Aldehydes and Ketones

• Carbonyls


  Compounds with a C=O bond
• Types of carbonyls


  • Aldehydes
  • Ketones
• Aldehyde


  C=O is on the end of the chain with an H attached
• Ketone


  C=O is in the middle of the chain
• Aldehyde names end in -al
• Ketone names end in -one
• Solubility of smaller carbonyls in water

  They can form hydrogen bonds with water
• Intermolecular forces in carbonyls

  • Pure carbonyls cannot hydrogen bond to themselves, but are attracted instead by permanent dipole forces
• In comparison to the C=C bond in alkenes, the C=O is stronger and does not undergo addition reactions easily
• Polarity of C=O bond

  The positive carbon atom attracts nucleophiles
• Aldehydes can be oxidized to carboxylic acids, but ketones cannot be oxidized
• Oxidation of aldehydes


  RCHO + [O] → RCO2H
• Potassium dichromate (VI) solution and dilute sulfuric acid are used as the reagent for the oxidation of aldehydes
• The orange dichromate ion (Cr2O7^2-) reduces to the green Cr^3+ ion during the oxidation of aldehydes
• Aldehydes can also be oxidized using Fehling's solution or Tollen's reagent
• Reduction of carbonyls


  Reducing agents such as NaBH4 or LiAlH4 will reduce carbonyls to alcohols
• Reduction of aldehydes


  Aldehydes will be reduced to primary alcohols
• Reduction of ketones


  Ketones will be reduced to secondary alcohols
• NaBH4 and LiAlH4 contain a source of nucleophilic hydride ions (:H-) which are attracted to the positive carbon in the C=O bond
• Nucleophilic addition of hydrogen cyanide to carbonyls

  Carbonyl + HCN → Hydroxynitrile
• Hydroxynitrile


  The CN becomes part of the main chain and carbon no 1
• Using NaCN or KCN is advantageous over HCN because they will completely ionize to provide a higher concentration of the CN- ion
• Nucleophilic addition of HCN to aldehydes and unsymmetrical ketones results in the formation of a racemate
• Mechanism for nucleophilic addition of HCN to carbonyls

  Nucleophilic attack by CN- on the carbonyl carbon

  2. Protonation by H+ from acid to form the hydroxynitrile