Alkanes

Edited by Jamie (ScienceAid Editor), Taylor (ScienceAid Editor)

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About 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
methane.jpg
Ethane C2H6
ethane.jpg
Propane C3H8
propane.jpg
Butane C4H10
butane.jpg

Combustion

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.

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catalytic conversion

Chlorination

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.

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chlorination of methane reaction

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.
    initiation of reaction
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  2. 2
    Propagation
    .
    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.
    propagation of methane
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  3. 3
    Termination
    .
    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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diagram of the bromine water test.

Referencing this Article

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Alkanes. (2017). In ScienceAid. Retrieved Jul 24, 2017, from https://scienceaid.net/chemistry/organic/alkanes.html

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Article Info

Categories : Organic

Recent edits by: Jamie (ScienceAid Editor)

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