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The old name for the alkanes is the paraffins, a word meaning unreactive. 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 the fact that they resist attack by concentrated acids and alkalis and powerful oxidising agents may convince you. They are often used to store reactive chemicals.
1. Combustion
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. Where there is sufficient oxygen, complete combustion produces carbon dioxide and water:
|
C8H18(l) + 12½O2(g) ⇒ 8CO2(g) + 9H2O(l) |
2. 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 fumes 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).
|
C6H14(l) + Br2(l) ⇒ C6H13Br(l) + HBr(g) |
3. Catalytic cracking
An alkene was detected by the fact that the product decolorized 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).
The reaction is complicated and a variety of products can result. The following is a possible equation:
| C10H22(l) ⇒ C8H18(l) + C2H4(g) |