Hydrocarbons
'Hydrocarbons are the simplest organic compounds — composed only of carbon and hydrogen — yet they are the foundation of fuels and organic chemistry.' — Organic Chemistry
1. Chapter Overview
HYDROCARBONS are compounds containing ONLY CARBON and HYDROGEN. They are classified as ALKANES (single bonds), ALKENES (double bonds), ALKYNES (triple bonds), and AROMATIC hydrocarbons (benzene and derivatives). This chapter covers the PREPARATION, PHYSICAL PROPERTIES, CHEMICAL REACTIONS, and MECHANISMS for each class, with special emphasis on the UNIQUE stability of aromatic compounds.
2. Alkanes (CₙH₂ₙ₊₂)
General
- sp³ hybridised carbon, tetrahedral geometry
- Saturated hydrocarbons (all single bonds)
- General formula: CₙH₂ₙ₊₂
Preparation
- From unsaturated hydrocarbons: Catalytic hydrogenation (Ni, Pt, or Pd catalyst)
- C₂H₄ + H₂ → C₂H₆ (nickel catalyst)
- From alkyl halides: Wurtz reaction (2RX + 2Na → R—R + 2NaX)
- 2CH₃Br + 2Na → CH₃—CH₃ + 2NaBr
- Limitation: For different alkyl halides, MIXTURE of products
- From carboxylic acids: Decarboxylation (soda lime, heat)
- CH₃COONa + NaOH → CH₄ + Na₂CO₃
- From Grignard reagents: R—Mg—X + H₂O → R—H + Mg(OH)X
Physical Properties
- Non-polar, INSOLUBLE in water, soluble in organic solvents
- Boiling point INCREASES with chain length
- Branched alkanes have LOWER boiling points than straight chain
- C₁-C₄: Gases; C₅-C₁₇: Liquids; C₁₈+: Solids
Chemical Reactions
-
Substitution (Free Radical Halogenation):
- CH₄ + Cl₂ → CH₃Cl + HCl (UV light or heat, 300°C)
- Mechanism: Chain reaction — Initiation (Cl₂ → 2Cl•), Propagation, Termination
- Reactivity: F₂ > Cl₂ > Br₂ > I₂
- Selectivity: Tertiary H > Secondary H > Primary H (for Br₂)
-
Combustion:
- CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O + HEAT
- Incomplete combustion: gives CO + C (soot)
-
Pyrolysis (Cracking):
- Higher alkanes heated → mixture of smaller hydrocarbons
3. Alkenes (CₙH₂ₙ)
General
- One or more C=C DOUBLE bonds (sp² hybridised)
- Unsaturated hydrocarbons
- General formula: CₙH₂ₙ
Preparation
-
Dehydrohalogenation of alkyl halides:
- R—CH₂—CH₂—X + alc. KOH → R—CH=CH₂ + KX + H₂O
- Follows SAYTZEFF rule: MORE substituted alkene is the MAJOR product
-
Dehydration of alcohols:
- R—CH₂—CH₂—OH + conc. H₂SO₄ → R—CH=CH₂ + H₂O (170°C)
-
From alkynes: Partial hydrogenation (Lindlar's catalyst → cis, Na/liq NH₃ → trans)
Chemical Reactions (Electrophilic Addition)
-
Addition of H₂ (Hydrogenation): C₂H₄ + H₂ → C₂H₆ (Ni catalyst)
-
Addition of Halogens (X₂):
- C₂H₄ + Br₂ → CH₂Br—CH₂Br (decolourises bromine water — TEST for unsaturation)
-
Addition of Hydrogen Halides (HX):
- Markovnikov's Rule: H adds to the C with MORE H atoms already; X adds to the other
- CH₃—CH=CH₂ + HBr → CH₃—CHBr—CH₃ (major, 2-bromopropane)
- Peroxide effect (Kharasch effect): With HBr + peroxide → ANTI-Markovnikov product
-
Addition of H₂O (Acid-catalysed):
- Follows Markovnikov's rule
- CH₃—CH=CH₂ + H₂O → CH₃—CHOH—CH₃
-
Oxidation:
- Cold KMnO₄ (Baeyer's reagent): Diol formation (cis-dihydroxylation)
- Hot KMnO₄/acid: Cleavage at C=C → ketones/carboxylic acids
Polymerisation
- nCH₂=CH₂ → (—CH₂—CH₂—)ₙ (Polythene, under high pressure/temperature)
4. Alkynes (CₙH₂ₙ₋₂)
General
- One or more C≡C TRIPLE bonds (sp hybridised)
- General formula: CₙH₂ₙ₋₂
Preparation
- Dehydrohalogenation of vicinal dihalides: alc. KOH, then NaNH₂
- From calcium carbide: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂
Chemical Reactions (Electrophilic Addition)
- Addition of H₂: Stepwise → alkene then alkane
- Addition of X₂: Decolourises Br₂ water faster than alkenes
- Addition of HX: Follows Markovnikov's rule
- HC≡CH + HCl → CH₂=CHCl (vinyl chloride, used for PVC)
- Addition of H₂O (Hydration): HgSO₄/H₂SO₄ catalyst → enol → KETONE (Keto-enol tautomerism)
- HC≡CH + H₂O → CH₃CHO (for ethyne: acetaldehyde, exception — terminal alkyne gives aldehyde)
Acidic Nature of Terminal Alkynes
- HC≡CH + NaNH₂ → HC≡C⁻Na⁺ + NH₃ (terminal H is ACIDIC)
- Reaction with AgNO₃/ammoniacal: White precipitate (identification test)
5. Aromatic Hydrocarbons
Benzene Structure
- C₆H₆, planar hexagonal, sp² hybridised
- 6 p-orbitals overlap to form a DELOCALISED π-system (aromatic sextet)
- Huckel's Rule: Aromatic if planar, cyclic, conjugated, with (4n+2)π electrons
Preparation
- From acetylene: 3C₂H₂ → C₆H₆ (cyclisation, red hot tube)
- From phenol: Reduction (Zn dust, heat)
Chemical Reactions (Electrophilic Substitution — SEAr)
- Nitration: C₆H₆ + HNO₃/H₂SO₄ → C₆H₅NO₂ + H₂O
- Halogenation: C₆H₆ + Br₂/FeBr₃ → C₆H₅Br + HBr
- Sulphonation: C₆H₆ + H₂SO₄ (fuming) → C₆H₅SO₃H + H₂O
- Friedel-Crafts Alkylation: C₆H₆ + RCl/AlCl₃ → C₆H₅R + HCl
- Friedel-Crafts Acylation: C₆H₆ + RCOCl/AlCl₃ → C₆H₅COR + HCl
Mechanism of SEAr
- Step 1: Formation of ELECTROPHILE (e.g., NO₂⁺ from HNO₃ + H₂SO₄)
- Step 2: Electrophile attacks π-system → σ-complex (arenium ion)
- Step 3: Loss of H⁺ → regeneration of aromaticity
Directive Effects
| Substituent | Ortho/Para directing (Activating) | Meta directing (Deactivating) |
|---|---|---|
| Strong | —OH, —NH₂, —OCH₃ | —NO₂, —CN |
| Moderate | —CH₃, —C₂H₅ | —CHO, —COOH |
| Weak | —F, —Cl, —Br, —I | —SO₃H, —COR |
6. Common Mistakes
- Wurtz reaction with mixed alkyl halides gives ALL three possible products: Not useful for preparing specific alkanes
- Markovnikov's rule is about H+ addition: The proton goes to the more substituted carbon (more H already)
- Benzene does NOT undergo addition reactions easily: It prefers substitution to preserve aromaticity
- Alkynes are more ACIDIC than alkenes and alkanes: Due to greater s-character in sp hybridisation
- Keto-enol tautomerism is EQUILIBRIUM, not resonance: It involves bond rearrangement, not electron delocalisation
7. CBSE Exam Focus
- Wurtz reaction, decarboxylation (3-mark)
- Markovnikov's and Saytzeff's rules — numericals/products (3-mark)
- Electrophilic substitution of benzene — mechanism (5-mark)
- Directive effects of substituents on benzene (3/5-mark)
- Distinction tests (alkane/alkene/alkyne/benzene)
- Free radical halogenation mechanism (3-mark)
8. Key Concepts
- Alkanes: SP³, sigma bonds, substitution reactions
- Alkenes: SP², pi bonds, addition reactions
- Alkynes: SP, pi bonds, acidic terminal H
- Aromatic: SP², (4n+2)π, substitution reactions
- Stability: Benzene > Alkane > Alkene > Alkyne (in terms of heat of hydrogenation per π-bond)
9. Self-Test (5+ Q&A)
Q1: Predict product of CH₃—CH=CH₂ + H₂O (H⁺ catalyst). A: Markovnikov addition. H⁺ adds to CH₂ (more H) → carbocation CH₃—C⁺H—CH₃. OH⁻ adds to carbocation. Product: PROPAN-2-OL.
Q2: Why does benzene undergo electrophilic substitution rather than addition? A: Addition would BREAK the aromatic π-system (lose 152 kJ/mol stabilisation energy). Substitution PRESERVES aromaticity.
Q3: Identify A and B: CH₃COONa + NaOH → A. A + Cl₂/UV → B. A: A = CH₄ (decarboxylation). B = CH₃Cl (free radical chlorination).
Q4: What is the peroxide effect? Give an example. A: In presence of organic peroxides, HBr adds to alkene AGAINST Markovnikov's rule (ANTI-Markovnikov). CH₃—CH=CH₂ + HBr (peroxide) → CH₃—CH₂—CH₂Br.
Q5: How would you distinguish between ethane, ethene, and ethyne? A: Baeyer's test (cold KMnO₄): Ethane → no reaction (purple persists). Ethene → KMnO₄ decolourised (diol). Ethyne → KMnO₄ decolourised faster. Further: Ethyne gives white ppt with ammoniacal AgNO₃ (terminal alkyne test).
10. Conclusion
Hydrocarbons are the SIMPLEST organic compounds but form the BASIS of organic chemistry. Alkanes are relatively INERT (paraffins = little affinity). Alkenes are REACTIVE due to the π-bond — they undergo addition reactions. Alkynes have TWO π-bonds and show ACIDIC character at terminal positions. Aromatic compounds have UNIQUE stability (aromaticity) and undergo substitution, not addition. Understanding hydrocarbons is ESSENTIAL for fuels (petroleum), polymers, and synthetic organic chemistry — the foundation for Class 12 organic chemistry.
