Dehydration reaction

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This article is about chemical reactions resulting in the loss of water from a molecule. For the removal of water from solvents and reagents, see desiccation.

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
Dienol benzene rearrangement

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.

[edit] See also

[edit] References

  1. ^ Margaret Jevnik Gentles, Jane B. Moss, Hershel L. Herzog, and E. B. Hershberg (1958). "The Dienol-Benzene Rearrangement. Some Chemistry of 1,4-Androstadiene-3,17-dione". J. Am. Chem. Soc. 80 (14): 3702 - 3705. doi:10.1021/ja01547a058.