Alkanes are the simplest family of hydrocarbons – compounds containing carbon and hydrogen only. They only contain carbon-hydrogen bonds and carbon-carbon single bonds. The first six are:
You can work out the formula of any of them using: CnH2n+2
All the alkanes with 4 or more carbon atoms in them show structural isomerism. This means that there are two or more different structural formulae that you can draw for each molecular formula.
For example, C4H10 could be either of these two different molecules:
These are called respectively butane and 2-methylpropane.
The boiling points shown are all for the “straight chain” isomers where there are more than one.
Notice that the first four alkanes are gases at room temperature. Solids don’t start to appear until about C17H36.
You can’t be more precise than that because each isomer has a different melting and boiling point. By the time you get 17 carbons into an alkane, there are unbelievable numbers of isomers!
Cycloalkanes have boiling points which are about 10 – 20 K higher than the corresponding straight chain alkane.
There isn’t much electronegativity difference between carbon and hydrogen, so there is hardly any bond polarity. The molecules themselves also have very little polarity. A totally symmetrical molecule like methane is completely non-polar.
This means that the only attractions between one molecule and its neighbours will be Van der Waals dispersion forces. These will be very small for a molecule like methane, but will increase as the molecules get bigger. That’s why the boiling points of the alkanes increase with molecular size.
Where you have isomers, the more branched the chain, the lower the boiling point tends to be. Van der Waals dispersion forces are smaller for shorter molecules, and only operate over very short distances between one molecule and its neighbours. It is more difficult for short fat molecules (with lots of branching) to lie as close together as long thin ones.
For example, the boiling points of the three isomers of C5H12are:
|boiling point (K)|
The slightly higher boiling points for the cycloalkanes are presumably because the molecules can get closer together because the ring structure makes them tidier and less “wriggly”!
What follows applies equally to alkanes and cycloalkanes.
Alkanes are virtually insoluble in water, but dissolve in organic solvents. The liquid alkanes are good solvents for many other covalent compounds.
Solubility in water
When a molecular substance dissolves in water, you have to
- break the intermolecular forces within the substance. In the case of the alkanes, these are Van der Waals dispersion forces.
- break the intermolecular forces in the water so that the substance can fit between the water molecules. In water the main intermolecular attractions are hydrogen bonds.
Breaking either of these attractions costs energy, although the amount of energy to break the Van der Waals dispersion forces in something like methane is pretty negligible. That isn’t true of the hydrogen bonds in water, though.
As something of a simplification, a substance will dissolve if there is enough energy released when new bonds are made between the substance and the water to make up for what is used in breaking the original attractions.
The only new attractions between the alkane and water molecules are Van der Waals. These don’t release anything like enough energy to compensate for what you need to break the hydrogen bonds in water. The alkane doesn’t dissolve.
Solubility in organic solvents
In most organic solvents, the main forces of attraction between the solvent molecules are Van der Waals – either dispersion forces or dipole-dipole attractions.
That means that when an alkane dissolves in an organic solvent, you are breaking Van der Waals forces and replacing them by new Van der Waals forces. The two processes more or less cancel each other out energetically – so there isn’t any barrier to solubility.
Alkanes contain strong carbon-carbon single bonds and strong carbon-hydrogen bonds. The carbon-hydrogen bonds are only very slightly polar and so there aren’t any bits of the molecules which carry any significant amount of positive or negative charge which other things might be attracted to.
For example, you will find (or perhaps already know) that many organic reactions start because an ion or a polar molecule is attracted to a part of an organic molecule which carries some positive or negative charge. This doesn’t happen with alkanes, because alkane molecules don’t have this separation of charge.
The net effect is that alkanes have a fairly restricted set of reactions.
- burn them – destroying the whole molecule;
- react them with some of the halogens, breaking carbon-hydrogen bonds;
- crack them, breaking carbon-carbon bonds.