4.1.12
Ek reaction mein bas electrons move karte hain. Atoms toh peeche-peeche khiñchte hain, lekin bonds kya tootenge aur kya banenge — yeh electrons decide karte hain. Curved arrows organic chemistry ki grammar hain — inse tum har electron ko track kar sakte ho, shuru se aakhir tak. Agar tum arrows sahi se push kar sako, toh aise products bhi predict kar sakte ho jo tune kabhi memorize nahi kiye.
Definition Reaction mechanismMechanism ek step-by-step account hai — kaunse bonds toote , kaunse bonds bane , aur electrons kaise move kiye reactants se products tak pahunchne ke liye.
Definition Curved-arrow notationEk curved arrow electrons ki movement dikhata hai, atoms ki nahi .
Ek double-barbed arrow (↷, full arrowhead) = ek electron pair ki movement (2 electrons).
Ek single-barbed / fishhook arrow (⇀, half arrowhead) = ek single electron ki movement (1 electron).
Arrow start hota hai electron source se (ek bond ya lone pair) aur point karta hai wahan jo jagah electrons jaate hain.
Ek covalent bond ek shared pair hota hai electrons ka (·:·). Jab bond toot ta hai, woh 2 electrons kahin na kahin jaate hain . Sirf do possibilities hain:
Definition Heterolysis (heterolytic cleavage)Bond unevenly toot ta hai — dono electrons EK atom ko jaate hain .
A : B ⟶ A + + : B − Double-barbed arrow use hota hai. Ions banate hain (ek cation aur ek anion). Jis atom ko electrons milte hain woh negative ho jaata hai; jo khota hai woh positive.
Definition Homolysis (homolytic cleavage)Bond evenly toot ta hai — har atom ek EK electron rakhta hai .
A : B ⟶ A ∙ + B ∙ Do fishhook arrows use hote hain. Free radicals banate hain (neutral species jisme ek unpaired electron hota hai).
Intuition KYO kaun sa hoga?
Heterolysis tab favored hota hai jab dono atoms ki electronegativity mein bahut fark ho (ek polar bond) aur/ya solvent ions ko stabilize kare (polar/protic). Zyada electronegative atom khushi se dono electrons le leta hai.Homolysis tab favored hota hai jab bond non-polar ho (similar electronegativity) aur/ya high energy input ho bina kisi ion-stabilizing help ke — jaise UV light ya gas phase mein heat. Evenly split karna charge separation se bachata hai jise koi stabilize nahi kar sakta.
Ek arrow hai hi ek moving electron pair. Tum electrons empty jagah se nahi move kar sakte — wahan kuch hoga hi nahi move karne ko. Isliye tail kisi aisi jagah honi chahiye jahan actually electrons ho.
Ek naya bond tab banta hai jab ek lone pair (ya π/σ bond) kisi electrophilic atom par attack karta hai. Arrowhead dono atoms ke beech mein land karta hai = naya shared pair = naya bond. Bond breaking aur bond forming ek hi act hai electron motion ka — arrow ke dono siron se dekha jaaye toh.
Worked example 1 — C–Br bond ki Heterolysis (carbocation banana)(CH 3 ) 3 C–Br ⟶ (CH 3 ) 3 C + + : Br − Arrow: tail C–Br bond par, head Br par.
Yeh step kyun? Br zyada electronegative hai, isliye woh dono electrons kheenchta hai. Arrow ka head Br par land karna use : B r − − 1 + 1 carbocation ).Charge check: reactant neutral (0) → products ( + 1 ) + ( − 1 ) = 0
Worked example 2 — Bond formation: nucleophile carbocation par attack karta hai(CH 3 ) 3 C + + : O − H ⟶ (CH 3 ) 3 C–OH Arrow: tail O H − C +
Yeh step kyun? Carbocation electron-poor hai (+ hai hi naya C–O bond.Charge check: ( + 1 ) + ( − 1 ) = 0
Worked example 3 — UV light se Cl–Cl ki HomolysisCl–Cl h ν Cl ∙ + Cl ∙ Arrows: do fishhooks, bond ka ek-ek electron har Cl ko.
Yeh step kyun? Cl–Cl non-polar hai (same atom, same electronegativity) — koi bhi side dono electrons nahi chahti, isliye evenly split hote hain. UV energy provide karta hai; gas phase mein charge stabilize karne ko kuch nahi hai isliye ions nahi bante.Result: do neutral chlorine radicals , har ek mein 7 electrons aur ek unpaired electron.
Common mistake Arrow electrons ki jagah atom se draw karnaKyun sahi lagta hai: hum atoms ke move hone ke terms mein sochte hain ("Cl leaves"), isliye arrow Cl atom se draw karte hain.
Fix: tail kisi bond ya lone pair par honi chahiye. "Cl leaves" ko model karte hain bond ke electrons Cl par jaate hue: tail C–Cl bond par, head Cl par.
Common mistake Radical reactions ke liye full arrow use karnaKyun sahi lagta hai: double-barbed arrow hi woh hai jo tum sabse zyada use karte ho.
Fix: radicals single electrons move karte hain → fishhook (half) arrows use karo, aur homolysis ke liye do chahiye.
Common mistake Bhool jaana ki net charge conserved hoti haiKyun sahi lagta hai: tum product skeleton banane par focus karte ho aur charges ignore karte ho.
Fix: dono sides ke charges sum karo. Agar match nahi karta, toh koi arrow galat hai ya missing hai.
Common mistake "Heterolysis hamesha carbocation banata hai"Kyun sahi lagta hai: pehli heterolysis jo tum seekhte ho woh C–Br → C⁺ hai.
Fix: kaun sa atom pair rakhega yeh electronegativity/stability par depend karta hai. C–MgBr heterolyze karke carbanion (C −
Recall Feynman: ek 12-saal ke bachche ko samjhaoDo bacche do candies share karte hain (yahi ek chemical bond hai). Jab woh alag hote hain toh do tarike hain:
Selfish split (heterolysis): ek bachcha dono candies le leta hai aur ameer (negative) hokar chala jaata hai, doosra khali haath (positive) reh jaata hai. Yeh tab hota hai jab ek bachcha lalchi ho (zyada electronegative).
Even split (homolysis): har bachcha ek candy leta hai — bilkul fair. Yeh tab hota hai jab bacche equal hon aur koi (Sun = UV light) unhe alag dhakele.
Curved arrow bas ek arrow hai jo dikhata hai candies kahan jaati hain . Full arrow = dono candies saath move karti hain; fishhook = ek candy move karti hai.
Mnemonic Splits yaad karo"HOMO = HOMOgeneous = same/even → radicals (har ek ko EK milta hai)."
"HETERO = different → ions (ek lalchi atom DONO le leta hai)."
Arrowheads: Full arrow = Full pair. Half (fishhook) arrow = pair ka Half (ek electron).
Kaun sa arrow ek single electron ki movement dikhata hai?
Heterolysis kaunsi species produce karta hai? Homolysis?
Molecule ke kaunse feature se arrow ka tail shuru hota hai?
Cl–Cl heterolysis ke bajaye homolysis kyun prefer karta hai?
Ek curved arrow kya represent karta hai (atoms nahi)? Electrons (full arrow ke liye ek electron pair, fishhook ke liye ek electron).
Ek double-barbed (full) arrow kitne electrons ki movement dikhata hai? Do (ek electron pair).
Ek single-barbed fishhook arrow kitne electrons ki movement dikhata hai? Ek electron.
Heterolysis define karo. Uneven bond cleavage jisme dono bonding electrons ek atom ko jaate hain, ek cation aur ek anion (ions) produce karte hue.
Homolysis define karo. Even bond cleavage jisme har atom ek electron rakhta hai, do neutral free radicals produce karte hue.
Heterolysis kaunsi species produce karta hai? Ions (ek cation + ek anion).
Homolysis kaunsi species produce karta hai? Free radicals (species jisme ek unpaired electron hota hai).
Curved arrow ka tail kahan se start hona chahiye? Electrons ke source par — ek lone pair ya ek bond.
Curved arrow ka head kahan point karta hai? Ek electron-poor site ki taraf (positive charge, δ⁺, ya ek atom jo electrons accept kar sake).
Homolysis kaunse conditions mein favored hota hai? Non-polar bonds, gas phase, high energy input jaise UV light ya heat bina ion stabilization ke.
Heterolysis kaunse conditions mein favored hota hai? Polar bonds (bada electronegativity difference) aur ion-stabilizing polar/protic solvents.
Ek mechanism mein hamesha kya conserved rehta hai? Total charge (aur total electrons ki sankhya).
C–Br heterolysis mein kaun sa atom negative banta hai aur kyun? Br, kyunki woh zyada electronegative hai aur dono bonding electrons rakhta hai → :Br⁻.
Homolysis ke liye kitne fishhook arrows chahiye? Do (bond ke har ek electron ke liye ek).
Kya heterolysis hamesha carbocation deta hai? Nahi — electrons rakhne wala atom electronegativity par depend karta hai; jaise C–Mg carbanion deta hai.
Carbocations — stability and structure Nucleophiles and Electrophiles Inductive effect and electronegativity Free radical substitution (halogenation of alkanes) Electrophilic addition to alkenes Bond dissociation energy Resonance and arrow pushing
Bond breaking and forming
Polar bond and polar solvent
Non-polar bond and UV or heat
Start at electrons, point to electron-poor, conserve charge
