Like water, alcohols are amphoteric, able to serve as either a proton donor or a proton receiver. The basicity of hydroxyl group, its ability to receive a proton, permits the conversion of the hydroxyl group into a leaving group. A protonated alcohol (the conjugate acid of the alcohol) is called an alkyloxionium ion, formed in the presence of a strong acid. Formation of alkyloxonium can be is followed by the departure water as a leaving group forming a carbocation. Dehydrohalogenation of an alcohol is an SN1 substitution mechanism where water is the leaving group and halide ion serves as the nucleophile.