Lecture Notes on Hybridization, Alkenes, and Benzene

Hybridization of Carbon

• sp3 Hybridization:
  • Carbon forms four bonds.
  • Shape: Tetrahedral.
  • Bond angles: 109.5°.

Benzene Structure

• Benzene Representation:

  • Six carbon atoms.
  • Planar, hexagonal shape.
  • Bond angles: 120° (Trigonal planar configuration).
  • Each carbon bonds to three atoms (8 electrons), with one unused p orbital forming a pi bond.
  • Delocalized pi electron cloud formed by overlapping p orbitals.

• Key Points:

  • Carbon in benzene has a unique hybridization (sp2).
  • The structure contributes to the stability and reactivity of benzene.
  • Most reactions are influenced by the pi electron cloud.

Comparison: Alkenes vs. Benzene

• Alkenes:

  • sp2 hybridized.
  • High electron density in the pi electron cloud (localized).
  • Electrophilic addition reactions (e.g., bromination).
  • Markovnikov's Rule applies to carbocation stability:
    • More stable with more alkyl chains.

• Benzene:

  • Lower electron density in pi cloud due to delocalization.
  • Requires a catalyst for reactions (e.g., electrophilic substitution).

Reactions of Alkenes

• Electrophilic Addition Reactions:
  • Bromination: Polarization of bromine leads to carbocation formation.
  • Hydrogenation and Hydration reactions with specific conditions (e.g., catalysts and temperature).
  • Oxidation:
    • Minor oxidation (cold dilute alkaline KMnO4).
    • Strong oxidation can lead to cleavage into carboxylic acids or ketones.

Electrophilic Substitution Reactions of Benzene

• Mechanism:

  • Requires a catalyst (e.g., FeBr3, AlCl3).
  • Electrophile generated from reaction with a halogen.
  • Positive charge formed on the carbon after losing an electron to the electrophile.

• Examples:

  • Nitration: Uses concentrated HNO3 and H2SO4, resulting in substitution by NO2.
  • Hydrogenation: Benzene reacts with H2 and nickel to form cycloalkane.

Derivative Compounds of Benzene

• Functional Groups:
  • Phenol, phenylamine, benzoic acid, benzaldehyde.
• 2,4,6 Directing Effects:
  • Electron donating groups enhance nucleophilicity at these positions.

Electrophilic Substitution Mechanism for Phenol

• Higher Reaction Rate: Due to lone pairs on oxygen interacting with the benzene electron cloud.
• Example: Reaction with bromine leads to 2,4,6-tribromophenol.

Comparison of Hydrolysis Reactions

• Chlorobenzene: No reaction with water.
• Halogenoalkanes: Moderate hydrolysis, reacts with NaOH.
• Acyl Chlorides: Vigorous hydrolysis due to strong electrophilic carbon.

Acid Strength Comparison

• Carboxylic Acids: Strongest due to electron-withdrawing effect of the carbonyl group.
• Phenols: Weaker than carboxylic acids but stronger than alcohols.
• Alcohols: Weakest acids due to electron-donating alkyl groups.

Summary of Reactions and Mechanisms

• Alkylation and Acylation: Friedel-Crafts reactions require catalysts.
• Esters: Formed by esterification; hydrolysis reversible under acid/base conditions.
• Azo Dyes: Formed via diazonium ion reactions with activated benzene.

Polymerization

• Polyesters: Formed from diols and dicarboxylic acids.
• Polyamides: Formed through condensation reactions with amino acids.
• Properties: Influenced by polarity, branching, and cross-linking in polymer chains.