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Part of OC-03 — Aromatic Hydrocarbons

Cheat Sheet — OC-03 Aromatic Hydrocarbons (Pinned)

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BENZENE BASICS

  • Structure: c1ccccc1 | All C-C = 1.39 Å | Planar hexagon | 6 π electrons delocalized
  • Bond comparison: C-C single = 1.54 Å | Benzene C-C = 1.39 Å | C=C double = 1.34 Å
  • Kekule structures = resonance contributors ONLY (not real alternating single-double bonds)

HUCKEL RULE — AROMATICITY

Aromatic=Planar+Cyclic+Conjugated+(4n+2)π electrons\text{Aromatic} = \text{Planar} + \text{Cyclic} + \text{Conjugated} + (4n+2)\pi \text{ electrons}

n(4n+2)Example
02Cyclopropenyl cation
16Benzene, Pyridine, C5H5−C_{5}H_{5}^{-}
210Naphthalene, Azulene
  • Anti-aromatic: 4n π e−e^{-} + planar + cyclic + conjugated (e.g., cyclobutadiene = 4 π e−e^{-})
  • Non-aromatic: Fails ANY criterion (e.g., COT = 8 π e−e^{-} but NON-PLANAR → non-aromatic)

EAS MECHANISM (3 Steps)

  1. Generate E+E^{+} (Lewis acid catalyst activates reagent)
  2. E+E^{+} attacks π cloud → Arenium ion (sigma complex / Wheland intermediate) — RDS; aromaticity lost
  3. H+H^{+} leaves → Product; aromaticity restored (thermodynamic driving force)

5 EAS REACTIONS — QUICK REFERENCE

ReactionReagentElectrophileKey Feature
HalogenationX2X_{2} / Lewis acidX+X^{+}Lewis acid required
NitrationHNO3HNO_{3} / H2SO4H_{2}SO_{4}NO2+NO_{2}^{+}H2SO4H_{2}SO_{4} generates NO2+NO_{2}^{+}
SulfonationFuming H2SO4H_{2}SO_{4}SO3SO_{3}ONLY REVERSIBLE EAS
FC AlkylationRCl / AlCl3AlCl_{3}R+R^{+}Rearrangement + polyalkylation
FC AcylationRCOCl / AlCl3AlCl_{3}RCO+CO^{+}No rearrangement, no polyacylation

DIRECTING GROUPS — MASTER TABLE

EffectGroupsDirectorActivity
+M, +I-OH, -NH2NH_{2}, -OR, -NR2NR_{2}o/pActivating
+I, hyperconj.-CH3CH_{3}, alkylo/pWeakly activating
-I (weak), +M-F, -Cl, -Br, -Io/pDEACTIVATING (unique!)
-M, -I-NO2NO_{2}, -CN, -CHO, -COOH, -CORmetaDeactivating

CRITICAL NEET TRAPS

  1. Halogens: o/p directors BUT deactivating — the only group with this combination
  2. COT: 8 π e−e^{-} (4n) but NON-aromatic (not anti-aromatic) because non-planar
  3. FC Acylation > Alkylation: Acylation = no rearrangement + no polysubstitution
  4. -NH3+NH_{3}^{+} (protonated aniline): becomes a META director (no lone pair for +M)
  5. Sulfonation is the ONLY reversible EAS — all others are irreversible

KEY SMILES

  • Benzene: c1ccccc1
  • Toluene: Cc1ccccc1
  • Chlorobenzene: Clc1ccccc1
  • Nitrobenzene: [O-][N+](=O)c1ccccc1
  • Phenol: Oc1ccccc1
  • Aniline: Nc1ccccc1
  • Acetophenone: CC(=O)c1ccccc1
  • Benzenesulfonic acid: OS(=O)(=O)c1ccccc1

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