Cheat sheet · Class 11 · CBSE · Science Chemistry
Hydrocarbons cheat sheet
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CBSE Class 11 · Chemistry
Cheat sheet
Chapter 9Hydrocarbons
Markovnikov: H adds to the carbon that already has more H (anti with peroxide for HBr).
1. Alkanes (CₙH₂ₙ₊₂)
- Preparation: hydrogenation of alkenes; Wurtz reaction (2RX + 2Na → R–R); decarboxylation.
- Reactions: free-radical halogenation, combustion, isomerisation.
- Conformations of ethane: staggered (more stable) and eclipsed.
2. Alkenes (CₙH₂ₙ)
- Preparation: dehydrohalogenation (alc. KOH), dehydration of alcohols (conc. H₂SO₄).
- Addition of H₂, X₂, HX, H₂O.
- Ozonolysis gives aldehydes/ketones — locates the double bond.
- Cold dilute KMnO₄ (Baeyer's reagent) decolourises: test for unsaturation.
3. Alkynes (CₙH₂ₙ₋₂)
- Ethyne from CaC₂ + H₂O.
- Terminal alkynes are acidic (sp C–H).
- Hydration (Hg²⁺/H₂SO₄): ethyne → ethanal.
4. Aromatic Hydrocarbons
Hückel rule: planar, cyclic, conjugated with electrons ⇒ aromatic.
| Reaction (benzene) | Reagent | Product |
|---|---|---|
| Nitration | conc. HNO₃ + H₂SO₄ | nitrobenzene |
| Halogenation | Cl₂ / FeCl₃ | chlorobenzene |
| Sulphonation | fuming H₂SO₄ | benzenesulphonic acid |
| Friedel–Crafts alkylation | CH₃Cl / AlCl₃ | toluene |
| Friedel–Crafts acylation | CH₃COCl / AlCl₃ | acetophenone |
Directing groups: o/p — –OH, –NH₂, –CH₃, halogens; m — –NO₂, –CHO, –COOH, –CN.
5. Key Points
- Kolbe electrolysis forms alkanes from carboxylate salts.
- Trans-alkenes are less polar than cis.
- Benzene prefers substitution over addition.
- Polynuclear hydrocarbons can be carcinogenic.
Remember: Markovnikov | ozonolysis | Hückel 4n + 2 | o/p vs m directors
Hydrocarbons: questions and notes
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