Dehydration Reaction Definition in Chemistry

A visual representation of a dehydration reaction

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A dehydration reaction is a chemical reaction between two compounds where one of the products is water. For example, two monomers may react where a hydrogen (H) from one monomer binds to a hydroxyl group (OH) from the other monomer to form a dimer and a water molecule (H2O). The hydroxyl group is a poor leaving group, so Bronsted acid catalysts may be used to help to protonate the hydroxyl to form -OH2+. The reverse reaction, where water combines with hydroxyl groups, is termed hydrolysis or a hydration reaction.

Chemicals commonly used as dehydrating agents include concentrated phosphoric acid, concentrated sulfuric acid, hot ceramic and hot aluminum oxide.

A dehydration reaction is the same as a dehydration synthesis. A dehydration reaction may also be known as a condensation reaction, but more properly, a dehydration reaction is a specific type of condensation reaction.

Dehydration Reaction Examples

Reactions that produce acid anhydrides are dehydration reactions. For example acetic acid (CH3COOH) forms acetic anhydride ((CH3CO)2O) and water by the dehydration reaction
2 CH3COOH → (CH3CO)2O + H2O
Dehydration reactions are also involved in the production of many polymers.

Other examples include:

  • Conversion of alcohols to ethers (2 R-OH → R-O-R + H2O)
  • Conversion of alcohols to alkenes (R-CH2-CHOH-R → R-CH=CH-R + H2O)
  • Conversion of amides to nitriles (RCONH2 → R-CN + H2O)
  • Dienol benzene rearrangement
  • the reaction of sucrose with concentrated sulfuric acid (a popular chemistry demonstration)