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The old name for the alkanes is the paraffins, a word meaning unreactive. We shall see in this sequence of experiments that the alkanes are particularly unreactive. This sometimes bothers students as they see the alkanes as rather dull. However, it is very important for chemists to have materials that do not react with others. If you were making a material for an artificial heart valve or underground piping you wouldn't want to use reactive materials. The combustion reaction of alkanes can be particularly violent - domestic gas explosions, which often demolish houses, are examples of such reactions. You may find it difficult to consider them as unreactive, but when you see the other sequence of reactions you should be convinced.
1 Combustion
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Alkanes generally burn with a clean, blue flame. However, larger alkanes do not readily vaporize, and so it is difficult to get enough oxygen to the reaction for complete combustion. You should have seen this in the video, where the poly(ethene) first melts and then burns less well than the hexane. Although the flames are yellow showing incomplete combustion, there is still little sign of soot coming from the flame. Where there is sufficient oxygen, complete combustion produces carbon dioxide and water. For example, with hexane: |
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C6H14(l) + 9½O2(g) ⇒ 6CO2(g) + 7H2O(l) |
2 Oxidation
Acidified potassium manganate(VII) is a powerful oxidizing agent. No colour change or temperature change is observed. It has no effect on either alkane.
3 Action of bromine
In the absence of light bromine does not react with alkanes. The bromine can be seen dissolving in the hexane, colouring it dark orange, but no chemical reaction takes place. This is often used to distinguish an alkane from its more reactive, alkene relative.
4 Action of bromine in sunlight
In the presence of ultra violet light (present in sunlight) a reaction does take place between bromine and alkanes. The bromine colour disappears in the sample kept in the light, showing that it must have reacted. Hydrogen bromide gas is produced which gave white smoke with ammonia. The organic product results from a substitution reaction in which a hydrogen atom is replaced by a bromine atom. The reaction is complicated as the position of substitution is variable and more than one hydrogen atoms can be replaced. You will need to know the mechanism of the reaction. No reaction takes place in the dark (although if you look carefully in the video you will see a trace of hydrogen bromide fuming with the ammonia).
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C6H14(l) + Br2(l) ⇒ C6H13Br(l) + HBr(g) |
5 Action of sulfuric acid
There is no sign of any reaction when concentrated sulfuric acid is added to an alkane. There is no change of temperature or colour change and the two substances do not mix.
6 Action of alkali
The substances do not mix and there is no sign of any reaction.
7 Catalytic cracking
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An alkene was detected by the fact that the product decolourised the bromine water. Also the combustion produced a luminous flame with a small amount of soot being formed on the mouth of the test tube - another sign of unsaturation. |
On heating alkanes, in the absence of air, they break up into smaller alkanes and alkenes. This process is called cracking. It is usually done in the presence of a catalyst, a process known as cat cracking. It is very important in the oil industry, where there is great demand for smaller alkenes (like ethene used to make polythene) and smaller alkanes (like octane in petrol). Some of the original paraffin and decomposition products distil over and appear as oily drops on the surface of the water.
The reaction is complicated and a variety of products are possible. The following is a possible equation:
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C10H22(l) ⇒ C8H18(l) + C2H4(g) |
