JEE Mains Chemistry · Hydrocarbons

Electrophilic Substitution: Reactivity and Directing Effects

Benzene keeps its aromatic ring by swapping an H for an electrophile made by an acid or a Lewis acid; a group already on the ring speeds or slows that swap and sends the new group ortho and para or meta.

All Hydrocarbons notes JEE Mains Chemistry strategy

Why this matters

Nineteen PYQs, fifteen of them multiple choice, and three from 2026. Five are about the electrophile itself: how HNO₃ and H₂SO₄ make NO₂⁺, what the Lewis acid does, and which rings cannot undergo Friedel–Crafts reactions. Seven classify groups as activating or deactivating and as ortho-para or meta directing. Seven put substituted benzenes in order of reactivity. Four of the nineteen ask for a number.

Concept 1 of 3: Making the electrophile, and when Friedel–Crafts fails

Benzene's π cloud is only weakly nucleophilic, so every aromatic substitution first builds a strong, positive electrophile. A proton acid or a Lewis acid does this. The electrophile then adds to the ring to give a carbocation (the arenium ion or σ-complex), and loss of H⁺ restores the aromatic ring. A Friedel–Crafts reaction fails when the ring is too electron-poor, or when a basic nitrogen on the ring grabs the Lewis acid.

Definition

Nitration: $\mathrm{HNO_3 + 2H_2SO_4 \rightleftharpoons NO_2^+ + H_3O^+ + 2HSO_4^-}$ . H₂SO₄ is the acid; HNO₃ accepts a proton, acting as a base.
Halogenation: $\mathrm{Cl_2 + AlCl_3 \rightarrow Cl^+ + AlCl_4^-}$ (or FeCl₃, FeBr₃).
Sulphonation: SO₃ from conc. H₂SO₄ or oleum.
Friedel–Crafts: $\mathrm{RCl + AlCl_3 \rightarrow R^+ + AlCl_4^-}$ ; $\mathrm{RCOCl + AlCl_3 \rightarrow RCO^+ + AlCl_4^-}$ . The catalyst is a Lewis ACID.
Friedel–Crafts fails with strongly deactivated rings (–NO₂, –CN, –COR, –SO₃H on the ring) and with –NH₂, –NHR, –NR₂ (the N lone pair binds AlCl₃ and the ring becomes positively charged).

Reaction	Reagents	Electrophile	Product from benzene
Nitration	Conc. HNO₃ + conc. H₂SO₄	$\mathrm{NO_2^+}$ (nitronium)	Nitrobenzene
Chlorination	Cl₂ with anhydrous AlCl₃ or FeCl₃	$\mathrm{Cl^+}$	Chlorobenzene
Sulphonation	Fuming H₂SO₄ (oleum)	$\mathrm{SO_3}$	Benzenesulphonic acid
Friedel–Crafts alkylation	CH₃Cl with anhydrous AlCl₃	$\mathrm{CH_3^+}$	Toluene
Friedel–Crafts acylation	CH₃COCl with anhydrous AlCl₃	$\mathrm{CH_3CO^+}$ (acylium)	Acetophenone

Every electrophile is made by an acid or a Lewis acid; the ring then loses H⁺ to stay aromatic.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2024 · 9 April 2024 · Q148Moderate

Example 1 · Hydrocarbons · Electrophilic Substitution: Reactivity and Directing Effects

Number of compounds from the following which cannot undergo Friedel-Crafts reactions is :______toluene, nitrobenzene, xylene, cumene, aniline, chlorobenzene, m-nitroaniline, m-dinitrobenzene

AlCl₃ is a Lewis acid, not a Lewis base

AlCl₃ has an empty orbital and accepts Cl⁻. A statement calling the Friedel–Crafts catalyst a Lewis base is wrong.

Chlorobenzene still reacts

A halogen deactivates the ring only weakly, so chlorobenzene does undergo Friedel–Crafts reactions (para mainly). Only strongly deactivated rings and rings bearing an amino N fail.

Drill 4 more on making the electrophile, and when friedel–crafts fails

Concept 2 of 3: Activating, deactivating, ortho-para and meta directors

A group that pushes electrons into the ring (by a lone pair, +R, or by alkyl groups, +I and hyperconjugation) makes the ring richer and the reaction faster, and the extra density sits at the ortho and para positions. A group that pulls electrons out (–R, –I) slows the reaction and leaves the meta position the least starved. Halogens are the one mixed case: they pull by –I, so they deactivate, but their lone pairs still send the electrophile ortho and para.

Definition

Activating, ortho-para: –NH₂, –NHR, –NR₂, –OH, –OCH₃, –NHCOCH₃, –CH₃ and other alkyl groups.
Deactivating, ortho-para: –F, –Cl, –Br, –I.
Deactivating, meta: –NO₂, –CN, –CHO, –COR, –COOH, –COOR, –SO₃H, –CF₃, –NR₃⁺.
With two groups, the stronger activator decides where the next group goes; nothing enters between two meta groups easily.
–NO₂ deactivates the ring to ELECTROPHILES but activates it (at ortho and para) to NUCLEOPHILIC substitution.

Group	Main electronic effect	Rate compared with benzene	Directs to
–NH₂, –NR₂	+R (strong)	Much faster	ortho and para
–OH, –OCH₃	+R (strong)	Much faster	ortho and para
–NHCOCH₃	+R (moderate; the lone pair is shared with C=O)	Faster	ortho and para
–CH₃, –C₂H₅	+I and hyperconjugation	Slightly faster	ortho and para
–Cl, –Br	–I stronger than +R	Slightly slower	ortho and para
–CHO, –COR, –COOH, –COOR	–R and –I	Slower	meta
–CN, –SO₃H, –CF₃	–R and –I (–CF₃ by –I only)	Much slower	meta
–NO₂	–R and –I (strongest)	Much slower	meta

Every activator directs ortho and para; every meta director deactivates; halogens deactivate yet direct ortho and para.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 24 Jan 2025 · Q114Moderate

Example 2 · Hydrocarbons · Electrophilic Substitution: Reactivity and Directing Effects

Identify correct statement/s : (A)

-OCH_{3}

and

-NHCOCH_{3}

are activating group (B) -CN and -OH are meta directing group (C) -CN and

-SO_{3}H

are meta directing group (D) Activating groups act as ortho - and para directing groups (E) Halides are activating groups Choose the correct answer from the options given below :

Halogens deactivate but direct ortho and para

A halogen is neither an activator nor a meta director. It slows the reaction (–I) but its lone pair steers the electrophile ortho and para.

–OH and –OCH₃ are never meta directors

Any group with a lone pair on the atom attached to the ring is ortho-para directing. –NHCOCH₃ is weaker than –NH₂ but is still activating.

Nitro activates for the other kind of substitution

–NO₂ makes the ring electron-poor. That hinders electrophiles but helps a nucleophile replace a halogen ortho or para to it.

Drill 6 more on activating, deactivating, ortho-para and meta directors

Concept 3 of 3: Ranking rings by rate of electrophilic substitution

The electron-richer the ring, the faster an electrophile attacks it. So rank each ring by its strongest group, then by how many groups it carries. Strong +R donors (–NR₂, –OH, –OR) come first, alkyl groups next (more alkyls, faster), plain benzene in the middle, halogens just below it, carbonyl groups lower, and nitro last.

Definition

Order of the groups, fastest first: –NR₂ > –NH₂ > –OH > –OCH₃ > –NHCOCH₃ > –CH₃ > –H > –Cl, –Br > –CHO, –COR > –CN > –NO₂.
More alkyl groups add up: 1,3,5-trimethylbenzene > xylene > toluene > benzene.
Two strong withdrawing groups (m-dinitrobenzene) make the ring slower still.

Compound	Group	Effect on the ring	Place in rate order
N,N-Dimethylaniline	–N(CH₃)₂	Strong +R	Fastest of this list
Anisole	–OCH₃	Strong +R	Second
Toluene	–CH₃	+I and hyperconjugation	Third
Benzene	–H	Reference	Fourth
Chlorobenzene	–Cl	–I beats +R	Fifth
Benzaldehyde	–CHO	–R and –I	Sixth
Benzonitrile	–CN	–R and –I	Seventh
Nitrobenzene	–NO₂	Strongest –R and –I	Slowest of this list

Rank by the strongest group on each ring; alkyl groups add up, and each extra withdrawing group slows the ring further.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2022 · 28 July 2022 · Q133Moderate

Example 3 · Hydrocarbons · Electrophilic Substitution: Reactivity and Directing Effects

Arrange the following in increasing order of reactivity towards nitration A. p-xylene B. bromobenzene C. mesitylene

D

, nitrobenzene E. benzene Choose the correct answer from the options given below

Halogenobenzenes are slower than benzene

It is tempting to put chlorobenzene or bromobenzene above benzene because halogens direct ortho and para. They are still deactivating, so they come just below benzene.

Count the alkyl groups

Each methyl adds electron density, so a trimethylbenzene beats a dimethylbenzene, which beats toluene.

Drill 6 more on ranking rings by rate of electrophilic substitution

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 (3)

Making the electrophile, and when Friedel–Crafts fails5 rows

Reaction	Reagents	Electrophile	Product from benzene
Nitration	Conc. HNO₃ + conc. H₂SO₄	$\mathrm{NO_2^+}$ (nitronium)	Nitrobenzene
Chlorination	Cl₂ with anhydrous AlCl₃ or FeCl₃	$\mathrm{Cl^+}$	Chlorobenzene
Sulphonation	Fuming H₂SO₄ (oleum)	$\mathrm{SO_3}$	Benzenesulphonic acid
Friedel–Crafts alkylation	CH₃Cl with anhydrous AlCl₃	$\mathrm{CH_3^+}$	Toluene
Friedel–Crafts acylation	CH₃COCl with anhydrous AlCl₃	$\mathrm{CH_3CO^+}$ (acylium)	Acetophenone

Every electrophile is made by an acid or a Lewis acid; the ring then loses H⁺ to stay aromatic.

Activating, deactivating, ortho-para and meta directors8 rows

Group	Main electronic effect	Rate compared with benzene	Directs to
–NH₂, –NR₂	+R (strong)	Much faster	ortho and para
–OH, –OCH₃	+R (strong)	Much faster	ortho and para
–NHCOCH₃	+R (moderate; the lone pair is shared with C=O)	Faster	ortho and para
–CH₃, –C₂H₅	+I and hyperconjugation	Slightly faster	ortho and para
–Cl, –Br	–I stronger than +R	Slightly slower	ortho and para
–CHO, –COR, –COOH, –COOR	–R and –I	Slower	meta
–CN, –SO₃H, –CF₃	–R and –I (–CF₃ by –I only)	Much slower	meta
–NO₂	–R and –I (strongest)	Much slower	meta

Every activator directs ortho and para; every meta director deactivates; halogens deactivate yet direct ortho and para.

Ranking rings by rate of electrophilic substitution8 rows

Compound	Group	Effect on the ring	Place in rate order
N,N-Dimethylaniline	–N(CH₃)₂	Strong +R	Fastest of this list
Anisole	–OCH₃	Strong +R	Second
Toluene	–CH₃	+I and hyperconjugation	Third
Benzene	–H	Reference	Fourth
Chlorobenzene	–Cl	–I beats +R	Fifth
Benzaldehyde	–CHO	–R and –I	Sixth
Benzonitrile	–CN	–R and –I	Seventh
Nitrobenzene	–NO₂	Strongest –R and –I	Slowest of this list

Rank by the strongest group on each ring; alkyl groups add up, and each extra withdrawing group slows the ring further.

Watch out for (7)

AlCl₃ is a Lewis acid, not a Lewis base
→ Making the electrophile, and when Friedel–Crafts fails
Chlorobenzene still reacts
→ Making the electrophile, and when Friedel–Crafts fails
Halogens deactivate but direct ortho and para
→ Activating, deactivating, ortho-para and meta directors
–OH and –OCH₃ are never meta directors
→ Activating, deactivating, ortho-para and meta directors
Nitro activates for the other kind of substitution
→ Activating, deactivating, ortho-para and meta directors
Halogenobenzenes are slower than benzene
→ Ranking rings by rate of electrophilic substitution
Count the alkyl groups
→ Ranking rings by rate of electrophilic substitution

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