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Oxidation of alcohols
Organic
AS
Alcohols
AQA Content
Write equations for these oxidation reactions (equations showing [O] as oxidant are acceptable)
Explain how the method used to oxidise a primary alcohol determines whether an aldehyde or carboxylic acid is obtained
Use chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent.
Specification Notes
Alcohols are classified as primary, secondary and tertiary.
Primary alcohols can be oxidised to aldehydes which can be further oxidised to carboxylic acids.
Secondary alcohols can be oxidised to ketones.
Tertiary alcohols are not easily oxidised.
Acidified potassium dichromate(VI) is a suitable oxidising agent.
Notes
Alcohols can be primary (1°) secondary (2°) 0r tertiary (3°). This designation depends on how many alkyl groups are bonded to the carbon that the hydroxyl group is attached to.
If there is 1 alkyl group attached to the carbon, it is a primary alcohol, for example - propan-1-ol. Two alkyl groups gives a secondary alcohol, for example propan-2-ol. Tertiary alcohols include 2-methylpropan-2-ol, where ther are 3 alkyl groups attached to the hydoxyl carbon.
There is a difference in the degree of oxidation for these alcohols:
Primary alcohols
These can be oxidised to form aldehydes. Further oxidation will lead to the formation of carboxylic acids.
Adding the 1° alcohol to an oxidising agent [O] and warming the mixture will give an aldehyde and water. The aldehyde, since its boiling point is lower than the alcohol, can be separated out through distillation. Refluxing with excess [O] leads to further oxidation and the formation of a carboxylic acid.
Secondary alcohols
These, when oxidised, will form ketones only so distillation is not needed (no further oxidation of the ketone).
Tertiary alcohols
These are very resistant to oxidation.
Oxidising agents
The two oxidising agents main are acidified potassium dichromate(VI) and acidified potassium permanganate(VII) (dilute sulfuric acid is often used to acidify).
Potassium dichromate(VI) is reduced by hydrogen ions supplied by the acid.
Orange dichromate ions are reduced to green chromium(III) ions during the oxidation.
(remember this one!)
Similarly for acidified potassium manganate(VII), purple manganate ions are reduced to colourless manganese(II) ions.
Testing for Aldehydes
This can be achieved using two alkaline oxidising solutions, while warming - Fehling's solution and Tollens’ reagent.
Fehling's solution
Fehling’s solution contains copper(II) ions. Acting as an oxidising agent, the copper(II) ions are reduced to copper(I) ions while the aldehyde is oxidised to a carboxylic acid. The carboxylic acid becomes a carboxylate ion that forms a salt
The solution is blue but with a positive test will form a red precipitate of copper(I) oxide. Ketones give a negative test (no change).
Tollens’ reagent.
Also known as ammoniacal silver nitrate solution (there's excess ammonia solution).
Again, the aldehyde is oxidised to a carboxylic acid and the silver(I) ions are reduced to elemental silver. The alkaline conditions leads to a carboxylate ion and salt formation. Silver is deposited forming a 'silver mirror'. Ketones give a negative test when warmed with Tollens’ reagent - they cannot be oxidised
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