Oxidation of Alcohols (DP IB Chemistry: SL): Revision Note

Oxidation of alcohols

Classification of alcohols

• Alcohols are compounds that contain at least one hydroxy (-OH) group

• The general formula of alcohols is C_nH_2n+1OH

• They can be classed as primary, secondary and tertiary

Primary alcohols

• A primary alcohol contains a carbon atom, bonded to the -OH group, which is attached to one other carbon atom (or alkyl group)

Secondary alcohols

• A secondary alcohol contains a carbon atom, bonded to the -OH group, that is attached to two other carbon atoms (or alkyl groups)

Tertiary alcohols

• A tertiary alcohol contains a carbon atom, bonded to the -OH group, that is attached to three other carbon atoms (or alkyl groups)

Oxidation of alcohols (general)

• Primary alcohols can be oxidised to form aldehydes which can undergo further oxidation to form carboxylic acids

• Secondary alcohols can be oxidised to form ketones only

• Tertiary alcohols do not undergo oxidation

• The oxidising agent of alcohols is acidified potassium dichromate(VI), K₂Cr₂O₇ 

• The colour change is from orange to green

  • For potassium dichromate(VI) to act as an oxidising agent, it itself needs to be reduced

  • This reduction requires hydrogen (H⁺) ions which are provided by an acid, typically sulfuric acid

  • When alcohols are oxidised, the orange dichromate ions (Cr₂O₇^2-) are reduced to green Cr³⁺ ions

Oxidation of primary alcohols

• The diagram below shows the two-step oxidation of a primary alcohol using [O] to represent the oxidising agent

  • The first oxidation step forms an aldehyde

  • The second oxidation step produces a carboxylic acid

Formation of a carboxylic acid (using ethanol)

• This reaction is performed using reflux apparatus

• A primary alcohol, such as ethanol, is added to the oxidising agent and warmed

CH₃CH₂OH (l) + 2[O] → CH₃COOH (g) + H₂O (l)

Formation of an aldehyde (using ethanol)

• This reaction is performed using distillation apparatus

• A primary alcohol, such as ethanol, is added to the oxidising agent and warmed

• The aldehyde product has a lower boiling point than the alcohol reactant so it can be distilled off as soon as it forms 

CH₃CH₂OH (l) + [O] → CH₃CHO (g) + H₂O (l)

Oxidation of secondary alcohols

• Secondary alcohols, such as propan-2-ol, are oxidised to ketones

• Since ketones cannot be further oxidised, the ketone product does not need to be distilled off straight away after it has been formed

CH₃CH(OH)CH₃ (l) + [O] → CH₃COCH₃ (g) + H₂O (l)

Oxidation of tertiary alcohols

• Tertiary alcohols do not undergo oxidation

  • This is because there must be a hydrogen on the functional group carbon, which breaks off to form water

  • There are no C-H bonds on the functional group carbon in a tertiary alcohol