Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Quick Summary
Hydrocarbons contain only carbon and hydrogen. They are the simplest organic compounds and the foundation for all other organic chemistry. The three main families — alkanes (only C−C), alkenes (C=C), and alkynes (C≡C) — have distinct reactions. Benzene introduces aromatic chemistry. The reactions and mechanisms here appear heavily in NEET.
Classification of Hydrocarbons
Alkanes (CₙH₂ₙ₊₂)
| Hybridisation: sp³ | Bond angle: 109.5° |
|---|
Reactions of Alkanes
Halogenation (Free Radical Substitution — FRS): CH₄ + Cl₂ → CH₃Cl + HCl (in presence of light/heat)
Mechanism steps: 1. Initiation: Cl₂ → 2Cl• (light) 2. Propagation: Cl• + CH₄ → HCl + CH₃•; CH₃• + Cl₂ → CH₃Cl + Cl• 3. Termination: Cl• + Cl• → Cl₂; CH₃• + Cl• → CH₃Cl
Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O (complete)
Alkenes (CₙH₂ₙ)
| Hybridisation: sp² | Bond angle: 120° |
|---|
Reactions of Alkenes
| Reaction | Reagent | Product |
|---|---|---|
| Hydrogenation | H₂ / Ni, Pt, Pd | Alkane |
| Halogenation | Cl₂ / Br₂ (CCl₄) | Vicinal dihalide; Br₂ decolourisation is test for alkene |
| Hydrohalogenation | HX | Alkyl halide (Markovnikov's rule) |
| Hydration | H₂O / H₂SO₄ | Alcohol (Markovnikov) |
| Ozonolysis | O₃, then Zn/H₂O | Aldehydes/Ketones |
Markovnikov's Rule: H adds to carbon with more H atoms; X goes to more substituted carbon.
Anti-Markovnikov (Peroxide effect): In presence of peroxide, HBr adds anti-Markovnikov (free radical mechanism).
Alkynes (CₙH₂ₙ₋₂)
| Hybridisation: sp | Bond angle: 180° |
|---|
Reactions of Alkynes
- Hydrogenation: Ethyne + 2H₂ → Ethane (Lindlar's catalyst gives cis-alkene)
- Halogenation: HC≡CH + 2Cl₂ → CHCl₂−CHCl₂
- Hydration: HC≡CH + H₂O → CH₃CHO (Markovnikov; Hg²⁺ catalyst)
- Acidic nature: Terminal alkynes react with Na: HC≡CH + Na → HC≡C⁻Na⁺ + ½H₂
Benzene (C₆H₆) — Aromatic Compound
Structure: Planar hexagonal ring; sp² hybridised carbons; 6 π electrons delocalised.
Hückel's rule: Aromatic if 4n+2 π electrons (n = 0,1,2…). Benzene: n = 1, 6π electrons ✓
Reactions of Benzene (Electrophilic Aromatic Substitution — EAS)
| Reaction | Reagent | Product |
|---|---|---|
| Halogenation | Cl₂ / FeCl₃ | Chlorobenzene + HCl |
| Nitration | Conc. HNO₃ + conc. H₂SO₄ | Nitrobenzene |
| Sulphonation | Fuming H₂SO₄ | Benzenesulphonic acid |
| Friedel-Crafts alkylation | RCl / AlCl₃ | Alkylbenzene |
| Friedel-Crafts acylation | RCOCl / AlCl₃ | Aryl ketone |
Worked Examples
Example 1: Write the mechanism for free radical chlorination of methane. (Initiation: Cl₂ → 2Cl•; Propagation: two steps; Termination: radical coupling) — see above.
Example 2: Predict the major product of propene + HBr. CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (2-bromopropane, Markovnikov).
Example 3: How is acetaldehyde prepared from acetylene? HC≡CH + H₂O → CH₃CHO (Hg²⁺/H₂SO₄ catalyst, Markovnikov addition, keto form of the enol).
Important Questions
One Mark
- What is the general formula of alkynes?
- State Markovnikov's rule.
- Which halogen decolourises bromine water test — alkene or alkane?
- What is the hybridisation of carbon in benzene?
Short Answer (3 marks)
- Explain the free radical mechanism of chlorination of methane.
- Write the products of ozonolysis of CH₂=CHCH₃. What does this tell you about the alkene structure?
- Why does benzene prefer substitution over addition reactions?
Long Answer (5 marks)
- Explain the electrophilic addition mechanism for the reaction of ethene with HBr. How does the peroxide effect change the product?
- Compare the reactions of alkanes, alkenes, and alkynes with bromine. What do the observations tell you about saturation?
Common Mistakes
- Markovnikov's rule: adding H to the more substituted carbon (wrong!) — H goes to the carbon with MORE hydrogen.
- Confusing free radical substitution (alkanes, light) with electrophilic addition (alkenes, no light needed).
- Benzene undergoes SUBSTITUTION, not addition — its aromatic stability prevents addition.
- Terminal alkyne acidity: forgetting that only terminal alkynes are acidic (sp carbon's higher s-character → stronger bond holding the H → actually more acidic, not less).
Board Exam Tips
- The free radical mechanism of halogenation of alkanes (all three steps) is a guaranteed 5-mark question.
- Markovnikov vs anti-Markovnikov (peroxide effect) appears as a 3-mark question.
- Electrophilic aromatic substitution mechanism (nitration or halogenation) is a 5-mark board staple.
- NEET: Reactions of alkenes (especially addition reactions with different reagents) appear in 2–3 questions per paper.