Aldehydes and Ketones

• General formula


  CnH2nO
• What group do they both contain?


  A carbonyl group c=o
• Functional group of aldehydes

  CHO

  
• Ketone functional group


  RCOR

  
• Aldehydes and ketones are functional group isomers of each other when they have the same number of carbon atoms
• 3 ways to oxidise aldehydes

  Aldehydes are readily oxidised to carboxylic acids

  • Warm with tollens’ reagent - silver mirror seen
  • warm with fehling’s solution- brick red ppt seen
  • H2SO4/K2Cr2O7- orange solution turns green
• Ketones cannot be further oxidised so will not react with tollens’ or fehlings
• Tollens’ reagent


  Contains the complex ion [Ag(NH3)2]+

  silver mirror seen if aldehyde is present

  aldehyde is oxidised to a carboxylic acid

  and silver ions reduced to silver metal

  Ag+ +e- -> Ag (s)
• Fehling’s solution


  Contains Cu2+ ions

  when warmed with an aldehyde a brick red ppt is seen

  aldehyde oxidised to a carboxylic acid

  and copper(II) ions are reduced to Cu+
• what reaction do aldehydes and ketones undergo?

  nucleophilic addition

  contain a polar c=o bond

  so they are susceptible to attacks by nucleophiles

  the nucleophile can attack the delta + c atom forming a new covalent bond, an addition reaction occurs across the double bond
• 1. reduction of aldehydes and ketones


  they can be reduced to alcohols

  reagent and conditions: aqueous NaBH4 (sodium borohydride)

  contains Na+ and H-

  the reducing agent is simplified to [H]

  aldehydes are reduced to primary alcohols

  ketones are reduced to secondary alcohols
• Reaction


  • Ethanal with sodium borohydride, NaBH(aq)
• Mechanism of ethanal and sodium borohydride

  Nucleophilic addition

  
• 3. Addition of HCN

  aldehydes and ketones can be converted to hydroxy nitriles by addition of HCN. This is nucleophilic addition

  reagents: KCN followed by dilute HCl

  nucleophile: CN-

  reacting an aldehyde or ketone with cyanide increases the length of the carbon chain
• Mechanism of ethanal with KCN and dilute HCl

  Forms 2-hydroxy propanenitrile

  
• why is HCN not used?

  in theory, HCN could be used as the source of the nucleophile but it is a toxic gas and it is hard to prevent it escaping into the lab.

  KCN is also toxic but it is an ionic solid and readily dissolves to form a solution
• When aldehydes and unsymmetrical ketones undergo reaction with KCN followed by dilute acid a racemic mixture of 2 enantiomers form
• How does the shape of the aldehyde/ unsymmetrical ketone lead to a product that is optically inactive?


  • the carbonyl group is trigonal planar
  • the nucleophile ( H or CN) is equally likely to attack from above and below the plane.
  • leading to a racemic mixture