Aldehydes and Ketones

• Aldehydes


  Compounds with general formula RCHO
• Ketones


  Compounds with general formula RCOR'
• Nomenclature of aldehydes and ketones


  • Select the longest continuous carbon chain
  • Start numbering the carbon from the end that has the functional group
  • Substituents are named in alphabetical orders
  • Whenever, there are more than one substituent of the same type, use di, tri, tetra or penta
• Aldehydes and ketones


  • Benzaldehyde
  • Methylphenylketone
  • 2-butanone
  • 2,4-dimethyl-3-pentanone
• Polar molecules


  Molecules that have a net dipole moment
• Aldehydes and ketones are polar molecules because there is charge separation
• Hydrogen bonding


  Aldehydes and ketones don't have an H atom attached to the carbonyl oxygen, so they cannot function as H-bond donors to one another or other molecules such as H2O and alcohols. They can only function as H-bond acceptors.
• Aldehydes and ketones boiling points


  Lower than those of alcohols of similar molecular weight because alcohols can act both as an H-bond donor and H-bond acceptor
• Aldehydes and ketones are good solvents for polar hydroxylic compounds such as alcohols
• As the hydrocarbon chain length increases, hydrophobicity increases and hence aldehydes and ketones become less soluble in water
• Preparation of aldehydes and ketones from alcohols


  1. Oxidation of primary and secondary alcohols

  2. Use of mild oxidants like PCC to stop at aldehyde stage

  3. Swern oxidation to oxidise primary alcohols to aldehydes
• Preparation of aldehydes and ketones from alkenes


  Ozonolysis of alkenes to yield an ozonide, which on treatment with dimethylsulphide yields a mixture of an aldehyde and ketones
• Nucleophilic addition


  Addition of a nucleophile and proton across the C=O double bond, causing the carbon atom to change hybridization from sp2 to sp3
• Aldehydes are more reactive than ketones towards nucleophilic addition due to both steric and electronic effects
• Reaction of aldehydes and ketones with Grignard reagents


  RMgX + HCHO -> RCH2OH + MgBrOH

  RMgX + RCHO -> RCHR'OH + MgBrOH

  RMgX + R2C=O -> RCR'R''OH + MgBrOH
• Reaction of aldehydes with HCN


  Yields a β-hydroxycyano compound, which can be hydrolysed to a β-hydroxycarboxylic acid
• Clemmensen reduction


  Reduces the carbonyl group of aldehydes and ketones to a methylene unit using Zn with Hg/HCl
• Wolff-Kishner reduction


  Reduces the carbonyl group of aldehydes and ketones to a methylene unit using hydrazine, N2H4 and a base
• Reaction of aldehydes and ketones with acidified water

  Yields a dihydroxy compound
• Reaction of aldehydes and ketones with alcohols


  Forms hemiacetals and acetals
• Reaction of aldehydes and ketones with ammonia and ammonia derivatives


  Forms condensation products called carbinolamines, which on dehydration yield Schiff's base type compounds
• Depending on the type of amine, the condensation products are given specific names (e.g. oxime, hydrazone, phenylhydrazone)
• Reaction of aldehydes with Tollens reagent


  Silver ion, Ag+ oxidises aldehydes selectively in a convenient functional group test for aldehydes