Lecture Notes: Carbonyl Reactivity

Overview

• Discussed carbonyl reactivity focusing on resonance structures.
• Examined how resonance affects carbonyl reactivity and polarity.
• Analyzed the impact of different substituents (R and Y groups) on carbonyl reactivity.

Carbonyl Structure

• Generic structure: R-C=O-Y
  • R: could be a benzene ring, alkyl group, etc.
  • Y: varies significantly; affects reactivity.
• Resonance structures:
  • Pi electrons move from carbon-oxygen pi bond to oxygen.
  • Carbon becomes electron poor (δ+), oxygen becomes electron rich (δ−).
• Importance of resonance:
  • Contributes to the molecule's overall reactivity.
  • Predicts electron distribution and chemical behavior.

Factors Affecting Reactivity

• R Group/Y Group:
  • Electron donating ability: determines if carbonyl is more or less electrophilic.
  • Examples include esters, aldehydes, ketones.
• Resonance Donation:
  • Methoxy group as a good electron donor via resonance.
  • Electronic donation reduces electrophilicity of carbonyl carbon.

Reactivity Ranking

• Aldehydes
  • Highly reactive due to lack of electron donation.
• Esters
  • Less electrophilic due to resonance donation.
• Ketones
  • Intermediate reactivity, electron donation via inductive effect.
• Amides
  • Least reactive; nitrogen is a better donor than oxygen.
• Carboxylates
  • Very low reactivity due to strong resonance and delocalization.
• Thioesters
  • More reactive than esters due to better leaving groups (S- vs. O-).

Mechanisms and Reactions

• Nucleophilic Attack:
  • Carbonyl carbon is the site of attack.
  • Formation of tetrahedral intermediates.
  • Importance of LUMO (Lowest Unoccupied Molecular Orbital) in reactions.
• Leaving Groups & Electrophilicity:
  • Better leaving groups result in higher reactivity.
  • Acid chlorides and anhydrides are highly reactive due to good leaving groups.

Practical Applications

• Biochemistry and Synthesis:
  • Importance in biological systems (e.g., peptide bonds in proteins).
  • Reactivity principles apply to metabolic pathways and synthesis.
• Laboratory Synthesis:
  • Conversion of more reactive carbonyls to less reactive forms is common.

Additional Concepts

• Acid-Base Reactions: Protonation affects electrophilicity.
• Molecular Orbital Theory: Insight into high energy electrons and orbitals.
• Resonance Structures and Protonation: Enhance understanding of reaction pathways.

These notes cover the key points discussed in the lecture on carbonyl reactivity. They include structural insights, factors affecting reactivity, detailed mechanisms, and practical implications in chemistry.