ketones and aldehydes

• Aldehydes


  Compounds with general formula RCHO
• Ketones


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

  • Select the longest continuous carbon chain
  • Indicate the position of the functional group, using the lowest number
  • 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
• Polar molecules


  Molecules that have a net dipole moment
• Aldehydes and ketones are polar molecules because there is charge separation
• Physical properties of aldehydes and ketones

  • Soluble in polar organic solvents, such as ethanol, water etc.
  • Cannot function as H-bond donors to one another or other molecules such as H2O and alcohols
  • Can only function as H-bond acceptor
  • Boiling points are lower than those of alcohols of similar molecular weight
  • Can act as H-bond acceptor with an hydrogen from H2O or from an alcohol molecule
  • Good solvents for polar hydroxylic compounds such as the alcohol
  • Remarkably soluble in water
  • 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 oxidant like PCC to stop at aldehyde stage

  3. Swern oxidation to oxidise primary alcohol to aldehyde
• Preparation of aldehydes and ketones via ozonolysis of alkenes


  1. Alkenes react with ozone to yield an ozonide

  2. Ozonide treated with dimethylsulphide yields a mixture of an aldehyde and ketones
• Reactions of aldehydes and ketones

  • Undergo nucleophilic addition
  • Aldehydes are more reactive than ketones towards nucleophilic addition due to steric and electronic effects
• Reaction with Grignard reagents

  1. RMgX + HCHO -> RCH2OH + MgBrOH

  2. RMgX + RCHO -> RCHR'OH + MgBrOH

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


  1. Aldehyde reacts with HCN to yield a b-hydroxycyano compound

  2. b-hydroxycyano compound can be hydrolysed to yield a b-hydroxycarboxylic acid
• Clemmensen reduction


  Carbonyl group of aldehydes and ketones reduced to a methylene unit using Zn with Hg/HCl
• Wolff Kishner reduction


  Carbonyl group of aldehydes and ketones reduced to a methylene unit using hydrazine, N2H4 and a base
• Reaction with H2O


  Aldehydes and ketones react with acidified water to yield a dihydroxy compound
• Reaction with alcohols


  1. Aldehydes react with one mole of an alcohol to form a hemi-acetal

  2. Aldehydes react with two moles of an alcohol to form an acetal
• Reaction with NH3 and NH3 derivatives

  1. Aldehydes and ketones react to form a condensation product called a carbinolamine

  2. Carbinol amine on dehydration yields a Schiff's base type compound
• Depending on the type of amine, the condensation products are given specific names
• Reaction with Tollens reagent


  Silver ion, Ag+ oxidises aldehydes selectively in a convenient functional group test for aldehydes
• ketone carbonyl group is more stable than an aldehyde
• alcohols are oxidised to aldehydes, which can be further oxidised to carboxylic acids
• Ketones do not undergo reduction by LiAlH4 or NaBH4
• Aldehydes are oxidised to carboxylic acids by KMnO4 or acidified dichromate(VI) solution.
• Acetoacetic ester (ethyl methyl ketone): used as solvent and starting material for synthesis of drugs such as aspirin
• Aldehydes are reduced to primary alcohols by LiAlH4 or NaBH4
• Ketones do not undergo this reaction as they have no hydrogen atoms attached to their carbon atom
• The reaction between alkyl halides and Grignard reagents gives secondary alcohols.
• The reaction between an alcohol and a strong base (NaOH/KOH) produces water and a metal salt (RO-)
• The reaction between an alcohol and conc. H2SO4 at high temperatures forms a mixture of alkenes and aromatic hydrocarbons via dehydrogenation
• Diethyl malonate: used as a starting material for the preparation of many organic compounds including pharmaceuticals
• Grignard reagent is an organometallic compound that contains magnesium, which can be prepared from the reaction of an alkyl halide with magnesium metal in ether.
• Secondary alcohols are oxidised to ketones with chromic acid/pyridine
• Primary alcohols are oxidised to aldehydes using chromic acid/pyridine
• Ethanal: used as a solvent and starting material for the production of plastics and resins
• The reaction between an alcohol and PCl5 in the presence of peroxide catalysts forms chloroformates, which can be converted into carbamates using ammonia
• The reaction between an alcohol and SO3 in pyridine forms sulfonic acids
• Aldehydes are oxidised by acidified potassium dichromate(VI) solution to form carboxylic acids.
• Carboxylic acids react with NaOH to give sodium salts and water.
• Aldehydes react with NaBH4 to give primary alcohols
• Reagent strip


  Also called a dipstick, contains absorbent pads impregnated with test reagents
• Reagent strip testing


  1. Dip strip in urine

  2. Chemical reactions result in colour changes on pads

  3. Compare colour against chart

  4. Interpret results (trace, 1+, 2+, etc)