Hydrocarbons are the simplest organic compounds composed exclusively of carbon and hydrogen atoms. Despite their simple composition, they are fundamental to all organic chemistry because they act as the structural and functional core of more complex molecules. They serve as fuels, industrial solvents, starting materials for synthesis, and precursors for polymers, pharmaceuticals, detergents, dyes, and petrochemicals. Understanding hydrocarbons teaches students the essential language of organic chemistry-hybridisation, bonding, structure, reactivity, mechanisms, and nomenclature. This expanded explanation strengthens conceptual clarity for both board exams and competitive exams.
Hydrocarbons are broadly classified based on saturation (single, double, triple bonds) and structure (chain, cyclic, aromatic). Their reactivity depends primarily on the nature of the bonds and electron distribution. NCERT Class 11 divides this unit into six major sections-classification, alkanes, alkenes, alkynes, aromatic hydrocarbons, and toxicity concerns. This comprehensive expansion covers all subtopics with mechanisms, illustrations, real‑world connections, and exam‑oriented explanations.
Classification of Hydrocarbons
Hydrocarbons can be classified into three major categories: saturated, unsaturated, and aromatic. This classification is essential because the presence of multiple bonds dramatically influences reactivity, geometry, and stability.
A. Saturated Hydrocarbons (Alkanes)
Saturated hydrocarbons contain only carbon–carbon single bonds (σ‑bonds). They are the least reactive hydrocarbons and form the base of petroleum and natural gas.
Key features:
- General formula: CₙH₂ₙ₊₂
- Tetrahedral geometry (109.5° bond angle)
- Composed of strong σ bonds → low reactivity
- Can be straight‑chain (n‑alkanes), branched (iso‑alkanes), or cyclic (cycloalkanes)
Saturated hydrocarbons undergo only limited reactions such as combustion and substitution because breaking a σ bond requires high energy.
B. Unsaturated Hydrocarbons
Unsaturated hydrocarbons have one or more multiple bonds (C=C or C≡C). The presence of π bonds makes them more chemically reactive.
1. Alkenes (C=C)
- General formula: CₙH₂ₙ
- Contain one or more double bonds
- Participate in electrophilic addition reactions
- Exhibit cis–trans (geometric) isomerism
2. Alkynes (C≡C)
- General formula: CₙH₂ₙ₋₂
- Contain one or more triple bonds
- Triple bond has one σ and two π bonds
- Terminal alkynes are weakly acidic
Unsaturated hydrocarbons have planar or linear geometry depending on hybridisation, and their reactivity centers around the π electrons, which are exposed and weak compared to σ bonds.
C. Aromatic Hydrocarbons
Aromatic compounds contain a benzene ring or similar cyclic π‑electron cloud. They are characterised by:
- Delocalised π electrons
- sp² hybridised carbon atoms
- Planar ring structure
- Resonance stability
- Obedience to Hückel’s rule (4n+2 π electrons)
- Electrophilic substitution reactions
Aromatic hydrocarbons are found in fuels, dyes, medicines, and industrial solvents.
Alkanes (Saturated Hydrocarbons)
Alkanes are the simplest hydrocarbons. They are relatively inert but extremely important industrially.
Structure and Bonding
- Each carbon atom is sp³ hybridised
- Bond angle = 109.5°
- Carbon atoms form tetrahedral geometry
- Chains may be straight or branched (affecting boiling point)
Preparation of Alkanes
Alkanes can be prepared by several synthetic routes:
1. Hydrogenation of Alkenes/Alkynes
Unsaturated hydrocarbons are reduced by adding hydrogen in the presence of catalysts like Ni, Pt, or Pd.
2. Wurtz Reaction
2R–X + 2Na → R–R + 2NaX
Used to prepare higher alkanes from alkyl halides.
3. Kolbe’s Electrolysis
Electrolysis of sodium/potassium salts of fatty acids gives alkanes with double the number of carbon atoms.
4. Reduction of Alkyl Halides
Zn/HCl or LiAlH₄ reduces halides to alkanes.
Physical Properties
- Non‑polar, insoluble in water
- Soluble in non‑polar solvents
- Boiling points increase with chain length and molecular mass
- Branched alkanes have lower boiling points than straight‑chain alkanes
Chemical Properties
Alkanes undergo few reactions due to strong σ bonds.
1. Combustion
CnH₂n₊₂ + O₂ → CO₂ + H₂O (with heat) Used as fuels because they release large amounts of energy.
2. Halogenation (Free Radical Substitution)
Occurs in the presence of sunlight or UV light.
Mechanism:
- Initiation: Cl₂ → 2Cl•
- Propagation: CH₄ + Cl• → CH₃• + HCl
- Termination: Combination of radicals
Multiple substitution yields CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄.
Uses of Alkanes
- LPG, CNG, petrol, diesel
- Solvents (hexane)
- Raw materials for petrochemicals and polymers
Alkenes (Unsaturated Hydrocarbons)
Alkenes contain double bonds and display higher reactivity because of their π bonds.
Structure and Bonding
- Carbon atoms are sp² hybridised
- Trigonal planar geometry (120°)
- Presence of π bond restricts rotation → geometric isomerism
Preparation of Alkenes
1. Dehydration of Alcohols
Alcohols → Alkenes using conc. H₂SO₄ or Al₂O₃.
2. Dehydrohalogenation of Alkyl Halides
Alkyl halides + alcoholic KOH → Alkenes (β‑elimination).
3. Cracking of Alkanes
Long‑chain alkanes break into alkenes + alkanes under heat/catalysts.
Physical Properties
- Non‑polar, lighter than water
- Insoluble in water
- Show increased reactivity compared to alkanes
Chemical Properties
1. Electrophilic Addition Reactions
- Addition of H₂ (hydrogenation)
- Addition of HX (Markovnikov and peroxide effect)
- Addition of halogens (bromine water test)
2. Oxidation
- Cold KMnO₄ → Vicinal diols
- Ozonolysis → Aldehydes/ketones
Tests for Alkenes
- Decolourisation of bromine water
- Baeyer’s test (pink KMnO₄ decolourises)
Alkynes (Unsaturated Hydrocarbons)
Alkynes contain triple bonds and show characteristic acidic behaviour when terminal.
Structure and Bonding
- Carbon atoms are sp hybridised
- Linear geometry (180°)
- Triple bond = 1 σ + 2 π bonds
Preparation of Alkynes
1. Dehydrohalogenation of Vicinal Dihalides
Double elimination forms alkynes.
2. From Calcium Carbide
CaC₂ + H₂O → HC≡CH + Ca(OH)₂
Calcium carbide reacts with water to form acetylene.
Chemical Properties
1. Electrophilic Addition
- Hydrogenation → Alkene → Alkane
- Addition of HX (follows Markovnikov rule)
2. Oxidation
Strong oxidants cleave triple bond → acids.
3. Acidic Nature of Terminal Alkynes
Terminal alkynes have acidic hydrogen.
NaNH₂ + HC≡CH → HC≡C⁻Na⁺ + NH₃
Uses
- Oxy‑acetylene flame for welding
- Starting materials in synthesis (vinyl halides, aldehydes)
Aromatic Hydrocarbons
Aromatic hydrocarbons represent one of the most important classes of organic compounds due to their exceptional stability and wide applications. The simplest aromatic hydrocarbon is benzene (C₆H₆), and its derivatives form the basis of numerous industrial chemicals, pharmaceuticals, dyes, polymers, and fuels.
Structure and Bonding in Benzene
Benzene exhibits:
- A planar hexagonal ring with six carbon atoms
- sp² hybridisation at each carbon
- 120° bond angles
- A delocalised π-electron cloud
Aromaticity and Hückel’s Rule
A compound is aromatic if it is cyclic, planar, fully conjugated, and contains (4n + 2) π electrons.
Electrophilic Substitution Reactions
Common ESR reactions include nitration, sulphonation, halogenation, and Friedel–Crafts reactions.
Directive Influence
- Ortho/para-directing: –OH, –NH₂, –CH₃
- Meta-directing: –NO₂, –CN, –COOH
Side Chain Reactions
Alkyl side chains undergo oxidation and benzylic halogenation.
Carcinogenicity and Toxicity
Polycyclic aromatic hydrocarbons (PAHs) can be carcinogenic due to their tendency to form DNA-reactive metabolites.
Toxicity of Benzene
Long-term exposure may cause bone marrow suppression and leukaemia.
Environmental Impact
Hydrocarbons contribute to smog and persistent pollution.
FAQs
Q1. Why do alkenes decolourise bromine water while alkanes do not?
Because alkenes undergo electrophilic addition across the C=C double bond.
Q2. Why is benzene unusually stable?
Due to delocalised π electrons providing resonance stabilisation.
Q3. How can you distinguish between an alkyne and an alkene?
Terminal alkynes react with ammoniacal silver nitrate; alkenes do not.
Q4. Why are PAHs harmful?
They form DNA-reactive intermediates that may trigger cancer.
Q5. What determines directing effects on benzene?
Electron-donating groups stabilise carbocations (ortho/para), while electron-withdrawing groups favour meta.
Conclusion
Hydrocarbons form the backbone of organic chemistry. By understanding saturated, unsaturated, and aromatic hydrocarbons, students gain the foundation required for advanced organic mechanisms. Deeksha Vedantu strengthens student mastery through clear explanations and exam-focused learning.
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