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Class XII · Chemistry · Chapter 4 · Interactive Lecture

Hydrocarbons

The complete lecture — hydrocarbons come alive in the live panel as you read. Scroll down; the animation keeps pace as bromine water decolourises across a C=C and benzene's electrons circulate.

1 — Classification of hydrocarbons
  • Hydrocarbons — made of carbon and hydrogen only. Aliphatic = open-chain (alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂); aromatic = benzene ring. Saturated = single bonds; unsaturated = has a C=C or C≡C.
2 — Alkanes & free-radical halogenation
Chain reaction (CH₄ + Cl₂, UV)Init: Cl₂ → 2 Cl•
Prop: Cl• + CH₄ → HCl + •CH₃ ; •CH₃ + Cl₂ → CH₃Cl + Cl•
Term: Cl• + •CH₃ → CH₃Cl

Alkanes are saturated and fairly unreactive; in sunlight a halogen substitutes an H by this three-stage radical chain.

3 — Alkenes: electrophilic addition
Addition across C=CCH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

The π electrons of the C=C attract electrophiles. With orange bromine water the colour disappears as the two Br atoms add to the carbons — the classic test for a double bond. Alkenes also add H₂ (Ni), HX and H₂O.

4 — Markovnikov's rule
Propene + HBrCH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃ (major)

The H adds to the doubly-bonded carbon that already has the most hydrogens. Reason: this forms the more stable (secondary) carbocation, so Br⁻ ends up on the middle carbon.

5 — Oxidation, ozonolysis & polymerisation
ReactionResult
cold KMnO₄diol (purple fades)
O₃ then Zn/H₂Otwo carbonyls (locates C=C)
n CH₂=CH₂–(CH₂CH₂)ₙ– polythene
6 — Alkynes
EthyneCaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂
HC≡CH + 2Br₂ → CHBr₂–CHBr₂

The H on a triple-bonded carbon is weakly acidic — terminal alkynes give a white silver-acetylide ppt with ammoniacal AgNO₃.

7 — Benzene: structure & aromaticity
  • Aromaticity — a flat ring of delocalised π electrons (benzene has 6). All C–C bonds are equal; the resonance hybrid is far more stable than a single Kekulé structure.

This delocalisation stabilises benzene, so it resists addition (which would break the ring) and prefers substitution.

8 — Benzene: electrophilic substitution
ReactionElectrophileProduct
NitrationNO₂⁺nitrobenzene
HalogenationCl⁺ (AlCl₃)chlorobenzene
SulphonationSO₃benzenesulphonic acid
Friedel–CraftsR⁺ / RCO⁺alkyl/aryl ketone
9 — Exam recap
  1. Classification: aliphatic/aromatic, saturated/unsaturated.
  2. Alkanes: free-radical halogenation, combustion.
  3. Alkenes: electrophilic addition & Markovnikov's rule.
  4. Oxidation, ozonolysis, addition polymerisation.
  5. Alkynes: acidic ≡C–H, addition.
  6. Benzene: aromaticity & electrophilic substitution.
Next: 📝 Practice · 🧪 Virtual lab
⚛ Live panelHydrocarbons
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