Aromatic Compounds – Benzene & Its Derivatives
(KEAM – Chemistry – Study Notes)
1. Simple Explanation
| Concept | What it means | Why it matters |
|---|---|---|
| Aromaticity | A cyclic, planar π‑electron system that follows Hückel’s rule (4 n + 2 π electrons) and is highly stable. | Gives benzene its unique physical & chemical properties (low reactivity towards addition, high stability). |
| Benzene | A 6‑membered ring of sp²‑hybridized carbons with one C=C bond delocalized over the ring. | Prototype aromatic system; basis for all substituted benzenes. |
| Resonance & Delocalization | Electron density is shared over all six C atoms, giving equal C–C bond lengths (~1.39 Å). | Explains the extra stability and the pattern of reactivity. |
| Substituted Benzenes | Benzene with one or more groups (R) attached to the ring. | The nature of R (electron‑donating or withdrawing) dictates orientation and reactivity in electrophilic substitution. |
2. Key Points
| Topic | Details |
|---|---|
| Hückel’s Rule | 4n + 2 π electrons → aromatic. For benzene, n = 1 → 6 π electrons. |
| Stability | Aromatic compounds are ≈ 36 kcal mol⁻¹ more stable than the corresponding 1,3-butadiene analogues. |
| Bonding & Geometry | All C–C bonds are equal (≈1.39 Å). Planar, sp²‑hybridized, 120° H–C–H angles. |
| Physical Properties | Low boiling points relative to similar alkanes, high symmetry, characteristic UV/Vis absorption (≈254 nm). |
| Electrophilic Aromatic Substitution (EAS) | Key reactions: nitration, sulfonation, halogenation, Friedel‑Crafts alkylation/acylation. |
| Orientation Rules | <ul><li>Activating groups (EDG) – donate electron density → ortho/para preference.</li><li>Deactivating groups (EWG) – withdraw electron density → meta preference.</li></ul> |
| Common Activating Groups | –OH, –OR, –NH₂, –NR₂, alkyl groups (–CH₃). |
| Common Deactivating Groups | –NO₂, –CN, –COOH, –SO₃H, –Cl, –Br, –F (strongly deactivating). |
| Reaction Conditions | 1. EAS requires a strong electrophile (generated in situ). 2. Friedel‑Crafts alkylation needs Lewis acid (AlCl₃). 3. Friedel‑Crafts acylation also uses AlCl₃. |
| Nomenclature | 1. Name substituents as prefixes; ring as “benzene.” 2. Use numbers to indicate positions (e.g., 4‑chloro‑2‑methyl‑benzene). |
| Heteroaromatic Rings | Pyridine, furan, thiophene, pyrrole. |
| Redox Reactions | 1. Reduction: H₂/Pd → cyclohexane (hydrogenation). 2. Oxidation: KMnO₄ (hot, conc.) → maleic acid (oxidative cleavage). |
| Special Notes for KEAM | • Look for orientation-based multiple-choice questions. • Pay attention to rate vs product question (e.g., nitration of toluene → p‑nitrotoluene). • Heteroaromatic naming and reactivity patterns. |
3. Worked Example
Problem (typical KEAM style)
Predict the major product and give the mechanism when toluene (CH₃C₆H₅) is nitrated using a mixture of conc. H₂SO₄ and conc. HNO₃.
Step‑by‑Step Solution
-
Generate Electrophile
- In the mixture, nitric acid is protonated by sulfuric acid:
[ \ce{HNO3 + H2SO4 -> [NO2⁺] + HSO4⁻ + H2O} ] - The nitronium ion ((\ce{NO2⁺})) is the active electrophile.
- In the mixture, nitric acid is protonated by sulfuric acid:
-
Orientation by the –CH₃ Group
- The methyl group is an activating, ortho/para‑directing substituent.
- It don