Alkanes

Definition and Structure of Alkanes

Alkanes are saturated hydrocarbons — they contain only hydrogen and carbon atoms and all carbon–carbon bonds are single covalent bonds (σ bonds).
Saturated means that each carbon atom is bonded to the maximum possible number of hydrogen atoms; there are no double or triple bonds.
General formula: CₙH₂ₙ₊₂, where n is the number of carbon atoms.
Examples:
- Methane: CH₄
- Ethane: C₂H₆
- Propane: C₃H₈
- Butane: C₄H₁₀

Type of bonding: Single covalent bonds between atoms (C–C and C–H) formed by the overlap of atomic orbitals to create σ bonds.
All bond angles are approximately 109.5° (tetrahedral geometry around carbon).
These bonds are non-polar because the electronegativity difference between C and H is very small.

Chemical Reactivity
- Alkanes are generally unreactive due to:
  - Strong C–C and C–H σ bonds.
  - Non-polar nature, meaning they don’t react easily with polar reagents.
- They mainly undergo combustion and substitution reactions.
Physical Properties
- Boiling point increases as the number of carbon atoms increases (due to stronger London dispersion forces).
- State at room temperature:
  - C₁–C₄: gases
  - C₅–C₁₇: liquids
  - C₁₈ and above: solids
- Solubility: Insoluble in water (non-polar), soluble in non-polar solvents.

Alkanes burn in oxygen to produce carbon dioxide and water.
Complete combustion (sufficient oxygen):CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion (limited oxygen):
- Produces carbon monoxide (CO), a toxic gas:2CH₄ + 3O₂ → 2CO + 4H₂O
- Or produces carbon (soot):CH₄ + O₂ → C + 2H₂O

In a substitution reaction, one atom or group in a molecule is replaced by another atom or group.
For alkanes, hydrogen atoms can be replaced by halogen atoms (e.g., chlorine) under certain conditions.

Type of reaction: Photochemical reaction — requires ultraviolet (UV) light to provide the activation energy (Eₐ) needed to break the strong bonds.
General equation (monosubstitution example):CH₄ + Cl₂ → CH₃Cl + HCl
(methane + chlorine → chloromethane + hydrogen chloride)
Conditions:
- UV light (from sunlight or a UV lamp).
- Presence of gaseous chlorine and an alkane.
Mechanism (radical substitution, simplified for syllabus):
1. Initiation:
  - UV light breaks the Cl–Cl bond into two chlorine radicals:Cl₂ → 2Cl•
2. Propagation:
  - Cl• attacks CH₄, forming HCl and a methyl radical (CH₃•):CH₄ + Cl• → CH₃• + HCl
  - The CH₃• reacts with Cl₂ to form CH₃Cl and another Cl•:CH₃• + Cl₂ → CH₃Cl + Cl•
3. Termination:
  - Radicals combine to form stable molecules.
Limitation for syllabus: Focus only on monosubstitution (one hydrogen atom replaced by one chlorine atom).
Products for monosubstitution:
- Structural formula: CH₃–Cl
- Displayed formula: