Part 1 of 5Bonding in Alkanes
The simplicity of the C–C and C–H bonds explains why alkanes are such unreactive molecules.
Saturated
- Alkanes are saturated hydrocarbons, containing only carbon and hydrogen joined by single bonds.
General formula
- Open-chain alkanes follow CₙH₂ₙ₊₂, while cycloalkanes follow CₙH₂ₙ.
Sigma bonds
- Each bond is a σ bond formed by direct overlap of orbitals along the line between the two nuclei.
Shape
- Every carbon has four bonding pairs, so the shape around it is tetrahedral with bond angles of about 109.5°.
Rotation
- Atoms can rotate freely about a single σ bond, so alkane molecules are flexible.
Part 2 of 5Polarity and Reactivity
Polarity and Reactivity
Non-polar bonds
- Carbon and hydrogen have very similar electronegativities, so the C–H bonds have no significant dipole.
No attack sites
- With no δ+ or δ− regions, alkanes attract neither nucleophiles nor electrophiles.
Strong bonds
- The σ bonds are strong and need a lot of energy to break, so alkanes resist most reagents.
Reactions possible
- Alkanes still burn in oxygen and react with halogens in UV light through radical mechanisms.
Part 3 of 5Boiling Points and Solubility
1
Alkane molecules are held together only by London forces, which arise from temporary induced dipoles.
2
A longer chain has more electrons and a larger surface area, so the London forces are stronger.
3
Stronger forces need more energy to overcome, so the boiling point increases as chain length increases.
4
Branched isomers have less contact between molecules, so they have weaker London forces and lower boiling points than straight chains.
5
Alkanes are insoluble in water because they cannot form hydrogen bonds with water molecules.
Part 4 of 5Examples of Alkanes
Methane
- CH₄ is the main component of natural gas and has the smallest alkane structure.
Ethane and propane
- C₂H₆ and C₃H₈ are gases at room temperature, with propane commonly sold as liquefied fuel.
Butane
- C₄H₁₀ exists as two structural isomers, butane and 2-methylpropane.
Hexane
- C₆H₁₄ is a liquid at room temperature and is used as a solvent.
| Alkane | Formula | Boiling point / °C (approx.) |
|---|---|---|
| Methane | CH₄ | −162 |
| Ethane | C₂H₆ | −89 |
| Propane | C₃H₈ | −42 |
| Butane | C₄H₁₀ | −1 |
| Pentane | C₅H₁₂ | 36 |
Part 5 of 5Exam Checklist & Tips
4.2.1 – Key Points for the Exam
- Alkane A saturated hydrocarbon with the general formula CₙH₂ₙ₊₂.
- Sigma bond A covalent bond formed by head-on overlap of orbitals along the bond axis.
- London forces Weak forces of attraction between temporary induced dipoles in neighbouring molecules.
- Saturated Containing only single bonds between carbon atoms.
Common Mistakes
- Saying alkanes are unreactive because their bonds are polar; the bonds are nonpolar.
- Explaining boiling point trends by breaking covalent bonds instead of overcoming London forces.
- Giving the shape of an alkane carbon as trigonal planar with 120° rather than tetrahedral 109.5°.
- Stating that alkanes dissolve in water because they contain hydrogen.