Dehydration reaction

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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

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.)

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

See also

References



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