Chemistry A-Level – Alkanes: Revision Notes
1. Introduction to Alkanes
• Definition: Alkanes are saturated hydrocarbons, meaning they contain only
single C–C and C–H bonds.
• General Formula: CₙH₂ₙ₊₂
• Homologous Series: A family of compounds with the same functional group
and similar chemical properties.
• Examples:
o Methane: CH₄
o Ethane: C₂H₆
o Propane: C₃H₈
o Butane: C₄H₁₀
2. Bonding and Structure
• Tetrahedral shape (109.5° bond angle) around each carbon atom.
• Sigma (σ) bonds form by overlap of orbitals directly between bonding atoms.
• Alkanes are non-polar due to small difference in electronegativity between C
and H.
3. Physical Properties
• Boiling points increase with chain length due to stronger London dispersion
forces.
• Branched alkanes have lower boiling points than straight-chain isomers (less
surface contact).
• Insoluble in water: non-polar molecules don't interact with polar water
molecules.
• Soluble in non-polar solvents.
4. Reactivity of Alkanes
• Low reactivity due to:
o Strong C–C and C–H bonds
, o Lack of polarity
• Main reactions:
o Combustion (complete and incomplete)
o Free radical substitution (e.g. with halogens)
5. Combustion of Alkanes
• Complete Combustion:
o Alkane + O₂ → CO₂ + H₂O
o Produces maximum energy
• Incomplete Combustion:
o Products: CO (carbon monoxide) or C (soot)
o Less energy released
o CO is toxic – binds to haemoglobin
Example:
• CH₄ + 2O₂ → CO₂ + 2H₂O (complete)
• CH₄ + 1.5O₂ → CO + 2H₂O (incomplete)
6. Free Radical Substitution
Occurs in presence of UV light with halogens (e.g. Cl₂):
Steps:
1. Initiation:
a. Cl₂ → 2Cl• (homolytic fission under UV)
2. Propagation:
a. CH₄ + Cl• → CH₃• + HCl
b. CH₃• + Cl₂ → CH₃Cl + Cl•
3. Termination:
a. 2Cl• → Cl₂
b. CH₃• + Cl• → CH₃Cl
c. CH₃• + CH₃• → C₂H₆
• Limitations:
o Multiple substitutions possible (e.g. CH₃Cl, CH₂Cl₂)
o Formation of a mixture of products
1. Introduction to Alkanes
• Definition: Alkanes are saturated hydrocarbons, meaning they contain only
single C–C and C–H bonds.
• General Formula: CₙH₂ₙ₊₂
• Homologous Series: A family of compounds with the same functional group
and similar chemical properties.
• Examples:
o Methane: CH₄
o Ethane: C₂H₆
o Propane: C₃H₈
o Butane: C₄H₁₀
2. Bonding and Structure
• Tetrahedral shape (109.5° bond angle) around each carbon atom.
• Sigma (σ) bonds form by overlap of orbitals directly between bonding atoms.
• Alkanes are non-polar due to small difference in electronegativity between C
and H.
3. Physical Properties
• Boiling points increase with chain length due to stronger London dispersion
forces.
• Branched alkanes have lower boiling points than straight-chain isomers (less
surface contact).
• Insoluble in water: non-polar molecules don't interact with polar water
molecules.
• Soluble in non-polar solvents.
4. Reactivity of Alkanes
• Low reactivity due to:
o Strong C–C and C–H bonds
, o Lack of polarity
• Main reactions:
o Combustion (complete and incomplete)
o Free radical substitution (e.g. with halogens)
5. Combustion of Alkanes
• Complete Combustion:
o Alkane + O₂ → CO₂ + H₂O
o Produces maximum energy
• Incomplete Combustion:
o Products: CO (carbon monoxide) or C (soot)
o Less energy released
o CO is toxic – binds to haemoglobin
Example:
• CH₄ + 2O₂ → CO₂ + 2H₂O (complete)
• CH₄ + 1.5O₂ → CO + 2H₂O (incomplete)
6. Free Radical Substitution
Occurs in presence of UV light with halogens (e.g. Cl₂):
Steps:
1. Initiation:
a. Cl₂ → 2Cl• (homolytic fission under UV)
2. Propagation:
a. CH₄ + Cl• → CH₃• + HCl
b. CH₃• + Cl₂ → CH₃Cl + Cl•
3. Termination:
a. 2Cl• → Cl₂
b. CH₃• + Cl• → CH₃Cl
c. CH₃• + CH₃• → C₂H₆
• Limitations:
o Multiple substitutions possible (e.g. CH₃Cl, CH₂Cl₂)
o Formation of a mixture of products
