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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

In organic (carbon) chemistry, the "rule of thumb" is that an organic compound has been oxidised if1:

  • it loses hydrogen

    OR

  • it gains oxygen

Oxidation of a Primary Alkanol

Using a suitable oxidising agent such as acidified dichromate solution, or, acidified permanganate solution, primary alkanols (alcohols) can be oxidised to alkanals (aldehydes), and, alkanals can then be oxidised to the alkanoic acid (carboxylic acid).

The oxidation of the alkanol to the alkanal is signified by the loss of hydrogen from the alkanol in order to produce a double bond to the oxygen.

The oxidation of the alkanal to the alkanoic acid is signified by the incorporation of more oxygen into the molecule.

  primary alkanol
(colourless)
oxidising agent
alkanal
(colourless)
oxidising agent
alkanoic acid
(colourless)
oxidation by dichromate
  H
|
 
R- C -OH
  |
H
 
H+/Cr2O72-
  H
|
 
R- C =O
     
H+/Cr2O72-
    OH
|
 
R- C =O
     
oxidation by permanganate
  H
|
 
R- C -OH
  |
H
 
H+/MnO4-
  H
|
 
R- C =O
     
H+/MnO4-
    OH
|
 
R- C =O
     

Note that it doesn't matter whether you use acidifed dichromate solution or acidified permangante solution as the oxidising agent, the organic products of the reaction will be the same.

Also note that the observable colour change is not due to the colour of organic reactants and products.
The alkanol, alkanal, and alkanoic acid are all colourless.

The observable colour changes are due to the reduction of the oxidising agent, that is, the oxidising agent gains electrons.

We can represent these changes in the simplified way shown below2:

Reduction of dichromate   Reduction of permanganate
oxidising agent gains
electrons

reduced species   oxidising agent gains
electrons

reduced species
dichromate ions gains
electrons

chromium(III) ions   permangante ions gains
electrons

manganese(II) ions
Cr2O72- gains
electrons

cr3+   MnO4- gains
electrons

Mn2+
orange gains
electrons

green   purple gains
electrons

colourless

Example: Oxidation of Primary Alcohol

Butan-1-ol (1-butanol) is a primary alkanol, the OH functional group is attached to a terminal (end) carbon atom.
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C - C -OH
  |
H
  |
H
  |
H
  |
H
 

Oxidation of a primary alkanol produces firstly an alkanal, and then the alkanal can be further oxidised to produce the alkanoic acid.

Oxidation of butan-1-ol (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
     
butan-1-ol
(1-butanol)
colourless
H+/Cr2O72-
butanal
colourless
H+/Cr2O72-
butanoic acid
colourless

In the presence of excess butan-1-ol (1-butanol), the reaction mixture should change colour from orange to green as the orange dichromate ions, Cr2O72-, are reduced to green chromium(III) ions, Cr3+.

Oxidation of a Secondary Alkanol

Using a suitable oxidising agent such as acidified dichromate solution, or, acidified permangante solution, secondary alkanols (alcohols) can be oxidised to alkanones (ketones).

The oxidation of the alkanol to the alkanone is signified by the loss of hydrogen from the alkanol in order to produce a double bond to the oxygen.

  secondary alkanol
(colourless)
oxidising agent
alkanone
(colourless)
oxidation by dichromate
  OH
|
 
R- C -R'
  |
H
 
H+/Cr2O72-
  O
||
 
R- C -R'
     
oxidation by permanganate
  OH
|
 
R- C -R'
  |
H
 
H+/MnO4-
  O
||
 
R- C -R'
     

Note that it doesn't matter whether you use acidifed dichromate solution or acidified permangante solution as the oxidising agent, the organic products of the reaction will be the same.

Also note that the observable colour change is not due to the colour of organic reactants and products.
The alkanol and the alkanone are both colourless.

The observable colour changes are due to the reduction of the oxidising agent, that is, the oxidising agent gains electrons.

Example: Oxidation of a Secondary Alcohol

Butan-2-ol (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
 

The oxidation of a secondary alkanol (alcohol) produces an alkanone (ketone).

Oxidation of butan-2-ol (2-butanol) by acidified dichromate solution produces butan-2-one (2-butanone or butanone):

  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
 
butan-2-ol
(2-butanol)
colourless
H+/Cr2O72-
butan-2-one
(2-butanone)
colourless

In the presence of excess butan-2-ol (2-butanol), the reaction mixture should change colour from orange to green as the orange dichromate ions, Cr2O72-, are reduced to green chromium(III) ions, Cr3+.

Oxidation of a Tertiary Alkanol

Tertiary alkanols can not be oxidised using oxidising agents such as acidified dichromate solution, nor acidified permanganate solution.

Since no reaction occurs, the oxidising agent is not reduced and the colour of the solution will not change. that is, if you use orange dichromate solution the solution will stay orange, if you use purple permanganate solution the solution will stay purple.

Example: Oxidation of a Tertiary Alcohol

2-methylpropan-2-ol (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-methylpropan-2-ol (2-methyl-2-propanol) will NOT react with acidified dichromate solution.

The orange dichromate ions will NOT be reduced so there will be no colour change.
The orange reaction mixture will remain orange.

Animated Tutorial


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1In organic (carbon) chemistry, it is not always easy to see which atoms have gained or lost electrons in order to determine whether a species has been oxidised or reduced.
The "rules of thumb" will help you quickly determine whether an organic (carbon) compound has been oxidised or reduced:
Oxidation: loss of hydrogen or gain of oxygen
Reduction: gain of hydrogen or loss of oxygen

2 The balanced half-reaction equations for the reduction of these oxidising agents in acidic aqueous solution is given below:

reduction of dichromate: Cr2O72-(aq) + 14H+(aq) + 6e- → 2Cr3+(aq) + 7H2O(l)
reduction of permanganate: MnO4-(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4H2O(l)

For more information, go to Writing Half Equations for Aqueous Solutions

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