Aldehydes and Ketones

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What Aldehydes and Ketones have in common

  • Both Aldehydes and Ketones are carbonyl compounds. This means that they have the functional group of C=O
  • They also have lower boiling points then their respective alcohols. This is because they don't have polar O-H bonds. This means that they can't hydrogen bond with themselves. 
  • However short chain aldehydes and ketones can dissolve in water. This is because the C=O bond is polar. This means the oxygen can use its lone pairs to form hydrogen bonds with the hydrogen in water molecules. 
  • They can also both react with hydrogen cyanide to produce a hydroxynitrile. This is an example of a nucleophiloc addition reaction. 
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Where the C=O group is placed and naming.

  • Aldehydes always have their C=O group at the end of the carbon chain. Their names end in -al. 
  • Ketones however have their C=O group in the middle of the carbon chain. Their names end in -one, moreover you may have to number the carbon where the carbonyl group is located. 
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Oxidising, Reducing and testing.

  • Aldehydes can be oxidised to carboxylic acids (such as vinegar).This is done under reflux with potassium dicromate. Ketones  however, can't be oxidsed as easily and thus won't produce anything more. 
  • Aldehydes can be reduced to primary alcohols. This is done with either lithium tetrahydridoaluminate (II) or sodium tetrahydridoborate. Ketones can also be reduced using these reducing agents but they are reduced to secondary alcohols. 
  • If you have 2 samples and know that one is an aldehyde and one is a ketone, you can use two tests to work out which is which. If you add tollens' to the aldehyde solution you will form a silver mirror, whilst if you add it to the ketone solution the solution will stay colourless. 
  • You can also use Fehling's solution. This starts as a blue solution and if heat with an aldehyde will produce a brick red precipitate. The ketone solution however will still remain blue. 
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