MHT-CET Chemistry · Aromatic Compounds

Electrophilic Substitution: the Reactions and Where the Group Goes

Benzene keeps its aromatic ring by substituting rather than adding: an electrophile (Cl⁺, NO₂⁺, R⁺, RCO⁺, HCO⁺) replaces a ring H, and a group already on the ring decides whether the new one goes ortho/para or meta.

All Aromatic Compounds notes MHT-CET Chemistry strategy

Why this matters

12 PYQs, none HARD. Seven are about direction — which group is ortho/para directing, which starting compound gives a stated o/p pair, how much para product forms; five are about the reactions and their reagents — Gattermann–Koch, iodination, hexachlorobenzene. Two cards.

Concept 1 of 2: The Substitution Reactions and Their Reagents

Every reaction here is the same three steps: make an electrophile with a Lewis acid, let it attack the ring to form a σ-complex, lose H⁺ to get the ring back. So the thing to learn for each is the reagent pair that makes the electrophile.

Definition

Halogenation: X₂ / FeX₃ or anhydrous AlCl₃. Excess Cl₂ gives hexachlorobenzene, C₆Cl₆.
Iodination is reversible — the HI formed reduces iodobenzene back — so it is not done directly.
Nitration: conc. HNO₃ + conc. H₂SO₄ (NO₂⁺).
Friedel–Crafts: RCl or RCOCl with anhydrous AlCl₃.
Gattermann–Koch: CO + HCl, anhydrous AlCl₃, pressure → benzaldehyde.

Reaction	Reagent	Product from benzene
Gattermann–Koch formylation	CO, HCl / anhyd. AlCl₃	C₆H₅CHOQ
Exhaustive chlorination	Excess Cl₂ / AlCl₃	C₆Cl₆Q
Iodination	I₂	Not possible — reversibleQ
Friedel–Crafts alkylation	CH₃Cl / anhyd. AlCl₃	Toluene
Nitration	conc. HNO₃ + conc. H₂SO₄	Nitrobenzene

Practice this conceptself-check · 2 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 10th May Shift 1 · Q81Moderate

Example 1 · Aromatic Compounds · Electrophilic Aromatic Substitution

Identify the product obtained in following reaction:

\text{C}_6\text{H}_6 \xrightarrow{\text{CO, HCl/Anhyd. AlCl}_3\text{, pressure}}

product

Hexachlorobenzene as C₆H₆Cl₆

C₆H₆Cl₆ is benzene hexachloride (BHC), made by ADDITION of Cl₂ in UV light. Hexachlorobenzene is SUBSTITUTION of all six H: C₆Cl₆.

Drill 4 more on the substitution reactions and their reagents

Concept 2 of 2: Directing Effects: Ortho/Para or Meta

A group that pushes electrons into the ring (–OH, –OCH₃, –NH₂, –CH₃, and the halogens through their lone pairs) makes the ortho and para positions richest, so the new group goes there — with para usually the major product because it is less crowded. A group that pulls electrons out (–NO₂, –CHO, –COOH, –CN) leaves meta the least deactivated.

Definition

o/p directors: –OH, –OCH₃, –NH₂, –R, –X (halogens deactivate but still direct o/p).
m directors: –NO₂, –CHO, –COOH, –COR, –CN, –SO₃H.
Para is the major product: chlorobenzene + Cl₂ → 1,4-dichlorobenzene; anisole + Br₂/AcOH → p-bromoanisole ~90%.
Working backwards: 2- and 4-substituted products from CH₃Cl or CH₃COCl → the start was chlorobenzene.
Phenol + conc. HNO₃/H₂SO₄ → 2,4,6-trinitrophenol (picric acid) — –OH activates all three positions.

Start	Reagent	Major product
Chlorobenzene	Cl₂ / AlCl₃	1,4-dichlorobenzeneQ
Anisole	Br₂ in acetic acid	p-bromoanisole (90%)Q
Chlorobenzene	CH₃Cl / anhyd. AlCl₃	2- and 4-chlorotolueneQ
Chlorobenzene	CH₃COCl / anhyd. AlCl₃	2- and 4-chloroacetophenoneQ
Phenol	conc. HNO₃ / conc. H₂SO₄	2,4,6-trinitrophenolQ

Practice this conceptself-check · 2 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 11th May Shift 2 · Q70Easy

Example 2 · Aromatic Compounds · Electrophilic Aromatic Substitution

Identify the ortho/para directing group in EAS reaction from following.

Toluene as the starting compound

Toluene with CH₃Cl gives xylenes, and with anything it cannot give a CHLORO product. The chlorine in both products must have been on the ring already — the start is chlorobenzene.

Mononitration of phenol

Dilute HNO₃ gives o- and p-nitrophenol. With CONCENTRATED HNO₃ and H₂SO₄ all three activated positions are nitrated.

Drill 6 more on directing effects: ortho/para or meta

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

The Substitution Reactions and Their Reagents5 rows

Reaction	Reagent	Product from benzene
Gattermann–Koch formylation	CO, HCl / anhyd. AlCl₃	C₆H₅CHOQ
Exhaustive chlorination	Excess Cl₂ / AlCl₃	C₆Cl₆Q
Iodination	I₂	Not possible — reversibleQ
Friedel–Crafts alkylation	CH₃Cl / anhyd. AlCl₃	Toluene
Nitration	conc. HNO₃ + conc. H₂SO₄	Nitrobenzene

Directing Effects: Ortho/Para or Meta5 rows

Start	Reagent	Major product
Chlorobenzene	Cl₂ / AlCl₃	1,4-dichlorobenzeneQ
Anisole	Br₂ in acetic acid	p-bromoanisole (90%)Q
Chlorobenzene	CH₃Cl / anhyd. AlCl₃	2- and 4-chlorotolueneQ
Chlorobenzene	CH₃COCl / anhyd. AlCl₃	2- and 4-chloroacetophenoneQ
Phenol	conc. HNO₃ / conc. H₂SO₄	2,4,6-trinitrophenolQ

Watch out for (3)

Hexachlorobenzene as C₆H₆Cl₆
→ The Substitution Reactions and Their Reagents
Toluene as the starting compound
→ Directing Effects: Ortho/Para or Meta
Mononitration of phenol
→ Directing Effects: Ortho/Para or Meta

Test yourself on Aromatic Compounds

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