3.8 Aldehydes and Ketones
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What is the carbonyl group?
C=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
Why is the addition of HCN important?
Increases the length of the carbon chain 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 carbon is planar, so the :CN- can attack from either above or below, forming two enantiomers.
What is the name of the product when HCN is added to a carbonyl compound?
Hydroxynitriles (have OH and CN groups)
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
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)
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 (sodium tetrahydridoborate (III)), releases an H- ion