📍 Chapter Overview
Aromatic Hydrocarbons — Complete Mind Map
Topics Covered: Aromaticity (Hückel's Rule) · Benzene Structure (Kekulé, resonance hybrid) · Preparation of Benzene (from acetylene, decarboxylation, destructive distillation) · EAS Mechanism (3 steps) · Types of EAS: Halogenation, Nitration, Sulphonation, Friedel-Crafts Alkylation, Friedel-Crafts Acylation · Directing Effects of substituents (o/p directors vs m directors) · Activating vs Deactivating Groups
🤖 AI Prompt — Chapter Mind Map:
Fiery orange-red themed mind map on dark background. Central node: benzene ring (hexagon with inner circle) labeled "BENZENE C₆H₆". Six branches: (1) "Aromaticity" — Hückel rule 4n+2 pi electrons, planar, conjugated ring; (2) "Benzene Structure" — Kekulé 1 and 2 alternating double bonds, resonance hybrid with all equal C-C bonds (1.40 Å), delocalized electrons shown as a shaded doughnut above and below the ring; (3) "Preparation" — 3 routes: 3 HC≡CH (charcoal, 600°C) → benzene; C₆H₅COONa + NaOH/CaO → C₆H₆; from coal tar; (4) "EAS Mechanism" — 3-step boxes: Step 1 E⁺ formation (catalyst), Step 2 E⁺ attacks ring → arenium ion (σ-complex), Step 3 H⁺ loss → benzene restored with substituent; (5) "Types of EAS" — 5 named reactions: Halogenation (Cl₂/FeCl₃), Nitration (conc.HNO₃+H₂SO₄), Sulphonation (oleum), FC Alkylation (RCl/AlCl₃), FC Acylation (RCOCl/AlCl₃); (6) "Directing Effects" — two-column table: o/p directors (EDG: −NH₂, −OH, −OR, halogens) vs m-directors (EWG: −NO₂, −CN, −CHO, −COOH). High resolution educational poster, bold ring structures.
1. Benzene Structure — The Most Famous Puzzle in Chemistry
Benzene Structure — Kekulé Structures, Resonance Hybrid, and Orbital Diagram
Draw three views of benzene (C₆H₆) structure side by side on white background. Left: "Kekulé Structure 1" — regular hexagon with alternating single and double bonds (3 double bonds). Label the C-C single bond length (1.54 Å, dashed) and C=C double bond length (1.34 Å, bold). Draw H atoms at each corner. Center: "Kekulé Structure 2" — same hexagon but double bonds on the OTHER set of three bonds (alternating positions). Show a double-headed resonance arrow (⇌) between the two Kekulé structures. Right: "Resonance Hybrid (actual benzene)" — regular hexagon with all bonds EQUAL (no alternating), with an inner filled circle representing the delocalized 6π electrons (doughnut of electron density). Label: all C-C = 1.40 Å (between single and double). Below the hybrid, draw an orbital perspective diagram: top view of benzene ring showing 6 p orbitals (one on each C), all aligned perpendicular to the ring plane, overlapping to form a continuous pi electron cloud (shaded torus/doughnut shape, both above and below the ring). Label: "π electrons delocalized over all 6 carbons — aromatic stability". White background, clean chemistry textbook quality, all bonds labeled.
Benzene (C₆H₆) key data:
- All C–C bond lengths = 1.40 Å (between single 1.54 Å and double 1.34 Å — delocalized)
- All bond angles = 120° (all C are sp², flat/planar molecule)
- All 6 C and 6 H atoms are coplanar
- Resonance energy = 150 kJ/mol (extra stability due to delocalization)
- Prefers substitution over addition — protecting the aromatic ring is more favorable than adding and disrupting aromaticity
2. Electrophilic Aromatic Substitution (EAS) — The Universal Mechanism
EAS General Mechanism — Arenium Ion (σ-complex / Wheland Intermediate)
Draw a detailed 3-step electrophilic aromatic substitution (EAS) mechanism diagram on white background. Show benzene (hexagon with inner circle, representing delocalized electrons) reacting with an electrophile E⁺. Step 1 (top of diagram): "Electrophile generation" — show a general equation: A-B + Lewis acid catalyst → E⁺ + anion. Example for nitration: HNO₃ + H₂SO₄ → NO₂⁺ (nitronium ion) + HSO₄⁻ + H₂O. Step 2 (middle, rate-determining, slow): Show benzene ring with a curved arrow — the π electron pair from the ring attacks E⁺. Product: the ARENIUM ION (Wheland intermediate / sigma-complex). Draw this intermediate as: a cyclohexadienyl cation — a 6-membered ring with the positive charge LOCALIZED at 3 positions (draw 3 resonance structures of the arenium ion, with + charge shown at ortho and para positions relative to where E attached). Show the E group attached to one ring carbon (sp3 now). The arenium ion loses aromaticity temporarily. Label: "σ-complex / Wheland intermediate / arenium ion — SLOW step". Step 3 (bottom, fast): Show a base (HSO₄⁻, Br⁻, etc.) removing the H⁺ from the carbon where E attached. Curved arrow from C-H bond to base. Product: Aromatic benzene ring restored with E replacing H. Label: "FAST — aromaticity restored". Overall equation at bottom: benzene + E-X (catalyst) → E-substituted benzene + HX. White background, clean curved arrows, all intermediates labeled, textbook quality.
3. Named EAS Reactions of Benzene
3.1 Halogenation
C₆H₆ + Cl₂ →(FeCl₃ or AlCl₃, Lewis acid catalyst)→ C₆H₅Cl + HCl
Electrophile generation: Cl₂ + FeCl₃ → Cl⁺ (or Cl–FeCl₃ complex that acts as Cl⁺) + FeCl₄⁻
FeCl₃ is the Lewis acid that activates Cl₂ by accepting electrons from one Cl, making the other Cl electrophilic.
Product: chlorobenzene (C₆H₅Cl). Further reaction possible → dichlorobenzenes (o, m, p depending on substituent already present).
3.2 Nitration
C₆H₆ + conc. HNO₃ →(conc. H₂SO₄, 55°C)→ C₆H₅NO₂ + H₂O
Electrophile: NO₂⁺ (nitronium ion) generated by: HNO₃ + H₂SO₄ → NO₂⁺ + H₂O + HSO₄⁻
H₂SO₄ protonates HNO₃ → nitronium ion NO₂⁺. The NO₂⁺ is the electrophile that attacks benzene.
Product: Nitrobenzene (yellow oily liquid). Further nitration → dinitrobenzene → trinitrobenzene (TNT relates to 2,4,6-trinitrotoluene, an explosive).
3.3 Sulphonation
C₆H₆ + H₂SO₄ (fuming, oleum, SO₃)→ C₆H₅SO₃H + H₂O
Electrophile: SO₃ (from oleum, fuming H₂SO₄ = H₂SO₄ + SO₃). SO₃ itself is the electrophile (S is electrophilic).
Unique feature: sulphonation is REVERSIBLE. Heating benzenesulphonic acid with steam (100°C) → benzene + H₂SO₄ (desulphonation). This reversibility is exploited as a protecting group strategy in synthesis.
Product: Benzenesulphonic acid (C₆H₅SO₃H). Very water-soluble. Salt: sodium benzenesulphonate (surfactant — detergent synthesis).
3.4 Friedel-Crafts Alkylation
C₆H₆ + R–Cl →(AlCl₃)→ C₆H₅–R + HCl
Electrophile: R⁺ carbocation (or AlCl₃·R–Cl complex). RCl + AlCl₃ → R⁺ + AlCl₄⁻
Problems with alkylation:
- Polyalkylation: Product (alkylbenzene) is more reactive than benzene (R is electron-donating, activating) → reacts again → di- and trialkylbenzene mixtures.
- Carbocation rearrangement: n-propyl chloride + AlCl₃ → n-propyl cation → rearranges to isopropyl cation (more stable 2°) → product is isopropylbenzene (cumene), NOT n-propylbenzene.
Example: C₆H₆ + CH₃Cl + AlCl₃ → toluene (C₆H₅CH₃)
3.5 Friedel-Crafts Acylation
C₆H₆ + R–CO–Cl →(AlCl₃)→ C₆H₅–CO–R + HCl (aryl ketone)
Electrophile: RCO⁺ (acylium ion). RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻
Why acylation is BETTER than alkylation:
- No rearrangement (acylium ion is stabilised by resonance R–C≡O⁺ ↔ RCO⁺ → linear charge delocalized — does not rearrange).
- No polyacylation: the C=O group is electron-withdrawing (deactivating) → product is LESS reactive than benzene → stops after mono-acylation.
- Product is a ketone (aryl ketone, acetophenone if R=CH₃).
Example: C₆H₆ + CH₃COCl + AlCl₃ → C₆H₅COCH₃ (acetophenone, methyl phenyl ketone)
4. Directive Effect of Substituents in EAS
When benzene already has a substituent (–G), where does the next EAS substituent go? The existing group –G directs the incoming electrophile to specific positions.
Substituent Directing Effects in EAS — o/p vs meta Directors on Benzene Ring
Draw a clear, informative chemistry diagram explaining directive effects in electrophilic aromatic substitution on white background. Central large section: a benzene ring with an existing substituent "G" at position 1 (top of ring). Label positions 2 and 6 as "ortho", positions 3 and 5 as "meta", position 4 as "para". SECTION A (left side, green background box): "Ortho/Para (o/p) Directors — Electron Donating Groups (EDG)": show benzene rings with groups listed: −NH₂ (amino), −NR₂, −OH (hydroxyl), −OR (ether), −OCOR, −R (alkyl), −halogens (F, Cl, Br, I — special case, deactivating but still o/p). For each, show the incoming electrophile E⁺ goes to ortho OR para positions (highlight those positions in green on the ring). Add note: "EDG donate electrons into ring → make ortho/para positions electron-rich → EAS goes there. Activate ring (faster than benzene) except halogens (halogens are deactivating but o/p directors due to +M)". SECTION B (right side, red background box): "Meta (m) Directors — Electron Withdrawing Groups (EWG)": show groups: −NO₂, −CN, −CHO, −COR, −COOH, −COOR, −SO₃H, −CCl₃, −NR₃⁺. For each, show E⁺ goes to META position (highlight meta positions in red). Add note: "EWG withdraw electrons from ring → make ortho/para positions electron-poor → EAS goes to meta (least electron-poor). Deactivate ring (slower than benzene)". At bottom, show an energy diagram comparing: EWG → meta arenium ion is more stable than o/p. Clean educational chemistry poster, white background, labeled diagrams.
Why do o/p directors direct to ortho and para?
An EDG like –NH₂ donates electrons into the ring via resonance (+M effect) → the resonance structures show increased electron density at the ortho and para positions → electrophile preferentially attacks those positions → more stable arenium ion (positive charge lands on more electron-rich positions).
Why do m directors direct to meta?
An EWG like –NO₂ withdraws electrons from the ring via resonance (−M effect) → ortho and para positions become especially electron-poor. Meta is the LEAST electron-poor → electrophile goes to meta.
Special case: Halogens (F, Cl, Br, I) are DEACTIVATING (−I effect, overall withdraw electrons) but are o/p DIRECTORS (because +M effect dominates the position selectivity even though +M is weaker than −I overall).
| Group | Effect on Ring | Directing Position | Example Product |
|---|
| −NH₂, −NR₂ | Strong activating (+M >> −I) | o & p | o- and p-aminobenzene |
| −OH, −OR | Activating | o & p | o- and p-nitrophenol |
| −R (alkyl) | Activating (+I) | o & p | o- and p-nitrotoluene |
| −F, −Cl, −Br, −I | Deactivating (−I) but o/p (+M wins) | o & p (major) | o- and p-dichlorobenzene |
| −NO₂ | Strong deactivating (−I & −M) | m | m-dinitrobenzene |
| −CN, −CHO, −COR | Deactivating (−M) | m | m-nitrobenzaldehyde |
| −COOH, −SO₃H | Deactivating (−M) | m | m-nitrobenzoic acid |
5. Combustion of Benzene and Important Aromatic Compounds
Combustion: C₆H₆ + 15/2 O₂ → 6CO₂ + 3H₂O (very sooty flame — high C:H ratio)
Toluene (methylbenzene, C₆H₅CH₃): The methyl group is an o/p director (activating by +I). More reactive than benzene in EAS. The CH₃ group can be oxidised: C₆H₅CH₃ + KMnO₄ (hot, acidic) → C₆H₅COOH (benzoic acid).
Xylenes (dimethylbenzene): 3 isomers (o-, m-, p-xylene). p-Xylene is used to make PET (polyethylene terephthalate — plastic bottles, Dacron fibres).
Naphthalene (C₁₀H₈): Two fused benzene rings. White shiny plates. BP 218°C. Used as a moth repellent.
Anthracene (C₁₄H₁₀): Three linearly fused benzene rings. Used to make anthraquinone dyes.
Polycyclic Aromatic Hydrocarbons — Naphthalene, Anthracene, Phenanthrene Structures
Draw structural formulas of three polycyclic aromatic hydrocarbons (PAHs) on white background, arranged in a row. Left: NAPHTHALENE (C₁₀H₈) — two benzene rings fused side by side sharing one C-C bond. Draw as two hexagons sharing one edge, with inner circles (delocalized electrons) in each ring. Label the shared bond and numbering (positions 1-8 and 4a, 8a for junction carbons). Label: "Naphthalene — moth repellent, BP = 218°C". Center: ANTHRACENE (C₁₄H₁₀) — three benzene rings fused in a straight line (linear fusion). Draw as three hexagons in a row sharing edges, each with inner circle. Label positions 1-10. Label: "Anthracene — three fused rings, used in dyes". Right: PHENANTHRENE (C₁₄H₁₀) — three benzene rings in an angled/bent arrangement (not linear). Draw the characteristic "bent" or "kinked" structure with three hexagons. Label: "Phenanthrene — angular fusion, same molecular formula as anthracene, different shape". All drawn in standard aromatic notation (hexagons with inner circles). White background, clean bold lines, educational chemistry textbook quality.
Ex 1 M: Give the major product(s) when toluene (C₆H₅CH₃) is nitrated with HNO₃/H₂SO₄.
Solution: –CH₃ is an o/p director (activating). Nitration will give o-nitrotoluene and p-nitrotoluene as major products, with m-nitrotoluene as only minor product. Major: 2-nitrotoluene + 4-nitrotoluene. In practice, p-nitrotoluene is the dominant product (less steric hindrance at para vs ortho).
Ex 2 M: Why does FC alkylation of benzene with n-propyl chloride give isopropylbenzene, not n-propylbenzene?
Solution: n-PrCl + AlCl₃ → n-propyl cation (CH₃CH₂CH₂⁺, primary). Primary carbocations are unstable → 1,2-H shift → isopropyl cation (CH₃–CH⁺–CH₃, secondary, more stable). This isopropyl cation attacks benzene → isopropylbenzene (cumene). Carbocation rearrangement is inevitable in FC alkylation.
Ex 3 H: Predict major product: bromination of nitrobenzene (C₆H₅NO₂) with Br₂/FeBr₃.
Solution: –NO₂ is a strong m-director (EWG, deactivating). Major product: m-bromonitrobenzene. Reaction will be slower than pure benzene (deactivated ring).
E Q1. Why is benzene more stable than expected from three double bonds (Kekulé)? What is the numerical value of this extra stability?
E Q2. Write the electrophile in: (a) bromination, (b) nitration, (c) Friedel-Crafts acylation of benzene.
M Q3. How many monobromo substitution products are possible for toluene? Name them and predict the major product using directive effects.
M Q4. Explain why sulphonation of benzene is reversible while nitration is not.
H Q5. Starting from benzene, how would you prepare p-nitrotoluene (minimising ortho isomer)? Consider the order of reactions carefully.
H Q6. Chlorobenzene has a higher MP than benzene but is LESS reactive in EAS than benzene. Explain both observations using the inductive and mesomeric effects of −Cl.
- Saying benzene undergoes addition reactions like alkenes — WRONG! Benzene strongly prefers SUBSTITUTION (EAS) to preserve aromaticity. Addition only under extreme conditions (hydrogenation with Ni at high T).
- Confusing activating/deactivating with o/p or m direction — halogens are deactivating (overall) but STILL o/p directors. These are SEPARATE concepts.
- Writing rearranged product for FC acylation — acylium ions do NOT rearrange (stabilized by resonance). Only alkylation gives rearranged products.
- Saying nitration product of toluene gives only one isomer — it gives BOTH ortho and para (with trace meta). Don't forget both o & p products for o/p directors.
- Writing H₂SO₄ as a reactant in nitration — H₂SO₄ is the catalyst (generates NO₂⁺). The actual reactant is HNO₃.
| EAS Reaction | Electrophile | Catalyst | Product |
|---|
| Halogenation | Cl⁺ (or Br⁺) | FeCl₃ or AlCl₃ (Lewis acid) | Chlorobenzene / Bromobenzene |
| Nitration | NO₂⁺ (nitronium) | Conc. H₂SO₄ | Nitrobenzene |
| Sulphonation | SO₃ | Oleum (fuming H₂SO₄) | Benzenesulphonic acid (reversible) |
| FC Alkylation | R⁺ (carbocation) | AlCl₃, anhydrous | Alkylbenzene (rearrangement possible) |
| FC Acylation | RCO⁺ (acylium) | AlCl₃, anhydrous | Aryl ketone (no rearrangement) |
