis base activity. The lone electron pairs on oxygen enable alcohols to serve as nucleophiles. Additionally, protonation may transform a hydroxyl group into a leaving group. The condensation of primary alcohols to form an ether begins by protonation of the hydroxyl group by strong acid. The condensation then occurs by SN2 substitution with water as the leaving group. With secondary and tertiary alcohols, though, the reaction that predominates under these conditions is E1 elimination.