Aromatic Hydrocarbons
World C — Carbon Chemistry
35.A — STRUCTURE & STABILITY
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R35.01 The Benzene Ring
- Parent compound is Benzene ($C_6H_6$).
- Kekulé proposed rapidly alternating double bonds.
- Modern view: A planar ring with a completely delocalized $\pi$ electron cloud.
4n + 2 = 6e⁻
R35.02 Hückel's Rule
- To be aromatic, a molecule must be planar, cyclic, and fully conjugated.
- Must have exactly $(4n + 2)$ $\pi$ electrons.
- Benzene has $n=1$, yielding 6 delocalized $\pi$ electrons.
R35.03 Resonance Energy
- Benzene is significantly more stable than the hypothetical "cyclohexatriene".
- This difference (~150 kJ/mol) is the Resonance Energy.
- Makes the ring highly resistant to breaking.
R35.04 Orbital Overlap
- All 6 carbon atoms are $sp^2$ hybridized.
- Un-hybridized $p$-orbitals sit perpendicular to the ring.
- They merge sideways to form continuous rings of electron density above and below.
35.B — REACTIONS & DERIVATIVES
R35.05 Electrophilic Substitution
- Unlike alkenes, aromatics undergo substitution, not addition.
- Addition would destroy the highly stable aromatic ring.
- An electrophile ($E^+$) replaces a Hydrogen atom.
- Examples: Nitration, Halogenation, Friedel-Crafts.
R
R35.06 Directing Groups
- An existing group (R) dictates where the next substituent goes.
- Ortho: Position 2 adjacent.
- Meta: Position 3 (one carbon away).
- Para: Position 4 (directly opposite).
-OH
-CH₃
-NH₂
R35.07 Common Derivatives
- Phenol: Benzene + OH (Antiseptics, plastics).
- Toluene: Benzene + $CH_3$ (Solvent, TNT precursor).
- Aniline: Benzene + $NH_2$ (Dyes, pharmaceuticals).
R35.08 Polycyclic Aromatics (PAH)
- Multiple benzene rings fused together.
- Naphthalene (2 rings - mothballs).
- Anthracene (3 rings - dyes).
- Many larger PAHs are formed by incomplete combustion.