CHEMCA
EXAM MASTER FORMULA SHEET
Aromatic Hydrocarbons: Benzene
1. Aromaticity & Structure
Benzene ($\ce{C6H6}$) is a planar, cyclic conjugated system with exceptional thermodynamic stability due to electron delocalization.
- • Cyclic & Planar
- • Complete Conjugation
- • $(4n + 2) \pi$ electrons
- ($n = 0, 1, 2 \dots \to 2, 6, 10\pi$)
- • Cyclic & Planar
- • Complete Conjugation
- • $4n \pi$ electrons
- ($n = 1, 2 \dots \to 4, 8, 12\pi$)
- • Fails any of the basic criteria.
- • Usually contains an $sp^3$ hybridized carbon that breaks the conjugation.
Due to resonance, all $\ce{C-C}$ bond lengths are identical ($139 \text{ pm}$), intermediate between single ($154 \text{ pm}$) and double bonds ($134 \text{ pm}$).
Resonance Energy $\approx 36 \text{ kcal/mol}$ or $150 \text{ kJ/mol}$
2. Methods of Preparation
1. Decarboxylation
Heating Sodium Benzoate with Soda Lime ($\ce{NaOH + CaO}$).
2. Phenol Reduction
Phenol vapors passed over heated Zinc Dust.
3. Cyclic Trimerization
Passing Ethyne through a Red Hot Iron tube.
4. Reduction of Diazonium Salt
Using mild reducing agents like Hypophosphorous acid or Ethanol.
3. Electrophilic Aromatic Substitution (EAS)
Benzene undergoes substitution rather than addition to preserve its aromaticity. The intermediate is the Sigma Complex (Wheland Intermediate), which is resonance stabilized but non-aromatic.
| Reaction | Standard Reagents | Active Electrophile ($E^+$) |
|---|---|---|
| Nitration | Conc. $\ce{HNO3}$ + Conc. $\ce{H2SO4}$ ($\Delta$) | $\ce{NO2+}$ (Nitronium ion) |
| Halogenation | $\ce{Cl2}$ + Anhyd. $\ce{FeCl3}$ / $\ce{AlCl3}$ | $\ce{Cl+}$ (Chloronium ion) |
| Sulphonation | Fuming $\ce{H2SO4}$ (Oleum) | $\ce{SO3}$ (Neutral Electrophile) |
| F.C. Alkylation | $\ce{R-Cl}$ + Anhyd. $\ce{AlCl3}$ | $\ce{R+}$ (Rearranges to stable Carbocation!) |
| F.C. Acylation | $\ce{R-COCl}$ + Anhyd. $\ce{AlCl3}$ | $\ce{R-C\equiv O+}$ (Acylium ion - No rearrangement) |
- Fails with strongly deactivated rings: Nitrobenzene, Benzoic Acid, etc., do not undergo F.C. reactions.
- Fails with Aniline: The basic $\ce{-NH2}$ group forms a complex with the Lewis acid ($\ce{AlCl3}$), severely deactivating the ring.
- Polyalkylation occurs: The alkyl group activates the ring, making the product more reactive than benzene, leading to polyalkylation. (Acylation does not suffer from this).
- Vinyl/Aryl halides fail: The $\ce{C-Cl}$ bond has partial double bond character (resonance), preventing the formation of carbocations.
4. Directing Effects of Substituents
Groups that donate electrons ($+M$ or $+H$), increasing electron density at ortho & para positions.
Groups that withdraw electrons ($-M$ or $-I$), decreasing density at ortho/para, leaving meta relatively richer.
However, they are Ortho/Para directing because their $+M$ effect stabilizes the ortho/para sigma complex intermediates.
5. Side-Chain & Addition Reactions
Side-Chain Oxidation
Any alkyl benzene with at least one benzylic hydrogen oxidizes entirely to Benzoic Acid, regardless of chain length.
*Tert-butylbenzene does NOT oxidize (no benzylic H).*
Addition of Chlorine (BHC)
High energy addition breaking aromaticity. Uses UV light.
Product: Benzene Hexachloride (Gammexane / Lindane)
Birch Reduction Regioselectivity
Reagent: $\ce{Na / \text{liq. } NH3 + EtOH}$. Benzene $\rightarrow$ 1,4-Cyclohexadiene (Isolated diene).
Repels electrons. The double bonds stay at the substituted carbon to avoid high electron density at the substituent.
Attracts electrons. The double bonds stay away from the substituted carbon, placing electron density at the substituent.
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