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Oxidation of Alcohols

Chemistry: AQA A Level, topic 5.5.2

Ketones and Aldehydes

Ketones and aldehydes are functional groups that can be produced by the oxidation of alcohols.

Ketones

Ketones

  • When you oxidise a secondary alcohol, the molecule you get is called a ketone.
  • Ketones have a carbon double bonded to an oxygen atom, and two other single carbon-carbon bonds.
  • The simplest ketone, propanone (acetone), is shown here.
Naming ketones

Naming ketones

  • We use the general name:
  • Alkan-X-one:
    • Propanone.
    • Butanone.
    • Pentan-2-one or pentan-3-one.
  • The numbers are used on longer alkanes to show on which carbon the ketone group lies.
Aldehydes

Aldehydes

  • You can oxidise a primary alcohol to an aldehyde.
  • An aldehyde has a carbon-oxygen double bond and at least one carbon-hydrogen bond.
  • The simplest aldehyde, methanal (formaldehyde), is shown here.
Naming aldehydes

Naming aldehydes

  • We use the general name:
  • Alkanal:
    • Propanal.
    • Butanal.
    • Pentanal.
  • Here, there is no need for any numbers as the aldehyde group must lie on one of the end carbons.
Carboxylic acids

Carboxylic acids

  • You can oxidise an aldehyde to a carboxylic acid.
  • You cannot oxidise a ketone to a carboxylic acid.
  • The simplest acid, methanoic acid (formic acid), is shown here.

Oxidation of Alcohols

There are different kinds of alcohols, and as always in chemistry, they all have slightly different reactions.

Oxidation

Oxidation

  • Each of the kinds of alcohol can be oxidised to different extents.
  • The rule is that you can break carbon-hydrogen bonds, but not a carbon-carbon bond.
  • Primary alcohols can be oxidised twice.
  • Secondary alcohols can be oxidised once.
  • Tertiary alcohols cannot be oxidised.
  • The usual oxidising agent, for all, is acidified potassium dichromate(VI).
Primary alcohols

Primary alcohols

  • Two possible oxidation reactions for primary alcohols:
  • They can be oxidised to aldehydes.
  • If you change the experimental conditions, you can also oxidise them to carboxylic acids.
Secondary alcohols

Secondary alcohols

  • One possible oxidation reaction for secondary alcohols:
  • You can oxidise them to a ketone.
    • You cannot oxidise it further without breaking carbon-carbon bonds.
Tertiary alcohols

Tertiary alcohols

  • You can’t oxidise a tertiary alcohol at all!
  • Tertiary alcohols have three carbon groups bonded to the alcohol carbon.
  • The only way to add more oxygen involves breaking carbon-carbon bonds.

Test yourself on Oxidation of Alcohols

  • What is the molecule CH3CH2CH2COCH3 called?

  • What type of alcohol is oxidised to form an aldehyde?

  • Which of the following can be oxidised to a carboxylic acid?

  • How many oxidations can a secondary alcohol undergo?

  • An aldehyde comes from the oxidation of which of the following?

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