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

4.2.9 · Chemistry › Hydrocarbons

EAS KYA HAI?

Addition kyun nahi, substitution KYUN? Addition ek non-aromatic diene/triene deta hai → high energy. Substitution aromaticity ko temporarily khoata hai (intermediate mein) lekin product mein wapas milta hai. Net result: aromatic prize bacha rehta hai.

KAISE: universal 2-step mechanism

Har EAS (nitration, halogenation, sulfonation, Friedel–Crafts) same skeleton follow karta hai. Ise ek baar seekho (80/20!).

Step 1 — Electrophile generate karo. Har reaction apna $E^{+}$ banata hai (yahi woh ek part hai jo badalta hai).

Step 2 — π electrons $E^{+}$ ko attack karte hain → arenium ion banta hai (ise σ-complex ya Wheland intermediate bhi kehte hain). Yeh slow, rate-determining step (RDS) hai kyunki aromaticity yahan toot ti hai.

Step 3 — $H^{+}$ lose karo $s p^{3}$ carbon se → rearomatize. Ek base proton remove karta hai, π system reform ho jaata hai.

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

Arenium ion (EAS ka dil)

Reaction-by-reaction: har $E^{+}$ kaise banta hai

Formula Har reaction ke liye Step 1 (electrophile generation)

Nitration — $H N O_{3} + H_{2} S O_{4}$ use karta hai: $H N O_{3} + 2 H_{2} S O_{4} \to nitronium ion N O_{2}^{+} + H_{3} O^{+} + 2 H S O_{4}^{-}$ $H_{2} S O_{4}$ , $H N O_{3}$ ko protonate karta hai, water nikalta hai → ==nitronium ion $N O_{2}^{+}$ == electrophile hai.

Halogenation (Cl, Br) — Lewis acid catalyst $F e X_{3}$ (ya $A l X_{3}$ ) chahiye: $X_{2} + F e X_{3} \to X^{+} \dots F e X_{4}^{-} (polarized X^{+})$ Lewis acid $X_{2}$ se electron density kheench leta hai, jisse ek $X$ electron-poor ( $X^{+}$ jaisa) ho jaata hai.

Sulfonation — fuming $H_{2} S O_{4}$ (oleum) / $S O_{3}$ use karta hai: $Electrophile = S O_{3} (or^{+} S O_{3} H)$ Sulfonation reversible hai — dilute acid ke saath heat karne par $- S O_{3} H$ hat jaata hai. (Blocking group ke roop mein useful!)

Friedel–Crafts Alkylation — alkyl halide + $A l C l_{3}$ : $R - C l + A l C l_{3} \to R^{+} + A l C l_{4}^{-} (carbocation)$

Friedel–Crafts Acylation — acyl halide + $A l C l_{3}$ : $R - C O - C l + A l C l_{3} \to acylium ion R - C \equiv O^{+} + A l C l_{4}^{-}$

Step 1 ke baad, paanchon same Step 2 → Step 3 mein plug ho jaate hain.

Worked Example 1 — Benzene ka Nitration

Reagents: conc. $H N O_{3}$ + conc. $H_{2} S O_{4}$ , ~50 °C → nitrobenzene.

$E^{+}$ banao: $H_{2} S O_{4}$ , $H N O_{3}$ ko protonate karta hai → water nikalta hai → $N O_{2}^{+}$ . Yeh step kyun? $N O_{2}^{+}$ neutral $H N O_{3}$ se kaafi zyada strong electrophile hai; $H_{2} S O_{4}$ proton-supplier hai jo water ko drive off karta hai.
Benzene π, $N O_{2}^{+}$ ko attack karta hai → arenium ion ( $+$ charge 3 ring C's par delocalized). Yeh step kyun? Electron-rich ring nucleophile hai; yeh RDS aromaticity todata hai.
$H S O_{4}^{-}$ proton remove karta hai → nitrobenzene + regenerated $H_{2} S O_{4}$ . Yeh step kyun? Rearomatization stable aromatic ring wapas deta hai; catalyst regenerate ho jaata hai.

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

Reagents: $C H_{3} C O C l$ + $A l C l_{3}$ → $C_{6} H_{5} C O C H_{3}$ .

$A l C l_{3}$ , acetyl chloride se $C l^{-}$ kheench leta hai → acylium ion $C H_{3} - C \equiv O^{+}$ banta hai (resonance-stabilized). Yeh step kyun? Acylium cation electrophile hai; resonance ( $R - C \equiv O^{+} \leftrightarrow R - C^{+} = O$ ) ise stable aur selective banata hai.
Ring acylium ko attack karti hai → arenium ion.
$H^{+}$ lose karo → acetophenone; $A l C l_{3}$ workup ke baad release hota hai.

Apni galtiyon ko steel-man karo

Flashcards

Saare EAS reactions kaun sa 2-step skeleton share karte hain?

(1) π,

E^{+}

ko attack karta hai → arenium ion (RDS), (2)

H^{+}

lose karo → rearomatize.

Benzene addition nahi, substitution kyun karta hai?

Substitution aromaticity restore karta hai (~150 kJ/mol resonance energy wapas milti hai); addition use destroy kar deta.

Nitration mein electrophile kya hai?

Nitronium ion

N O_{2}^{+}

N O_{2}^{+}

kaise generate hota hai?

H N O_{3} + 2 H_{2} S O_{4} \to N O_{2}^{+} + H_{3} O^{+} + 2 H S O_{4}^{-}

Benzene ki chlorination/bromination ke liye kaun sa catalyst chahiye?

Ek Lewis acid:

F e X_{3}

A l X_{3}

Friedel–Crafts acylation mein electrophile kya hai?

Acylium ion

R - C \equiv O^{+}

EAS intermediate ka naam batao.

Arenium ion (σ-complex / Wheland intermediate).

EAS mein rate-determining step kaun sa hai?

Arenium ion ka formation (aromaticity wahan toot ti hai).

Friedel–Crafts alkylation ke do drawbacks batao.

Polyalkylation (R ring ko activate karta hai) aur carbocation rearrangement.

Sulfonation special kyun hai?

Yeh reversible hai —

- S O_{3} H

ko dilute acid se heat karke remove kiya ja sakta hai (accha blocking group hai).

Acylation over-substitute kyun NAHI karta?

- C O R

group deactivating hai, toh product starting benzene se kam reactive hota hai.

Friedel–Crafts nitrobenzene par kyun fail karta hai?

- N O_{2}

group ring ko strongly deactivate karta hai (EAS ke liye bahut zyada electron-poor).

Recall Feynman: ek 12-saal ke bachche ko explain karo

Socho benzene ek khush ring of friends hai jo ek circle mein haath pakad ke khade hain — woh circle super stable hai aur unhe bahut pasand hai. Ek bully ( $E^{+}$ ) aata hai aur ek jagah chahta hai. Ek dost briefly haath chhodata hai bully ko pakadne ke liye (ring abhi unhappy hai — yeh mushkil, slow part hai). Khush circle wapas paane ke liye, ring ek chhote hydrogen ( $H^{+}$ ) ko bahar nikaalti hai aur bully ko poori tarah join karne deti hai. Ab sab haath pakad ke hain — circle restore, bully included. Woh poora circle kabhi nahi todenge (isliye koi "addition" nahi) — woh bas ek member swap karte hain.

Connections

Benzene and aromaticity (Hückel 4n+2 rule) — ring itni stable kyun hai aur addition kyun resist karti hai.
Directing effects (ortho/para vs meta directors) — doosra group kahan jaata hai; same arenium ion use karta hai.
Resonance and delocalization — arenium ion stability explain karta hai.
Carbocation stability and rearrangements — Friedel–Crafts alkylation problem explain karta hai.
Reactions of alkenes — electrophilic addition — contrast: addition vs substitution.
Lewis acids and catalysis — $A l C l_{3}$ , $F e B r_{3}$ ka role.