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Oxidation of Alkanols

Key Concepts

Oxidising agents are reduced:

  • purple permanganate ion, MnO4-, is reduced to colourless Mn2+.

  • orange dichromate ion, Cr2O72-, is reduced to green chromium(III) ions, Cr3+.

  • NAD+ (a biological oxidising agent) is reduced to NADH

Animated Tutorial

Examples

Oxidation of a Primary Alkanol

1-butanol is a primary alkanol, the OH functional group is attached to an end carbon atom.
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C - C -OH
  |
H
  |
H
  |
H
  |
H
 
Oxidation of 1-butanol by acidified dichromate solution:
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C - C -OH
  |
H
  |
H
  |
H
  |
H
 
H+/Cr2O72-
       
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C - C =O
  |
H
  |
H
  |
H
     
H+/Cr2O72-
       
  H
|
  H
|
  H
|
  OH
 
H- C - C - C - C =O
  |
H
  |
H
  |
H
     

1-butanol
H+/Cr2O72-
       

butanal
H+/Cr2O72-
       

butanoic acid

In the presence of excess 1-butanol, the reaction mixture should change colour from orange to green.

Oxidation of a Secondary Alkanol

2-butanol is a secondary alkanol, the OH functional group is attached to a carbon atom which is itself bonded to two other carbon atoms:
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C - C -H
  |
H
  |
OH
  |
H
  |
H
 
Oxidation of 2-butanol by acidified dichromate solution:
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C - C -H
  |
H
  |
OH
  |
H
  |
H
 
H+/Cr2O72-
       
  H
|
      H
|
  H
|
 
H- C - C - C - C -H
  |
H
  ||
O
  |
H
  |
H
 
2-butanol H+/Cr2O72-
       
butanone

In the presence of excess 2-butanol, the reaction mixture should change colour from orange to green.

Oxidation of a Tertiary Alkanol

2-methyl-2-propanol is a tertiary alkanol, the OH functional group is attached to a carbon atom bonded to 3 other carbon atoms:
      H
|
     
  H
|
  H-C-H
|
  H
|
 
H- C - C - C -H
  |
H
  |
OH
  |
H
 
2-methyl-2-propanol will not react with acidified dichromate solution, so the reaction mixture will remain orange.


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