Dehydration reaction

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⁺.

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In organic synthesis, there are many examples of dehydration reactions:

• 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 carboxylic acids to acid anhydrides

2 RCO₂H → (RCO)₂O + H₂O

• Conversion of amides to nitriles

RCONH₂ → R-CN + H₂O

• in this rearrangement reaction called the dienol benzene rearrangement ^[1]:

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.[1]

• Sugar (sucrose) is dehydrated[2]:

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.)

Common dehydrating agents; concentrated sulfuric acid, concentrated phosphoric acid, hot aluminium oxide, hot ceramic.

See also

• Hydration reaction

References