Mechanisms

• What type of reaction can carbonyl groups undergo?

  Nucleophilic addition reactions
• What influences the reactivity of aldehydes and ketones?

  The nature of the carbon-oxygen double bond.
• What is the C=O bond made up of?

  Both a sigma bond and a pi bond. The pi bond is above and below the plane of the carbon and oxygen atoms, formed by the sideways overlap of p-oritals
• How are pi bonds formed?

  Above and below the plane of the central bond, formed by the sideways overlap of p-orbitals
• Why do carbonyl compound and alkenes not react in the same way?

  C=C Bond in alkenes in non-polar whereas the C=O Bond in carbonyl compounds is polar. This is because oxygen is more electronegative than carbon
• What makes the C=O bond polar?

  The electron density in the double bond lies closer to the oxygen atom than to the carbon atom, as it is more electronegative. This makes the carbon end slightly positive and the oxygen end slightly negative.
• What does the polarity of the C=O bond mean?

  Aldehydes and ketones react with some nucleophiles because the nucleophile is attracted to and attacks the slightly positive carbon atom resulting in addition across the C=O bond
• Why can aldehydes and ketones react with some nucleophiles?

  Due to the polarity of the C=O bond
• What reaction do C=C bonds undergo?

  The react with electrophiles by electrophilic addition
• Reaction of carbonyl groups with NaBH4?

  Sodium tetrahydridoborate (III), is used as a reducing agent to reduce aldehydes and ketones to alcohols. The aldehyde or ketone is usually warmed with NaBH4 reducing agent in aqueous solution.
• What are aldehydes reduced to?

  Primary alcohols
• What are ketones reduced to?

  Secondary alcohols
• Reaction of carbonyl compounds with HCN?

  Hydrogen cyanide adds across the C=O bond.
• What is hydrogen cyanide?

  A colourless, extremely poisonous liquid that boils slightly above room temperature - cannot be used safely in an open lab.
• What is used to provide the hydrogen cyanide in the reaction?

  Sodium cyanide and sulfuric acid - reaction still potentially hazardous.
• Why is the reaction of carbonyl compounds with HCN useful?

  Provides a means of increasing the length of the carbon chain
• What is formed from the reaction of an aldehyde with HCN?

  An organic compound containing 2 functional groups:

  A hydroxyl group: -OH

  A nitrile group C-N triple bond

  These compounds are classed as hydroxynitriles (cyanohydrins)
• The carbon atom in the C=O bond is electron deficient and attracts nucleophiles. Aldehydes and ketones react by nucleophilic addition to form alcohols.
• Mechanism for the reaction with NaBH4:

  1. the lone pair of electrons from the hydride ion is attracted to and donated to the delta + carbon atom in the C=O bond
  2. a dative covalent bond is formed between the hydride ion and the carbon atom
  3. the pi bond in the C=O bond breaks by heterolytic fission forming a negatively charged intermediate
  4. the O atom of the intermediate donates a lone pair of electrons to a H atom in a water molecule
  5. The intermediate has been protonated to form an alcohol
• What acts as the nucleophile in the reaction of aldehyde or ketone with NaBH4?

  NaNH4 can be considered as containing the hydride ion, which acts as the nucleophile
• Mechanism for the reaction with NaCN/H+?

  1. the lone pair of electrons from the cynaide ion attacks the electron-deficient carbon atom of the C=O bond - a dative covalent bond forms
  2. the pi bond in the C=O bond breaks by heterolytic fission, forming a negatively charged intermediate
  3. the intermediate is protonated by donating a pair of electrons to a hydrogen ion, to form the product
  4. the product is a hydroxynitrile
• In the first stage of the reaction of an aldehyde or ketone with NaCN/H+, the cyanide ion attacks the electron deficient caron atom in the aldehyde or ketone