Hydrocarbons
Why This Chapter Matters
Hydrocarbons are the foundation of organic JEE chemistry — 6-10 marks. Alkane reactions, alkene addition mechanisms, and aromaticity are all tested.
Core Concepts
1. Alkanes (CnH2n+2) — Saturated
Reactions:
Halogenation (free radical substitution): CH₄ + Cl₂ →(hν) CH₃Cl + HCl
Combustion: CnH2n+2 + O₂ → CO₂ + H₂O (complete); CO + H₂O (incomplete)
Cracking: long chain → shorter chain (industrial process)
Reactivity of H towards substitution: 3° > 2° > 1° > CH₄ (stability of radicals formed)
2. Alkenes (CnH2n) — One C=C Double Bond
Addition reactions (electrophilic):
Hydrogenation: CH₂=CH₂ + H₂ →(Ni/Pt) CH₃-CH₃
Halogenation: CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br (adds across double bond, decolorises Br₂ water — test for unsaturation)
HX addition (Markovnikov): H adds to carbon with more H → more stable carbocation
Acid-catalysed hydration: CH₂=CH₂ + H₂O →(H⁺) CH₃CH₂OH
Anti-Markovnikov (peroxide effect): In presence of ROOR, radical mechanism. H adds to less substituted C.
Ozonolysis: C=C + O₃ then Zn/H₂O → aldehydes/ketones (used to locate double bond)
Test: Baeyer's reagent (KMnO₄/OH⁻, cold, dilute) — decolorises (gives diol)
3. Alkynes (CnH2n-2) — One C≡C Triple Bond
More reactive than alkenes (to nucleophiles due to acidity of terminal H).
Acidic character: sp C-H bond. HC≡CH + NaNH₂ → HC≡CNa + NH₃
Addition reactions similar to alkenes (two steps possible).
Hydration: HC≡CH + H₂O →(HgSO₄/H₂SO₄) CH₃CHO (Markovnikov)
4. Arenes — Benzene (C₆H₆)
Aromaticity conditions (Hückel's rule):
Planar, cyclic, conjugated, 4n+2 π electrons (n = 0,1,2...)
Benzene: 6 π electrons (n=1) ✓
Electrophilic Aromatic Substitution (EAS):
ArH + E⁺ → Ar-E + H⁺
Types: halogenation (FeCl₃ catalyst), nitration (HNO₃/H₂SO₄), sulphonation (SO₃/H₂SO₄), Friedel-Crafts alkylation and acylation (AlCl₃ catalyst)
Directing effects:
o,p-directors (activate ring): -OH, -OR, -NH₂, -NHR, -NR₂, -R (alkyl), halogens
m-directors (deactivate ring): -NO₂, -CN, -CHO, -COOH, -SO₃H, -COR
PYQs
2024: Which reaction product of propene + HBr is the major product (no peroxide)?
CH₃-CH=CH₂ + HBr →(Markovnikov) CH₃-CHBr-CH₃ (2-bromopropane, more stable 2° carbocation)
2023: Which compound decolorises Br₂ in CCl₄ but NOT KMnO₄/OH⁻?
Alkyne in presence of peroxide... Actually: alkanes don't decolorise either.
Compounds decolorising Br₂/CCl₄ (unsaturated): alkenes, alkynes, benzene (slow without catalyst).
2022: Toluene reacts with Cl₂/FeCl₃. Which positions are the main products?
CH₃ is o,p-director → chlorination occurs at ortho and para positions.