4.3.1
Intuition Poori kahani ek saansh meinEk alkyl halide hota hai R − X X electronegative hai, C–X bond polar hota hai: carbon δ + X δ − δ +
Ek nucleophile carbon par attack karta hai → substitution (Nu, X ki jagah le leta hai).
Ek base β-hydrogen ko pakad leta hai → elimination (C=C banta hai, X bahar nikal jaata hai).
Dono ke do-do flavours hain — ek one-step version (bimolecular: SN2 / E2) aur ek two-step version jo carbocation se guzarta hai (unimolecular: SN1 / E1). Neeche jo bhi hai woh sirf yeh batata hai ki kaun jeetataa hai, aur kyun .
Alcohols se (sabse common):
R − O H + H X → R − X + H 2 O H I > H B r > H C l SOCl 2 R C l + S O 2 + H C l P X 3 P X 5 Alkenes se : Markovnikov addition of H X H B r anti-Markovnikov .Alkanes se : free-radical halogenation R − H + X 2 h ν R − X C l 2 Halide exchange (Finkelstein) : R − C l + N a I acetone R − I + N a C l ↓ N a C l
SOCl 2 H C l SOCl 2 O H − O S O C l S O 2 H C l gas ke roop mein nikal jaate hain, isliye reaction irreversible ho jaati hai aur product ko purify karna aasaan hota hai.
Intuition Do cheezein sab decide karti hain(a) Leaving group ability: ek accha leaving group ek weak base hota hai (apne electrons ke saath akele stable rehta hai). Toh I − > B r − > C l − ≫ F − (b) Carbocation stability: 3 ∘ > 2 ∘ > 1 ∘ > methyl + carbocation banati hain (SN1, E1) unhe 3 ∘ carbon tak pahunchna hota hai (SN2) unhe unhindered 1 ∘
Ek concerted step: nucleophile leaving group ke bilkul opposite side se carbon par attack karta hai; bond banana aur bond todna dono saath-saath hote hain ek 5-coordinate transition state ke zariye.
Intuition Stereochemistry — Walden inversionKyunki Nu X ke opposite se aata hai, baaki teen bonds ek aandhi mein umbrella ki tarah palat jaate hain. Ek single stereocentre invert ho jaata hai (R → S
Substrate reactivity ka order: methyl > 1 ∘ > 2 ∘ ≫ 3 ∘ steric hindrance ), isliye 3 ∘
Yeh favoured hota hai: strong/charged nucleophile, polar aprotic solvent (DMSO, acetone — nucleophile ko "naked" aur reactive chhod deta hai), accha leaving group.
Do steps: (1) R − X → R + + X − (2) R + + Nu → R − Nu
Intuition Stereochemistry — racemisationCarbocation flat (sp2 hota hai. Nu dono taraf se equally attack kar sakta hai → inversion + retention ka mixture → racemic (aksar thodi zyada inversion hoti hai kyunki leaving group thodi der ke liye ek face ko shield karta hai).
Reactivity ka order: 3 ∘ > 2 ∘ > 1 ∘ Yeh favoured hota hai: polar protic solvent (paani, alcohols — ions ko solvate aur stabilise karte hain), weak/neutral nucleophile, accha leaving group.
E2: ek step. Strong base ek β-H hataata hai jab X leave karta hai; rate = k [ R − X ] [ base ] Anti-periplanar geometry chahiye (H aur X 18 0 ∘ E1: do steps, SN1 jaisa hi carbocation, phir base R + = k [ R − X ]
Intuition E2 ke liye anti-periplanar kyun?Naye π parallel hona chahiye taaki unke electrons developing π anti (18 0 ∘ wajah hai ki kuch cyclohexyl halides dheere eliminate karte hain — H ko axial hona chahiye.
Factor
SN2
SN1
E2
E1
Substrate
1 ∘ 3 ∘ 2 ∘ , 3 ∘ 3 ∘
Reagent
strong Nu, weak base
weak/neutral
strong bulky base
weak base
Rate law
k [ R X ] [ N u ] k [ R X ] k [ R X ] [ B ] k [ R X ]
Solvent
polar aprotic
polar protic
—
polar protic
Stereo
inversion
racemisation
anti-periplanar
mixture
Worked example Worked decision 1 —CH 3 CH 2 Br + NaCN
Step: substrate identify karo = 1 ∘ Kyun? 1 ∘ Step: C N − Kyun? elimination ke mukable substitution ko favour karta hai, aur SN1 ke mukable SN2 ko.Answer: clean SN2 → CH 3 CH 2 CN = k [ R B r ] [ C N − ]
Worked example Worked decision 3 — 2-bromobutane +t
Step: 2 ∘ strong bulky base + heat. Kyun? heat aur strong base elimination ko push karte hain; bulk Hofmann ko push karta hai.Step: E2; bulky base sabse accessible β-H ko pakadta hai.Answer: major = 1-butene (Hofmann) , minor = 2-butene. Chote O E t −
Common mistake "Zyada nucleophile → faster, toh SN1 bhi speed up ho jaayega."Kyun sahi lagta hai: zyada attacking species ka matlab usually faster hota hai. Fix: SN1 ka slow step ionisation hai, jisme nucleophile hota hi nahi, isliye [ Nu ] appear nahi karta . Sirf SN2/E2 reagent concentration se accelerate hote hain.
Common mistake "SN1 bhi SN2 jaisi full inversion deta hai."Kyun sahi lagta hai: dono X ko replace karte hain. Fix: SN1 ek flat carbocation se guzarta hai jis par dono taraf se attack hota hai → racemisation ; sirf SN2 ka backside attack clean inversion deta hai.
Common mistake "Zyada stable (Zaitsev) alkene HAMESHA major hoti hai."Kyun sahi lagta hai: thermodynamics stable product ko favour karta hai. Fix: bulky base ke saath product stability se zyada transition state matter karta hai — base crowded β-H tak nahi pahunch sakta, isliye Hofmann (less substituted) alkene jeet jaati hai.
3 ∘ Kyun sahi lagta hai: 3 ∘ Fix: yeh SN1/E1 mein help karta hai, SN2 mein nahi. SN2 ko backside access chahiye, jo 3 ∘ 3 ∘
Recall Feynman: ek 12-saal ke bachche ko samjhaaoSocho ek bachcha (carbon) ek bhaari backpack (halogen) pakde hua hai. SN2 game mein, ek naya dost peeche se dodata hua aata hai aur ek smooth motion mein backpack dhakka de ke gira deta hai — bachcha ghoom jaata hai (inversion). SN1 game mein, bachcha pehle khud backpack girata hai (slow), wahan flat-footed khada rehta hai, aur phir ek naya dost uske kisi bhi haath ko pakad sakta hai. Agar backpack lene ki jagah dost bachche ka belt loop (ek nearby hydrogen) kheenchta hai, toh bachche ki shirt ek naye shape mein phat jaati hai — yahi elimination hai, double bond banta hai. Ek gentle chota dost aasaan belt loop pakadta hai; ek bada clumssy dost sirf baahri wala pakad sakta hai (Hofmann).
Mnemonic Chaaron yaad rakho"2 for 2, 1 for 1" — SN2 /E2 ke rate laws 2nd order ke hain (ek partner chahiye, ek step); SN1 /E1 1st order ke hain (ek akela substrate ionise hota hai). Aur "Strong Bulky Base = Bye, Hofmann" (bulky base → Hofmann). "Aprotic for SN2, Protic for SN1."
SN1 first order kyun hai jabki do molecules product banate hain?
Kaun sa stereochemical outcome prove karta hai ki reaction SN2 se gayi?
Hofmann Zaitsev ko kab haraata hai?
What is the rate law for SN2 and why? rate = k [ R X ] [ N u ]
What is the rate law for SN1 and why? rate = k [ R X ] [ N u ]
Stereochemistry of SN2? Backside attack → Walden inversion of configuration.
Stereochemistry of SN1? Planar carbocation dono faces se attack hota hai → racemisation .
Substrate order for SN2? methyl > 1° > 2° ≫ 3° (steric hindrance 3° ko block karta hai).
Substrate order for SN1? 3° > 2° > 1° (carbocation stability).
Solvent preference: SN1 vs SN2? SN1 polar protic (ions ko stabilise karta hai); SN2 polar aprotic (nucleophile ko free karta hai).
Geometry required for E2? H aur leaving group anti-periplanar (180° dihedral) orbital overlap ke liye.
Zaitsev rule? Major product more substituted (zyada stable) alkene hoti hai.
When is the Hofmann (less substituted) alkene major? Bulky bases (jaise t-BuO⁻) ke saath ya bulky leaving groups ke saath — base sirf less hindered β-H tak pahunch sakta hai.
Why is SOCl₂ preferred over HCl for R-OH → R-Cl? By-products SO₂ aur HCl gas ke roop mein nikal jaate hain → irreversible, clean product.
Finkelstein reaction? R-Cl + NaI in acetone → R-I + NaCl↓ (NaCl precipitation se drive hota hai).
What conditions push elimination over substitution? Strong/bulky base, high temperature, hindered substrate.
Leaving group ability order among halides? I⁻ > Br⁻ > Cl⁻ ≫ F⁻ (weaker base = better leaving group).
Carbocations — stability, hyperconjugation, rearrangements Nucleophilicity vs Basicity Markovnikov & anti-Markovnikov addition Alcohols — preparation and reactions Stereochemistry — R/S, optical activity, racemisation Reaction kinetics — molecularity vs order
from alcohols, alkenes, alkanes
irreversible, gas by-products
delta+ carbon electrophilic
Good leaving group is weak base
Carbocation stability 3>2>1
