Carbonyls

• A carbonyl group is a carbon to oxygen double bond C=O

  The C=O bond is polar, the C being partially positive an the oxygen being partially negative.

  The two carbonyl groups seen so far are:

  ketones [R-(C=O)-R]

  aldehydes [R-(C=O)-H]
• To test for an aldehyde or ketone, the compound is reacted with 2,4-dinitrophenylhydrazine (2,4-DNPH).

  Compounds containing a carbonyl group will under go a condensation reaction, forming a deep orange precipitate.

  Aldehydes and ketones can be differentiated by purifying the precipitate and measuring the melting point and comparing to literature values.

  Only aldehydes and ketones will give a positive result, carboxylic acids and esters will not.
• Fehlings solution is a copper ion containing oxidising agent. When it is warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid. This reduces the Cu(II) ions to Cu(I) ions, causing a colour chang of blue to red.

  Ketones do not react and the solution stays blue.
• Tollens' reagent contain aqueous silver nitrate in excess ammonia. When warmed with an aldehyde, the aldehyde is oxidized reducing the silver ions to silver metal forming a silver mirror.

  Ketones do not react with Tollens' reagent so no silver mirror is formed.
• As the C=O bond is polar, carbonyl compounds can undergo nucleophilic addition. What are the mechanisms for both aldehydes and ketones?


  Aldehydes:

  Ketones:

  
• As the C=O bond is polar, carbonyl compounds can undergo nucleophilic addition.
• What is the general mechanism for aldehyde nucleophilic addition?

  1. Nucleophile donates electrons to C, breaking the C=O double bond. The shared pair of electrons are accepted by the oxygen.
  2. The negative oxygen shares its pair of electrons with a hydrogen atom forming an OH.
  3. Final compound is formed containing a hydroxyl group.

  
• What is the general mechanism for ketone nucleophilic addition?

  1. Nucleophile attacks the carbon, bonding with it and breaking the C=O bond from which the electrons are accepted by the oxygen.
  2. The negative oxygen forms a bond with a H ion from the solvent.
  3. Final product containing a hydroxyl group is formed.

  
• What is the typical reducing agent for carbonyls?

  Sodium Tetraborate
• In order to reduce carbonyls you need a reducing agent and water in a reflux setup.
• What nucleophile is formed by sodium tetraborate?

  H- / hydride ion