This MCQ module is based on: Aromatic Mechanisms Pollution
TOPIC 12 OF 13
Aromatic Mechanisms Pollution
🎓 Class 11
Chemistry
CBSE
Theory
Ch 9 – Hydrocarbons
⏱ ~14 min
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Aromatic Reaction Mechanisms and Carcinogenic Pollution
9.7 Chemical Reactions of Aromatic Hydrocarbons
Benzene reacts with electrophiles by electrophilic substitution rather than addition because retaining the aromatic 6π system saves ~150 kJ/mol of resonance energy.
9.7.1 General Mechanism (Arenium Ion / σ-Complex Pathway)
All aromatic substitutions follow a common three-step pattern:
1
Generation of the electrophile (E⁺) from the reagent + Lewis acid catalyst.
2
Attack of E⁺ on the π cloud → arenium ion (σ-complex / Wheland intermediate): the π electrons of benzene act as a nucleophile and form a σ bond with E⁺. The intermediate is a non-aromatic, positively charged cyclohexadienyl cation (resonance-stabilised over three positions).
3
Loss of H⁺ from the sp³ carbon → restoration of aromaticity: a base (often the conjugate base of the catalyst) abstracts H⁺ from the ring, regenerating the 6π aromatic system and releasing the catalyst.
9.7.2 Specific Reactions
(i) Nitration
Benzene + concentrated HNO₃/H₂SO₄ at 323–333 K → nitrobenzene.
C₆H₆ + HNO₃ →conc H₂SO₄ C₆H₅NO₂ + H₂OElectrophile generation:
HNO₃ + 2 H₂SO₄ → NO₂⁺ + H₃O⁺ + 2 HSO₄⁻ (NO₂⁺ = nitronium ion)(ii) Halogenation
Benzene + Cl₂ (or Br₂) in presence of anhydrous FeCl₃ (or FeBr₃, AlCl₃) → chlorobenzene.
C₆H₆ + Cl₂ →anhyd. FeCl₃ C₆H₅Cl + HClElectrophile: Cl–Cl + FeCl₃ → Cl⁺ + [FeCl₄]⁻. Note: in absence of catalyst, benzene is too unreactive — F₂ is too violent and I₂ too unreactive.
(iii) Sulphonation
Benzene heated with oleum (H₂SO₄ + SO₃) gives benzenesulphonic acid:
C₆H₆ + H₂SO₄ (oleum) ⇌ C₆H₅SO₃H + H₂OElectrophile: SO₃ itself or HSO₃⁺. Reaction is reversible — heating sulphonic acid with dilute H₂SO₄/H₂O reverts to benzene.
(iv) Friedel–Crafts Alkylation
Benzene + alkyl halide + anhydrous AlCl₃ → alkylbenzene.
C₆H₆ + CH₃Cl →anhyd AlCl₃ C₆H₅CH₃ + HCl (toluene)Electrophile: CH₃Cl + AlCl₃ → CH₃⁺ + [AlCl₄]⁻ (carbocation). Limitations: (1) Polyalkylation occurs because product is more reactive than starting benzene. (2) Carbocations rearrange — e.g. n-propyl chloride gives mostly isopropylbenzene (cumene) due to 1° → 2° rearrangement. (3) Doesn't work on strongly deactivated rings (e.g. nitrobenzene).
(v) Friedel–Crafts Acylation
Benzene + acyl chloride (or acid anhydride) + anhydrous AlCl₃ → aryl ketone.
C₆H₆ + CH₃COCl →anhyd AlCl₃ C₆H₅COCH₃ + HCl (acetophenone)Electrophile: CH₃COCl + AlCl₃ → CH₃CO⁺ + [AlCl₄]⁻ (acylium ion). Advantage over alkylation: the acyl product is a deactivator — stops at mono-substitution (no over-acylation). Acylium ions also do not rearrange.
(vi) Addition Reactions of benzene
Although less favoured, benzene does undergo addition under forcing conditions:
C₆H₆ + 3 H₂ →Ni/Pt, 473–573 K C₆H₁₂ (cyclohexane)C₆H₆ + 3 Cl₂ →UV light C₆H₆Cl₆ (BHC, benzene hexachloride / Lindane — earlier insecticide)
(vii) Combustion
Aromatic hydrocarbons burn in air with a sooty (luminous) flame — high C/H ratio:
C₆H₆ + 15/2 O₂ → 6 CO₂ + 3 H₂O ΔH = –3267.6 kJ/mol9.8 Directive Influence of Substituents
When benzene already has a substituent (mono-substituted benzene), the new electrophile does not enter randomly — it goes preferentially to particular positions. The existing substituent directs the incoming group to either (a) the ortho & para positions, or (b) the meta position. It also activates or deactivates the ring.
9.8.1 Ortho-/Para-Directing Activators
Groups with lone pairs on the atom directly attached to the ring (–OH, –OR, –NH₂, –NHR, –NR₂, –NHCOR), as well as alkyl groups (+I effect: –CH₃, –CH₂CH₃) activate the ring (increase reactivity) and direct incoming electrophiles to ortho and para positions. Reason: the lone pair (or hyperconjugating C–H) can donate electron density to the ring, particularly to the ortho/para carbons (resonance structures place negative charge on these positions). Halogens (–F, –Cl, –Br, –I) are an exception — they are deactivators but still ortho/para directors (–I dominates over +M for activation, but resonance still controls position).
9.8.2 Meta-Directing Deactivators
Strong electron-withdrawing groups (–NO₂, –CN, –CHO, –COR, –COOH, –COOR, –SO₃H, –CF₃, –NR₃⁺) deactivate the ring (reduce reactivity) and direct incoming electrophiles to the meta position. Reason: these groups withdraw electron density via resonance/induction, making ortho and para positions especially electron-poor; meta is the "least bad" location.
| Existing group on benzene | Effect on ring | Directs to |
|---|---|---|
| –OH, –OR, –NH₂, –NHR, –NR₂ | Strong activator | ortho, para |
| –NHCOR, –OCOR | Weak activator | ortho, para |
| –CH₃, –C₂H₅, –CR₃ (alkyl) | Weak activator (+I, hyperconjugation) | ortho, para |
| –F, –Cl, –Br, –I | Weak deactivator | ortho, para (exception) |
| –NO₂, –CN, –COOH, –COOR, –CHO, –COR, –SO₃H | Strong deactivator | meta |
| –NR₃⁺, –CCl₃, –CF₃ | Strong deactivator | meta |
Resonance argument for ortho/para direction by –OH (phenol): when the electrophile attaches at the ortho or para position, a resonance structure of the σ-complex places the +ve charge on the ring carbon bearing –OH, which can be stabilised by the lone pair donation from O (forming a stable oxocarbenium-like structure). At meta, no such structure is possible. Hence o/p attack gives a more stable σ-complex → faster.
Substitution Position Predictor
Pick the substituent already on the benzene; the predictor tells you where the next E⁺ goes and whether the ring is activated or deactivated.
Effect: Strong activator
Directs to: ortho & para
The –OH lone pair donates into the ring, activating o/p positions to electrophilic attack.
9.9 Carcinogenicity and Toxicity
Polynuclear aromatic hydrocarbons (PAHs) containing more than two benzene rings — especially benzo[a]pyrene, dibenz[a,h]anthracene, 3-methylcholanthrene — are known carcinogens. They are produced by the incomplete combustion of organic materials such as tobacco, coal, petroleum, fats and meats. PAHs enter the body through the lungs (smoke) or skin (soot, tar) and are converted in the liver into reactive epoxides that bind covalently to DNA, triggering cancer (typically lung, skin, bladder).
Public-health relevance: India's Ministry of Health classifies benzo[a]pyrene as a Group 1 carcinogen. Exposure pathways: tobacco smoke (active and passive), grilled/charred food, automobile exhaust (especially diesel), industrial coal-tar workers. Wearing masks during heavy traffic, avoiding burnt food, and eliminating smoking dramatically lower exposure.
9.9.1 Other Air-Quality Concerns
Hydrocarbon emissions from vehicles and industry contribute to:
- Photochemical smog — unburnt hydrocarbons + NOₓ + sunlight → ozone, PAN, aldehydes (lung & eye irritants).
- Greenhouse effect — methane is a potent GHG (~28× more warming than CO₂ per molecule over 100 years).
- Ground-level ozone — secondary pollutant, asthma trigger.
- Soot & particulate matter (PM₂.₅) — incomplete hydrocarbon combustion → fine carbon particles deep into lungs.
Activity 9.4 — Predict the major nitration product
Setup: Three students nitrate (a) toluene, (b) chlorobenzene, (c) nitrobenzene under identical conditions (HNO₃/H₂SO₄, 50 °C).
Predict: For each substrate, where will the new –NO₂ group enter? Rank the three substrates in order of reactivity.
(a) Toluene: –CH₃ is a weak activator and o/p director → o-nitrotoluene + p-nitrotoluene (major mixture, p slightly favoured due to less steric crowding). Reactivity ~25× benzene.
(b) Chlorobenzene: –Cl is a weak deactivator BUT o/p director (lone pair resonance still places electron density at o/p) → o-chloronitrobenzene + p-chloronitrobenzene. Reactivity ~0.03× benzene.
(c) Nitrobenzene: –NO₂ is strong deactivator and meta director → m-dinitrobenzene. Reactivity ~10⁻⁷× benzene — needs very harsh conditions (fuming HNO₃, high T).
Reactivity order: toluene > chlorobenzene >>> nitrobenzene.
Worked Example 1: Nitration mechanism
Write the complete mechanism for nitration of benzene with HNO₃/H₂SO₄.
Step 1 — Generation of NO₂⁺:
HNO₃ + H₂SO₄ → H₂NO₃⁺ + HSO₄⁻
H₂NO₃⁺ → NO₂⁺ + H₂O
Step 2 — Attack of NO₂⁺ on benzene:
The π electrons of benzene attack NO₂⁺, forming the arenium ion (σ-complex) — a positively charged cyclohexadienyl cation in which one ring carbon is sp³ (bears H and NO₂).
Step 3 — Loss of H⁺ to restore aromaticity:
HSO₄⁻ removes the H⁺ from the sp³ ring carbon, regenerating H₂SO₄ and the aromatic ring.
Net: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O.
HNO₃ + H₂SO₄ → H₂NO₃⁺ + HSO₄⁻
H₂NO₃⁺ → NO₂⁺ + H₂O
Step 2 — Attack of NO₂⁺ on benzene:
The π electrons of benzene attack NO₂⁺, forming the arenium ion (σ-complex) — a positively charged cyclohexadienyl cation in which one ring carbon is sp³ (bears H and NO₂).
Step 3 — Loss of H⁺ to restore aromaticity:
HSO₄⁻ removes the H⁺ from the sp³ ring carbon, regenerating H₂SO₄ and the aromatic ring.
Net: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O.
Worked Example 2: Friedel-Crafts limitation
Why is direct Friedel-Crafts alkylation a poor route to n-propylbenzene?
The reaction CH₃CH₂CH₂Cl + AlCl₃ is intended to give n-propylbenzene + HCl. But the AlCl₃ generates n-propyl cation, which rapidly rearranges by 1,2-hydride shift to the more stable secondary isopropyl cation:
CH₃CH₂CH₂⁺ → CH₃CH⁺CH₃ (more stable, secondary)
The 2° cation then attacks benzene, giving cumene (isopropylbenzene) as the major product, NOT n-propylbenzene.
Workaround: Use Friedel-Crafts acylation with propionyl chloride (CH₃CH₂COCl) → propiophenone, then reduce the C=O to CH₂ by Clemmensen (Zn-Hg/HCl) or Wolff-Kishner (NH₂NH₂/KOH). This 2-step strategy avoids carbocation rearrangement.
CH₃CH₂CH₂⁺ → CH₃CH⁺CH₃ (more stable, secondary)
The 2° cation then attacks benzene, giving cumene (isopropylbenzene) as the major product, NOT n-propylbenzene.
Workaround: Use Friedel-Crafts acylation with propionyl chloride (CH₃CH₂COCl) → propiophenone, then reduce the C=O to CH₂ by Clemmensen (Zn-Hg/HCl) or Wolff-Kishner (NH₂NH₂/KOH). This 2-step strategy avoids carbocation rearrangement.
Worked Example 3: Direct of substitution
Predict the product when (a) phenol, (b) nitrobenzene is brominated with Br₂/Fe.
(a) Phenol: –OH is a strong activator and o/p director. Bromination is so fast that even Br₂/H₂O without Fe gives 2,4,6-tribromophenol (white precipitate). With controlled conditions (dilute Br₂/CS₂), the major monobrominated products are o-bromophenol + p-bromophenol (p favoured).
(b) Nitrobenzene: –NO₂ is a strong deactivator and meta director. Bromination is slow and requires high temperature with Fe catalyst → m-bromonitrobenzene (1-bromo-3-nitrobenzene). The yield is moderate; mono-substitution dominates because the product is even more deactivated than the starting nitrobenzene.
(b) Nitrobenzene: –NO₂ is a strong deactivator and meta director. Bromination is slow and requires high temperature with Fe catalyst → m-bromonitrobenzene (1-bromo-3-nitrobenzene). The yield is moderate; mono-substitution dominates because the product is even more deactivated than the starting nitrobenzene.
Competency-Based Questions
Q1. The electrophile in nitration of benzene is: L1 Remember
Answer: (c) NO₂⁺ (nitronium ion). Generated by protonation of HNO₃ by H₂SO₄ followed by loss of H₂O.
Q2. Explain why the methyl group of toluene activates the ring towards electrophilic substitution and directs to ortho/para positions. L4 Analyse
Answer: The –CH₃ group donates electrons to the ring through (i) the +I effect (weak σ donation along the C–C bond) and (ii) hyperconjugation (overlap of C–H σ orbitals with the ring π system). These effects increase electron density at all positions, making the ring more nucleophilic. The donated density is concentrated at the ortho and para carbons (resonance structures put a partial negative charge there); meta gets little extra density. Consequently the σ-complex from o/p attack is more stable, and o/p products dominate.
Q3. Write the products formed when (a) toluene and (b) nitrobenzene undergo monosulphonation. L3 Apply
(a) Toluene + oleum → o-toluenesulphonic acid + p-toluenesulphonic acid (mixture; p favoured at higher T because o is more reversible).
(b) Nitrobenzene + fuming H₂SO₄ → m-nitrobenzenesulphonic acid (–NO₂ is a strong deactivator and meta director; conditions are harsh).
(b) Nitrobenzene + fuming H₂SO₄ → m-nitrobenzenesulphonic acid (–NO₂ is a strong deactivator and meta director; conditions are harsh).
Q4. Critically compare Friedel-Crafts alkylation and acylation as synthetic methods. List two advantages of acylation. L5 Evaluate
Advantages of acylation:
(1) No carbocation rearrangement — the acylium ion (R-CO⁺) is stabilised by resonance and does not rearrange; alkyl carbocations often do.
(2) Stops at mono-substitution — the aryl ketone product is deactivated towards further EAS, so only one acyl group enters; alkylation tends to over-react (poly-alkylation).
(3) Combined with Clemmensen/Wolff-Kishner reduction, acylation effectively delivers a "clean" alkyl group to the ring without rearrangement.
Disadvantages: Requires more than 1 equivalent of AlCl₃ (forms complex with ketone product); reducing the ketone is an extra step.
(1) No carbocation rearrangement — the acylium ion (R-CO⁺) is stabilised by resonance and does not rearrange; alkyl carbocations often do.
(2) Stops at mono-substitution — the aryl ketone product is deactivated towards further EAS, so only one acyl group enters; alkylation tends to over-react (poly-alkylation).
(3) Combined with Clemmensen/Wolff-Kishner reduction, acylation effectively delivers a "clean" alkyl group to the ring without rearrangement.
Disadvantages: Requires more than 1 equivalent of AlCl₃ (forms complex with ketone product); reducing the ketone is an extra step.
Q5. HOT (Create): Plan a complete synthetic route from benzene to p-nitrotoluene that maximises selectivity. L6 Create
Strategic plan: Methylate first, then nitrate.
Step 1: Friedel-Crafts methylation: C₆H₆ + CH₃Cl/AlCl₃ → C₆H₅CH₃ (toluene). Methyl carbocations don't rearrange.
Step 2: Nitration: C₆H₅CH₃ + HNO₃/H₂SO₄, low T → o- + p-nitrotoluene. Para-isomer is favoured at lower temperature and recovered by fractional crystallisation.
Why this order? Reverse order (nitrate first, then alkylate) fails: nitrobenzene is so deactivated that Friedel-Crafts alkylation does not proceed at all on it. Always install activating groups before deactivating ones in a multi-step EAS synthesis.
Step 1: Friedel-Crafts methylation: C₆H₆ + CH₃Cl/AlCl₃ → C₆H₅CH₃ (toluene). Methyl carbocations don't rearrange.
Step 2: Nitration: C₆H₅CH₃ + HNO₃/H₂SO₄, low T → o- + p-nitrotoluene. Para-isomer is favoured at lower temperature and recovered by fractional crystallisation.
Why this order? Reverse order (nitrate first, then alkylate) fails: nitrobenzene is so deactivated that Friedel-Crafts alkylation does not proceed at all on it. Always install activating groups before deactivating ones in a multi-step EAS synthesis.
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Phenol undergoes bromination so easily that even bromine water (no Fe catalyst) gives 2,4,6-tribromophenol.
R: The –OH group is a strong activator that pushes electron density into the ring through resonance.
Answer: (A). Both true; R correctly explains A. The lone pair on O delocalises into the ring (especially at o/p), making phenol ~10⁶× more reactive than benzene.
A: Nitration of nitrobenzene gives mainly meta-dinitrobenzene.
R: The –NO₂ group withdraws electrons by both –I and –M, deactivating the ring most at the ortho and para positions and leaving meta as the relatively electron-richer site.
Answer: (A). Both true; R correctly explains A. Resonance structures of the σ-complex from o/p attack place the +ve charge directly on the ring carbon bearing –NO₂ — extremely destabilising.
A: Friedel-Crafts alkylation works on aniline.
R: The –NH₂ group is a strong activator that should accelerate EAS.
Answer: (D). Assertion is FALSE — F-C alkylation does NOT work on aniline. The Lewis-basic –NH₂ group strongly coordinates the AlCl₃ Lewis acid, deactivating it (forming a –NH₂AlCl₃ ammonium salt). This makes the ring effectively meta-directing and inert. Reason is TRUE on its own — but the lone pair complexation overrides the activating effect. Workaround: protect –NH₂ as –NHCOCH₃ (acetanilide) which is still an o/p director but does not poison AlCl₃.
Frequently Asked Questions — Aromatic Reaction Mechanisms and Carcinogenic Pollution
What is the mechanism of electrophilic aromatic substitution?
Electrophilic aromatic substitution (EAS) proceeds by a three-step mechanism in NCERT Class 11 Chemistry Chapter 9: (1) generation of electrophile (e.g., NO₂⁺ from HNO₃/H₂SO₄, Cl⁺ from Cl₂/FeCl₃); (2) attack of electrophile on the π-electron cloud forming a positively-charged, non-aromatic arenium ion (sigma complex or Wheland intermediate); (3) loss of a proton from the sp³ carbon by a base, restoring aromaticity and giving the substituted product. The aromatic system's resonance stabilisation is briefly lost in the intermediate but quickly regained on deprotonation, making the substitution kinetically favourable.
What are activating and deactivating groups?
Substituents on an aromatic ring affect the rate of EAS: (1) activating groups donate electron density (by +R or +I effect), making the ring more reactive than benzene — examples: -NH₂, -OH, -OR, -CH₃, -R; (2) deactivating groups withdraw electron density (by -R or -I effect), making the ring less reactive — examples: -NO₂, -CN, -SO₃H, -COOH, -COR. NCERT Class 11 Chemistry Chapter 9 explains that activating groups stabilise the arenium ion intermediate, lowering the activation energy. Halogens (-F, -Cl, -Br, -I) are special — slightly deactivating (-I) but ortho/para directing (+R).
What are ortho/para and meta directing groups?
Substituents direct the incoming electrophile to specific positions on the aromatic ring. NCERT Class 11 Chemistry Chapter 9: (1) ortho/para directors — usually activating groups (-NH₂, -OH, -OR, -CH₃) plus halogens; they stabilise the arenium ion with positive charge at ortho and para positions through resonance; (2) meta directors — deactivating groups (-NO₂, -CN, -SO₃H, -COOH, -COR); they destabilise ortho and para attack so meta is preferred (least destabilising). Predicting the orientation of disubstituted products is a key skill: o/p ratios depend on steric and electronic factors. Useful in planning multi-step syntheses.
Why are some polynuclear aromatic hydrocarbons carcinogenic?
Some polynuclear aromatic hydrocarbons (PAHs) with three or more fused benzene rings are carcinogenic — they cause cancer in living organisms. NCERT Class 11 Chemistry Chapter 9 examples: benzo[a]pyrene (in cigarette smoke, charred meat, vehicle exhaust), 3,4-benzpyrene, 1,2-benzanthracene, dibenzanthracene. PAHs are produced by incomplete combustion of organic matter and are widespread environmental pollutants. They become carcinogenic after metabolic activation in the body — cytochrome P450 enzymes convert them to electrophilic epoxides that bind to DNA bases, causing mutations leading to cancer. Even short-term inhalation can be harmful.
What is the combustion of benzene and aromatic compounds?
Benzene and other aromatic hydrocarbons undergo combustion in air to form CO₂ and H₂O, releasing energy. Equation: C₆H₆ + 15/2 O₂ → 6 CO₂ + 3 H₂O, ΔH_c° = −3268 kJ/mol. However, due to the high carbon-to-hydrogen ratio, aromatic compounds burn with a smoky, sooty yellow flame due to incomplete combustion and unburnt carbon. NCERT Class 11 Chemistry Chapter 9 contrasts this with alkanes (clear blue flame) and alkenes (slightly smoky). Aromatic compounds also undergo controlled oxidation (with V₂O₅ catalyst) to give maleic anhydride from benzene — an industrially important reaction.
How is benzene a major environmental and health hazard?
Benzene is a serious health and environmental hazard. NCERT Class 11 Chemistry Chapter 9 notes that benzene is: (1) volatile and easily inhaled — exposure causes acute toxic effects on the central nervous system (dizziness, headaches, confusion); (2) a confirmed human carcinogen (Group 1, IARC) — chronic exposure causes leukaemia; (3) widely used as a solvent and chemical feedstock — strict workplace exposure limits apply; (4) emitted from vehicle exhaust, cigarette smoke, industrial processes — major urban air pollutant. Substitutes like toluene and xylene are preferred where possible. Always handle benzene with proper PPE in well-ventilated areas.
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