Lecture on Structure of Benzene

Introduction to Benzene

• Lecturer: Rajneesh Kumar, Assistant Professor, NIT
• Topics covered: Structure of benzene, evidence for different benzene structures, aromatic compounds.

Organic Chemistry Basics

• Organic Chemistry: Deals with organic compounds containing carbon.
  • Organic Compounds: Can be classified as aliphatic or aromatic.
  • Aliphatic Compounds: Can be cyclic or acyclic (open chain structure).
  • Aromatic Compounds: Known for pleasant smells, classified as benzenoids or non-benzenoids.
  • Benzenoids: Contain benzene ring.
  • Non-benzenoids: Aromatic without benzene ring.

Benzene: An Aromatic Compound

• Aromatic compounds have carbon and hydrogen in ring structures with delocalized pi electrons.
• Benzene is considered a parent aromatic compound.

Historical Background of Benzene

• Origin: Name derived from "gum benzoin," known in the 16th century.
• Michael Faraday (1825): Isolated benzene, named it "bicarburet of hydrogen."
• Ilhard Misch-Cherlis (1834): Established benzene's formula as C6H6.
• August Kekule (1858): Proposed ring structure inspired by a dream of a snake.

Benzene's Structure and Stability

• All carbon-carbon bonds in benzene are equivalent, with a bond length of 1.39 Å.
• Benzene is unusually stable due to delocalized pi electrons.
• Represented as a hexagon with a circle (indicating shared pi electrons).

Representation of Benzene

• Kekulé Structure: Six carbon atoms forming a ring.
• Resonance Structure: Delocalization of pi bonds results in a hybrid structure.

Hybridization in Benzene

• Benzene Structure: Planar with sp² hybridized carbons.
• Hybridization: Results in trigonal planar geometry for carbon atoms.
• Sigma Bonds: Formed from linear overlapping of sp² orbitals.

Analytical and Chemical Evidence Supporting Benzene's Structure

• Analytical Evidence:
  • Elemental analysis shows carbon and hydrogen are in a 1:1 ratio (C6H6).
  • Molecular weight determination: 78 g/mol.
• Chemical Behavior:
  • Benzene does not readily undergo addition reactions (unlike alkenes).
  • Prefers electrophilic substitution reactions.

Substitution Reactions

• Mono-substitution: Benzene reacts with bromine to form mono-bromo benzene.
• Di-substitution: Results in ortho, meta, and para isomers.

Stability and Resonance

• Heat of Hydrogenation: Benzene's heat is much less than predicted, indicating high stability.
• Resonance Energy: Further supports benzene’s stability.

Conclusion

• Benzene's cyclic structure and stability explained.
• Next lecture will cover molecular orbital theory of benzene.

Thank you for your attention. This concludes the lecture.