Noesis
4.2.4Hydrocarbons

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

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1. Preparation of Alkenes (β-elimination)

1a. Dehydration of alcohols (–H and –OH removed)

WHAT: Alcohol →\to alkene + water, using a strong acid catalyst (conc. H2SO4\text{H}_2\text{SO}_4 or H3PO4\text{H}_3\text{PO}_4, heat).

WHY a catalyst? OH−\text{OH}^- is a terrible leaving group (strong base). The acid protonates −OH-\text{OH} into −O+H2-\overset{+}{\text{O}}\text{H}_2, turning it into excellent leaving group water.

HOW — derive the E1 mechanism from scratch:

  1. Protonation: R-OH+H+→R-O+H2\text{R-OH} + \text{H}^+ \rightarrow \text{R-}\overset{+}{\text{O}}\text{H}_2 (Why? make a good leaving group.)
  2. Loss of water → carbocation: R-O+H2→R++H2O\text{R-}\overset{+}{\text{O}}\text{H}_2 \rightarrow \text{R}^+ + \text{H}_2\text{O} (Why? slowest step; the C now wants to share electrons.)
  3. Loss of β-H: a neighbouring C–H bond's electrons swing in to form the π bond, releasing H+\text{H}^+ (regenerating catalyst). (Why? this neutralises the + charge and forms the stable double bond.)

1b. Dehydrohalogenation of haloalkanes (–H and –X removed)

WHAT: Alkyl halide + alcoholic KOH →\to alkene + KX + water.

WHY alcoholic KOH (not aqueous)? In alcohol, OH−/OR−\text{OH}^-/\text{OR}^- acts as a base (grabs β-H → elimination). In water it acts as a nucleophile (attacks C → substitution gives alcohol). Same reagent, different solvent, different job!

1c. Zaitsev's (Saytzeff) Rule

WHY does substitution = stability? More alkyl groups → more hyperconjugation (more α C–H bonds overlapping with the π system) and more +I electron donation, lowering the alkene's energy.

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

2. Addition Reactions of Alkenes

2a. Addition of H₂ (hydrogenation)

CH2=CH2+H2→Ni/Pt/PdCH3-CH3\text{CH}_2{=}\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni/Pt/Pd}} \text{CH}_3\text{-CH}_3 Catalyst adsorbs H₂ on its surface; adds across the π bond (syn). Converts unsaturated → saturated (basis of making vegetable ghee).

2b. Addition of halogen (X₂)

CH2=CH2+Br2→BrCH2-CH2Br\text{CH}_2{=}\text{CH}_2 + \text{Br}_2 \rightarrow \text{BrCH}_2\text{-CH}_2\text{Br} Test for unsaturation: reddish-brown Br2\text{Br}_2 water decolourises → alkene present. Why? π electrons attack Br₂ forming a cyclic bromonium ion, then Br⁻ opens it (anti addition).

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

WHY (derive it): Mechanism is electrophilic, via the carbocation.

  1. H⁺ adds to one alkene carbon → carbocation on the other.
  2. Markovnikov product comes from the more stable carbocation (3° > 2° > 1°).

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

2e. Addition of water (acid-catalysed hydration)

CH3CH=CH2+H2O→H+CH3CH(OH)CH3\text{CH}_3\text{CH=CH}_2 + \text{H}_2\text{O}\xrightarrow{\text{H}^+}\text{CH}_3\text{CH(OH)CH}_3 Markovnikov addition of H–OH → OH on more substituted C → 2° alcohol.

Recall Feynman: explain to a 12-year-old

Imagine two kids (carbons) holding hands twice — that double hand-hold is the double bond, springy and ready to grab. Making it: take a kid who's holding a heavy backpack (–OH or –Br) and a friend holding a snack (an H) next to him; pull off the backpack and the snack, and the two kids grab hands a second time → a double bond forms. We always pull from neighbours (β). Reacting it: that springy double hand-hold is greedy. When a new toy (HBr) floats by, the H jumps onto the kid who already has the most hands free, leaving Br for the busier kid — because that arrangement is the comfiest (most stable). That comfort rule is Markovnikov. Zaitsev: when there's a choice of which neighbour's snack to pull, we pull the one that makes the double bond have the most friends (alkyl groups) around it — friends make it cozy and stable.

Flashcards

What does β-elimination remove and from where?
Two groups (H and a leaving group X/OH) from adjacent carbons, forming a C=C bond.
Why is conc. H₂SO₄ needed to dehydrate alcohols?
It protonates –OH into –OH₂⁺, converting a bad leaving group into water (a good one).
Order of ease of dehydration of alcohols?
3° > 2° > 1° (follows carbocation stability).
Why alcoholic KOH and not aqueous KOH for making alkenes?
Alcoholic → OH⁻ acts as a base → elimination (alkene). Aqueous → nucleophile → substitution (alcohol).
State Zaitsev's rule.
The major elimination product is the more highly substituted (more stable) alkene.
Why is a more substituted alkene more stable?
More alkyl groups give more hyperconjugation and +I donation, lowering its energy.
State Markovnikov's rule.
In HX addition, H goes to the carbon with more hydrogens; X goes to the more substituted carbon.
What is the mechanistic reason for Markovnikov orientation?
Addition proceeds via the more stable carbocation (3°>2°>1°).
What is the peroxide (Kharasch) effect and which reagent shows it?
Anti-Markovnikov addition of HBr only, via a free-radical mechanism (Br• adds to give the more stable radical).
How do you test an alkene with bromine water?
The reddish-brown Br₂ water gets decolourised → unsaturation present.
Markovnikov vs Zaitsev — key difference?
Markovnikov = addition (where H/X go). Zaitsev = elimination (which alkene forms). Opposite reactions.
Product of propene + HBr (no peroxide)?
2-bromopropane (Markovnikov, via 2° cation).
Product of propene + HBr (with peroxide)?
1-bromopropane (anti-Markovnikov, radical).
Why doesn't the peroxide effect work with HCl or HI?
H–Cl bond too strong and H–I too weak to sustain the radical chain.

Connections

  • Alkanes — preparation and properties (saturation via hydrogenation)
  • Alkynes — preparation and addition (similar electrophilic addition, two π bonds)
  • Haloalkanes — SN1, SN2, E1, E2 (elimination vs substitution competition)
  • Carbocations — stability and rearrangement (heart of E1 & Markovnikov)
  • Hyperconjugation and Inductive effect (why substitution = stability)
  • Aromatic hydrocarbons — electrophilic substitution (contrast: addition vs substitution)

Concept Map

made by

reacts by

remove H and OH

remove H and X

needs

makes good LG

via

stability order 3>2>1

due to

uses

multiple products

multiple products

favours

Alkene C=C pi bond

beta-elimination

Addition reactions

Dehydration of alcohols

Dehydrohalogenation

Acid catalyst H2SO4

Water leaves

E1 carbocation

Ease of reaction

Hyperconjugation and induction

Alcoholic KOH as base

Zaitsev rule

More substituted alkene

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, alkene ka matlab hai ek molecule jisme C=C double bond hota hai. Yeh double bond ke π electrons thode loosely held hote hain — isliye yeh do tarah se important hai. Banane ke liye hum elimination karte hain: padosi (adjacent) carbons se do cheezein nikaalte hain — ek H aur ek leaving group (OH ya halogen) — taaki ek naya double bond ban jaaye. Iss process ko β-elimination kehte hain.

Alcohol se alkene banao to conc. H₂SO₄ chahiye, kyunki OH⁻ bahut bekaar leaving group hai; acid use protonate karke paani (good leaving group) bana deta hai, phir carbocation banta hai. Yaad rakho: dehydration ki ease 3°>2°>1° hoti hai, kyunki zyada substituted carbocation zyada stable. Alkyl halide se alkene banane ke liye alcoholic KOH lagta hai (aqueous KOH nahi — woh alcohol de dega substitution se). Zaitsev's rule kehta hai: jab do alkene possible ho, to zyada substituted (zyada stable) wala major banega.

Reactions ki side dekho to alkene addition karta hai, kyunki π electrons electrophile ko pakad lete hain. HBr add karo to Markovnikov rule: H usko milega jiske paas pehle se zyada H hain, aur Br more substituted carbon pe — kyunki mechanism more stable carbocation se jata hai. Lekin agar peroxide ho (sirf HBr ke saath), to radical mechanism se anti-Markovnikov product banta hai.

Sabse common confusion: Zaitsev aur Markovnikov ko same mat samajhna. Zaitsev = elimination (kaunsa alkene banega), Markovnikov = addition (H aur X kahan jayenge) — bilkul opposite directions. Aur peroxide effect sirf HBr ke liye, HCl/HI ke liye nahi. Yeh chhote points hi exam me number dilate hain!

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