Because they are saturated compounds, the organic, hydrocarbon compounds, alkanes, tend to show very little reactivity. However, there are a few reactions that alkanes can and do undergo under specific conditions. These reactions include combustion, substitution and cracking. The reactions of alkanes are very important in everyday life.
Combustion of Alkanes
Combustion is the most important reaction of alkanes. Alkanes burn in oxygen to form carbon dioxide and water vapor. If there is insufficient oxygen for the reaction, carbon monoxide and sometimes even carbon is formed instead of carbon dioxide. In actuality, there is always a certain amount of carbon monoxide formed in the process.
This is the reaction that occurs in internal combustion engines. It is the production of these gases, carbon dioxide and water vapor, and even the carbon monoxide, that allows the useful work of engines.
An example of the combustion of an alkane is:
CH4(g) + 4O2(g) —————–> CO2(g) + 2H2O(g)
Cracking of Alkanes
Very large alkanes are not as useful to human as smaller, short-chained ones. As a result, large alkanes are often cracked to produce smaller, more useful alkanes. During this process, alkenes and hydrogen gas may also be formed. The cracking of alkanes is carried out at 450 degrees celcius over an aluminum oxide or silicon oxide catalyst.
An example of the cracking of an alkane is:
C15H32(l) —————> 2C2H4(g) + C3H6(g) + C8H18(l)
Substitution Reactions of Alkanes
Alkanes can be converted to halogenoalkanes via substitution. This reaction only occurs in light or at high temperatures. As the name suggests, a replacement is made. One or more hydrogen atoms on the alkane is replaced by one or more halogen atoms. This reaction occurs mainly with chlorine, bromine and iodine. The halogen, fluorine, however, does not form halogenoalkanes having reacted with alkanes. Instead, carbon and hydrogen halides are formed as in the following reaction. This reaction is explosive and occurs even in the cold and dark.
CH4(g) + 2F2(g) —————-> C(s) + 4HF(g)
Halogenoalkanes are very useful compounds. They may be used as refrigerants, as propellants for aerosols, for generating foamed plastics and as solvents for dry cleaning.
An example of a substitution reaction of alkanes is:
CH4(g) + Cl2(g) —————-> CH3Cl(g) + HCl(g)
Because of their saturated nature, alkanes are generally very inert compounds. However, under certain conditions alkanes can undergo some chemical reactions. These reactions include combustion, substitution and cracking. Some of these reactions play very significant roles in the lives of people.