Conditions for Substitution vs Elimination

Hydroxide ions can react with halogenoalkanes by nucleophilic substitution or by elimination. The reaction conditions decide which reaction happens most.Warm aqueous conditions favour substitution. Here the hydroxide ion acts as a nucleophile and the main product is an alcohol.Hot ethanolic conditions favour elimination. Here the hydroxide ion acts as a base and the main product is an alkene. A higher temperature also favours elimination.The structure of the halogenoalkane matters too. Primary halogenoalkanes tend to undergo substitution, while tertiary halogenoalkanes tend to undergo elimination.In practice both reactions usually happen at the same time, so a mixture of products is often formed.