Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Chapter Introduction
Hydrocarbons are compounds composed solely of carbon and hydrogen. This is one of the most important chapters in Class 11 Chemistry — it covers alkanes, alkenes, alkynes, and aromatic hydrocarbons, along with key reactions like halogenation, addition, elimination, ozonolysis, and electrophilic aromatic substitution. Mastering this chapter builds the foundation for Class 12 organic chemistry. Expect 5–8 mark questions on reaction mechanisms, Markovnikov's rule, conformational analysis, and benzene reactions in every CBSE exam.
Key Definitions
| Term | Definition |
|---|---|
| Hydrocarbons | Compounds containing only carbon and hydrogen |
| Alkanes | Saturated hydrocarbons with the general formula CₙH₂ₙ₊₂ (all single bonds) |
| Alkenes | Unsaturated hydrocarbons with C=C, general formula CₙH₂ₙ |
| Alkynes | Unsaturated hydrocarbons with C≡C, general formula CₙH₂ₙ₋₂ |
| Aromatic hydrocarbons | Cyclic, planar, fully conjugated hydrocarbons satisfying Hückel's rule (4n+2 π-electrons) |
| Conformation | Different spatial arrangements of atoms resulting from rotation around a C—C single bond |
| Markovnikov's rule | In addition of HX to an unsymmetrical alkene, H adds to the carbon with more H atoms |
| Anti-Markovnikov addition | Addition where the nucleophile adds to the more substituted carbon (peroxide/Kharasch effect) |
| Ozonolysis | Cleavage of C=C by ozone to give carbonyl compounds |
| Catenation | Self-linking property of carbon to form chains and rings |
| Pyrolysis | Thermal decomposition of a compound at high temperature (cracking) |
| Aromaticity | Property of cyclic, planar, conjugated systems with (4n+2) π-electrons (Hückel's rule) |
| Electrophilic aromatic substitution (EAS) | Reaction where an electrophile replaces a hydrogen on an aromatic ring |
Classification of Hydrocarbons
| Comparison | Alkanes | Alkenes | Alkynes |
|---|---|---|---|
| General formula | CₙH₂ₙ₊₂ | CₙH₂ₙ | CₙH₂ₙ₋₂ |
| Hybridisation of C | sp³ | sp² | sp |
| C—C bond | Single (σ) | Double (1σ + 1π) | Triple (1σ + 2π) |
| Bond length (C—C) | 154 pm | 134 pm | 120 pm |
| Reactivity | Low (substitution) | High (addition) | High (addition) |
| Burns with | Non-sooty flame | Sooty flame | Very sooty flame |
| Bromine water test | No decolourisation | Decolourises | Decolourises |
Alkanes
Preparation Methods
| Method | Reaction | Notes |
|---|---|---|
| Wurtz reaction | 2R—X + 2Na → R—R + 2NaX | Joins two alkyl groups; best for symmetrical alkanes; gives even-number alkanes |
| Reduction of alkyl halides | R—X + Zn/H⁺ → R—H + ZnX₂ | Or use LiAlH₄, NaBH₄ |
| From carboxylic acids (decarboxylation) | R—COONa + NaOH/CaO → R—H + Na₂CO₃ | Soda lime decarboxylation; removes one C |
| Kolbe's electrolysis | 2RCOONa + 2H₂O → R—R + 2CO₂ + H₂ + 2NaOH | Electrolysis of sodium carboxylate |
| Hydrogenation of alkenes | CₙH₂ₙ + H₂ (Ni/Pt/Pd) → CₙH₂ₙ₊₂ | Catalytic hydrogenation |
| From Grignard reagent | RMgX + H₂O → RH + Mg(OH)X | Grignard reacts with any acidic H |
Wurtz reaction details:
- 2CH₃CH₂Br + 2Na → CH₃CH₂CH₂CH₃ (butane) + 2NaBr
- Limitation: Cannot be used to prepare methane; gives mixtures with two different alkyl halides
- Mechanism: Free radical
Properties and Reactions of Alkanes
Halogenation (Free Radical Substitution)
General: R—H + X₂ → R—X + HX (in presence of UV light or heat)
Mechanism (chlorination of methane): 1. Chain initiation: Cl₂ → 2Cl• (homolytic cleavage by hv) 2. Chain propagation:
3. Chain termination: Cl• + Cl• → Cl₂; •CH₃ + Cl• → CH₃Cl; •CH₃ + •CH₃ → C₂H₆
- Cl• + CH₄ → HCl + •CH₃
- •CH₃ + Cl₂ → CH₃Cl + Cl•
Order of reactivity: F₂ > Cl₂ > Br₂ > I₂ (fluorination is explosive; iodination is reversible)
Order of H-abstraction: 3° > 2° > 1° (tertiary H is most easily abstracted)
Selectivity: Bromine is more selective than chlorine (Br₂ prefers 3° H; Cl₂ is less selective)
Combustion
CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O + heat
Alkanes burn with a non-sooty blue flame (complete combustion) or yellow flame (incomplete combustion).
Pyrolysis (Cracking)
Alkanes decompose at high temperatures (600–800 K) in the absence of air:
C₁₀H₂₂ → C₅H₁₂ (pentane) + C₅H₁₀ (pentene)
Higher alkanes → lower alkanes + alkenes (used in petroleum refining)
Conformational Analysis
Ethane Conformations
When one carbon of ethane rotates relative to the other around the C—C bond, different conformations result:
| Conformation | Dihedral Angle | Energy | Stability |
|---|---|---|---|
| Staggered | 60° (H atoms offset) | Minimum (12.5 kJ/mol less than eclipsed) | More stable |
| Eclipsed | 0° (H atoms aligned) | Maximum | Less stable |
In Newman projections:
- Staggered: Front and back H atoms are maximally apart (60°)
- Eclipsed: Front and back H atoms are aligned (0°)
The energy barrier (12.5 kJ/mol) is too small to isolate conformers at room temperature.
n-Butane Conformations
n-Butane (CH₃—CH₂—CH₂—CH₃) has three important staggered conformations:
| Conformation | Arrangement | Relative Energy | Stability |
|---|---|---|---|
| Anti | Two CH₃ groups 180° apart | Lowest (most stable) | Most stable |
| Gauche | Two CH₃ groups 60° apart | Intermediate | Less stable than anti |
| Eclipsed | CH₃ groups aligned | Highest | Least stable |
Anti is the most stable because the bulky methyl groups are maximally separated, minimising steric strain.
Alkenes
Preparation Methods
| Method | Reaction | Notes |
|---|---|---|
| Dehydrohalogenation | R—CH₂—CH₂—X + KOH (alc.) → R—CH=CH₂ + KX + H₂O | E2 elimination; β-elimination |
| Dehydration of alcohols | R—CH₂—CH₂—OH → R—CH=CH₂ + H₂O | Acid-catalysed (H₂SO₄, 170°C); follows Saytzeff rule |
| Dehalogenation | R—CHBr—CH₂Br + Zn → R—CH=CH₂ + ZnBr₂ | Vicinal dihalide reduction |
| Kolbe's electrolysis | 2CH₃COOK + 2H₂O → CH₂=CH₂ + 2CO₂ + H₂ + 2KOH | Less practical for alkenes |
Saytzeff's rule: In elimination reactions, the more substituted alkene (more stable) is the major product.
Example: Dehydration of 2-butanol gives 80% but-2-ene (more substituted) and 20% but-1-ene.
Properties and Reactions of Alkenes
Addition of Hydrogen (Hydrogenation)
R—CH=CH₂ + H₂ → R—CH₂—CH₃ (catalyst: Ni, Pd, or Pt)
Used in hydrogenation of vegetable oils (liquid → solid, e.g., vanaspati ghee).
Addition of Halogens
CH₂=CH₂ + X₂ → CH₂X—CH₂X (X = Cl, Br)
- With Br₂/CCl₄: decolourisation of bromine water — test for unsaturation
- Anti-addition mechanism (via bromonium ion intermediate)
Addition of Hydrogen Halides (Markovnikov's Rule)
Markovnikov's rule: When HX adds to an unsymmetrical alkene, H adds to the carbon with more H atoms (less substituted carbon).
CH₃—CH=CH₂ + HBr → CH₃—CHBr—CH₃ (2-bromopropane) — Markovnikov product
Mechanism: Formation of the more stable secondary carbocation intermediate.
Anti-Markovnikov addition (Peroxide effect / Kharasch effect):
In the presence of organic peroxides (e.g., R—O—O—R), HBr adds in an anti-Markovnikov fashion:
CH₃—CH=CH₂ + HBr (peroxide) → CH₃—CH₂—CH₂Br (1-bromopropane) — anti-Markovnikov product
Important: The peroxide effect works ONLY with HBr, NOT with HCl or HI. This is because:
- H—Cl bond is too strong (peroxide radical cannot abstract H)
- H—I bond is too weak (iodine radical recombines before addition)
Addition of Water (Hydration)
CH₂=CH₂ + H₂O (H⁺) → CH₃—CH₂—OH (ethanol)
Follows Markovnikov's rule: H adds to the less substituted carbon.
CH₃—CH=CH₂ + H₂O (H⁺) → CH₃—CH(OH)—CH₃ (propan-2-ol)
Ozonolysis
Ozone (O₃) cleaves the C=C bond:
Step 1: Alkene + O₃ → Ozonide (molozonide → ozonide rearrangement)
Step 2: Ozonide + Zn/H₂O → Two carbonyl compounds
| Alkene | Ozonolysis Products |
|---|---|
| CH₂=CH₂ | HCHO + HCHO (2 formaldehyde) |
| CH₃—CH=CH₂ | HCHO + CH₃CHO (formaldehyde + acetaldehyde) |
| CH₃—CH=CH—CH₃ | 2 CH₃CHO (2 acetaldehyde) |
| (CH₃)₂C=CH₂ | (CH₃)₂C=O + HCHO (acetone + formaldehyde) |
| (CH₃)₂C=C(CH₃)₂ | 2 (CH₃)₂C=O (2 acetone) |
Exam trick: If ozonolysis gives only one product, the alkene is symmetrical. If it gives two different products, the alkene is unsymmetrical. Formaldehyde (HCHO) indicates a terminal =CH₂ group.
Polymerisation
n CH₂=CH₂ → (—CH₂—CH₂—)ₙ (polyethylene)
n CH₂=CHCl → (—CH₂—CHCl—)ₙ (PVC)
n CH₂=CHCN → (—CH₂—CHCN—)ₙ (polyacrylonitrile / PAN)
Oxidation with KMnO₄
Cold, dilute KMnO₄ (Baeyer's test): Alkenes give vicinal diols
CH₂=CH₂ + [O] + H₂O → HOCH₂—CH₂OH (ethane-1,2-diol) — syn addition
Hot, concentrated KMnO₄: Cleaves C=C to give ketones/carboxylic acids
| Alkene | Product with hot KMnO₄ |
|---|---|
| R—CH=CH₂ | RCOOH + CO₂ |
| R—CH=CH—R' | RCOOH + R'COOH |
| R₂C=CR₂ | 2 R₂C=O |
Alkynes
Preparation Methods
| Method | Reaction | Notes |
|---|---|---|
| From CaC₂ | CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂ | Industrial method |
| Dehydrohalogenation | CH₃—CHCl₂ + 2KOH (alc.) → CH≡CH + 2KCl + 2H₂O | Vicinal or geminal dihalide |
| From 1,2-dibromoethane | CH₂Br—CH₂Br + 2KOH (alc.) → CH≡CH + 2KBr + 2H₂O | Double elimination |
Properties and Reactions of Alkynes
Acidic Character of Terminal Alkynes
Alkynes with terminal ≡C—H are weakly acidic (sp carbon is more electronegative):
CH≡CH + NaNH₂ → CH≡C⁻Na⁺ + NH₃ (acetylide salt)
CH≡CH + 2Na → NaC≡CNa + H₂ (sodium acetylide)
This reaction is possible because the sp-hybridised carbon is more electronegative (50% s-character) than sp² (33%) or sp³ (25%) carbons.
Addition Reactions
| Reaction | Equation | Notes |
|---|---|---|
| + H₂ | CH≡CH + 2H₂ → CH₃—CH₃ | Full hydrogenation; Lindlar catalyst gives cis-alkene |
| + X₂ | CH≡CH + 2Br₂ → CHBr₂—CHBr₂ | Tetrahalide |
| + HX | CH≡CH + HCl → CH₂=CHCl; CH₂=CHCl + HCl → CH₃—CHCl₂ | Markovnikov addition |
| + H₂O | CH≡CH + H₂O (HgSO₄/H₂SO₄) → CH₃CHO | Kucherov reaction; vinyl alcohol tautomerises to acetaldehyde |
| Ozonolysis | CH≡CH + 2O₃ → 2HCOOH (glyoxal) | Triple bond gives two carboxylic acids (or diketones) |
Polymerisation
Linear polymerisation: n CH≡CH → (—CH=CH—)ₙ (polyacetylene)
Cyclic trimerisation: 3 CH≡CH → C₆H₆ (benzene, Ziegler catalyst)
Aromatic Hydrocarbons
Benzene Structure
- Molecular formula: C₆H₆
- Kekulé structure: Alternating single and double bonds (1865) — but benzene does NOT behave like a typical alkene
- Resonance: Benzene is a resonance hybrid of two Kekulé structures; all C—C bonds are equal (139 pm, between single and double bond length)
- Hückel's rule: A compound is aromatic if it is cyclic, planar, fully conjugated, and has (4n+2) π-electrons (n = 0, 1, 2, ...)
- Benzene: 6 π-electrons (n=1) → aromatic ✓
- Cyclobutadiene: 4 π-electrons (n=0.5) → anti-aromatic ✗
- Cyclooctatetraene: 8 π-electrons → non-aromatic ✗
Evidence for resonance: 1. All C—C bond lengths are equal (139 pm) 2. Resonance energy = 150 kJ/mol (benzene is more stable than expected) 3. Benzene does not give typical alkene reactions (no addition of Br₂/CCl₄ without catalyst)
Electrophilic Aromatic Substitution (EAS)
In EAS, an electrophile replaces a hydrogen atom on the benzene ring. The general mechanism is:
1. Electrophile generation 2. Attack on benzene ring → carbocation intermediate (arenium ion / sigma complex) 3. Loss of H⁺ → restored aromaticity
Key EAS Reactions
| Reaction | Reagents | Product | Electrophile |
|---|---|---|---|
| Nitration | Conc. HNO₃ + Conc. H₂SO₄ (323–333 K) | Nitrobenzene (C₆H₅NO₂) | NO₂⁺ (nitronium ion) |
| Sulfonation | Conc. H₂SO₄ (373 K) or fuming H₂SO₄ | Benzenesulfonic acid (C₆H₅SO₃H) | SO₃ |
| Friedel-Crafts alkylation | R—Cl + AlCl₃ (anhydrous) | Alkylbenzene (C₆H₅R) | R⁺ (carbocation) |
| Friedel-Crafts acylation | RCOCl + AlCl₃ (anhydrous) | Acylbenzene (C₆H₅COR) | RCO⁺ (acylium ion) |
| Halogenation | X₂ + FeX₃ or AlCl₃ | Halobenzene (C₆H₅X) | X⁺ (halonium ion) |
Friedel-Crafts limitations: 1. Cannot be used with strongly deactivating groups (—NO₂, —CHO, —COOH) — the ring is too unreactive 2. Polyalkylation occurs (alkyl groups activate the ring for further substitution) 3. Alkylation can undergo carbocation rearrangement (e.g., n-propyl → isopropyl rearrangement) 4. Acylation does NOT undergo rearrangement (acylium ion is more stable)
Directive Influence of Substituents
The substituent already present on the benzene ring determines where the next electrophile attacks:
Ortho/Para Directors (Activating Groups)
| Substituent | Nature | Reason | Major Products |
|---|---|---|---|
| —OH | Strongly activating | +M effect donates electrons to ortho/para | Ortho + Para |
| —NH₂ | Strongly activating | +M effect | Ortho + Para |
| —OR | Activating | +M effect | Ortho + Para |
| —CH₃ (alkyl) | Weakly activating | +I and hyperconjugation | Ortho + Para |
| —Cl, —Br | Weakly deactivating but ortho/para directing | +M > —I (net donation to o/p) | Ortho + Para |
Why ortho/para?: Resonance structures show increased electron density at ortho and para positions, making them more nucleophilic.
Meta Directors (Deactivating Groups)
| Substituent | Nature | Reason | Major Products |
|---|---|---|---|
| —NO₂ | Strongly deactivating | —M and —I withdraw electrons from ring | Meta |
| —CN | Strongly deactivating | —M and —I | Meta |
| —CHO | Moderately deactivating | —M and —I | Meta |
| —COOH | Moderately deactivating | —M and —I | Meta |
| —SO₃H | Moderately deactivating | —M and —I | Meta |
Why meta?: Resonance structures show electron withdrawal from ortho and para positions, leaving meta position relatively less electron-poor.
Halogens are an exception: —Cl and —Br are deactivating (—I effect dominates overall reactivity) but ortho/para directing (+M effect dominates regioselectivity).
Important Reaction Summary
| Reaction | Equation | Type |
|---|---|---|
| Wurtz reaction | 2R—X + 2Na → R—R + 2NaX | Coupling |
| Kolbe's electrolysis | 2CH₃COONa → C₂H₆ + 2CO₂ + H₂ | Electrolysis |
| Decarboxylation | RCOONa + NaOH/CaO → RH + Na₂CO₃ | Elimination |
| Dehydration | R—OH (H₂SO₄, 170°C) → Alkene + H₂O | Elimination |
| Dehydrohalogenation | R—X + KOH (alc.) → Alkene + KX + H₂O | E2 elimination |
| Markovnikov addition | CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ | Addition |
| Anti-Markovnikov | CH₃CH=CH₂ + HBr (peroxide) → CH₃CH₂CH₂Br | Free radical addition |
| Ozonolysis | RCH=CHR' + O₃ → RCHO + R'CHO | Oxidative cleavage |
| Friedel-Crafts alkylation | C₆H₆ + CH₃Cl/AlCl₃ → C₆H₅CH₃ | EAS |
| Friedel-Crafts acylation | C₆H₆ + CH₃COCl/AlCl₃ → C₆H₅COCH₃ | EAS |
| Nitration | C₆H₆ + HNO₃/H₂SO₄ → C₆H₅NO₂ | EAS |
| Sulfonation | C₆H₆ + H₂SO₄ → C₆H₅SO₃H | EAS |
Common Mistakes to Avoid
1. Confusing Markovnikov and anti-Markovnikov: Markovnikov addition is the default (H adds to the carbon with more H). Anti-Markovnikov ONLY occurs with HBr in the presence of peroxides — NOT with HCl or HI. 2. Incorrect conformational drawing: In Newman projections, staggered = 60° dihedral angle, eclipsed = 0°. Draw front and back carbons correctly. 3. Saying "benzene undergoes addition": Benzene predominantly undergoes substitution (EAS), NOT addition. It does not decolourise Br₂/CCl₄ under normal conditions. 4. Wrong ozonolysis products: Terminal alkene (RCH=CH₂) gives one aldehyde (HCHO) and one ketone/aldehyde. Symmetrical alkene gives identical products. Always reduce with Zn/H₂O to prevent further oxidation. 5. Forgetting Friedel-Crafts limitations: Cannot alkylate/acylate a benzene ring with —NO₂ or other deactivating groups present. 6. Halogen as ortho/para director: Do NOT say halogens are meta directors. They are deactivating but ortho/para directing because +M > —I for regioselectivity. 7. Polyalkylation: In Friedel-Crafts alkylation, the alkyl group activates the ring further, leading to polyalkylation. Acylation avoids this issue.
Exam Tips
- Most-asked reactions: Markovnikov addition (2–3 marks), ozonolysis (2 marks), Friedel-Crafts (2–3 marks), Wurtz reaction (1–2 marks)
- Conformational analysis: Practise drawing Newman projections for ethane (staggered and eclipsed) and butane (anti, gauche, eclipsed). Know the energy order: anti < gauche < eclipsed.
- EAS mechanisms: Always draw the arenium ion intermediate (sigma complex) with three resonance structures.
- Directive influence: Prepare a table of ortho/para vs meta directors with reasons. This is a guaranteed 2–3 mark question.
- Comparison tables: Be ready to compare alkanes vs alkenes vs alkynes in terms of hybridisation, bond order, reactivity, and characteristic reactions.
- Ozonolysis trick: Given the products, work backwards to identify the alkene. The number of products = number of C=C bonds + 1 (for each double bond).
- Hückel's rule: Always check: cyclic + planar + fully conjugated + (4n+2) π-electrons. All four conditions must be satisfied for aromaticity.
Quick Revision Notes
Alkanes (CₙH₂ₙ₊₂)
- Preparation: Wurtz, decarboxylation, Kolbe's electrolysis, hydrogenation
- Reactions: Free radical halogenation (3° > 2° > 1° H), combustion, pyrolysis
- Conformations: Staggered (more stable) > eclipsed (less stable); anti > gauche in butane
Alkenes (CₙH₂ₙ)
- Preparation: Dehydration, dehydrohalogenation, dehalogenation
- Reactions: Addition of H₂, X₂, HX (Markovnikov), H₂O, O₃, KMnO₄; polymerisation
- Markovnikov: H adds to C with more H; anti-Markovnikov: peroxide effect (HBr only)
- Ozonolysis: Identifies C=C position; gives aldehydes/ketones
Alkynes (CₙH₂ₙ₋₂)
- Preparation: CaC₂ + H₂O, dehydrohalogenation
- Reactions: Addition (H₂, X₂, HX, H₂O → aldehyde via tautomerism), acidic H (with NaNH₂)
- Terminal alkynes: form acetylides with Na, NaNH₂, AgNO₃/NH₃
Aromatic Hydrocarbons
- Benzene: 6 π-electrons, resonance-stabilised, all C—C bonds equal (139 pm)
- EAS: Nitration, sulfonation, Friedel-Crafts (alkylation, acylation), halogenation
- Directive influence: o/p directors (—OH, —NH₂, —OR, —CH₃, —Cl) activate; meta directors (—NO₂, —CN, —CHO, —COOH) deactivate
- Hückel's rule: (4n+2) π-electrons, cyclic, planar, conjugated