Chapter 8: Aliphatic Hydrocarbons – Chemistry 2nd Year Notes

This chapter covers Aliphatic Hydrocarbons from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Aliphatic (open-chain) hydrocarbons are of three kinds: alkanes (saturated, only single bonds), alkenes (one carbon to carbon double bond) and alkynes (one carbon to carbon triple bond). These are the simplest organic compounds and the starting point for making almost every other organic substance.

Here you will learn how each type is prepared and how they react, including free-radical substitution of alkanes, the addition reactions of alkenes, Markownikoff rule, and tests for unsaturation.

Learning Objectives

• Distinguish alkanes, alkenes and alkynes and give their general formulas.
• Describe methods of preparing alkanes, alkenes and alkynes.
• Explain the free-radical substitution (halogenation) of alkanes.
• Describe the addition reactions of alkenes and alkynes.
• State and apply Markownikoff rule.
• Explain the tests that distinguish saturated from unsaturated hydrocarbons.

Key Concepts

The Three Families

Alkanes (CnH2n+2) are saturated with only single bonds; alkenes (CnH2n) contain one double bond; alkynes (CnH2n-2) contain one triple bond. The name endings are -ane for single, -ene for double and -yne for triple bonds.

Alkanes and Free-Radical Halogenation

Alkanes are rather unreactive (the old name paraffin means little affinity) because their bonds are strong and non-polar. Their main reactions are combustion (used as fuels) and substitution by halogens in sunlight. In sunlight methane reacts with chlorine by a free-radical chain mechanism with three stages: initiation (chlorine splits into radicals), propagation (radicals react to make products and new radicals) and termination (radicals combine).

Alkenes and Addition Reactions

The double bond is a region of high electron density, so alkenes readily undergo addition reactions in which the pi bond breaks and atoms add to the two carbons. Common additions include hydrogenation (H2 with nickel gives an alkane), halogenation (X2 gives a dihalide), addition of hydrogen halides (HX gives a haloalkane), hydration (water with acid gives an alcohol) and polymerisation (many alkene molecules join to form a polymer such as polythene).

Markownikoff Rule and Tests for Unsaturation

Markownikoff rule states that when a hydrogen halide adds to an unsymmetrical alkene, the hydrogen goes to the carbon that already has more hydrogen atoms and the halogen to the carbon with fewer; for example propene plus HBr gives 2-bromopropane as the major product. Alkenes and alkynes decolourise bromine water (orange to colourless) and dilute alkaline potassium permanganate (Baeyer reagent), which are simple tests for a multiple bond.

Alkynes

Alkynes contain a triple bond and, like alkenes, undergo addition reactions. Ethyne (acetylene) is prepared from calcium carbide and water (CaC2 + 2H2O gives C2H2 + Ca(OH)2) and is used in oxy-acetylene welding because it burns with a very hot flame. The hydrogen on a terminal triple-bonded carbon is weakly acidic, so ethyne reacts with sodium to release hydrogen.

Important Definitions

Formulas & Rules

Item	Fact
Formulas	alkane CnH2n+2, alkene CnH2n, alkyne CnH2n-2
Alkanes	combustion and free-radical substitution
Alkenes	addition (H2/Ni, X2, HX, H2O/H+, polymerisation)
Markownikoff	H to the carbon with more H
Ethyne	CaC2 + 2H2O gives C2H2 + Ca(OH)2

Diagrams & Illustrations

Alkane homologous series: the first members methane, ethane, propane, butane and pentane, each differing by a CH2 unit.

Addition reactions of alkenes: a map showing hydrogenation, halogenation, addition of hydrogen halides, hydration and polymerisation around a central carbon to carbon double bond.

Free-radical substitution: the three steps of the halogenation of methane, namely initiation, propagation and termination.

Solved Examples & Numericals

Example 1: Markownikoff rule

Predict the major product of propene plus HCl. By Markownikoff rule, hydrogen adds to the CH2 (more hydrogens) and chlorine to the middle carbon, giving 2-chloropropane.

Example 2: Test for unsaturation

How would you distinguish ethane from ethene? Add bromine water; ethene decolourises it quickly while ethane does not. Alkaline potassium permanganate can be used the same way.

Example 3: Preparation

How is ethyne prepared from calcium carbide? Calcium carbide reacts with water: CaC2 + 2H2O gives C2H2 + Ca(OH)2, producing ethyne gas.

Short Questions & Answers

Long Questions & Answers

Q1: Describe the free-radical halogenation of methane.

When methane and chlorine are mixed in sunlight, the hydrogen atoms of methane are replaced one by one by chlorine in a substitution that proceeds by a free-radical chain mechanism. In the initiation step, ultraviolet light splits a chlorine molecule into two chlorine radicals. In the propagation steps a chlorine radical removes a hydrogen atom from methane to give a methyl radical and hydrogen chloride, and the methyl radical then reacts with another chlorine molecule to give chloromethane and a new chlorine radical, so the chain continues. In the termination steps two radicals combine, ending the chain. Further substitution can give dichloromethane, trichloromethane and tetrachloromethane.

Q2: Discuss the addition reactions of alkenes.

Because the carbon to carbon double bond is a region of high electron density, alkenes readily undergo addition reactions in which the weaker pi bond breaks and new atoms add to the two carbons. With hydrogen and a nickel catalyst an alkene is hydrogenated to the corresponding alkane. With halogens such as bromine it forms a dihalide and bromine water is decolourised, a test for unsaturation. With hydrogen halides it forms haloalkanes, and for unsymmetrical alkenes the product follows Markownikoff rule. With water and an acid catalyst it undergoes hydration to give an alcohol. Under suitable conditions many alkene molecules add to one another in polymerisation, as when ethene forms polythene.

Q3: Compare alkanes, alkenes and alkynes in structure and reactivity.

Alkanes (CnH2n+2) are saturated with only single bonds; their strong non-polar bonds make them relatively unreactive, so their main reactions are combustion and free-radical substitution. Alkenes (CnH2n) contain one double bond and alkynes (CnH2n-2) one triple bond; both are unsaturated and much more reactive, undergoing addition reactions across the multiple bond. Alkenes and alkynes decolourise bromine water and potassium permanganate, whereas alkanes do not, which provides a simple test to tell them apart. A special feature of terminal alkynes is the weak acidity of the triple-bonded carbon to hydrogen bond, which lets ethyne react with sodium to release hydrogen.

MCQs with Answers

Quick Revision Summary

• Alkanes CnH2n+2 (single), alkenes CnH2n (double), alkynes CnH2n-2 (triple).
• Alkanes: unreactive; combustion plus free-radical substitution.
• Alkenes: reactive; addition (H2/Ni, X2, HX, H2O/H+, polymerisation).
• Markownikoff: hydrogen adds to the carbon with more hydrogen atoms.
• Unsaturation tests: decolourise bromine water and potassium permanganate.
• Ethyne from calcium carbide; used in welding.

Exam Tips

• Write the free-radical mechanism with all three steps; a frequent long question.
• Apply Markownikoff rule carefully and name the major product.
• Learn the two unsaturation tests and their colour changes.
• Remember the catalyst Ni for hydrogenation and the ethyne preparation from calcium carbide.
• Do not confuse -ene (double) with -yne (triple) endings.