Comprehensive Notes on Electrophilic Aromatic Substitution

Overview of Electrophilic Aromatic Substitution in Non-Symmetrical Aromatic Compounds

  • The discussion revolves around the complexity of synthesizing molecules, particularly focusing on electrophilic aromatic substitution (EAS) reactions for non-symmetrical aromatic compounds, including heterocycles.

Importance of Complicated Molecules

  • A notable example of a complex molecule is git definitive, used in the treatment of non-small cell lung cancer.
  • Statistical Insight:
    • More individuals die from lung cancer than from all other cancers combined, highlighting the severe public health impact of lung cancer.
  • Implication: The ability to advance our knowledge in synthesizing complicated molecules can potentially extend life significantly, influencing personal and familial relationships.

Introduction to Electrophilic Aromatic Substitution (EAS)

  • EAS is common in both symmetrical and non-symmetrical aromatic compounds.
  • Focus will be on heterocycles, which are cyclic compounds containing at least one non-carbon atom in the aromatic ring.

Heterocycles Overview

  • Definition of Heterocycles:
    • Heterocycles are rings that contain atoms of at least one element other than carbon.
  • Examples of Heterocycles:
    • Pyrrole: A five-membered ring with a nitrogen atom.
    • Aromaticity: Aromatic due to nitrogen's lone pair residing in a p orbital.
    • Pyridine: A six-membered ring that includes a nitrogen atom.
    • Aromaticity: Nitrogen’s lone pair does not contribute to aromaticity as it is located in an sp² hybridized orbital.

Reactivity of Different Rings

  • Reactivity of Pyrrole and Pyridine:
    • Pyrrole: More reactive in EAS compared to benzene due to its structure.
    • Pyridine: Less reactive in EAS than benzene.
  • Benzopyrrole (fused benzene and pyrrole):
    • The site of substitution will favor the more reactive pyrrole position (five-membered ring).
  • Quinoline (fused benzene and pyridine):
    • The site of substitution is more favorable on the benzene ring side due to its higher reactivity compared to pyridine.

Example: Substituting on Furan

  • Furan is a five-membered aromatic compound with two distinct carbon atoms:
    • Alpha Carbon: The carbon directly bonded to oxygen.
    • Beta Carbon: The carbon one position removed from oxygen.
  • Resonance Analysis:
    • To determine if the electrophile will go to the alpha or beta position, conduct resonance analysis for both positions.
    • Tiebreakers include:
    • Greater number of resonance structures indicates preference.
    • Stability of canonical forms (more stable forms preferred).
  • Chlorination Example:
    • Chlorination yields 70% alpha substitution and 30% beta substitution, highlighting the resonance analysis's role in predicting product distribution.

Quinoline Reaction Analysis

  • Numbering in Quinoline:
    • Atoms are numbered based on potential substituent sites.
    • Key positions are measured primarily between carbon 5 (alpha top) and carbon 8 (alpha bottom).
  • Resonance Structure for Substitution:
    • Analyzing resonance structures for both carbon positions leads to identification of the more stable product.
    • Major product arises from the carbon 8 substitution due to resonance stability, as nitrogen remains stable without a positive charge.

Exploring Other Heterocycles

  • Additional heterocycles such as thiophene and pyrropyridine can be explored using similar resonance analysis principles.
  • Tasks may include predicting major products and rationalizing data provided in problems.

Student Guidance and Office Hours

  • Emphasized the importance of resonance structures in determining the sites of substitution in aromatic systems.
  • Personal engagement: Instructor will be available for questions to assist with EAS problems, emphasizing the learning process to resolve difficulties encountered in earlier exams.