Properties of alkanes

• Alkanes are amongst the most stable organic compounds. Their lack or reactivity has allowed crude oil deposits to remain in the earth for millions of years
• They are saturated hydrocarbons. Each carbon atom in an alkane is joined to four other atoms by single covalent bonds. These are sigma bonds
• A sigma bond is the result of the overlap of two orbitals. Each overlapping orbital contains one electron so the sigma bond has two electrons that are shared between the bonding atoms. The sigma bond is positioned on a line directly between bonding atoms
• Repulsion between the electrons in the sigma bonds of the alkane results in a 3D tetrahedral arrangement around each carbon atom because they each are surrounded by four areas of electron density. each bond angle is therefore 109.5 degrees approx
• Sigma bonds in alkanes act as axes around which the atoms can rotate freely so the shapes of alkanes are not rigid
• The boiling points of alkanes increase as their chain length increases.
• Oil refineries separate crude oil into fractions by fractional distillation in a distillation tower. Each fraction contains a range of alkanes due to the boiling points that vary with chain length
• London forces act between molecules that are in close surface contact. As the chain length increases, the molecules have larger surface area, so more surface contact is possible between molecules. The London forces will be greater and so more energy will be needed to overcome the forces. (so boiling point increases)
• Branched isomers have lower boiling points. This is because there are fewer surface points of contact between molecules of the branched alkanes, giving fewer London forces. The branches also get in the way and prevent the branched molecules getting as close together as straight-chained molecules, decreasing the intermolecular forces further