3.3.8 - Aldehydes and Ketones

• How can you tell the difference between aldehydes and ketones?

  1. Tollen's Reagent - Warm with ammoniacal silver nitrate.

  • Aldehydes form a silver mirror
  • Ketones show no change

  2. Fehling's Solution: Warm with Fehling's Solution.

  • Aldehydes form brick red precipitate.
  • Ketones show no change
• Aldehydes are reduced to primary alcohols

  
  

  Ketones are reduced to secondary alcohols
• Are aldehydes or ketones are more reactive?

  • Alkyl groups push electrons away, so the δ+ charge on the C is reduced; 
  • makes C more stable and less reactive
  • Positive inductment effect
  • Aldehydes are therefore more reactive that ketones 
• What is the name of the product when HCN is added to a carbonyl compound? Hydroxynitriles (have OH and CN groups)
• Why is is preferable to use KCN over HCN?

  • Both are toxic, due to the cyanide ions. 
  • However, HCN is far more difficult to control as it's a gas.
• What is the carbonyl group? C=O
• What is the functional group and general formula for an aldehyde? RCHO (C double bonded to O, single bond to H and R)
• What is the functional group for a ketone? RCOR’ (C double bonded to O)
• What kind of intermolecular forces do molecules with the carbonyl group have? Why? Permanent dipole-dipole due to the polar C=O bond (O is delta -)
• Which bond in carbonyl compounds is usually involved in reactions?

  • C=O
  • Because to the polarity of the bond (large difference in electronegativity between C and O)
• What is the strongest bond in carbonyl compounds? C=O
• What happens when a ketone is added to Fehling’s solution? No visible change → stays blue
• What happens when an aldehyde is added to Tollen’s reagent? Silver mirror forms as Ag+ reduced to Ag (s)
• What is a reducing agent for aldehydes and ketones? What ions does this release in solution?

  
  

  NaBH4 (sodium tetrahydridoborate (III)), releases an H- ion
• Cabonyl group is soluble in water. They form hydrogen bonds between water molecules and oxygen of C=O. As C chain length increases, solubility decreases.
• hat is Fehling’s solution? What colour is it? Copper complex ions, blue
• What happens when an aldehyde is added to Fehling’s solution? Reduced to Cu+ ions → colour change to brick red ppt
• What is in Tollens’ reagent? Silver complex ions, colourless solution
• What is a reducing agent for aldehydes and ketones? What ions does this release in solution? NaBH4 (sodium tetrahydridoborate (III)), releases an H- ion