GCSE OCR Gateway Organic Chemistry C6.2c Oxidation reactions

GCSE OCR Gateway Organic Chemistry C6.2c Oxidation reactions

C6.2c To be able to predict the formulae and structures of products of reactions of the first four and other given members of the homologous series of alkanes, alkenes and alcohols

Reactions to include combustion; addition of bromine and hydrogen across a double bond; oxidation of alcohols to carboxylic acids using potassium manganate(VII)

Oxidation of alcohols to aldehydes and carboxylic acids

Wine is a very delicate drink.  You wouldn’t leave a half filled bottle for too long either with the cork removed or with it in the bottle.

Either way, leaving wine exposed to oxygen has very bad effects on the quality of the drink in the bottle.

If left for long enough you would also notice a profound worsening of the smell of the liquid in there as well as the taste becoming very sour.  The smell would increasingly seem like vinegar. 

What is going on in the wine when it is exposed to air and oxygen?

Air will oxidise the alcohols in the wine to aldehydes and carboxylic acids.  The major alcohol is ethanol and that molecule is oxidised to ethanal and then to ethanoic acid.  Ethanoic acid is the main constituent of malt vinegar and has a pungent smell noticeable from the wine if left for long enough.

Here are the equations for the reactions involved:

These reactions can also be achieved in the lab using a powerful oxidising agent like purple potassium manganate(VII) (KMnO4)

The reaction is carried out in a flask with a condenser fitted to reflux the reactants back into the flask see below:

The potassium manganate(VII) has to be acidified usually with sulphuric acid.

You would heat the flask contents for around half an hour to completely oxidise the ethanol.

The ethanoic acid can be distilled out of the reaction flask at the end of the reaction.

Similar reactions will occur with other alcohols such as methanol (CH₃OH), propan-1-ol (CH₃CH₂CH₂OH) and butan-1-ol (CH₃CH₂CH₂CH₂OH) since in these reactions all that is changing is the number of carbon atoms in the carbon chain.  All these are liquid alcohols at rtp and fairly soluble in aqueous solution.

So the equation for the complete oxidation of propan-1-ol is as follows:

CH₃CH₂CH₂OH      +    [O]       =        CH₃CH₂COOH   +   H₂O

The products are propanoic acid and water.

[O] represents the oxygen required from the oxidising agent.