Explain the role of hydroxide ions as nucleophile and base.

In an E2 elimination, the hydroxide ion (OH⁻) acts simultaneously as a strong base and a good nucleophile. The base component abstracts a β‑hydrogen from the halogenoalkane, forming a C=C double bond, while the nucleophilic part donates its lone pair to the electron‑poor carbon bearing the leaving halogen, facilitating C–X bond cleavage. This concerted mechanism requires a strong base to deprotonate and a nucleophile capable of stabilising the transition state, which explains why OH⁻ is effective in E2 reactions. The dual role of OH⁻ ensures both proton removal and halogen departure occur in a single step, leading to efficient alkene formation.