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Syllabus
🚀 IIT JEE
Subject:Chemistry Syllabus Structure

Hydrocarbons

Syllabus Unit: Organic Chemistry

Why it matters

Chapter overview

Hydrocarbons are the simplest organic compounds, built purely from carbon and hydrogen, and JEE tests them through the lens of reaction mechanism: alkanes react by free-radical substitution, alkenes and alkynes by electrophilic addition (governed by Markovnikov's rule and carbocation stability), and aromatic rings by electrophilic substitution that preserves the special stability granted by Hückel's (4n+2) π-electron rule.

Chemistry Syllabus Structure Organic Chemistry ~34% weightage
Live Animation1 / 4
1Initiation: X2 → 2X•
2Propagation: R-H + X• → R• + HX
3Propagation: R• + X2 → R-X + X•
4Termination: radicals combine

1Alkanes: Preparation & Properties

Alkanes (CnH2n+2) are saturated hydrocarbons made by reduction, decarboxylation or the Wurtz coupling, and react almost exclusively via free-radical substitution because their strong, non-polar sigma bonds resist electrophiles and nucleophiles alike.

Learning Objectives

  • Name three laboratory preparation methods for alkanes: Wurtz reaction, decarboxylation, and Kolbe's electrolysis.
  • State why the Wurtz reaction is best suited to making symmetrical alkanes.
  • Outline the three-step free-radical chain mechanism (initiation, propagation, termination) for alkane halogenation.
  • Explain why alkanes resist most reagents except halogens (under UV/heat) and O2 (combustion).

Key Terms

Wurtz reaction2 R-X + 2Na (dry ether) → R-R + 2NaX; couples two alkyl halides, best for making symmetrical alkanes.
DecarboxylationHeating the sodium salt of a carboxylic acid with soda lime (NaOH + CaO) removes CO2, giving an alkane with one fewer carbon.
Kolbe's electrolysisElectrolysis of a concentrated carboxylate salt solution; at the anode, carboxylate radicals lose CO2 and couple to form an alkane.
Free-radical substitutionThe characteristic reaction mechanism of alkanes (e.g. halogenation), proceeding via homolytic bond cleavage and radical chain propagation.

Alkanes, CnH2n+2, are saturated hydrocarbons in which every carbon is sp3-hybridised and bonded by strong, non-polar sigma bonds — making them the least reactive class of organic compounds.

Preparation

  • Wurtz reaction — 2R-X + 2Na (dry ether) → R-R + 2NaX; best for symmetrical alkanes, since mixed alkyl halides give a mixture of products.
  • Decarboxylation — the sodium salt of a carboxylic acid heated with soda lime loses CO2, giving an alkane with one carbon fewer than the acid.
  • Kolbe's electrolysis — electrolysis of a concentrated carboxylate salt solution; at the anode, RCOO- loses an electron and CO2 to form R•, which dimerises to R-R.

Free-radical halogenation mechanism

  • Initiation — X2 absorbs UV/heat and homolytically splits into 2 X• radicals.
  • Propagation — X• abstracts an H from the alkane, forming R• and HX; R• then reacts with X2 to give R-X and regenerate X•, continuing the chain.
  • Termination — two radicals combine (X•+X•, R•+R•, or R•+X•) to end the chain.
Common MistakeFree-radical halogenation is rarely fully selective — JEE tests which C-H bond is abstracted preferentially: 3° > 2° > 1°, because more substituted radicals are more stable.

Alkanes are non-polar and insoluble in water, undergoing combustion (complete: CO2+H2O; incomplete: CO/soot) and free-radical halogenation, while resisting addition, nucleophilic and electrophilic attack due to the absence of π-bonds or polar sites.