ochem 2/20

Introduction to Reaction Mechanisms

  • Discussion of vicinal dibromides in organic synthesis.

  • Use of strong bases and high temperatures in reactions.

Cyclization and Resonance Contributors

  • Triple bond formation can occur at the ends of carbon chains.

  • Example Structure: CH2-CH=CH2-CH3.

    • Importance of resonance contributors explained,

    • Low temperature promotes radical participation.

  • Products of reactions and the significance of conjugated double bonds.

    • Each conjugated double bond increases light absorption by 30-40 nm.

    • Alkyl groups only increase absorption by 5 nm.

Aromaticity and Nonaromatic Systems

  • Understanding Hückel's rule for aromaticity.

  • Conditions for a system to be considered aromatic:

    • Continuous circle of p orbitals.

    • Each atom must have p orbitals and be planar.

    • Carbon atoms need to be sp2 hybridized.

  • Nonaromatic characteristics explained:

    • Presence of sp3 hybridized carbons disrupts aromatic character.

    • Example: Sterically hindered cyclic structures can't maintain planarity.

  • Differentiation between aromatic, antiaromatic, and nonaromatic.

Pi Electron Delocalization

  • Role of hybridization in pi bond formation:

    • Example of a cation with delocalized pi electrons over a framework of p orbitals.

    • Mention of stabilizing aromaticity in certain cation systems.

    • Examining delocalization of pi electrons for heteroatoms in heterocycles.

Heterocycles and Hybridization

  • Analysis of nitrogen-containing heterocycles:

    • The role of lone electron pairs in determining aromaticity.

    • Examples given for nitrogen hybridization (sp2 and sp3).

    • Programmatic choices about such compounds are crucial.

  • Explanation of the nature of bases and how they alter aromatic systems.

Carcinogens from Combustion Products

  • Overview of polynuclear aromatic hydrocarbons (PAHs):

    • Importance of knowing PAHs as they can act as carcinogens.

    • Common examples: Pyrene, dibenzopyrene, benzopyrene.

  • Mechanism of DNA damage through epoxidation by PAHs in living systems:

    • Details on how epoxides lead to mutations in DNA.

  • Caution against smoking and risks associated with PAHs from tobacco.

Stability and Energy of Aromatic Compounds

  • Resonance energy comparison:

    • Benzene (36 kcal/mol) vs. Naphthalene (30 kcal/mol) vs. Anthracene (28 kcal/mol).

    • Increasing complexity reduces stability and enhances reactivity.

IUPAC Naming and Substitution Patterns

  • Systematic approach to nomenclature in aromatic chemistry:

    • Naming of substituted benzene derivatives (e.g., toluene, phenol).

  • Explanation of ortho, para, meta nomenclature for substituted benzene:

    • Importance of lowest numbers in nomenclature despite alphabetical order.

Reaction Mechanisms Involving Aromacy

  • Different types of electrophilic aromatic substitutions:

    • Halogenation involves adding halogens, accompanied by HX as a byproduct.

    • Nitration using concentrated HNO3 and H2SO4 as catalysts.

    • Sulfonation enabling hydrogen removal to regain aromaticity.

  • Various reaction energetics:

    • Stabilization and resonance important in sustaining aromaticity while undergoing substitutions.

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