Hydrocarbons

 

Hydrocarbons – Detailed Notes with Mechanisms & Electronic Effects
(NCERT Class XI Chemistry)

 

INTRODUCTION

Hydrocarbons are organic compounds containing only carbon and hydrogen. They are the fundamental compounds of organic chemistry and are mainly obtained from petroleum, coal, and natural gas.

CLASSIFICATION OF HYDROCARBONS

1. Saturated hydrocarbons – Alkanes

2. Unsaturated hydrocarbons

   (a) Alkenes

   (b) Alkynes

3. Aromatic hydrocarbons

 

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ALKANES (PARAFFINS)

General formula: CₙH₂ₙ₊₂

All carbon atoms are sp³ hybridised and linked by single covalent (σ) bonds.

 

PREPARATION OF ALKANES

1. Hydrogenation of Alkenes/Alkynes

   CH₂=CH₂ + H₂  →  CH₃–CH₃  (Ni/Pt)

 

2. Wurtz Reaction

   2CH₃–Cl + 2Na  →  CH₃–CH₃ + 2NaCl

 

3. Kolbe’s Electrolysis

   2CH₃COO⁻ → C₂H₆ + 2CO₂ + 2e⁻

 

CHEMICAL REACTIONS OF ALKANES

1. Halogenation (Free Radical Substitution)

   CH₄ + Cl₂  →  CH₃Cl + HCl  (Sunlight)

 

MECHANISM OF HALOGENATION

 

 

 

 

 

 

(i) Initiation:

    Cl₂ → 2Cl·

(ii) Propagation:

    Cl· + CH₄ → CH₃· + HCl

    CH₃· + Cl₂ → CH₃Cl + Cl·

(iii) Termination:

    Cl· + Cl· → Cl₂

 

 

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ALKENES (OLEFINS)

General formula: CₙH₂ₙ

Contain one double bond (one σ and one π bond).

 

PREPARATION OF ALKENES

1. Dehydration of Alcohols

   CH₃–CH₂OH → CH₂=CH₂ + H₂O  (Conc. H₂SO₄)

 

2. Dehydrohalogenation of Alkyl Halides

   CH₃–CH₂Br + KOH → CH₂=CH₂ + KBr + H₂O

 

CHEMICAL REACTIONS OF ALKENES

1. Addition of Hydrogen (Hydrogenation)

   CH₂=CH₂ + H₂ → CH₃–CH₃

 

2. Addition of Halogens

   CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

 

3. Addition of HX (Markovnikov’s Rule)

   CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃

 

MECHANISM OF ELECTROPHILIC ADDITION (HBr)

 

ALKYNES

General formula: CₙH₂ₙ₋₂

Contain one triple bond (one σ and two π bonds).

 

PREPARATION OF ALKYNES

From calcium carbide:

CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂

 

PROPERTIES

• Terminal alkynes show acidic nature

• Undergo addition reactions

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AROMATIC HYDROCARBONS

Benzene formula: C₆H₆

STRUCTURE OF BENZENE

• Planar hexagonal ring

• All carbon atoms are sp² hybridised

• Resonance stabilised structure

 

RESONANCE STRUCTURES OF BENZENE

Two equivalent Kekulé structures explain the stability of benzene.

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IMPORTANT ELECTRONIC EFFECTS

1. INDUCTIVE EFFECT (–I and +I)

Permanent displacement of σ electrons due to electronegativity difference.

• –I effect: –NO₂, –Cl, –COOH

• +I effect: –CH₃, –C₂H₅

 

2. HYPERCONJUGATION

Delocalisation of σ electrons of C–H bond with adjacent π system.

• Increases stability of alkenes and carbocations

• More substituted alkene → More hyperconjugation → More stable

 

3. RESONANCE EFFECT (+R and –R)

Delocalisation of π electrons across conjugated systems.

• +R groups: –OH, –NH₂

• –R groups: –NO₂, –CN

ENVIRONMENTAL ASPECTS

• Incomplete combustion causes air pollution

• Benzene is carcinogenic

 

IMPORTANT NCERT POINTS

• Alkanes are least reactive

• Reactivity of alkenes and alkynes is due to π bonds

• Aromatic compounds are highly stable due to resonance