Alkanes
Properties of alkanes
- Alkanes have the general formula CnH2n+2.
- Alkanes form bonds with as many other atoms as they possibly — they are saturated hydrocarbons.
- There are only single bonds (sigma bonds) connecting the atoms together, formed from the head-on overlap of s orbitals.
- There is free rotation about the C-C sigma bond.
Since each carbon atom is connected to four other atoms, there's a tetrahedral geometry around each carbon atom with bond angles of 109.5o.
This shape means that the bonding electrons are the maximum distance apart, to minimise repulsion between them.
As the chain length increases, the boiling point of alkanes also increases. This is because as the molecule gets bigger, there are more electrons which mean that the molecule forms stronger London dispersion forces with other molecules. These stronger forces of attraction require more energy to overcome.
Reactions of alkanes
Alkanes are fairly unreactive:
There’s not much difference in electronegativity between carbon and hydrogen, so there is no bond polarity. This means that alkanes are unable to attract or react with polar or charged molecules.
C-C and C-H bonds also have high bond enthalpies and require a lot of energy to break.
Combustion of alkanes
Alkanes react with oxygen in combustion reactions. These reactions are very exothermic and release a lot of heat energy, which is why they make good fuels. The products of a combustion reaction are carbon dioxide and water.
If there's a lack of oxygen, we’ll get incomplete combustion.
Instead of producing carbon dioxide (CO2), carbon monoxide (CO) is formed, due to the limited availability of oxygen. Carbon monoxide is poisonous because it binds to haemoglobin in our red blood cells, preventing it from carrying oxygen. Less oxygen in our blood means that our cells don’t have the oxygen they need to respire, making carbon monoxide poisoning deadly.
Sometimes, if oxygen is really limited, we may get no oxygen atoms bonding at all to the carbon atoms. In this case carbon is produced as a single product and appears as fine black particles, often called soot.
Free radical substitution
Alkanes can react with halogens, but only in the presence of UV light. The mechanism for this reaction is called free radical substitution, because the radicals will substitute for hydrogen on the alkane.
UV light causes the halogen-halogen covalent bond to break by homolytic fission, creating two free radicals. This is the initiation step.
The alkane then reacts with the free radical in a series of propagation steps, where further free radicals are produced.
The reaction ends with the termination steps, which use up all of the free radicals.
This reaction will continue until all of the hydrogens have been replaced by chlorine. This is called multiple substitution and will form dichloromethane, trichloromethane and finally tetrachloromethane. The other halogens, such as bromine and iodine will react with alkanes in the same way.
Multiple substitution makes it tricky to use free radical substitution as a way of synthesising a halogenoalkane with just one halogen atom attached, since the reaction will continue until every single hydrogen has been replaced.
The other issue with synthesising organic compounds in this way is that the halogenoalkane is not the only product formed. You can see from the termination steps above that you’ll also get a larger alkane (formed from the two alkyl radicals) and a halogen molecule (from the two halogen radicals). You would then need to separate the mixture to obtain the halogenoalkane.