12.1 Properties of the Alkanes

The main components of natural gas and crude oil. Amongst the most stable organic compounds and their lack of reactivity has allowed crude oil deposits to remain in the Earth for many millions of years

As fuels, exploiting their reaction with oxygen to generate heat

CnH(2n+2)

Saturated Carbon Hydrogen Single

σ- bond Each carbon atom in an alkane is joined to four other atoms by single covalent bonds

Sigma bond The result of the overlap of two orbitals, one from each bonding atom Each overlapping orbital contains oe electron so the σ- bond has two electrons that are shared between the bonding atoms

Four Either C-C or C-H

Tetrahedral arrangement Each bond angle around 109.5

By fractional distillation in a distillation tower

Because the boiling points of the alkanes are different, increasing as their chain length increases

The weak, intermolecular London forces hold molecules together in solids and liquids but once broken, the molecules move apart from each other and the alkane becomes a gas. The greater the intermolecular forces, the higher the boiling point.

London forces act between molecules that are in close surface contact, as chain length increases, the molecules have a larger surface area, so more surface contact is possible between molecules London forces between will be greater so more energy required to overcome the forces

Greater surface area of contact Stronger London forces

…molecular mass

You find that the branched isomers have lower boiling points

There are fewer surface points of contact between molecules of the branched alkanes, giving fewer London forces. Branches also get in the way and prevent branched molecules getting as close together as straight-chain molecules, decreasing the intermolecular forces further