Producing Aldehydes & Ketones (CIE AS Chemistry)

• Aldehydes and ketones are carbonyl compounds containing a C=O group
• They can be prepared from the oxidation of primary and secondary alcohols respectively

Oxidising agents

• The oxidising agents used to prepare aldehydes and ketones from alcohols include acidified potassium dichromate (K₂Cr₂O₇) and acidified potassium manganate (KMnO₄)
• Acidified with dilute sulfuric acid, potassium dichromate(VI), K₂Cr₂O_7, is an orange oxidising agent
  • When the alcohols are oxidised the orange dichromate ions (Cr₂O₇^2-) are reduced to green Cr³⁺ ions

• Acidified with dilute sulfuric acid, potassium manganate(VII), KMnO₄ is a purple oxidising agent
  • When the alcohols are oxidised the purple manganate ions (MnO₄^-) are reduced to colourless Mn²⁺ ions

 The colour change in common oxidising agents

The oxidising agents change colour when they oxidise an alcohol and get reduced themselves

Synthesis of aldehydes

• To make an aldehyde, warm primary alcohol is slowly added to the oxidising agent
• The formed aldehyde has a lower boiling point than the alcohol and can therefore be distilled off as soon as it forms
• The aldehyde is then condensed into a liquid and collected

Using distillation to oxidise a primary alcohol to an aldehyde

Aldehydes are formed from the oxidation of primary alcohols

Synthesis of ketones

• To make a ketone, warm secondary alcohol is slowly added to the oxidising agent
• Since the formed ketone cannot be further oxidised it does not need to be distilled off straightaway after it has been formed

 

Oxidation of propan-2-ol

Ketones are formed from the oxidation of secondary alcohols