CIE A Level Chemistry复习笔记3.5.1 Production of Aldehydes & Ketones

• 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), K2Cr2O7, is an orange oxidising agent
  • When the alcohols are oxidised the orange dichromate ions (Cr2O72-) 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 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

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

Ketones are formed from the oxidation of secondary alcohols