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Question: SNAr rexn with mechm in hinglish explaine in detail adv. level/...

SNAr rexn with mechm in hinglish explaine in detail adv. level/

Answer

SNAr (Nucleophilic Aromatic Substitution) involves nucleophilic attack on an electron-deficient aromatic ring, replacing a leaving group.

Conditions:

  1. Strong Electron-Withdrawing Groups (EWGs) at ortho or para positions to the leaving group.
  2. Good leaving group.
  3. Strong nucleophile.

Mechanism (Addition-Elimination via Meisenheimer Complex):

  1. Slow Step (Rate-determining): Nucleophile attacks the carbon bearing the leaving group, forming a resonance-stabilized anionic Meisenheimer Complex (σ-complex). EWGs stabilize this negative charge.
  2. Fast Step: Leaving group departs from the Meisenheimer complex, restoring aromaticity and forming the substituted product.

Factors Affecting Rate:

  • EWGs: Stronger and more EWGs (ortho/para) increase rate.
  • Leaving Group: F > Cl > Br > I (opposite to SN1/SN2 due to stabilization of intermediate).
  • Nucleophile: Stronger nucleophiles increase rate.
  • Solvent: Polar aprotic solvents favor SNAr.
Explanation

Solution

SNAr reaction, yaani Nucleophilic Aromatic Substitution, ek aisa reaction hai jismein ek nucleophile (नाभिकरागी) aromatic ring par attached ek leaving group (छोड़ने वाला समूह) ko replace karta hai. Ye reaction alag hai SN1 aur SN2 se, jo aliphatic compounds mein hote hain. Aromatic compounds mein direct SN1/SN2 mushkil hota hai kyunki ring ki electron density high hoti hai aur nucleophile ko repel karti hai, aur SP2 hybridised carbon par backside attack bhi possible nahi hota.

SNAr Reaction Ki Conditions (शर्तें)

SNAr reaction tabhi hota hai jab:

  1. Strong Electron-Withdrawing Groups (EWGs): Aromatic ring par strong electron-withdrawing groups (जैसे -NO₂, -CN, -CHO, -COR, -SO₃H, -COOH) present hon. Ye groups ortho ya para position par hone chahiye leaving group ke respect mein. Meta position par EWGs ka effect bohot kam hota hai. Inka kaam hai ring ki electron density ko kam karna, jisse nucleophile ka attack easy ho sake aur banne wale intermediate (मध्यवर्ती) ko stabilize karna.
  2. Good Leaving Group: Ek good leaving group (जैसे -Cl, -Br, -I, -F, -OR, -SR, -NR₃⁺) attached ho. Halogens generally ache leaving groups hote hain.
  3. Strong Nucleophile: Reaction ke liye ek strong nucleophile ki zaroorat hoti hai.
  4. Polar Protic/Aprotic Solvent: Reaction ke liye suitable polar solvent (जैसे DMSO, DMF, इथेनॉल) use kiya jata hai.

SNAr Reaction Ka Mechanism (क्रियाविधि)

SNAr reaction ka mechanism addition-elimination mechanism ke through hota hai, jismein ek highly resonance-stabilized anionic intermediate banta hai jise Meisenheimer Complex ya σ-complex kehte hain.

Mechanism do steps mein hota hai:

Step 1: Nucleophilic Attack aur Meisenheimer Complex Ka Formation (नाभिकरागी का आक्रमण और मेइसनहाइमर कॉम्प्लेक्स का बनना)

  • Ye step slow aur rate-determining step (वेग-निर्धारक पद) hota hai.
  • Nucleophile (Nu⁻) aromatic ring ke us carbon par attack karta hai jisse leaving group (L) attached hai.
  • Is attack se carbon-carbon double bond break hota hai aur negative charge adjacent carbon par shift ho jata hai.
  • Ek tetrahedral intermediate banta hai jismein carbon SP2 se SP3 hybridised ho jata hai. Is intermediate ko Meisenheimer Complex kehte hain.
  • Ye complex resonance-stabilized hota hai, jiska negative charge ortho aur para positions par delocalize hota hai. Electron-withdrawing groups (EWGs) jo ortho ya para par hote hain, is negative charge ko effectively stabilize karte hain, jisse intermediate ki stability badhti hai aur reaction fast hota hai.

Example (Para-nitrophenyl chloride par hydroxide ka attack):

\begin{center}
\includegraphics[width=0.8\textwidth]{images/SNAr_mechanism.png} % This is a placeholder, actual image would be generated.
\end{center}

Imagine a benzene ring with -Cl and -NO₂ (para to Cl). A nucleophile (OH⁻) attacks the carbon bearing -Cl. The double bond shifts, creating a negative charge on the ortho carbon, which then delocalizes to the para carbon (where -NO₂ is) and then to the other ortho carbon. The -NO₂ group helps stabilize this negative charge through resonance.

Step 2: Leaving Group Ka Elimination (छोड़ने वाले समूह का निष्कासन)

  • Ye step fast hota hai.
  • Meisenheimer complex se leaving group (L⁻) expel ho jata hai.
  • Aromaticity restore ho jati hai, aur final substituted product ban jata hai.

Overall Reaction:

\begin{center}
\text{Ar-L} + \text{Nu}^- \longrightarrow \text{Ar-Nu} + \text{L}^-
\end{center}

Factors Affecting SNAr Reaction Rate (SNAr अभिक्रिया की दर को प्रभावित करने वाले कारक)

  1. Nature of Electron-Withdrawing Groups (EWGs):

    • Jitne strong EWG honge aur jitne zyada EWG ortho ya para position par honge, utni hi reaction rate fast hogi.
    • Order of EWG effectiveness: -NO₂ > -CN > -SO₂R > -CHO > -COR > -COOR > -COOH.
    • Reason: Ye groups intermediate Meisenheimer complex ke negative charge ko effectively delocalize aur stabilize karte hain.

  2. Nature of the Leaving Group:

    • SNAr mein, leaving group ki ability nucleophilicity se directly related nahi hoti.
    • Halogens ke case mein, reactivity order generally: F > Cl > Br > I.
    • Reason: Ye order SN1/SN2 se opposite hai. Fluorine sabse acha leaving group hai SNAr mein kyunki carbon-fluorine bond strong hone ke bawajood, fluorine ki high electronegativity Meisenheimer complex mein negative charge ko stabilize karne mein help karti hai, especially in the rate-determining step, jahan C-L bond break nahi hota, balki C-Nu bond form hota hai. C-F bond ka break hona second step mein hota hai jo fast hota hai.

  3. Nature of the Nucleophile:

    • Strong nucleophiles reaction rate ko increase karte hain.
    • Nucleophilicity order generally follows: RO⁻ > RNH₂ > OH⁻ > H₂O.

  4. Solvent Effect:

    • Polar aprotic solvents (जैसे DMSO, DMF, HMPA) SNAr reactions ko favour karte hain kyunki ye nucleophile ko solvate nahi karte, jisse uski reactivity badh jaati hai.

Summary (सारांश)

SNAr reaction ek important pathway hai aromatic compounds mein substitution ke liye. Iski pehchan EWGs ki presence (especially ortho aur para positions par) aur Meisenheimer complex ka formation hai. Ye reaction organic synthesis mein naye C-C, C-N, C-O bonds banane ke liye use hota hai.