14.3 Properties of Alkanes

This topic explains the physical and chemical properties of alkanes.

Main Idea

Alkane molecules are essentially non-polar. They are less dense than water and do not dissolve in it.

Chemically, alkanes do not react with most ionic compounds. The lack of reactivity makes them useful solvents.

• Alkanes containing up to four carbon atoms are colourless and odourless gases.
• Alkanes containing five to seventeen carbon atoms are colourless and odourless liquids.
• Higher alkanes are solids that are also colourless and odourless.

Simple explanation: Alkanes are non-polar, insoluble in water, and unreactive — making them useful as solvents and fuels.

Core Concept

Physical Properties

Chemical Properties

Detailed Explanation

Physical Properties of Alkanes

• Alkane molecules are essentially non-polar.
• This is because carbon and hydrogen have similar electronegativities.
• So alkanes do not dissolve in water (a polar solvent).

• Alkanes are less dense than water.
• They float on water.

• Alkanes do not dissolve in water.
• They dissolve in non-polar solvents (like other alkanes, benzene, etc.).

4. States at room temperature:

• All alkanes are colourless.
• All alkanes are odourless.

Simple explanation: Alkanes are colourless, odourless, non-polar, and float on water.

Chemical Properties of Alkanes

• C–C and C–H bonds are strong and non-polar.
• They do not have functional groups.
• They do not react with most ionic compounds.

• The lack of reactivity makes them useful solvents.
• For example, hexane is used to extract vegetable oils from corn, soybeans, cotton seeds, etc.

Simple explanation: Alkanes are unreactive — they don't react with most chemicals. This makes them good solvents.

Key Reaction 1: Halogenation

Although unreactive towards ionic substances, alkanes readily react with halogens in sunlight.

A chemical reaction that takes place in the presence of sunlight is called a photochemical reaction.

General reaction:

CH₄ + Cl₂ sunlight→ CH₃Cl + HCl

• Methane + Chlorine → Chloromethane + Hydrogen chloride
• This is a substitution reaction — one atom (H) is replaced by another (Cl).

Simple explanation: In sunlight, alkanes react with halogens — one hydrogen is replaced by a halogen.

Key Reaction 2: Combustion

Alkanes burn in oxygen to produce carbon dioxide, water, and heat.

General reaction:

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

Simple explanation: Alkanes burn to produce CO₂, H₂O, and heat — this is why they are used as fuels.

Uses Based on Properties

Intermediate Level

Why Alkanes Are Non-Polar

• Carbon and hydrogen have similar electronegativities.
• So C–H bonds are essentially non-polar.
• C–C bonds are also non-polar.
• So the whole molecule is non-polar.

Simple explanation: Similar electronegativities → non-polar bonds → non-polar molecule.

Why Alkanes Are Insoluble in Water

• Water is a polar solvent.
• Alkanes are non-polar.
• "Like dissolves like" — non-polar substances dissolve in non-polar solvents, not in polar ones.
• So alkanes are insoluble in water.

Simple explanation: Non-polar alkanes don't mix with polar water.

Physical States and Chain Length

Simple explanation: Bigger molecules → stronger intermolecular forces → more likely to be solid.

Halogenation — In Detail

Definition: A chemical reaction that takes place in presence of sunlight is called a photochemical reaction.

General reaction:

R–H + X₂ sunlight→ R–X + HX

• R–H = alkane
• X₂ = halogen (Cl₂, Br₂)
• R–X = haloalkane
• HX = hydrogen halide

Example:

CH₄ + Cl₂ sunlight→ CH₃Cl + HCl

Simple explanation: In sunlight, a halogen replaces a hydrogen atom in the alkane.

Combustion — In Detail

Complete combustion (excess oxygen):

CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion (limited oxygen):

2CH₄ + 3O₂ → 2CO + 4H₂O

- Incomplete combustion produces carbon monoxide (CO) — a toxic gas.

Simple explanation: With enough oxygen, alkanes burn cleanly to CO₂ and H₂O. With limited oxygen, they produce toxic CO.

Uses of Alkanes Based on Properties

Important Points

• - Alkanes are essentially non-polar.
• - Less dense than water — they float on water.
• - Insoluble in water — they dissolve in non-polar solvents.
• - States:
• - C₁–C₄ → gases
• - C₅–C₁₇ → liquids
• - C₁₈+ → solids
• - Colourless and odourless.
• - Chemically unreactive — do not react with most ionic compounds.
• - Useful as solvents — e.g., hexane for extracting vegetable oils.
• - Halogenation: reaction with halogens in sunlight (photochemical reaction).
• - CH₄ + Cl₂ → CH₃Cl + HCl
• - Combustion: burning in oxygen → CO₂ + H₂O + heat.
• - CH₄ + 2O₂ → CO₂ + 2H₂O

Remember

• - **Alkanes** = non-polar.
• - Less dense than water → float.
• - Insoluble in water.
• - States: gases (C₁–C₄), liquids (C₅–C₁₇), solids (C₁₈+).
• - Colourless and odourless.
• - Unreactive → useful as solvents.
• - Halogenation: in sunlight (CH₄ + Cl₂ → CH₃Cl + HCl).
• - Combustion: burns to CO₂ + H₂O.

Common Mistakes

• 1. Thinking alkanes are polar. Alkanes are non-polar.
• 2. Thinking alkanes dissolve in water. They are insoluble in water.
• 3. Thinking alkanes are reactive. They are relatively unreactive.
• 4. Forgetting the role of sunlight in halogenation. Halogenation is a photochemical reaction.
• 5. Confusing complete and incomplete combustion. Complete → CO₂ + H₂O; incomplete → CO + H₂O.
• 6. Thinking all alkanes are gases. Only C₁–C₄ are gases; C₅–C₁₇ are liquids; C₁₈+ are solids.

Quick Revision

• Physical properties of alkanes:
• Alkane molecules are essentially non-polar.
• They are less dense than water and do not dissolve in it.
• Alkanes containing up to four carbon atoms are colourless and odourless gases.
• Alkanes containing five to seventeen carbon atoms are colourless and odourless liquids.
• Higher alkanes are solids that are also colourless and odourless.
• Chemical properties of alkanes:
• Alkanes do not react with most ionic compounds.
• The lack of reactivity makes them useful solvents.
• Halogenation: alkanes readily react with halogens in sunlight (photochemical reaction).
• CH₄ + Cl₂ → CH₃Cl + HCl
• Combustion: alkanes burn in oxygen to produce CO₂, H₂O, and heat.
• CH₄ + 2O₂ → CO₂ + 2H₂O
• Uses:
• Non-polar solvent (hexane for vegetable oil extraction).
• Fuels (natural gas, LPG, petrol, diesel).
• Lubricants, waxes.

60-Second Revision

• Alkanes = non-polar, less dense than water, insoluble in water.
• States: gases (C₁–C₄), liquids (C₅–C₁₇), solids (C₁₈+).
• Colourless and odourless.
• Unreactive → useful as solvents.
• Halogenation in sunlight; combustion to CO₂ + H₂O.
• Warning: alkanes are non-polar — they do not dissolve in water.