L13 - Carbonyl Chemistry

• Carbonyls

  
  

  
  

  (what is it? what are the properties of the atoms and what are they susceptible to? What's the hybridisation? what bond breaks first and why?)

  C=O

  
  

  - O electronegative

  - C electrophilic, so susceptible to nucleophilic attack

  
  

  sp3 hybridised

  
  

  nucleophile breaks pi bond first as pi bond is weaker - further from atomic nucleus so less electrostatic attraction

  
• Rank order of reactivity

  
  

  (What is it? What is it due to?)

  acid chloride - acid anhydride - aldehyde - ketone - ester - amide

  
  

  Due to mesomeric and inductive effects

  
• Compare the reactivity of acid chlorides and acid anhydrides

  Acid chlorides are more reactive as they have a strong inductive effect pulling e-s towards the Cl, making the carbon more delta positive/electrophilic so more susceptible to nucleophilic attack.

  There is potential for mesomeric effect, but it's poor as Cl is a big atom.

  
  

  Cl is more electronegative than the -OOR group and is a better leaving group. It's corresponding acid is HCl with a pka of -7.

  
• Compare the reactivity of aldehydes and ketones

  Aldehydes are more reactive than ketones as they only have one inductive effect pulling e-s towards the oxygen, whereas ketones have two inductive effects that pull e-s, making the carbon less electrophilic, so less susceptible to nucleophilic attack and less reactive.

  
• Compare the reactivity of amides and esters

  
  

  (basicity of amides)

  Esters are more reactive than amides because amides have the mesomeric effect - the lone pair of the N is pulled into the p orbital, making the carbon less delta positive and less susceptible to nucleophilic attack. Esters also exhibit the mesomeric effect but to a smaller extent.

  
  

  Amides are not basic due to the mesomeric effect - lone pair is unavailable.

  
  

  N is less inductive than O, so has a bigger mesomeric effect. Mesomeric effects cancel out inductive effects here.

  
• What does carbonyl reactivity depend on?

  - the electron density at the carbonyl carbon

  - the more electropositive, the more reactive as the more it will attract nucleophiles

  
• Why do you get cis/trans rotamers of amides?

  Amides exhibit some double bond characteristics due to the mesomeric effects, but have restricted rotation.

  
  

  (so we need to keep temp below 40degrees - more temp = more energy = faster rotation)