Hydrocarbons

Syllabus Unit: Organic Chemistry

Why it matters

Chapter overview

Hydrocarbons split into three reactivity classes that each follow a distinct mechanism: alkanes react by free-radical substitution, alkenes and alkynes react by electrophilic addition through a carbocation intermediate, and benzene — despite having formal double bonds — reacts by electrophilic substitution because its delocalised aromatic system resists addition. NEET builds entire questions around picking the right mechanism, applying Markovnikov's rule (and its peroxide-driven reversal), and using Hückel's rule to judge aromaticity.

Chemistry Syllabus Organic Chemistry ~50% weightage

Live Animation1 / 4

1InitiationCl2 → 2Cl• under UV light

2Propagation 1Cl• + CH4 → HCl + •CH3

3Propagation 2•CH3 + Cl2 → CH3Cl + Cl•

4TerminationRadical-radical combination ends the chain

1Alkanes & Free Radical Reactions

Alkanes (CnH2n+2) have no reactive pi bonds or lone pairs, so their signature reaction — halogenation — proceeds by a self-sustaining free-radical chain instead of ionic attack.

Learning Objectives

Describe the three steps of free radical halogenation — initiation, propagation, termination — using methane chlorination as the example.
Explain why free radical substitution occurs preferentially at a more substituted (3° > 2° > 1°) C–H bond in higher alkanes.
State the Wurtz reaction as a method for preparing symmetrical alkanes from alkyl halides.
Write the general combustion equation for an alkane and its formula CnH2n+2.

Key Terms

Free Radical SubstitutionA reaction mechanism in which a radical (an atom/group with an unpaired electron) replaces a hydrogen atom via a self-sustaining chain of initiation, propagation and termination steps.

Initiation StepThe step that generates the first radicals, typically by homolytic bond cleavage under UV light or heat (e.g. Cl2 → 2Cl•).

Propagation StepThe repeating pair of steps that consumes one radical and produces another, sustaining the chain reaction and generating product each cycle.

Wurtz ReactionPreparation of a symmetrical alkane by treating an alkyl halide with sodium metal in dry ether: 2R–X + 2Na → R–R + 2NaX.

Alkanes (general formula CnH2n+2) contain only strong, non-polar C–C and C–H sigma bonds and no pi bonds or lone pairs, making them relatively unreactive except toward free radical halogenation and combustion.

Free radical chlorination of methane

Initiation: Cl2 → 2Cl• (homolysis under UV light or heat, generating the first radicals).
Propagation (step 1): Cl• + CH4 → HCl + •CH3.
Propagation (step 2): •CH3 + Cl2 → CH3Cl + Cl• (regenerates Cl•, sustaining the chain).
Termination: any radical-radical combination removes radicals from the cycle, e.g. Cl• + •CH3 → CH3Cl, or 2Cl• → Cl2.

Key PointIn higher alkanes, substitution occurs preferentially at 3° > 2° > 1° C–H bonds because the resulting free radical is stabilised the same way a carbocation is — by hyperconjugation from adjacent C–H bonds — so a more substituted C–H is abstracted faster despite being statistically less abundant.

Preparation and combustion

Wurtz reaction: 2R–X + 2Na →(dry ether) R–R + 2NaX | Combustion: CnH2n+2 + (3n+1)/2 O2 → n CO2 + (n+1) H2O