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Key Concepts

  • Dehydration of alkanols (alcohols) is an elimination reaction.
  • Water is eliminated from the alkanol resulting in an alkene.
  • Any concentrated strong acid can cause the dehydration of an alkanol, but hot, concentrated sulfuric acid is the most commonly used.
  • The reagent used to dehydrate the alkanol is known as a dehydrating agent.
  • The dehydration of a primary alkanol (alcohol) produces the 1-alkene
      H
    |    		 H
    |    		  
    R- C- C- OH
      |
    H    		 |
    H    		  
    		hot conc. H2SO4
    -------------->
      H
    |    		 H
    |    		  
    R- C= C  
      |
    H    		 |
    H    		  
    		+ H-O-H
  • The dehydration of a secondary alkanol or a tertiary alkanol (alcohol) may produce a mixture of alkene structural isomers*.
    *Saytseff's Rule predicts that the alkene with the greatest number of alkyl groups on the doubly-bonded carbon atoms will be the major product.

Animated Tutorial

Examples

Dehydration of a Primary Alkanol

Ethanol, C2H5OH, (ethyl alcohol) is a primary alcohol.
At 180oC, concentrated sulfuric acid will dehydrate ethanol to produce ethene, C2H4, (ethylene) and water.
ethanol
(ethyl alcohol)		 conc. H2SO4
------------>
180oC		 ethene
(ethylene)		 + water
CH3CH2OH conc. H2SO4
------------>
180oC		 CH2=CH2 + H2O
  H
|		 H
|		  
H- C- C- OH
  |
H		 |
H		  
conc. H2SO4
------------>
180oC
  H
|		 H
|		  
  C= C  
  |
H		 |
H		  
+ H-O-H

Dehydration of a Secondary Alcohol

2-propanol, (CH3)2CHOH, is a symmetrical secondary alcohol.
At 100oC, concentrated sulfuric acid will dehydrate 2-propanol to produce propene (propylene) and water.
2-propanol
 conc. H2SO4
------------>
100oC		 propene
(propylene)		 + water
(CH3)2CHOH conc. H2SO4
------------>
100oC		 CH3CH=CH2 + H2O
  H
|		 OH
|		 H
|		  
H- C- C- C- H
  |
H		 |
H		 |
H		  
conc. H2SO4
------------>
100oC
  H
|		   H
|		  
H- C- C= C  
  |
H		 |
H		 |
H		  
+ H-O-H

2-pentanol, CH3CH2CH2CHOHCH3, is an unsymmetrical secondary alcohol.
Hot concentrated sulfuric acid will dehydrate 2-pentanol to produce water and two possible isomers of pentene, 2-pentene and 1-pentene.

2-pentanol
 hot conc. H2SO4
------------->		 2-pentene + 1-pentene
CH3CH2CH2CHOHCH3 hot conc. H2SO4
------------->		 CH3CH2CH=CHCH3 + CH3CH2CH2CH=CH2
  H
|		 H
|		 H
|		 OH
|		 H
|		  
H- C- C- C- C- C- H
  |
H		 |
H		 |
H		 |
H		 |
H		  
hot conc. H2SO4
------------->
  H
|		 H
|		     H
|		  
H- C- C- C= C- C -H
  |
H		 |
H		 |
H		 |
H		 |
H		  
+
  H
|		 H
|		 H
|		   H
|		  
H- C- C- C- C= C  
  |
H		 |
H		 |
H		 |
H		 |
H		  
    (major product)   (minor product)
 
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