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
• 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
• C=O bond in carbonyls

  • It is stronger and does not undergo addition reactions easily
  • It is polarised because O is more electronegative than carbon, so the positive carbon atom attracts nucleophiles
• Oxidation of alcohols and aldehydes

  1. Primary alcohols are oxidised to aldehydes

  2. Aldehydes are oxidised to carboxylic acids

  3. Secondary alcohols are oxidised to ketones

  4. Tertiary alcohols do not oxidise
• Oxidising agent

  Potassium dichromate (VI) solution and dilute sulfuric acid
• Oxidation of aldehydes

  RCHO + [O] → RCO2H
• Observation: the orange dichromate ion (Cr2O7^2-) reduces to the green Cr^3+ ion
• Aldehydes can also be oxidised using

  • Fehling's solution
  • Tollen's reagent
• Reduction of carbonyls

  1. Reducing agents such as NaBH4 or LiAlH4 will reduce carbonyls to alcohols

  2. Aldehydes will be reduced to primary alcohols

  3. Ketones will be reduced to secondary alcohols
• Nucleophilic addition mechanism for carbonyl reduction

  NaBH4 contains a source of nucleophilic hydride ions (:H-) which are attracted to the positive carbon in the C=O bond

  and the aqueous/ethanolic solution provides H+ ions for the addition
• Catalytic hydrogenation of carbonyls

  1. Reagent: hydrogen and nickel catalyst

  2. Conditions: high pressure

  3.product: alcohol
• Observation with Tollens' reagent

  Aldehydes form a silver mirror coating the inside of the test tube, ketones result in no change
• Observation with Fehling's solution

  Blue Cu2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react.
• Addition of hydrogen cyanide to carbonyls

  Carbonyl + HCN → Hydroxynitrile
• Mechanism for addition of HCN to carbonyls

  Nucleophilic addition, the NaCN supplies the nucleophilic CN- ions and the H2SO4 acid supplies H+ ions needed in the second step
• Naming hydroxynitriles

  The CN becomes part of the main chain and carbon no 1
• we use NaCN and KCN for the nucleophilic addition mechanism for the reduction of carbonyls to form hydroxynitriles because HCN is toxic and hard to contain although KCN and NaCN is toxic due to the CN- ions, it has a higher concentration of CN- ions and so will completely ionise whereas HCN is a weak acid and will only partially ionise
• Nucleophilic addition of HCN to aldehydes and unsymmetrical ketones results in the formation of a racemate because its planar
• Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, using NaBH4 in aqueous solution. These reduction reactions are examples of nucleophilic addition
• The nucleophilic addition reactions of carbonyl compounds with KCN, followed by dilute acid, to produce hydroxynitriles
• draw the nucleophilic addition mechanism of NaBH4 to a carbonyl

  reduction of carbonyl

  
• draw the nucleophilic addition mechanism for the reduction of a carbonyl to a hydroxynitrile using NaCN

  reduction of a carbonyl

  
• nucleophilic addition of carbonyls to form hydroxynitriles


  reagent : NaCN or KCN in dilute sulfuric acid

  condition: room temperature and pressure

  product: hydroxynitrile
• Aldehydes can also be oxidised using Fehling’s solution or Tollen’s reagent. These are used as tests for the presence of aldehyde groups