L16 - Carbonyl Chem: Enols and Enolates

• Resonance form


  Same arrangement of atoms but different arrangements of electrons
• Tautomers

  
  

  (What are they? What are the keto and enol tautomers?)

  Constitutional isomers that readily interconvert by changing the location of a proton.

  
  

  Atoms arranged differently

  
• Constitutional isomers


  structural isomers - same Mr, different arrangement of atoms
• Enol formation

  
  

  (what conditions mucst be present? why? what's the mechanism?)

  Acid conditions bc need to add a proton

  
• Enolate formation

  
  

  (what conditions are needed? what's the mechanism?)

  Basic conditions

  
• Enol/Enolate Formation

  
  

  (what's acidity of alpha protons related to? what's the rank order of carbonyls? what happens to their pKa as reactivity decreases and why?)

  - acidity of alpha protons is related to rank order of reactivity

  
  

  - acyl chlorides, acyl anhydrides, aldehydes, ketones, ester/COOH, amides

  
  

  - as reactivity decreases, pKa increases. The smaller the pKa, the more acidic the alpha proton and the more reactive

  
• Why do we draw the oxanion not carbanion for enolates?

  - O is moreelectronegativethan C- so O is morestablethan C- so C is morereactivethan O

  
• How do enolates react?

  - Enolates have regions of high e- density so are nucleophiles: they react through the C

  
  

  - nucleophiles that react with electrophiles

  
• Example: Keto reacting with Bromine under acidic conditions

  
  

  (what's the mechanism? What's the product?)

  - form enolate first (or can be protonates in acid to give enol)

  
  

  - product is alpha bromoketone

  
  

  - (eg this occurs in salbutamol synthesis)

  
• Eg Iodoform reaction

  
  

  (what's the iodoform test used for? What conditions? What is a base?)

  - used to distinguish between methyl ketones and other ketones - pale yellow ppt forms in their presence

  
  

  - react iodine with enolate in basic conditions

  
  

  - base is a nucleophile that attacks a proton

  
• Aldol reaction

  
  

  (what does it do? why is it named this? what's the mechanism?)

  - forms C-C bonds

  - start withALDehyde, end with alcohOL

  
• Aldol reaction: What happens if you add excess acid?

  acid catalysed dehydration via E1/E2.

  
  

  Elimination of water