← Labs|Hydrocarbon Reactivity — Alkanes, Alkenes, Alkynes, Benzene

Alkenes (Unsaturated)

Hybridization

sp² (planar, 120°) — π bond perpendicular

Reactivity

High — π electrons available for electrophile attack

Physical

Similar to alkanes but slightly higher bp (π bond interaction)

Acidic H

Vinylic H: pKa ≈ 44. Not easily removed.

Electrophilic Addition — HX (Markovnikov)

HBr, HCl, HI

→

Alkyl halide

Via carbocationRT, no peroxide

H adds to less-substituted C (more H's) → more stable 3° carbocation on other C → X attacks. Regioselectivity: Markovnikov. Rearrangements possible.

Anti-Markovnikov (HBr/ROOR)

HBr + peroxide (ROOR)

→

Anti-Markovnikov bromide

Free radical additionROOR initiator or hν

Peroxide generates Br• radical. Br• adds to less hindered C → more stable secondary radical → H abstraction. ONLY HBr (not HCl or HI — thermodynamics).

Hydration (addition of H₂O)

H₂O + H₂SO₄ (cat.)

→

Alcohol (Markovnikov)

Via carbocationH₂SO₄, heat

Markovnikov addition of water. OH goes to more substituted C. Alternative: oxymercuration-demercuration (Markovnikov, no rearrangement) or hydroboration-oxidation (anti-Markovnikov, syn addition).

Halogenation (X₂)

Cl₂ or Br₂ (in CCl₄)

→

Vicinal dihalide

Via bromonium ionDark, CCl₄ solvent (not water)

Cyclic bromonium ion intermediate → anti addition of X⁻. Br₂/CCl₄ is TEST for unsaturation (decolourises instantly). Anti-addition = trans product (stereospecific).

Ozonolysis

O₃, then Zn/H₂O or H₂O₂

→

Aldehydes/ketones (reductive) or acids (oxidative)

[3+2] cycloaddition−78°C (O₃), then workup

Reductive (Zn/H₂O): RCHO + R'CHO. Oxidative (H₂O₂): RCOOH. Used to LOCATE double bond in unknown structure. =CH₂ → HCHO (formaldehyde).

JEE Key Facts

• Markovnikov addition: H to more-H carbon. Carbocation stability drives regioselectivity.

• Anti addition: Br₂, Cl₂, peracids (epoxidation). Bromonium ion → trans-dihalide.

• Syn addition: H₂/Pd (hydrogenation), BH₃ (hydroboration). Both groups add to same face.

• Hydroboration-oxidation: anti-Markovnikov, syn-addition, no rearrangement (BH₃·THF, then H₂O₂/NaOH)

• Diels-Alder: conjugated diene + dienophile → cyclohexene (syn, stereospecific, [4+2])