Properties of Alkanes
Specification Reference 4.1.2 (a)–(c)
Quick Notes
- Alkanes are saturated hydrocarbons with only single covalent bonds (σ-bonds).
- σ-bonds result from direct orbital overlap between bonding atoms.
- σ-bonds have free rotation.
- Tetrahedral shape around each carbon atom with bond angles ≈ 109.5°.
- Boiling points of alkanes:
- Increase with longer carbon chains (stronger London forces from more electrons and larger surface area).
- Decrease with branching (reduces contact area and London forces).
Full Notes
Bonding in Alkanes – σ-Bonds
Alkanes contain only single covalent bonds called sigma (σ) bonds, which are formed from the direct overlap of orbitals between two bonding atoms.
σ-bonds are free to rotate (free rotation). This contrasts with π-bonds (as in alkenes), which prevent rotation (see alkenes).
Shape and Bond Angles in Alkanes
Each carbon atom in an alkane is surrounded by four bonding pairs of electrons.
According to electron pair repulsion theory (see shapes of molecules), these pairs repel one another and adopt a tetrahedral geometry to minimise repulsion.
The bond angle around each carbon is approximately 109.5°.
Boiling Points of Alkanes
The boiling points of alkanes vary depending on chain length and branching:
- Chain length:
As the number of carbon atoms increases, there are more electrons in the molecules, leading to stronger induced dipole–dipole interactions (London forces). This increases boiling point. - Branching:
Branched isomers have fewer points of surface contact, leading to weaker London forces and therefore lower boiling points compared to straight-chain isomers.
Summary
- Alkanes are saturated hydrocarbons with only σ-bonds.
- σ-bonds allow free rotation around C–C single bonds.
- Each carbon forms a tetrahedral shape with bond angles of 109.5°.
- Boiling points increase with chain length and decrease with branching.