Properties of alkanes

• Alkanes are saturated hydrocarbons, containing only carbon and hydrogen atoms joined together by a single covalent bonds.
• Each carbon atom in an alkane is joined to four other atoms by single covalent bonds these covalent bonds are called a sigma bond.
• A sigma bond is the result of the direct overlap of two orbitals one from each bonding atom.
• Each carbon atom is surrounded by four electron pairs in four sigma bonds repulsion between these electron pairs results in a tetrahedral arrangement around each carbon atom with a bond angle of 109.5°.
• The sigma bonds act as axes around which the atoms can rotate freely, so these shapes are not rigid.
• Effect of chain length on boiling point

  As chain length increases, the molecules have a larger surface area, so more surface contact is possible between the molecules. The London forces between the molecules will be greater and so more energy is needed to overcome the forces.
• Effect of branching on boiling point

  There are fewer surface points of contact between molecules of the branch alkanes, giving fewer London forces. The branches get in the way and prevent the branch molecules from getting as close together as straight chain molecules decreasing the intermolecular forces further.