Alkenes — preparation (dehydration, dehydrohalogenation, Zaitsev's rule), addition reactions

4.2.4 · Chemistry › Hydrocarbons

1. Alkenes ki Preparation (β-elimination)

1a. Alcohols ki Dehydration (–H aur –OH hatate hain)

KYA: Alcohol $\to$ alkene + water, ek strong acid catalyst ke saath (conc. $H_{2} SO_{4}$ ya $H_{3} PO_{4}$ , heat).

Catalyst kyun? $OH^{-}$ ek bahut bekar leaving group hai (strong base). Acid $- OH$ ko protonate karke $- O + H_{2}$ bana deta hai, jisse wo ek zabardast leaving group — water — ban jaata hai.

KAISE — E1 mechanism ko scratch se samjho:

Protonation: $R-OH + H^{+} \to R- O + H_{2}$ (Kyun? ek accha leaving group banane ke liye.)
Water ka jaana → carbocation: $R- O + H_{2} \to R^{+} + H_{2} O$ (Kyun? sabse slow step; C ab electrons share karna chahta hai.)
β-H ka jaana: ek paas ke C–H bond ke electrons swing karke π bond banate hain, aur $H^{+}$ release hota hai (catalyst regenerate hota hai). (Kyun? yeh + charge neutralize karta hai aur stable double bond banaata hai.)

1b. Haloalkanes ki Dehydrohalogenation (–H aur –X hatate hain)

KYA: Alkyl halide + alcoholic KOH $\to$ alkene + KX + water.

Alcoholic KOH kyun (aqueous kyun nahi)? Alcohol mein, $OH^{-} / OR^{-}$ base ki tarah kaam karta hai (β-H pakadta hai → elimination). Paani mein yeh nucleophile ki tarah kaam karta hai (C par attack karta hai → substitution se alcohol banta hai). Ek hi reagent, alag solvent, alag kaam!

1c. Zaitsev's (Saytzeff) Rule

Substitution = stability kyun? Zyada alkyl groups → zyada hyperconjugation (zyada α C–H bonds π system ke saath overlap karte hain) aur zyada +I electron donation, jo alkene ki energy kam karta hai.

Figure — Alkenes — preparation (dehydration, dehydrohalogenation, Zaitsev's rule), addition reactions

2. Alkenes ki Addition Reactions

2a. H₂ ka Addition (hydrogenation)

$CH_{2} = CH_{2} + H_{2} Ni/Pt/Pd CH_{3} -CH_{3}$ Catalyst apni surface par H₂ ko adsorb karta hai; π bond ke across add karta hai (syn). Unsaturated → saturated mein convert karta hai (vanaspati ghee banane ka basis).

2b. Halogen ka Addition (X₂)

$CH_{2} = CH_{2} + Br_{2} \to BrCH_{2} -CH_{2} Br$ Unsaturation ka test: lal-bhoora $Br_{2}$ water decolourised ho jaata hai → alkene present hai. Kyun? π electrons Br₂ par attack karte hain aur ek cyclic bromonium ion banate hain, phir Br⁻ use kholti hai (anti addition).

2c. HX ka Addition (HCl, HBr, HI) — Markovnikov's Rule

Kyun (derive karo): Mechanism electrophilic hai, carbocation ke through.

H⁺ ek alkene carbon par add hota hai → doosre par carbocation banta hai.
Markovnikov product zyada stable carbocation (3° > 2° > 1°) se aata hai.

2d. Anti-Markovnikov (Peroxide / Kharasch effect)

2e. Water ka Addition (acid-catalysed hydration)

$CH_{3} CH=CH_{2} + H_{2} O H^{+} CH_{3} CH(OH)CH_{3}$ H–OH ka Markovnikov addition → OH zyada substituted C par → 2° alcohol.

Common mistake Common errors ko acche se samjho

Galti 1: "Aqueous KOH bhi alkene deta hai." Kyun sahi lagta hai: KOH toh KOH hai, hai na? Fix: Solvent role decide karta hai. Aqueous → OH⁻ nucleophile ki tarah → substitution (alcohol). Alcoholic → base ki tarah → elimination (alkene).

Galti 2: "Markovnikov aur Zaitsev ek hi rule hain." Kyun sahi lagta hai: dono "major" product chunte hain aur dono substitution pasand karte hain. Fix: Zaitsev = elimination (kaunsi alkene banti hai). Markovnikov = addition (H/X kahan jaate hain). Reaction ki ulti directions!

Galti 3: "Peroxide HCl aur HI ko bhi reverse karta hai." Kyun sahi lagta hai: general lagta hai. Fix: Peroxide (anti-Markovnikov) sirf HBr ke liye kaam karta hai. Bond energetics HCl aur HI ke liye radical chain fail kara deti hai.

Galti 4: "Carbon par zyada H matlab zyada stable carbocation." Kyun sahi lagta hai: plausible lagta hai. Fix: Yeh ulta hai — zyada alkyl groups (kam H) = zyada stable cation, hyperconjugation/+I ki wajah se.

Recall Feynman: ek 12 saal ke bachhe ko samjhao

Socho do bachche (carbons) do baar haath pakde hue hain — yeh double haath-pakad hi double bond hai, springy aur kuch pakadne ko taiyar. Banana: ek bachcha hai jo bhaari bag (–OH ya –Br) utha ke chal raha hai aur uske saath wala dost snack (ek H) pakde hua hai; bag aur snack dono kheeench lo, aur do bachche dobaara haath pakad lete hain → ek double bond banta hai. Hum hamesha padosiyon se (β) kheenchte hain. React karna: woh springy double haath-pakad lalchi hai. Jab ek naya khilona (HBr) paas aata hai, H us bachche par kood jaata hai jiske paas pehle se sabse zyada haath khali hain, Br busy bachche ke liye rehta hai — kyunki woh arrangement sabse comfy hai (zyada stable). Yeh comfort rule hai Markovnikov. Zaitsev: jab choice hoti hai ki kis padosi ka snack kheenchein, hum woh kheenchte hain jo double bond ke saath sabse zyada dost (alkyl groups) banata hai — dost usse cozy aur stable banate hain.

Flashcards

β-elimination kya hataata hai aur kahan se?

Do groups (H aur ek leaving group X/OH) adjacent carbons se, jisse C=C bond banta hai.

Alcohols dehydrate karne ke liye conc. H₂SO₄ kyun chahiye?

Yeh –OH ko –OH₂⁺ mein protonate karta hai, ek bekar leaving group ko water (ek accha leaving group) mein convert karta hai.

Alcohols ki dehydration ki aasaani ka order?

3° > 2° > 1° (carbocation stability follow karta hai).

Alkenes banane ke liye alcoholic KOH kyun, aqueous kyun nahi?

Alcoholic → OH⁻ base ki tarah → elimination (alkene). Aqueous → nucleophile → substitution (alcohol).

Zaitsev's rule batao.

Major elimination product zyada highly substituted (zyada stable) alkene hoti hai.

Zyada substituted alkene zyada stable kyun hoti hai?

Zyada alkyl groups zyada hyperconjugation aur +I donation dete hain, uski energy kam karte hain.

Markovnikov's rule batao.

HX addition mein, H us carbon par jaata hai jis par zyada hydrogens hain; X zyada substituted carbon par jaata hai.

Markovnikov orientation ki mechanistic wajah kya hai?

Addition zyada stable carbocation (3°>2°>1°) ke through hoti hai.

Peroxide (Kharasch) effect kya hai aur kaunsa reagent yeh dikhata hai?

Sirf HBr ka anti-Markovnikov addition, free-radical mechanism ke through (Br• add hokar zyada stable radical deta hai).

Bromine water se alkene ka test kaise karte hain?

Lal-bhoora Br₂ water decolourised ho jaata hai → unsaturation present hai.

Markovnikov vs Zaitsev — main fark?

Markovnikov = addition (H/X kahan jaate hain). Zaitsev = elimination (kaunsi alkene banti hai). Ulte reactions.

Propene + HBr (bina peroxide) ka product?

2-bromopropane (Markovnikov, 2° cation ke through).

Propene + HBr (peroxide ke saath) ka product?

1-bromopropane (anti-Markovnikov, radical).

Peroxide effect HCl ya HI ke saath kyun kaam nahi karta?

H–Cl bond bahut strong hai aur H–I bahut weak, radical chain sustain nahi hoti.

Connections

Alkanes — preparation and properties (hydrogenation se saturation)
Alkynes — preparation and addition (similar electrophilic addition, do π bonds)
Haloalkanes — SN1, SN2, E1, E2 (elimination vs substitution competition)
Carbocations — stability and rearrangement (E1 aur Markovnikov ka core)
Hyperconjugation and Inductive effect (substitution = stability kyun)
Aromatic hydrocarbons — electrophilic substitution (contrast: addition vs substitution)