CHEMCA
EXAM MASTER REVIEW SHEET
Aromatic Hydrocarbons: Benzene
1. Aromaticity & Structure
Benzene ($C_6H_6$) is a planar, cyclic conjugated system with exceptional stability.
A compound is aromatic if it is Cyclic, Planar, Fully Conjugated, and has:
$(4n + 2) \pi \text{ electrons}$
($n = 0, 1, 2, \dots$ i.e., $2, 6, 10, 14 \dots$ electrons)
All $C-C$ bond lengths are identical ($139 \text{ pm}$), intermediate between single and double bonds.
Resonance Energy $\approx 36 \text{ kcal/mol}$
2. Methods of Preparation
Decarboxylation
Sodium Benzoate + Soda Lime ($NaOH + CaO$) / $\Delta$.
$\text{C}_6\text{H}_5\text{COONa} \xrightarrow{NaOH/CaO} \text{C}_6\text{H}_6 + \text{Na}_2\text{CO}_3$
Phenol Reduction
Phenol distilled with Zinc Dust.
$\text{C}_6\text{H}_5\text{OH} + \text{Zn} \xrightarrow{\Delta} \text{C}_6\text{H}_6 + \text{ZnO}$
3. Electrophilic Aromatic Substitution (EAS)
The most characteristic reaction of benzene. Proceed via Sigma Complex (Wheland Intermediate).
| Reaction | Reagents | Electrophile ($E^+$) |
|---|---|---|
| Nitration | Conc. $HNO_3$ + Conc. $H_2SO_4$ | $NO_2^+$ (Nitronium ion) |
| Halogenation | $Cl_2 + \text{Anhyd. } FeCl_3$ | $Cl^+$ (Chloronium ion) |
| Friedel-Crafts Alkylation | $R-Cl + \text{Anhyd. } AlCl_3$ | $R^+$ (Carbocation - Rearranges!) |
| Friedel-Crafts Acylation | $RCOCl + \text{Anhyd. } AlCl_3$ | $R-C \equiv O^+$ (Acylium ion) |
4. Directing Effects of Substituents
Groups that donate electrons ($+M$ or $+I$).
$-NH_2 > -OH > -OR > -NHCOCH_3 > -R$
*Halogens ($-Cl, -Br$) are O/P directing but Deactivating.*
Groups that withdraw electrons ($-M$ or $-I$).
$-NO_2 > -CN > -CHO > -COOH > -SO_3H$
5. Addition & Side Chain Reactions
Addition of Chlorine
Benzene + $3Cl_2$ in presence of UV Light/Sunlight.
Benzene Hexachloride (BHC) / Gammexane / 666
Birch Reduction
Reagent: $Na / \text{liq. } NH_3 + \text{Ethanol}$.
Benzene $\rightarrow$ 1,4-Cyclohexadiene (Non-conjugated).
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