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.