Oxidation of Alcohols/Reduction of the Carbonyl Group

Oxidation

As mentioned on the 'Alcohol' page, the hydroxyl group can be oxidised to give a carbonyl group. The type of carbonyl group formed depends on the classificatioon of the alcohol used as the reagent. This is shown in the diagram below:

The products formed from the oxidation of alcohols

The scheme above shows that the primary alcohols undergo the greatest oxidation and these reactions can be shown by the following equations respectively. The [O] repreasents the oxidising agent.

1. CH₃CH₂OH + [O] → CH₃CHO + H₂O
2. CH₃CHO + [O] → CH₃COOH

The aldehyde can be made in practice by dripping the ethanol into warm acidified potassium dichromate(VI). The ethanal has a lower boiling point of 23°C than both the ethanol (78°C) and ethanoic acid (118°C). This means it can be distilled off easily to give a pure product. The acid is produced by heating the ethanol with the acidified potassium dichromate under reflux. This ensures the complete oxidation of the ethanol, without the ethanal evaporating off.

The apparatus used in the reflux of Ethanol

The scheme also shows the oxidation of secondary alcohols. These do not undergo the extreme oxidation the primary alcohols do. This is because they only have one proton attached to the hydroxyl carbon that can be removed during oxidation. The equation for the reaction is shown below.

CH₃CH(OH)CH₃ + [O] → CH₃C=OCH₃ + H₂O

Tertiary alcohols are not oxidised at all

Reduction

The carbonyl groups produced can also be converted back in to the original alcohols. This can be done one of two ways:

1. Catalytic hydrogenation:
   This uses hydrogen with a nickel or platinum catalyst. It is a very effective reducing agent and will also reduce any alkenes back to alkanes

2. Sodium tetrahydridoborate(III) (NaBH₄)in methanol.
   This is a milder reducing agent and the mechanism for this reaction will be covered in the 'Aldehydes and Ketones' section

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