Alkanes are aliphatic saturated hydrocarbons (no C=C double bonds, C and H atoms only). They are identified by having a –ane name ending.
The alkanes have similar chemistry to one another because they are a homologous series: a series of organic compounds having the same functional groups, each successive member differing by –CH2–
Their general formula is CnH2n+2 where n is the number of carbon atoms in the alkane.
We need to recognise the first 10 alkanes by name and formula, and be able to recognise chains attached to an organic molecule as being alkyl groups:
Alkane Molecular formula Alkyl group Alkyl- structural formula
e.g. methane CH4 methyl- CH3–
pentane C5H12 pentyl- CH3(CH2)4–
The cycloalkanes are alicyclic saturated hydrocarbons, having two less H atoms, and a ring rather than a chain structure. e.g. cyclohexane C6H12 compared to hexane C6H14
For the straight chain alkanes:
- Low molecular weight alkanes (C1 to C4) are gases at room temperature (~ 300K)
- Heavier molecular weight alkanes (C5 – C17) are liquids at room temperature
- High molecular weight alkanes (C18 and above) are solids at room temperature
Trend in melting and boiling points:
As the number of carbons increases, the melting ■ and boiling ♦ points get higher.
We say that the volatility of the alkanes decreases with increasing number of carbons in the alkane molecule.
volatility: the ease with which a substance turns into a vapour
The lack of polarity of alkane molecules means that only weak induced dipole-dipole forces (London forces) are present between molecules. There are no permanent dipole-dipole interactions or hydrogen bonds. As the number of electrons in the molecule increases (with increasing number of atoms) the strength of the London forces also increases. More energy is needed to separate the molecules, so melting and boiling points rise.
Effect of branching:
For isoelectronic alkanes, the melting and boiling points decrease as the chain gets more branched.
As the molecule gets more ‘ball shaped’ and less ‘long and thin’ the molecules can’t get so close together. There are fewer points of contact (less area of surface contact) and so the London forces are weaker, since the strength of these forces decreases with increasing distance between the molecules. A vapour is formed from the liquid more easily, and so the boiling point is lower.