14.1 Alkanes

This topic introduces alkanes — the simplest family of hydrocarbons — and explains their bonding, structure, and general formula.

Definition

Alkanes are saturated hydrocarbons.

Simple explanation: Alkanes are hydrocarbons that contain only single bonds between carbon atoms.

Main Idea

• Alkanes are saturated hydrocarbons.
• They have the general formula CₙH₂ₙ₊₂.
• Each carbon atom forms four bonds.
• Each hydrogen atom forms only one bond.
• The simplest alkane is methane (CH₄) — the main component of natural gas.

Simple explanation: Alkanes are hydrocarbons with only single bonds — they are "saturated" with hydrogen.

Core Concept

General Formula

C_nH_2n+2

• n = number of carbon atoms
• 2n + 2 = number of hydrogen atoms

Simple explanation: The general formula tells you how many hydrogen atoms are in an alkane with n carbon atoms.

Examples of Alkanes

Simple explanation: The first four alkanes are methane, ethane, propane, and butane.

Methane — The Simplest Alkane

• Carbon forms four bonds.
• Hydrogen forms only one bond.
• So carbon bonds with four hydrogen atoms.

Electron dot and cross structure for methane:

The book shows:

H : C : H

with H atoms around the C atom.

Structural formula for methane:

H–C–H

with H atoms around the C atom.

Simple explanation: Methane has one carbon bonded to four hydrogens.

Ethane — The Second Alkane

Ethane (C₂H₆) has the following structure:

H–C–C–H

with H atoms around each C atom.

Simple explanation: Ethane has two carbons bonded to each other, with hydrogens filling the remaining bonds.

Detailed Explanation

What Are Alkanes?

• Saturated hydrocarbons — they contain only single bonds (C–C and C–H).
• The simplest family of organic compounds.
• Also called paraffins (from Latin: parum affinis = little affinity).

• Each carbon atom forms four single bonds.
• Each hydrogen atom forms one single bond.
• They have the maximum number of hydrogen atoms possible for the number of carbon atoms.

Simple explanation: Alkanes are "saturated" because they hold as many hydrogen atoms as possible.

Structural Formula — What It Shows

Recall that a structural formula shows which atoms are bonded to each other.

For methane (CH₄):

\beginarrayc H \\ | \\ H–C–H \\ | \\ H \endarray

- Carbon is bonded to four hydrogen atoms.

For ethane (C₂H₆):

\beginarrayc H H \\ | | \\ H–C–C–H \\ | | \\ H H \endarray

- Each carbon is bonded to three hydrogen atoms and one carbon atom.

Simple explanation: A structural formula shows exactly which atoms are connected.

The First Four Alkanes

Simple explanation: The first four alkanes are all gases at room temperature.

Isomerism in Alkanes

Alkanes with four or more carbon atoms can have structural isomers.

• n-Butane: CH₃–CH₂–CH₂–CH₃ (straight chain)
• Iso-butane: CH₃–CH(CH₃)–CH₃ (branched chain)

Both have the same molecular formula (C₄H₁₀) but different structures.

Simple explanation: Butane can be straight or branched — these are isomers.

Naming Alkanes

• A prefix indicating the number of carbon atoms.
• The suffix -ane.

Simple explanation: The name tells you how many carbons are in the chain.

Intermediate Level

Structure of Alkanes

• Each carbon atom forms four single bonds.
• Each hydrogen atom forms one single bond.
• Carbon atoms form straight chains, branched chains, or rings (cycloalkanes).
• All bonds are sigma (σ) bonds — strong and stable.

Simple explanation: Alkanes have a "backbone" of carbon atoms with hydrogens attached.

Examples of Alkanes

Simple explanation: Alkanes can be written as full structures or condensed formulas.

Physical Properties of Alkanes

Simple explanation: Alkanes are colourless and odourless; they are gases, liquids, or solids depending on chain length.

Chemical Properties of Alkanes

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

• Combustion — burning in oxygen to produce CO₂ and H₂O.
• Halogenation — reaction with halogens in sunlight (substitution).
• Cracking — breaking large alkanes into smaller ones.

Simple explanation: Alkanes are unreactive except for combustion and halogenation.

Isomerism in Alkanes

Alkanes with four or more carbon atoms can have structural isomers.

Examples:

Simple explanation: More carbons means more ways to arrange them → more isomers.

Uses of Alkanes

Simple explanation: Alkanes are used as fuels and as raw materials for making other chemicals.

Important Points

• - Alkanes: saturated hydrocarbons.
• - General formula: CₙH₂ₙ₊₂.
• - Bonding: only single bonds (C–C and C–H).
• - Simplest alkane: methane (CH₄) — main component of natural gas.
• - Examples:
• - Methane: CH₄
• - Ethane: C₂H₆
• - Propane: C₃H₈
• - Butane: C₄H₁₀
• - Naming: ends in -ane.
• - Physical properties: colourless, odourless; gases/liquids/solids depending on chain length; insoluble in water.
• - Chemical properties: relatively unreactive; undergo combustion and halogenation.
• - Isomerism: alkanes with 4+ carbons can have isomers.
• - Uses: fuels (natural gas, LPG, petrol, diesel) and raw materials.

Remember

• - Alkanes = saturated hydrocarbons.
• - General formula: CₙH₂ₙ₊₂.
• - Only single bonds (C–C and C–H).
• - Simplest alkane: methane (CH₄).
• - Examples:
• - Methane: CH₄
• - Ethane: C₂H₆
• - Propane: C₃H₈
• - Butane: C₄H₁₀
• - Naming: ends in -ane.
• - Relatively unreactive.
• - Uses: fuels and raw materials.

Common Mistakes

• 1. Thinking alkanes are reactive. Alkanes are relatively unreactive except for combustion and halogenation.
• 2. Confusing alkanes with alkenes. Alkanes have only single bonds; alkenes have at least one double bond.
• 3. Forgetting the general formula. Alkanes: CₙH₂ₙ₊₂.
• 4. Thinking methane is not an alkane. Methane (CH₄) is the simplest alkane.
• 5. Thinking all alkanes are gases. Only C₁–C₄ are gases; larger alkanes are liquids or solids.
• 6. Confusing structural isomers with different compounds. Isomers have the same molecular formula but different structures.

Quick Revision

• Alkanes: saturated hydrocarbons.
• General formula: CₙH₂ₙ₊₂.
• Each carbon atom forms four bonds; each hydrogen atom forms only one bond.
• The simplest alkane is methane (CH₄) — the main component of natural gas.
• Examples:
• Methane: CH₄
• Ethane: C₂H₆
• Propane: C₃H₈
• Butane: C₄H₁₀
• Structural formula: shows which atoms are bonded to each other.
• Naming: ends in -ane (methane, ethane, propane, butane, pentane, hexane, etc.).
• Physical properties: colourless, odourless; gases (C₁–C₄), liquids (C₅–C₁₇), solids (C₁₈+); insoluble in water.
• Chemical properties: relatively unreactive; undergo combustion and halogenation.
• Isomerism: alkanes with 4+ carbons can have structural isomers.
• Uses: fuels (natural gas, LPG, petrol, diesel) and raw materials for making other chemicals.

60-Second Revision

• Alkanes = saturated hydrocarbons.
• General formula: CₙH₂ₙ₊₂.
• Simplest alkane: methane (CH₄).
• Examples: CH₄, C₂H₆, C₃H₈, C₄H₁₀.
• Relatively unreactive.
• Warning: alkanes have only single bonds; alkenes have at least one double bond.