In chemistry and the biological sciences, a dehydration reaction is usually defined as a chemical reaction that involves the loss of water from the reacting molecule. Dehydration reactions are a subset of elimination reactions. Because the hydroxyl group (-OH) is a poor leaving group, having an Brønsted acid catalyst often helps by protonating the hydroxyl group to give the better leaving group, -OH2+.
In organic synthesis, there are many examples of dehydration reactions:
Some dehydration reactions can be mechanistically complex, for instance the reaction of a sugar with concentrated sulphuric acid (experiment with video) to form carbon involves formation of carbon carbon bonds.
The reaction is driven by the strongly exothermic reaction sulfuric acid has with water. (Beware that this reaction produces dangerous sulfuric-acid containing steam, and should only be performed in a fume-hood or well ventilated area.)