In-Depth Notes on Arenes and Benzene

Introduction to Benzene

  • Arenes: Hydrocarbons with one or more benzene rings; simplest arene is benzene.
  • Molecular Formula: Benzene has the formula C6H6.
  • Aromatic Hydrocarbons: Named due to properties of compounds with pleasant aromas containing benzene rings.
  • Stability Due to Aromaticity: Benzene undergoes reactions differently than alkenes (substitution over addition) because of its extra stability from delocalised electrons.

Hückel's Rule for Aromaticity

  • Requirements:
    1. Must be planar, cyclic with continuous p-orbital overlap.
    2. Total of (4n + 2) π electrons (where n is an integer).

Proposed Structure of Benzene

  • Early Theories:
    • Presented by August Kekulé in 1865 as cyclohexa-1,3,5-triene (Kekulé structure).
    • Suggested alternating single and double C-C bonds.
  • Evidence Against Kekulé Structure:
    • Equal Bond Lengths: All C-C bonds have equal length 0.139 ext{ nm}, contradicting different bond types in Kekulé’s model.
    • Reactivity: Benzene does not undergo addition reactions like alkenes; instead, it undergoes substitution reactions.
    • Thermochemical Data: Hydrogenation of benzene is -208 ext{ kJ mol}^{-1}—indicating a more stable structure due to delocalisation.

Actual Structure of Benzene

  • Delocalised Electrons:
    • Six π electrons are spread over carbon atoms forming a stable delocalised electron cloud.
  • Bonding:
    • Hybridisation: Each carbon is sp^2 hybridised with trigonal planar geometry (120°).
    • σ Bonds: Formed from head-on overlap of hybrid orbitals.
    • π Bonds: Form from the sideways overlap of 2p orbitals creating a delocalised π electron cloud above and below the ring.

Nomenclature of Benzene

  • Mono-Substituted Benzenes: Substituent named as a prefix; benzene is the stem. Examples:
    • Methylbenzene: -CH_3 as the substituent.
    • Bromobenzene: -Br substituent.
    • Nitrobenzene: -NO_2 substituent.
  • Priority of Functional Groups: Higher priority groups (e.g. hydroxyl -OH, carboxyl -COOH) lead to naming benzene as a prefix (e.g., Phenyl).

Electrophilic Substitution Reactions

  • Benzene reacts via electrophilic substitution instead of addition due to stability from delocalisation.
  • Examples of Reactions:
    • Chlorination and Bromination (using Lewis acid as catalysts).
    • Nitration with concentrated nitric acid using concentrated sulfuric acid as a Brønsted-Lowry acid catalyst.
    • Friedel-Crafts Alkylation using halogenoalkanes with Lewis acid catalysts.

Chemistry of Side-Chain in Benzene

  • Free-Radical Substitution: Occurs with chlorine and bromine.
  • Complete Oxidation: Yields benzoic acid.
  • Halogenation Predictions: Depends on reaction conditions whether the reaction occurs in the side-chain or aromatic nucleus.

Environmental Consequences and Recycling

  • Recognise the impact of emission gases (carbon monoxide, nitrogen oxides, hydrocarbons) from combustion engines.
  • Importance of recycling due to finite resources of petroleum as chemical feedstocks.