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Chemistry · Hydrocarbons

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is the defining chemistry of benzene and its homologues. NCERT Class 11 Chemistry §9.5.5 lists five named reactions of arenes — nitration, halogenation, sulphonation and the Friedel-Crafts alkylation and acylation — all proceeding by a common three-step route through an arenium-ion intermediate. Because these reactions recur in NEET almost every year, mastering the electrophile of each reagent and the mechanism that preserves aromaticity is essential for the Hydrocarbons unit.

Why arenes substitute, not add

Benzene contains a closed loop of six delocalised pi electrons that satisfies the Hückel rule and gives the molecule its exceptional aromatic stability. This delocalisation is the reason an alkene readily adds bromine across its double bond while benzene does not. An addition reaction would force two ring carbons into sp3 geometry and permanently break the six-electron sextet, which is energetically costly.

Substitution offers a way out. When an electrophile attacks the ring, aromaticity is lost only transiently in the intermediate; the subsequent loss of a proton regenerates the aromatic sextet. NCERT therefore states that arenes are characterised by electrophilic substitution reactions, undergoing addition and oxidation only under special, forcing conditions such as high temperature, pressure, a nickel catalyst, or ultraviolet light.

The attacking species in every case is an electrophile, written generically as $\ce{E+}$. The ring, being electron-rich, behaves as the nucleophile. The net transformation simply replaces one ring hydrogen by the electrophile: $\ce{C6H6 + E+ -> C6H5E + H+}$.

Figure 1 · Substitution vs addition

Why benzene keeps its ring intact

Energy aromatic arenium ion aromatic restored Substitution: aromaticity recovered

The intermediate sits at the top of the barrier; deprotonation rolls the system back down to an aromatic product, so substitution is favoured over addition.

The five named reactions

NCERT lists five common electrophilic substitution reactions of arenes. Each differs only in the reagent and the electrophile it generates; the ring's role is identical throughout.

Nitration

A nitro group is introduced when benzene is heated with a nitrating mixture of concentrated nitric acid and concentrated sulphuric acid. The electrophile is the nitronium ion, $\ce{NO2+}$.

$$\ce{C6H6 + HNO3 ->[\text{conc. } H2SO4][\Delta] C6H5NO2 + H2O}$$

Halogenation

Arenes react with chlorine or bromine in the presence of a Lewis acid such as anhydrous $\ce{FeCl3}$, $\ce{FeBr3}$ or $\ce{AlCl3}$ to give haloarenes. The Lewis acid polarises the halogen to deliver the electrophilic halogen cation, e.g. $\ce{Cl+}$.

$$\ce{C6H6 + Cl2 ->[anhyd.~FeCl3] C6H5Cl + HCl}$$

Sulphonation

Replacement of a ring hydrogen by a sulphonic acid group is sulphonation, carried out by heating benzene with fuming sulphuric acid (oleum). The effective electrophile is sulphur trioxide, $\ce{SO3}$.

$$\ce{C6H6 + SO3 ->[H2SO4] C6H5SO3H}$$

Friedel-Crafts alkylation and acylation

When benzene is treated with an alkyl halide and anhydrous $\ce{AlCl3}$, an alkylbenzene is formed; the electrophile is the alkyl carbocation $\ce{R+}$. With an acyl halide or acid anhydride and $\ce{AlCl3}$, an acylbenzene results, the electrophile being the resonance-stabilised acylium ion $\ce{RC+=O}$.

$$\ce{C6H6 + CH3Cl ->[anhyd.~AlCl3] C6H5CH3 + HCl}$$

$$\ce{C6H6 + CH3COCl ->[anhyd.~AlCl3] C6H5COCH3 + HCl}$$

If an excess of the electrophilic reagent is used, more than one ring hydrogen may be replaced successively; for example, benzene with excess chlorine over anhydrous $\ce{AlCl3}$ can be chlorinated all the way to hexachlorobenzene, $\ce{C6Cl6}$.

Master table: reaction, reagent and electrophile

The single most examined fact in this subtopic is the identity of the electrophile in each reaction. Commit the following table to memory; NEET stems frequently quote a reagent and ask for the electrophile or product, or the reverse.

Reaction Reagent / conditions Electrophile Product
Nitration conc. HNO3 + conc. H2SO4, heat $\ce{NO2+}$ (nitronium ion) Nitrobenzene, $\ce{C6H5NO2}$
Halogenation $\ce{Cl2}$ / $\ce{Br2}$, anhyd. FeCl3 / FeBr3 / AlCl3 $\ce{Cl+}$ / $\ce{Br+}$ Chlorobenzene / bromobenzene
Sulphonation fuming $\ce{H2SO4}$ (oleum), heat $\ce{SO3}$ Benzenesulphonic acid, $\ce{C6H5SO3H}$
Friedel-Crafts alkylation $\ce{R-X}$, anhyd. AlCl3 $\ce{R+}$ (alkyl carbocation) Alkylbenzene, $\ce{C6H5R}$
Friedel-Crafts acylation $\ce{RCOCl}$ / $\ce{(RCO)2O}$, anhyd. AlCl3 $\ce{RC+=O}$ (acylium ion) Acylbenzene (aryl ketone), $\ce{C6H5COR}$
Keep building

Where the second group goes is decided by the group already present — see Directive influence of functional groups.

The three-step mechanism

Experimental evidence shows that electrophilic substitution ($S_E$, where S = substitution and E = electrophilic) proceeds through three steps that are common to all five reactions: generation of the electrophile, formation of the carbocation intermediate, and removal of a proton from that intermediate.

Step (b): the arenium ion or sigma complex

Once the electrophile is available, it attacks the pi system of the ring. This forms the sigma complex, also called the arenium ion — a carbocation in which the carbon that has bonded to the electrophile becomes sp3 hybridised. Delocalisation of electrons stops at this sp3 carbon, so the arenium ion has lost its aromatic character. The positive charge is not localised; it is spread over the three remaining ring carbons by resonance, which stabilises the intermediate.

Figure 2 · General EAS mechanism

Electrophile attack → arenium ion → proton loss

aromatic + E⁺ attack E H ⊕ arenium ion (sp³ carbon, not aromatic) − H⁺ E aromatic product

The solid teal inner ring marks an intact aromatic sextet; the dashed coral arc marks the arenium ion where delocalisation is interrupted at the sp3 carbon. Loss of $\ce{H+}$ from that carbon restores aromaticity.

Step (c): removal of the proton

To restore the aromatic character, the sigma complex releases the proton from the sp3 hybridised carbon. The base that removes it is the counter-ion generated alongside the electrophile: $\ce{[AlCl4]-}$ in halogenation, alkylation and acylation, and $\ce{[HSO4]-}$ in nitration. With the proton gone, the six-electron delocalised system reforms and the substituted arene emerges as the stable product.

Generating the electrophile

Step (a) — making the electrophile — is where the reagents do their distinctive work. In chlorination, alkylation and acylation, anhydrous $\ce{AlCl3}$ acts as a Lewis acid and combines with the attacking reagent to release $\ce{Cl+}$, $\ce{R+}$ and the acylium ion $\ce{RC+=O}$ respectively.

$$\ce{Cl2 + AlCl3 -> Cl+ + [AlCl4]-}$$

$$\ce{R-Cl + AlCl3 -> R+ + [AlCl4]-}$$

For nitration the nitronium ion is produced when sulphuric acid protonates nitric acid; the protonated nitric acid then loses water. NCERT highlights that here sulphuric acid serves as the acid and nitric acid as the base, so the generation is a simple acid-base equilibrium.

$$\ce{HNO3 + 2H2SO4 <=> NO2+ + H3O+ + 2HSO4-}$$

NEET Trap

Nitronium-ion equilibrium and the common-ion effect

The nitronium-ion equilibrium also produces $\ce{HSO4-}$. If a large amount of $\ce{KHSO4}$ is added, it dissociates to give extra $\ce{HSO4-}$, which by the common-ion effect drives the equilibrium backward and lowers the $\ce{NO2+}$ concentration — so the rate of nitration becomes slower, not faster. This is exactly the trap set in NEET 2016.

More $\ce{HSO4-}$ → less $\ce{NO2+}$ → slower nitration.

Friedel-Crafts pitfalls

Two recurring exam points concern the Friedel-Crafts reactions specifically. The first is the limitation of the substrate; the second is the difference between the alkyl and acyl electrophiles.

Because the alkyl electrophile is a free carbocation $\ce{R+}$, it can rearrange to a more stable carbocation before attacking the ring. This is why treating benzene with 1-chloropropane gives isopropylbenzene rather than n-propylbenzene: the primary $\ce{CH3CH2CH2+}$ rearranges to the secondary $\ce{(CH3)2CH+}$. The acylium ion $\ce{RC+=O}$ is resonance-stabilised and does not rearrange, so acylation delivers a single, clean product.

NEET Trap

When Friedel-Crafts simply fails

Friedel-Crafts alkylation and acylation do not work on strongly deactivated rings. A powerful electron-withdrawing group such as $\ce{-NO2}$ drains so much electron density from the ring that it can no longer act as the nucleophile, so nitrobenzene gives no Friedel-Crafts product. Separately, remember that alkylation risks carbocation rearrangement while acylation avoids it — the acylium ion does not rearrange.

No Friedel-Crafts on strongly deactivated arenes; acylation > alkylation for a rearrangement-free product.

Worked example

Identify the electrophile and product when benzene is heated with fuming sulphuric acid.

Fuming sulphuric acid (oleum) supplies the electrophile $\ce{SO3}$. It attacks the ring to form the arenium ion, which loses a proton to give benzenesulphonic acid: $\ce{C6H6 + SO3 -> C6H5SO3H}$. The reaction is sulphonation, the replacement of a ring hydrogen by the $\ce{-SO3H}$ group.

Quick Recap

Electrophilic aromatic substitution in one screen

  • Arenes are characterised by electrophilic substitution; addition is only forced under special conditions because substitution restores the aromatic sextet.
  • Five named reactions: nitration ($\ce{NO2+}$), halogenation ($\ce{Cl+}$/$\ce{Br+}$ via Lewis acid), sulphonation ($\ce{SO3}$), Friedel-Crafts alkylation ($\ce{R+}$) and acylation ($\ce{RC+=O}$).
  • Mechanism is three steps: generate electrophile → form arenium ion (sp3 carbon, non-aromatic, resonance-stabilised) → lose proton to restore aromaticity.
  • $\ce{AlCl3}$ generates $\ce{Cl+}$, $\ce{R+}$ and acylium ions; nitronium ion comes from an $\ce{H2SO4}$/$\ce{HNO3}$ acid-base equilibrium.
  • Alkylation can rearrange the carbocation; acylation does not. Friedel-Crafts fails on strongly deactivated rings.

NEET PYQ Snapshot — Electrophilic Aromatic Substitution

Questions drawn from official NEET papers on EAS reactions, the nitronium-ion equilibrium and substitution on substituted rings.

NEET 2016

Consider the nitration of benzene using mixed conc. $\ce{H2SO4}$ and $\ce{HNO3}$. If a large amount of $\ce{KHSO4}$ is added to the mixture, the rate of nitration will be:

  1. slower
  2. unchanged
  3. doubled
  4. faster
Answer: (1) slower

The equilibrium $\ce{H2SO4 + HNO3 <=> NO2+ + HSO4- + H2O}$ also produces $\ce{HSO4-}$. Added $\ce{KHSO4}$ supplies extra $\ce{HSO4-}$; by the common-ion effect the backward reaction is favoured, the $\ce{NO2+}$ concentration falls, and nitration slows.

NEET 2018

The compound $\ce{C7H8}$ (toluene) undergoes the sequence $\ce{C7H8 ->[3Cl2/\Delta] A ->[Br2/Fe] B ->[Zn/HCl] C}$. The product C is:

  1. m-bromotoluene
  2. o-bromotoluene
  3. 3-bromo-2,4,6-trichlorotoluene
  4. p-bromotoluene
Answer: (1) m-bromotoluene

$\ce{Br2/Fe}$ is electrophilic ring bromination ($\ce{Br+}$ electrophile). The bromination follows the chlorination pattern, and reduction by $\ce{Zn/HCl}$ removes the chlorines to leave the meta-bromotoluene framework as product C.

NEET 2025

Which one of the following reactions does NOT give benzene as the product?

  1. (reaction 1)
  2. (reaction 2)
  3. (reaction 3)
  4. (reaction 4)
Answer: (1)

Several routes regenerate benzene; one of the listed reactions does not. Recognising which transformations leave the aromatic ring intact is the same skill that underlies predicting EAS products — the ring survives substitution but not every named reaction targets the parent benzene.

Concept

Benzene is treated with 1-chloropropane and anhydrous $\ce{AlCl3}$. Identify the major product and the reason.

  1. n-propylbenzene; no rearrangement occurs
  2. isopropylbenzene; the primary carbocation rearranges to a secondary one
  3. cyclopropylbenzene; ring closure of the electrophile
  4. no reaction; the ring is deactivated
Answer: (2) isopropylbenzene

In Friedel-Crafts alkylation the electrophile is the carbocation $\ce{R+}$. The primary $\ce{CH3CH2CH2+}$ rearranges to the more stable secondary $\ce{(CH3)2CH+}$, so the major product is isopropylbenzene — a classic NCERT exercise point.

Concept

Which species is the actual electrophile in the sulphonation of benzene with oleum?

  1. $\ce{SO3H+}$
  2. $\ce{SO3}$
  3. $\ce{HSO4-}$
  4. $\ce{SO2}$
Answer: (2) $\ce{SO3}$

Fuming sulphuric acid (oleum) supplies $\ce{SO3}$, which attacks the ring to form the arenium ion; loss of a proton then gives benzenesulphonic acid, $\ce{C6H5SO3H}$.

FAQs — Electrophilic Aromatic Substitution

The conceptual questions examiners reuse most often on benzene's substitution chemistry.

Why does benzene undergo substitution rather than addition?
Benzene possesses a delocalised six-electron pi system that confers aromatic stability. An addition reaction would permanently destroy this delocalisation, whereas substitution sacrifices aromaticity only briefly in the arenium-ion intermediate and then restores it by loss of a proton. Because the aromatic sextet is regenerated, substitution is energetically favoured and arenes characteristically undergo electrophilic substitution rather than addition.
What is the arenium ion (sigma complex) in EAS?
The arenium ion, also called the sigma complex, is the positively charged carbocation intermediate formed when the electrophile bonds to a ring carbon. That carbon becomes sp3 hybridised, delocalisation of electrons stops at it, and the ion loses its aromatic character. The positive charge is spread over the three remaining carbons by resonance, which stabilises the intermediate before a proton is lost to restore aromaticity.
How is the nitronium ion generated in nitration?
In the nitrating mixture of concentrated nitric and sulphuric acids, sulphuric acid protonates nitric acid to give protonated nitric acid, which then loses water to furnish the nitronium ion NO2+. Here sulphuric acid acts as the acid and nitric acid as the base, so the step is a simple acid-base equilibrium. The nitronium ion is the actual electrophile that attacks the benzene ring.
Why is acylation often preferred over alkylation in Friedel-Crafts reactions?
In Friedel-Crafts alkylation the electrophile is an alkyl carbocation, which can rearrange to a more stable carbocation, so treating benzene with 1-chloropropane gives isopropylbenzene rather than n-propylbenzene. The acylium ion in acylation is resonance-stabilised and does not rearrange, so it gives a single, predictable acyl product. Acylation therefore avoids the carbocation-rearrangement problem of alkylation.
Why does adding KHSO4 slow down the nitration of benzene?
The nitronium ion is generated in an equilibrium that also produces HSO4 minus ions. Potassium hydrogen sulphate dissociates to give additional HSO4 minus, and by the common-ion effect this pushes the equilibrium backward. The concentration of the nitronium-ion electrophile falls, so the rate of nitration becomes slower, as tested in NEET 2016.
Can Friedel-Crafts reactions be carried out on any benzene ring?
No. Friedel-Crafts alkylation and acylation fail on strongly deactivated rings, such as nitrobenzene, because a strongly electron-withdrawing group lowers the electron density of the ring so much that it can no longer attack the electrophile effectively. Strongly deactivated arenes therefore do not undergo Friedel-Crafts reactions, a point repeatedly exploited in NEET traps.
Related Topics
Hydrocarbons Back to the full chapter hub for alkanes, alkenes, alkynes and arenes. Structure of Benzene The delocalised ring whose stability dictates substitution over addition. Directive Influence of Groups How an existing substituent steers the next electrophile to o-, p- or m-. Aromaticity & Hückel's Rule The (4n+2) pi-electron criterion that defines aromatic character.