Alkanes: Structure, Properties, and Reactions

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Topic 2 — Key Facts for Kenyatta University (Kenya) Core concept: Alkanes are saturated hydrocarbons with only single bonds (C–C and C–H); they are relatively inert due to the strength of C–H and C–C bonds and the non-polar nature of the molecule High-yield point: Know the complete combustion equation, substitution reactions with halogens, and the differences between the various isomers of butane and pentane ⚡ Exam tip: Alkane exam questions frequently ask about the mechanism of free-radical halogenation — be able to describe the initiation, propagation, and termination steps with appropriate equations

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Alkanes: Saturated Hydrocarbons

Alkanes are the simplest family of organic compounds — hydrocarbons containing only single bonds. Their general formula is CₙH₂ₙ₊₂ for acyclic (open-chain) alkanes. The C–C bonds in alkanes are all sigma bonds formed by sp³ hybrid orbital overlap, and the C–H bonds are formed by sp³–1s overlap.

Nomenclature of Alkanes

The IUPAC names for the first ten straight-chain alkanes:

Formula	IUPAC Name	Boiling Point (°C)	Physical State (25°C)
CH₄	Methane	−162	Gas
C₂H₆	Ethane	−88	Gas
C₃H₈	Propane	−42	Gas
C₄H₁₀	Butane	−0.5	Gas
C₅H₁₂	Pentane	36	Liquid
C₆H₁₄	Hexane	69	Liquid
C₇H₁₆	Heptane	98	Liquid
C₈H₁₈	Octane	126	Liquid
C₉H₂₀	Nonane	151	Liquid
C₁₀H₂₂	Decane	174	Liquid

Structural Isomerism in Alkanes

As the number of carbon atoms increases, the possibility of branched structures (structural isomerism) increases:

Butane (C₄H₁₀) — 2 isomers:

• Butane (n-butane): CH₃–CH₂–CH₂–CH₃ (straight chain)
• 2-Methylpropane (isobutane): (CH₃)₂CH–CH₃ (branched)

Pentane (C₅H₁₂) — 3 isomers:

• Pentane: CH₃–CH₂–CH₂–CH₂–CH₃
• 2-Methylbutane: CH₃–CH(CH₃)–CH₂–CH₃
• 2,2-Dimethylpropane: C(CH₃)₄

Physical Properties of Alkanes

Boiling Points:

• Boiling point increases with molecular size (more carbon atoms → stronger London dispersion forces)
• Branched alkanes have lower boiling points than their straight-chain isomers due to reduced surface area (less effective Van der Waals contact)

Solubility:

• Alkanes are non-polar
• They are insoluble in water (which is polar and can form hydrogen bonds)
• They are soluble in non-polar organic solvents (hexane, benzene, dichloromethane)

Density:

• All alkanes have densities less than 1.0 g/cm³ (they float on water)
• Density increases with increasing molecular size

Flammability:

• Alkanes burn in excess oxygen to produce CO₂ and H₂O
• Complete combustion: CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O

⚡ Exam Tip: For any alkane combustion question, balance the equation first. Always include the state symbols (g for gas) if the question asks for a chemical equation.

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🔴 Extended — Deep Study (3mo+)

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Chemical Reactions of Alkanes

1. Combustion Reactions

Alkanes undergo combustion reactions when ignited in the presence of excess oxygen. This is the basis for their use as fuels.

Complete Combustion (excess O₂):

CH₄ + 2O₂ → CO₂ + 2H₂O    ΔH = −890 kJ/mol
C₂H₆ + 7/2 O₂ → 2CO₂ + 3H₂O    ΔH = −1560 kJ/mol

Incomplete Combustion (limited O₂):

2CH₄ + 3O₂ → 2CO + 4H₂O    (carbon monoxide formation)
CH₄ + O₂ → C + 2H₂O        (soot/carbon formation)

⚠️ Safety Note: Incomplete combustion produces carbon monoxide (CO), a colourless, odourless toxic gas. In poorly ventilated spaces, hydrocarbon fuel combustion can be fatal. CO binds to haemoglobin, reducing oxygen-carrying capacity.

2. Halogenation (Substitution Reactions)

Alkanes react with halogens (Cl₂, Br₂) in the presence of UV light (or heat) via a free-radical substitution mechanism.

Mechanism of Chlorination of Methane:

Step 1 — Initiation:

Cl₂ → 2Cl•     (homolytic bond cleavage, requires UV light)

Step 2 — Propagation:

Cl• + CH₄ → HCl + CH₃•     (hydrogen abstraction)
CH₃• + Cl₂ → CH₃Cl + Cl•  (chlorine abstraction)

Step 3 — Termination:

Cl• + Cl• → Cl₂
CH₃• + CH₃• → C₂H₆
CH₃• + Cl• → CH₃Cl

The overall reaction: CH₄ + Cl₂ → CH₃Cl + HCl

Multi-halogenation: Since the product (CH₃Cl) still contains hydrogen atoms that can be further substituted, chlorination does not stop at monochlorination. A mixture of products is obtained:

• CH₃Cl (chloromethane)
• CH₂Cl₂ (dichloromethane)
• CHCl₃ (chloroform)
• CCl₄ (carbon tetrachloride)

⚡ Exam Tip: Fluorination is too violent to control. Bromination is much slower and more selective. The reactivity order for halogenation is: F₂ > Cl₂ > Br₂ > I₂.

Selectivity in Halogenation: Bromination is more selective than chlorination because the hydrogen abstraction step (Step 2) has a higher activation energy, making it more selective for weaker C–H bonds:

• 3° C–H bond (weakes): most reactive in bromination
• 2° C–H bond (intermediate)
• 1° C–H bond (strongest): least reactive

This means: Tertiary hydrogen is replaced preferentially over secondary, which is replaced preferentially over primary.

3. Isomerisation and Cracking

Isomerisation: Straight-chain alkanes can be converted to branched alkanes under catalytic conditions (e.g., with AlCl₃ and HCl at 100°C). This is important in the petroleum industry because branched alkanes have higher octane ratings.

Cracking: Large alkane molecules are broken down into smaller, more useful molecules:

• Thermal cracking: High temperature (500–700°C) and pressure breaks C–C bonds randomly
• Catalytic cracking: Zeolite catalyst at lower temperatures produces more branched products

Example: Decane cracking:

C₁₀H₂₂ → C₆H₁₄ + C₄H₈

4. Aromatisation (Reforming)

Naphtha and gasoline-range alkanes can be converted to aromatic compounds (benzene, toluene, xylene) through catalytic reforming, a key process in the petrochemical industry.

Cycloalkanes

Cycloalkanes have the general formula CₙH₂ₙ (same as alkenes). They contain rings of carbon atoms with only single bonds.

Key Cycloalkanes:

• Cyclopropane (C₃H₆): Triangle structure, bond angle 60° (highly strained, very reactive)
• Cyclobutane (C₄H₈): Square structure, bond angle 90°
• Cyclopentane (C₅H₁₀): Pentagon, bond angle 108° (relatively stable)
• Cyclohexane (C₆H₁₂): Chair conformation most stable, no angle strain

Cyclohexane Conformations:

• Chair conformation: Most stable; all bond angles approximately 109.5°; alternating axial and equatorial positions
• Boat conformation: Less stable due to steric clash between flagpole hydrogens
• In the chair, each carbon has one axial (up/down) and one equatorial (outward) hydrogen

** cis- and trans- Cyclohexane:** Substituents on cyclohexane rings can be classified as:

• Cis: Both substituents on the same face (both up or both down)
• Trans: Substituents on opposite faces

⚡ Exam Tip: In examination questions involving cyclohexane conformations, always label which hydrogens are axial and which are equatorial. Students frequently lose marks by not specifying the positions accurately.

Uses of Alkanes

Alkane	Major Uses
Methane	Natural gas fuel, hydrogen production, CO₂ capture
Propane/Butane	LPG (liquefied petroleum gas) for heating and cooking
Octane	Fuel component; branched isomers have high octane ratings
Paraffin wax	Candles, waxed paper, waterproofing
Gasoline	Fuel for internal combustion engines

Octane Rating: A measure of a fuel’s resistance to knocking (premature combustion). n-Heptane has an octane rating of 0; isooctane (2,2,4-trimethylpentane) has a rating of 100. High-compression engines require high octane fuel to prevent knocking.

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