Oxidation of alcohols

The simplest way the oxidise alcohols is to burn them

The C-C AND C-H bonds are broken as the alcohol is completely oxidised to make CO₂ and H₂O. This is a combustion reaction. e.g. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Ethanol has a pale blue flame.

How much an alcohol can be oxidised depends on its structure

You can use acidified dichromate(VI) [K₂Cr₂O₇]. Primarys are oxidised to aldehydes and then to carboxylic acids. Secondary are oxidised to ketones only. Tertiary won’t be oxidised.

Aldehydes and ketones

Aldehydes have a hydrogen and one alkyl group attached to the carbonyl carbon atom.

Ketones have to alkyl groups attached to the carbonyl carbon atom.

Their general formula is C_nH_2nO.

Primary alcohols will oxidise to aldehydes and carboxylic acids

Primary alcohols can be oxidised twice.

=O =O

R — CH₂ — OH + [O] —> R — C — H + H₂O + [O] —> R — C — OH

How to distil for an aldehyde and reflux for carboxylic acid

1) Gently heating ethanol with potassium dichromate solution and sulfuric acid in a test tube should produce “apple” smelling ethanal (an aldehyde). However, it’s really tricky to control the amount of heat and the aldehyde is usually oxidised to form “vinegar” smelling ethanoic acid.

2) To get just the aldehyde, you need to get it out of the oxidising solution as soon as it’s formed. You can do this by gently heating excess alcohol with a controlled amount of oxidising agent in distillation apparatus, so the aldehyde is distilled off immediately.

3) To produce the carboxylic acid, the alcohol has to be vigorously oxidised. The alcohol is mixed with excess oxidising agent and heated under reflux.

Secondary alcohols will oxidise to ketones

1) Refluxing a secondary alcohol will produce a ketone

2) Ketones can’t be oxidised easily, so even prolonged refluxing won’t produce anything more.

Tertiary alcohols can’t be oxidised easily

1) Tertiary alcohols don’t react with potassium di chromate at all — the solution stays orange

2) The only way to oxidise tertiary alcohols is by burning them