3.08 Aldehydes and Ketones

• 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-)
• How soluble are they in water? What influences solubility?


  • Yes - form hydrogen bonds between water molecules and oxygen of C=O.
  • As C chain length increases, solubility decreases
• Which bond in carbonyl compounds is usually involved in reactions? Why?

  C=O, due to the polarity of the bond (large difference in electronegativity between C and O)
• What is the strongest bond in carbonyl compounds?

  C=O
• Draw a mechanism for the nucleophilic addition of HCN to a carbonyl compound.


  .

  
• Why is the addition of HCN important?

  Increases the length of the carbon by one carbon atom - very useful
• Will the product of HCN added to a carbonyl compound have optical isomers? Why?

  Yes they will.

  • In the aldehyde/ketone, the carbonyl group is planar, so the :CN- can attack from either above or below, forming 2 enantiomers.
• What is the name of the product when HCN is added to a carbonyl compound?

  Hydroxynitriles (OH+CN)
• What 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

  • CH3CHO + 2Cu2+ + 5OH- → CH3COO- + Cu2O + 3H2O
• What happens when a ketone is added to Fehling's solution?

  No visible change → stays blue
• What is in Tollens' reagent?

  Silver complex ions, colourless solution
• What happens when an aldehyde is added to Tollen's reagent?

  Silver mirror forms as Ag+ reduced to Ag(s)

  • CH3CHO + 2Ag+ + H2O → CH3COOH + 2Ag + 2H+
• What happens when a ketone is added to Tollen's reagent?

  No visible change
• What is another oxidising agent for alcohols and aldehydes? What change in colour does this undergo?

  Acidified potassium dichromate VI - H2SO4 and K2Cr2O7. Colour change from orange to green.
• What is a reducing agent for aldehydes and ketones? What ions does this release in solution?

  NaBH4.

  • This releases a H- ion
• Draw and name a mechanism for the reduction of an aldehyde.

  Nucleophilic addition.

  
• Which types of carbonyl compounds can form optical isomers?

  • All aldehydes
  • Unsymmetrical ketones (4 carbons and longer)