In chemistry, 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:
- 2 R-OH → R-O-R + H2O
- Conversion of alcohols to alkenes
- R-CH2-CHOH-R → R-CH=CH-R + H2O
- 2 RCO2H → (RCO)2O + H2O
- RCONH2 → R-CN + H2O
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.
- Sugar (sucrose) is dehydrated:
- C12H22O11 + 98% Sulfuric acid → 12 C (graphitic foam) + 11 H2O steam + Sulfuric acid/water mixture
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.)