CHM_211_Lecture_1

Page 1: Introduction to Organic Chemistry

• Course Title: CHM 211: Organic Chemistry I

• Instructor: Dr. Bilkisu A.A

• Content Overview:

  • Chemistry of aromatic compounds

  • Structures of simple sugars, starch, cellulose, peptides, and proteins

  • Mechanisms of substitution, elimination, addition, and rearrangement reactions

Chemistry of Aromatic Compounds

• Definition: Aromatic compounds are hydrocarbons, often with pleasant smells.

  • The term "aromatic" comes from the Greek word for pleasant smell.

  • Structural characteristics:

    • Primarily consisting of carbon and hydrogen

    • Presence of sigma bonds and delocalized pi electrons in a cyclic structure.

• Aromatic compounds are classified as:

  • Benzenoids: Compounds that contain a benzene ring

  • Nonbenzenoids: Compounds that do not contain a benzene ring, e.g., furan.

Structure of Benzene

• History:

  • Isolated by Michael Faraday in 1825

  • Molecular formula: C6H6 shows high unsaturation

  • Stability and formation of triozonide indicates presence of double bonds.

• Kekulé Structure: Proposed in 1865

  • Cyclic arrangement of six carbon atoms with alternating single and double bonds.

  • Each carbon has one hydrogen attached.

Page 2: Further Insights into Benzene

• Revising the Kekulé Structure:

  • Found to form only one ortho disubstituted product suggesting double bonds are oscillating.

  • Led to resonance theory for a more accurate representation of benzene.

• Hybridization:

  • All six carbon atoms are sp² hybridized.

  • Overlapping of sp² orbitals forms C—C and C—H sigma bonds, while unhybridized p orbitals form delocalized π bonds.

Page 3: The Concept of π Bonding

• Delocalization of Electrons:

  • Six π electrons are freely delocalized across the carbon nuclei.

  • Benzene is planar with equal C—C bond lengths, contradicting cyclohexatriene's hypothetical unstable resonance.

  • Aromatic Stability Reasons:

  • Presence of resonance provides reluctance for addition reactions, leading to exceptional stability.

Aromaticity

• Definition:

  • Describes the chemical property of a cyclic, planar molecule with resonance bonds, exhibiting stability.

• Properties of Aromatics:

  • High resonance energy, undergo substitution reactions instead of addition, have delocalized pi-electrons.

Page 4: Criteria for Aromaticity

• Four Main Criteria:

  1. Cyclic Structure: Molecule must be cyclic.

  2. Planarity: Must allow π electron density delocalization.

  3. Complete Conjugation: Every atom must have a p orbital.

  4. Hückel’s Rule: Must contain (4n + 2) π electrons for aromaticity.

Page 5: Hückel’s Rule Explained

• Hückel’s Contribution:

  • Determined that cyclic planar molecules with 4n+2 π electrons are aromatic.

• Examples of π electron counts:

  • For n=0, 2 electrons; n=1, 6 electrons; etc.

• Categories:

  • Aromatic: 4n + 2 π electrons

  • Antiaromatic: 4n π electrons (less stable)

  • Nonaromatic: Neither aromatic nor antiaromatic due to structural abnormalities.

Page 6: Stability Comparison

• Stability Hierarchy:

  • Aromatic > Nonaromatic > Antiaromatic.

Molecular Orbital Theory

• Orbital Arrangement in Benzene:

  • Six sp2 carbon atoms form bonding and anti-bonding molecular orbitals.

  • Electrons occupy lower energy orbitals first per Hund's law.

Page 7: Frost's Circle Method

• Procedure for Aromaticity Evaluation:

  • Draw a circle and position a polygon to represent the molecule, marking vertices for energy levels.

  • Assign electrons starting from the lowest energy orbitals.

Page 8: Prediction Considerations

• Key Takeaways:

  • Aromatic: All filled orbitals.

  • Antiaromatic: Incomprately filled orbitals.

  • Antiaromatic species can change into nonaromatic due to lower stability.

Page 9: Examples of Aromatic Compounds

• Benzene (6 π electrons) and Derivatives:

  • Naphthalene (10 π electrons), phenanthrene, anthracene (14 π electrons).

  • Cyclopentadiene: Not aromatic until it forms a cyclopentadienyl anion upon losing H+.

Page 10: Aromatic Heterocycles

• Heterocycles Defined:

  • Cyclic compounds with heteroatoms (O, N, S), can be aromatic.

• Examples:

  • Pyridine: Nitrogen participates in resonance, basic.

  • Pyrrole: Five-membered ring with nitrogen.

  • Azulene: Nonbenzenoid aromatic compound.