Electrophilic Aromatic Substitution

8 MCQs9-step worked example

Source: NCERT Aldehydes, Ketones and Carboxylic AcidsPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

Benzene does not undergo addition reactions the way alkenes do — its aromatic sextet is too stable to break. Instead, benzene reacts by electrophilic aromatic substitution (EAS): an electrophile replaces one hydrogen while the aromatic ring is preserved.

The general mechanism has three steps:

Electrophile generation. A Lewis acid catalyst (AlCl₃, FeBr₃, anhydrous AlCl₃ for Friedel-Crafts) activates the reagent to produce a strong electrophile (NO₂⁺, Cl⁺, R⁺, RCO⁺).
Electrophilic attack. The π-electron cloud of benzene attacks the electrophile, forming a non-aromatic carbocation intermediate (sigma complex / arenium ion). This step is rate-determining.
Proton loss. The sigma complex loses H⁺ to restore aromaticity, yielding the substituted product.

The key named reactions under EAS are:

Reaction	Electrophile	Catalyst/Reagent	Product
Nitration	NO₂⁺	Conc. HNO₃ + conc. H₂SO₄	Nitrobenzene
Halogenation	Cl⁺ or Br⁺	Anhydrous AlCl₃ or FeBr₃	Chloro-/Bromobenzene
Friedel-Crafts alkylation	R⁺ (carbocation)	Anhydrous AlCl₃	Alkylbenzene
Friedel-Crafts acylation	RCO⁺ (acylium ion)	Anhydrous AlCl₃	Aryl ketone
Sulphonation	SO₃ / SO₃H⁺	Fuming H₂SO₄ (oleum)	Benzenesulphonic acid

High-frequency trap for NEET: confusing the electrophile identity. Students pick Cl₂ as the electrophile instead of Cl⁺, or write HNO₃ instead of NO₂⁺ (nitronium ion). The catalyst's job is specifically to generate the active electrophile — the neutral reagent itself is not the attacking species.

A second common confusion: Friedel-Crafts alkylation can give polyalkylation (the alkyl group activates the ring for further substitution), while Friedel-Crafts acylation stops at monosubstitution (the acyl group is deactivating). NEET questions exploit this by asking which Friedel-Crafts reaction gives a cleaner monosubstituted product.

NCERT reference: Class 11 Chemistry Chapter 10 (Hydrocarbons), Part 2, page 26.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

In the nitration of benzene, the electrophile that attacks the benzene ring is:

MCQ 2Easy RecallPractice

Which catalyst is used in the Friedel-Crafts alkylation of benzene?

MCQ 3Easy RecallPractice

Sulphonation of benzene is carried out using:

MCQ 4Direct ApplicationPractice

During electrophilic aromatic substitution, the rate-determining step is:

MCQ 5Direct ApplicationPractice

Friedel-Crafts acylation of benzene gives a monosubstituted product cleanly, while Friedel-Crafts alkylation often gives polysubstituted products. The reason is:

MCQ 6Direct ApplicationPractice

In the bromination of benzene using Br₂/FeBr₃, the role of FeBr₃ is to:

MCQ 7Concept TrapPractice

Benzene undergoes electrophilic substitution rather than electrophilic addition because:

MCQ 8CalculationPractice

Consider the following sequence: benzene is first treated with CH₃Cl/anhydrous AlCl₃, and the product is then treated with Br₂/FeBr₃. The methyl group on the ring is an ortho,para-director. Which statement about the final major product is correct?

Worked Example

1
Given
- Substrate: benzene (C₆H₆) - Reagents: conc. HNO₃ + conc. H₂SO₄ (nitrating mixture)
2
Required
Identify: (a) the electrophile, (b) the rate-determining step, (c) the major organic product.
3
Concept
This is electrophilic aromatic substitution — nitration. Conc. H₂SO₄ protonates HNO₃ to generate the nitronium ion (NO₂⁺), which is the true electrophile.
4
Mechanism outline
1. H₂SO₄ + HNO₃ → NO₂⁺ + HSO₄⁻ + H₂O 2. NO₂⁺ attacks benzene π cloud → sigma complex (arenium ion) — **rate-determining step** 3. Sigma complex loses H⁺ → nitrobenzene + aromaticity restored
5
Substitution (identification, not numerical)
- Electrophile: NO₂⁺ (nitronium ion) - The slow step is sigma complex formation (Step 2 above)
6
Reasoning
The sigma complex is a non-aromatic cyclohexadienyl cation. Its formation requires overcoming the aromatic stabilisation barrier (~150 kJ/mol resonance energy), making it the highest-energy transition state in the pathway. The subsequent proton loss is fast because it restores the thermodynamically stable aromatic sextet.
7
Final answer
(a) Electrophile: **NO₂⁺** (nitronium ion) (b) Rate-determining step: **formation of the sigma complex** (electrophilic attack on the ring) (c) Major product: **nitrobenzene** (C₆H₅NO₂)
8
Common trap
Students often write HNO₃ or NO₂ as the electrophile. HNO₃ is the reagent; NO₂ is a neutral radical (nitrogen dioxide gas). The electrophile must be a cationic species — NO₂⁺.
9
Similar NEET-style question
"When benzene is treated with Br₂ in the presence of anhydrous AlCl₃, identify the electrophile and the organic product formed." (Answer: electrophile = Br⁺; product = bromobenzene.) ---

Before solving, remember these

Key Fact

Electrophilic aromatic substitution

(1) Halogenation (Cl₂/FeCl₃, Br₂/FeBr₃). (2) Nitration (HNO₃/H₂SO₄). (3) Sulphonation (oleum, SO₃). (4) Friedel-Crafts alkylation (RX/AlCl₃) and acylation (RCOX/AlCl₃).

-- NCERT, p. 26

Formulas

Markovnikov's rule (and anti-Markovnikov)

H^{+} to C with more H

Without peroxide: ionic mechanism — H goes to carbon with MORE hydrogens (carbocation stability rule). With peroxide (HBr only, Kharasch): radical mechanism — anti-Markovnikov.

Symbol	Quantity	SI Unit
H,X	added atoms	-

Valid when

Asymmetric alkene
H-X with X = Cl, Br, I
Without peroxide for Markovnikov

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Markovnikov reverses with peroxide

Category: Organic Reaction Conditions

HBr addition to alkene: WITHOUT peroxide → Markovnikov (H to C with more H). WITH peroxide (Kharasch effect) → anti-Markovnikov (radical mechanism). Specific to HBr only — not HCl, HI.

When it triggers

Question gives HX addition to alkene with explicit peroxide condition or hints (e.g. ROOR, light).

How to avoid

Without peroxide: ionic mechanism, carbocation stability → Markovnikov. With peroxide: radical mechanism, radical stability → anti-Markovnikov. Effect ONLY for HBr (HCl too strong, HI too weak).

Trap

Solvent changes product distribution

Category: Organic Reaction Conditions

Same starting materials give different products depending on solvent. Polar protic (water, alcohols): SN1/E1 favoured. Polar aprotic (DMSO, DMF): SN2 favoured. Affects substitution vs elimination.

When it triggers

Question contrasts product when solvent is changed; or specifies solvent type.

How to avoid

Polar protic stabilises carbocation → SN1/E1 (3° preferred). Polar aprotic doesn't solvate nucleophile → strong SN2 nucleophile (1°/2° preferred). Bulky base (t-BuOK) favours E2 over SN2.

Mistake

Confuses ortho/para directors with meta directors.

Root cause: concept gap

Correction

o,p-directors (activators except halogens): -OH, -OR, -NH₂, -NHR, alkyl. m-directors (deactivators): -NO₂, -CN, -COOH, -CHO. Halogens: o,p-directors but DEACTIVATORS.

Mistake