13'7 Aldehydes and Ketones

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13.7Aldehydes and Ketones
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Structures.

• Two series of organic compounds containing CO
  carbonyl group.
• Aldehydes have a hydrogen attached to the
  carbonyl group.
• Two groups react differently and can be
  distinguished.

aldehyde
ketone
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Aldehydes
Now try these-

• Named using suffix -al.
• E.g. HCHO methanal
• CH3CHO ethanal

propanal
pentanal
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Ketones

• Name use suffix -one.
• E.g. CH3COCH3 propanone
• Ketones with 5 or more carbons have structural
  isomers eg.
• CH3COCH2CH2CH2CH3
• CH3CH2COCH2CH2CH3
• Draw the structures and name these isomers.

• Now try these
• CH3CH2CH2CH2COCH2CH2CH3
• CH3CH2CH2COCH2CH2CH3
• CH3CH2COCH2CH2CH2CH2CH3
• CH3COCH2CH3
• CH3CH2COCH2CH2CH2CH3

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Preparation of aldehydes and ketones

• Oxidation of alcohols
• Acidified potassium dichromate - heat under
  reflux.
• Orange dichromate Cr2O72- is reduced to green
  Cr3 in solution.
• Primary alcohol ? aldehyde ? carboxylic acid
• Secondary alcohol ? ketone.

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Redox reactions

• Oxidation of alcohols are redox reactions.
• Oxidation of ethanol
• CH3-CH2-OH ?CH3-CHO 2H 2e-
• CH3-CHO H2O ?CH3COOH 2H 2e-
• reduction
• Cr2O72-(aq) 14H(aq) 6e- ?2Cr3 (aq) 7H2O
• orange ? green

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Reactions - oxidation

• Aldehydes have a hydrogen atom next to the
  carbonyl group - easily oxidised.
• Fehlings solution - Cu2(aq) ions (in alkali)
  reduced to Cu - blue to brick red.
• Aldehyde oxidised to carboxylic acid.
• Ketones not oxidised easily by either Fehlings
  solution or acidified dichromate.

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Reactions - reduction

• Powerful reducing agent needed.
• NaBH4 - sodium tetrahydridoborate (III)
• Aldehydes reduced to primary alcohols.
• Ketones reduced to secondary alcohols.

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Reactions - addition

• Carbonyl groups can undergo addition reactions.
• HCN in presence of alkali.
• Adds across CO to form 2 hydroxynitriles
  (cyanohydrins)
• Nucleophilic addition