CIE AS Chemistry (9701) 2019-2021

Revision Notes

3.5.1 Aldehydes & Ketones

Production of Aldehydes & Ketones

Formation of aldehydes & ketones

  • Aldehydes and compounds are carbonyl compound which contain a C=O group
  • They can be prepared from the oxidation of primary and secondary alcohols
  • Acidified potassium dichromate (K2Cr2O7) is an orange oxidising agent
  • When the alcohols are oxidised the orange dichromate ions (Cr2O72-) are reduced to green Cr3+ ions
  • Secondary alcohols are oxidised to 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 straightway after it has been formed
    • Eg. propan-2-ol will get oxidised to propanone
  • Primary alcohols are oxidised to 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
    • If the aldehyde formed is not distilled off, further refluxing with excess oxidising agent will oxidise the aldehyde to a carboxylic acid
    • Eg. propan-1-ol will get oxidised to propanal

Reduction of aldehyde & ketones

  • Aldehydes and ketones can be reduced by reducing agents such as NaBH4 or LiAlH4
  • Aldehydes are reduced to primary alcohols
    • Eg. butanal is reduced to butan-1-ol
  • Ketones are reduced to secondary alcohols
    • Eg. butanone is reduced to butan-2-ol

Reaction of aldehyde & ketones with hydroxynitrile

  • Aldehydes and ketones can undergo nucleophilic addition with hydrogen cyanide, HCN
  • The cyanide ion, CN, acts as a nucleophile and adds across the C-O bond
  • Aldehydes and ketones react with HCN and NaCN (or KCN) as the catalyst and heat to produce hydroxynitriles
    • Hydroxynitriles are nitriles containing a hydroxy, -OH, group
    • Eg. propanal reacts with HCN to form 2-hydroxybutanenitrile

Exam Tip

The nucleophilic addition of HCN to an aldehyde or ketone increases the length of the carbon chain by one carbon atom!

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