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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
- 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
-
- 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
References
- ^ 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:.
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