Alkanes

Alkanes are saturated hydrocarbons, since they only have single covalent bonds. Examples of Alkanes, are the gases produced by fractional distillation.

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

These products make very good fuels as their combustion is exothermic. Here are two ways combustion takes place, depending on how much oxygen there is.

Complete Combustion

Complete combustion takes place when there is a plentiful supply of oxygen. Its products are carbon dioxide and water; see the example below. C₄H₁₀ + 6½O₂ ==>> 4CO₂ + 5H₂O.

Incomplete Combustion

'Incomplete combustion takes place when only a limited supply of oxygen is available. 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. Fortunately, we have a way of removing the more harmful chemicals - the 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 that act as a catalyst to convert the harmful gases into less harmful ones.

This is the process where chloromethane (methyl chloride) is produced from chlorine and methane. If you did this experiment outside at night, nothing would happen; but if you perform the experiment in a sun bed, 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.

1. 1
   Initiation

   .

   The first stage is initiation, where the UV light breaks the bond in the Chlorine Molecule to form two chlorine free radicals. This means it has an unpaired electron and is therefore very reactive.

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2. 2
   Propagation

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   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.

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3. 3
   Termination

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   The final step is termination. 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 CH₃ to join and make ethane.

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Know 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 color of bromine water) and stay that way while shaking.
• Alkenes however will turn red-brown, but then the color disappears while shaking. This is because the bromine has reacted with the alkene.

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