Aldehydes and Ketones

• Aldehydes have the functional group CHO which contains a carbonyl group (C=O)
• Aldehydes are produced from the initial oxidation and distillation of primary alcohols
• Aldehydes can be oxidised further and will produce carboxylic acids in the presence of acidified potassium dichromate.
• Ketones have the carbonyl group C=O. They are produced from the oxidation of secondary alcohols with acidified potassium dichromate.

  
• Ketones never have a carbonyl group at the end of their carbon chain.

  
• Aldehydes use the -anal suffix and never need to add a postion number
• Ketones use the -anone suffix
• Aldehydes always have carbonyl groups at the end of the carbon chain

  
• Tollens' reagent can be used to test for aldehydes as it acts as an oxidising agent.

  Aldehydes form a silver mirror

  Ketones no change
• Fehling's solution can be used to test aldehydes and ketones.

  Aldehydes change from blue to red

  Ketones no change - stays blue
• Aldehydes can be reduced to primary alcohols.

  Ketones can be reduced to secondary alcohols.
• A molecule is oxidised when a carbon forms a bond with a more electronegative element. In most cases this element is oxygen
• Reduction is when a carbon forms a bond with a less electronegative element such as hydrogen
• Aldehydes and ketones can be reduced into alcohols using the reducing agent sodium borohydride (NaBH4) which acts as a source of H- ions
• [STEP 1] In the reduction of an aldehyde, the hydride ion forms a C-H bond with the aldehyde. The C=O bond breaks and the lone pair goes to the oxygen, giving it a negative charge

  
• [STEP 2] In the reduction of aldehydes, the O- on the aldehyde bonds with H+ ions from the water which forms an OH group

  
• Reduction of ketones (same mechanism for aldehydes)

  1. C-H bond forms and C=O bond breaks, and the oxygen gains a lone pair
  2. OH bond forms between oxygen on the ketone and hydrogen from water
  3. forms a secondary alcohol
• A carbonyl group (C=O) is polar, so the oxygen is partially negative and the carbon is partially positive. Due to this, hydride ions will bond with partially positive carbon atoms.
• when writing the equation for the reduction of aldehydes and ketones, we write 2 atoms of hydrogen in square brackets

  
• Molecules such as butan-2-ol have optical isomers as they have an asymmetrical/chiral carbon atom

  
• to identify optical isomers, we can shine plane polarised light through the sample and see which way the light rotates. However the light will not rotate as there is a 50/50 split of both enantiomers, forming a racemic mixture where the light does not rotate