Alkanes
Alkanes
Alkanes are saturated hydrocarbons since they only have single covalent bonds. Examples of Alkanes, are the gases produced by fractional distilation.
Alkanes have the general formula of CnH2n+2, below is a table of the first four alkanes in increasing order of carbon atoms.
| Name | Formula | Structure |
|---|---|---|
| Methane | CH4 |  |
| Ethane | C2H6 |  |
| Propane | C3H8 |  |
| Butane | C4H10 |  |
Combustion
These products make very good fuels as their combustion is very exothermic. There are two way that a combustion might take place depending on how much oxygen there is.
Complete combustion takes place in a plentiful supply of oxygen, its products are carbon dioxide and water; see the example below.
C4H10 + 6½O2 ® 4CO2 + 5H2O
Incomplete combustion takes place in a limited supply of oxygen, its products are water, and carbon or carbon monoxide water, an example of the reaction is.
CH4 + O2 ® C + 2H2O
CH4 + 1½O2 ® CO + 2H2O
However, if you think about this as happening in your car engine, then because of impurities in the fuel, other chemicals are produced. Notably nitrous oxides and sulphur dioxide, which is responsible for acid rain. Fortuneately we have a way of removing the more harmful chemicals, by use of a catalytic converter.
The catalytic converter removes carbon monoxide, nitrogen oxide and hydrocarbons from the exhaust. They contain a thin layer of metals such as platinum and rhodium they act as a catalyst to convert the harmful gases into less harmful ones.
Chlorination
This is the process where chloromethane is produced from chlorine and methane. If you did this experiment outside at night, nothing would happen; but do it in a sun bed and it will react explosively. This is because UV light provides the energy to break the Cl - Cl bonds.
The overall equation of this reaction is as above, however there are three stages to it. And at each one we shall study the mechanism.
The first stage is initiation where the UV light breaks the bond in the Chlorine Molecule to form two chlorine free radicals this merely means it has an unpaired electron and is therefore very reactive.
The second step is propagation where a hydrogen is removed from the methane and bonds with the free radical; but this in turn produces a methyl free radical.
The final step is termination and this is when two free radicals combine. Mostly this means the Cl and CH3 because these collisions are more probable. However, it is also possible for two oCH3 to join and make ethane.
Telling the Difference
You can tell the difference between an alkane and alkene by adding bromine water to the substance and giving the tube a shake.
When bromine water is added to an alkane the solution will turn red-brown (the colour of bromine water) and stay that way on shaking. Alkenes however will turn red-brown, but then the colour disappears on shaking, this is because the bromine has reacted with the alkene.
