Aldehydes and Ketones

HideShow resource information
View mindmap
  • Aldehydes and Ketones
    • Contain a carbonyl group (C=O)
    • Hydrogen bonding
      • Can't hydrogen bond with themselves
        • Lower boiling points than alcohols with similar mass
        • Don't have polar OH bonds
      • Can hydrogen bond with water
        • Small aldehydes and ketones can dissolve in water
        • Larger molecules don't dissolve as they have stronger london forces   which are stronger than the hydrogen bonds
    • Reaction with Hydrogen Cyanide
      • React to produce hydroxynitriles (with CN and OH groups)
      • HCN is a weak acid so partially dissociates in water to form H+ and CN- ions
      • CN- ion attacks the slightly positive carbon and donates a pair of electrons to it
        • Both electrons from the double bond transfer to the oxygen
          • H+ bonds to the oxygen to form the hydroxyl group
      • HCN is a dangerous highly toxic gas - when in labs this reaction is normally carried out with acidified KCN (in a fume cupboard) instead to reduce the risk
      • Forms a racemic mixture as the carbonyl group is planar
    • Standard Reactions and Tests
      • Brady's Reagent
        • 2,4-dinitrophenyl hydrazine
        • Forms a bright orange precipitate in the presence of a carbonyl group
        • Harmful, flammable and can be explosive when dry
      • Melting Point
        • The melting point of the molecule can be used to identify it
      • Tollen's Reagent
        • Colourless solution of silver nitrate dissolved in aqueous ammonia
        • When heated in a test tube with an aldehyde a silver mirror is formed
          • The silver ions are reduced to form solid silver
          • Ag(NH3)2 (aq)  + e- -> Ag (s) + 2NH3 (aq)
        • No reaction with ketones
      • Fehling's and Benedict's solution
        • Fehlings is a blue solution of copper (II) ions dissolved in sodium hydroxide
          • Benedicts is exactly the same except the copper (II) ions are dissolved in sodium carbonate instead
        • Form a brick-red precipitate of copper (I) oxide in the presence of an aldehyde
          • The copper ions are reduced
          • Cu 2+ (aq) + e- -> Cu + (s)
        • No reaction with ketones
      • Oxidising with dichromate
        • When an aldehyde is heated with acidified dichromate  (VI) ions a carboxylic acid is formed
        • Potassium dichromate (VI) with dilute sulphuric acid is often used
        • Orange -> green
        • Ketones aren't oxidised by dichromate (VI) ions as is isn't a strong enough reducing agent
      • Reducing
        • Using a reducing agent [H] you can...
          • Reduce an aldehyde into a primary alcohol
          • Reducce a ketone into a secondary alcohol
        • Reducing agent can be LiAlH4 (lithium aluminium hydride) in dry diethyl ether
          • It is a very strong reducing agent so can only fully reduce back down to alcohols
      • Iodoform reaction
        • Carbonyls containing a methyl carbonyl group react when heated with iodine in the presence of an alkali
          • Can also use potassium iodide solution and sodium chlorate (I)
        • If there is a methyl carbonyl group a yellow precipitate is formed (triiodomethan CHI3) and there is an antiseptic smell
          • Ethanal or a ketone with at least one methyl group


No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Functional Groups resources »