This page gives you the facts and a simple, uncluttered mechanism for the elimination reaction between a simple halogenoalkane like 2-bromopropane and hydroxide ions (from, for example, sodium hydroxide) to give an alkene like propene.
The elimination reaction involving 2-bromopropane and hydroxide ions
2-bromopropane is heated under reflux with a concentrated solution of sodium or potassium hydroxide in ethanol. Heating under reflux involves heating with a condenser placed vertically in the flask to avoid loss of volatile liquids. Propene is formed and, because this is a gas, it passes through the condenser and can be collected.
Everything else present (including anything formed in the alternative substitution reaction) will be trapped in the flask.
In elimination reactions, the hydroxide ion acts as a base - removing a hydrogen as a hydrogen ion from the carbon atom next door to the one holding the bromine. The resulting re-arrangement of the electrons expels the bromine as a bromide ion and produces propene.
Jim Clark (Chemguide.co.uk)