Electrophilic aromatic substitution (EAS) — nitration, halogenation, sulfonation, Friedel-Crafts alkylation - acylation;

WHAT is EAS?

WHY substitution, not addition? Addition gives a non-aromatic diene/triene → high energy. Substitution loses aromaticity temporarily (in the intermediate) but regains it in the product. Net: keeps the aromatic prize.

HOW: the universal 2-step mechanism

Every EAS (nitration, halogenation, sulfonation, Friedel–Crafts) follows the same skeleton. Learn it once (80/20!).

Step 1 — Generate the electrophile. Each reaction makes its own $E^+$ (this is the only part that changes).

Step 2 — π electrons attack $E^+$ → forms the arenium ion (also called the σ-complex or Wheland intermediate). This is the slow, rate-determining step (RDS) because aromaticity is broken here.

Step 3 — Lose $H^+$ from the $sp^3$ carbon → rearomatize. A base removes the proton, π system reforms.

Figure — Electrophilic aromatic substitution (EAS) — nitration, halogenation, sulfonation, Friedel-Crafts alkylation - acylation;

The arenium ion (heart of EAS)

Reaction-by-reaction: how each $E^+$ is made

Formula Step 1 (electrophile generation) for each

Nitration — uses $HNO_3 + H_2SO_4$ : $HNO_3 + 2H_2SO_4 \rightarrow \underbrace{NO_2^+}_{\text{nitronium ion}} + H_3O^+ + 2HSO_4^-$ $H_2SO_4$ protonates $HNO_3$ , water leaves → ==nitronium ion $NO_2^+$ == is the electrophile.

Halogenation (Cl, Br) — needs a Lewis acid catalyst $FeX_3$ (or $AlX_3$ ): $X_2 + FeX_3 \rightarrow X^+ \cdots FeX_4^- \quad(\text{polarized } X^+)$ The Lewis acid pulls electron density off $X_2$ , making one $X$ electron-poor ( $X^+$ -like).

Sulfonation — uses fuming $H_2SO_4$ (oleum) / $SO_3$ : $\text{Electrophile} = SO_3 \ (\text{or } {}^+SO_3H)$ Sulfonation is reversible — heating with dilute acid removes $-SO_3H$ . (Useful as a blocking group!)

Friedel–Crafts Alkylation — alkyl halide + $AlCl_3$ : $R\text{–}Cl + AlCl_3 \rightarrow R^+ + AlCl_4^- \quad(\text{carbocation})$

Friedel–Crafts Acylation — acyl halide + $AlCl_3$ : $R\text{–}CO\text{–}Cl + AlCl_3 \rightarrow \underbrace{R\text{–}C{\equiv}O^+}_{\text{acylium ion}} + AlCl_4^-$

After Step 1, all five plug into the same Step 2 → Step 3.

Worked Example 1 — Nitration of benzene

Reagents: conc. $HNO_3$ + conc. $H_2SO_4$ , ~50 °C → nitrobenzene.

Make $E^+$ : $H_2SO_4$ protonates $HNO_3$ → loses water → $NO_2^+$ . Why this step? $NO_2^+$ is a much stronger electrophile than neutral $HNO_3$ ; $H_2SO_4$ is the proton-supplier that drives off water.
Benzene π attacks $NO_2^+$ → arenium ion ( $+$ charge delocalized over 3 ring C's). Why this step? The electron-rich ring is the nucleophile; this RDS breaks aromaticity.
$HSO_4^-$ removes the proton → nitrobenzene + regenerated $H_2SO_4$ . Why this step? Rearomatization gives back the stable aromatic ring; catalyst regenerated.

Worked Example 2 — Friedel–Crafts acylation (making acetophenone)

Reagents: $CH_3COCl$ + $AlCl_3$ → $C_6H_5COCH_3$ .

$AlCl_3$ pulls $Cl^-$ off acetyl chloride → acylium ion $CH_3\text{–}C{\equiv}O^+$ (resonance-stabilized). Why this step? The acylium cation is the electrophile; resonance ( $R\text{–}C{\equiv}O^+ \leftrightarrow R\text{–}C^+{=}O$ ) makes it stable and selective.
Ring attacks acylium → arenium ion.
Lose $H^+$ → acetophenone; $AlCl_3$ released after workup.

Steel-man your mistakes

Flashcards

What 2-step skeleton do all EAS reactions share?

(1) π attacks

E^+

→ arenium ion (RDS), (2) lose

H^+

→ rearomatize.

Why does benzene undergo substitution not addition?

Substitution restores aromaticity (regains ~150 kJ/mol resonance energy); addition would destroy it.

What is the electrophile in nitration?

Nitronium ion

NO_2^+

How is

NO_2^+

generated?

HNO_3 + 2H_2SO_4 \to NO_2^+ + H_3O^+ + 2HSO_4^-

What catalyst is needed for chlorination/bromination of benzene?

A Lewis acid:

FeX_3

AlX_3

What is the electrophile in Friedel–Crafts acylation?

The acylium ion

R\text{–}C{\equiv}O^+

Name the EAS intermediate.

Arenium ion (σ-complex / Wheland intermediate).

Which step is rate-determining in EAS?

Formation of the arenium ion (aromaticity is broken there).

Give two drawbacks of Friedel–Crafts alkylation.

Polyalkylation (R activates ring) and carbocation rearrangement.

Why is sulfonation special?

It is reversible —

-SO_3H

can be removed by heating with dilute acid (good blocking group).

Why does acylation NOT over-substitute?

The

-COR

group is deactivating, so product is less reactive than starting benzene.

Why does Friedel–Crafts fail on nitrobenzene?

The

-NO_2

group strongly deactivates the ring (too electron-poor for EAS).

Recall Feynman: explain to a 12-year-old

Imagine benzene is a happy ring of friends holding hands in a circle — that circle is super stable and they love it. A bully ( $E^+$ ) comes wanting a spot. One friend briefly lets go to grab the bully (the ring is unhappy now — this is the hard, slow part). To get the happy circle back, the ring kicks out a tiny hydrogen ( $H^+$ ) and lets the bully fully join. Now everyone is holding hands again — circle restored, bully included. They never break the whole circle apart (that's why no "addition") — they just swap one member.

Connections

Benzene and aromaticity (Hückel 4n+2 rule) — why the ring is so stable and resists addition.
Directing effects (ortho/para vs meta directors) — where the second group goes; uses the same arenium ion.
Resonance and delocalization — explains arenium ion stability.
Carbocation stability and rearrangements — explains the Friedel–Crafts alkylation problem.
Reactions of alkenes — electrophilic addition — contrast: addition vs substitution.
Lewis acids and catalysis — role of $AlCl_3$ , $FeBr_3$ .

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, benzene ek special ring hai jisme 6 π electrons poore ring par delocalized hote hain — isiliye yeh bahut stable hai (isko aromaticity kehte hain). Ye electron-rich cloud electrophile ( $E^+$ , yaani electron-bhookha species) ko attract karta hai. Lekin benzene alkene ki tarah addition nahi karta, kyunki addition se uska pyaara aromatic system toot jaayega aur ~150 kJ/mol stability chali jaayegi. Isliye benzene substitution karta hai — ek $H$ ko nikaal ke uski jagah $E$ laga deta hai, aur aromaticity wapas aa jaati hai. Yahi pure chapter ka core idea hai.

Mechanism har reaction me same hai (yeh 80/20 trick hai): Step 1 — apna electrophile banao (yahi part har reaction me badalta hai). Step 2 — ring ke π electrons $E^+$ par attack karte hain → arenium ion (sigma-complex) banta hai. Yeh slow, rate-determining step hai kyunki yahan aromaticity temporarily break hoti hai. Step 3 — $H^+$ nikal jaata hai aur ring fir se aromatic ho jaati hai. Bas isi skeleton me alag-alag $E^+$ daalo: Nitration me $NO_2^+$ , halogenation me $X^+$ (with $FeX_3$ Lewis acid), sulfonation me $SO_3$ , Friedel-Crafts alkylation me carbocation $R^+$ , aur acylation me acylium ion $RCO^+$ .

Do important warnings exam ke liye: (1) Friedel-Crafts alkylation problematic hai — $R$ group ring ko aur reactive bana deta hai (polyalkylation) aur carbocation rearrange ho jaata hai (1-chloropropane se isopropylbenzene). Isliye acylation zyada clean hai kyunki acylium rearrange nahi karta aur $-COR$ deactivating hone se over-substitution nahi hota. (2) Plain benzene $Br_2$ ke saath react nahi karta — Lewis acid catalyst zaroori hai. In points ko yaad rakho, exam me yahi ghuma ke poochte hain.

Electrophilic aromatic substitution (EAS) — nitration, halogenation, sulfonation, Friedel-Crafts alkylation - acylation;

WHAT is EAS?

HOW: the universal 2-step mechanism

The arenium ion (heart of EAS)

Reaction-by-reaction: how each $E^+$ is made

Worked Example 1 — Nitration of benzene

Worked Example 2 — Friedel–Crafts acylation (making acetophenone)

Steel-man your mistakes

Flashcards

Connections

Concept Map

Hinglish (regional understanding)

Go deeper — visual, from zero

Test yourself — Hydrocarbons

Connections