HydrocarbonsHydrocarbons are compounds of carbon and hydrogen. The natural sources of hydrocarbons are petroleum (crude oil), natural gas and coal. Petroleum is found under the crust of the earth and is preserved in nature by being enclosed in suitable rocks. Petroleum is obtained by drilling wells at places where petroleum is known to occur. The property of catenation, that of forming long chains of carbon atoms, accounts for the possibility of a large number of hydrocarbons.
Hydrocarbons are of various types. These are classified as shown in the chart.
Saturated Hydrocarbons-AlkanesAlkanes are open-chained with the general formula CnH2n+2. Cycloalkanes too are saturated hydrocarbons but contain a ring and have the general formula, CnH2n where, n is the total number of carbon atoms.
Example: If the number of carbon atoms 'n' is 6 the corresponding alkane and cycloalkane will be C4H14 (hexane) and C2H12 (cycloalkane) respectively.
Other examples of saturated compounds are:
While writing down the structures of organic compounds the following points should be kept in mind.
- Carbon is always tetravalent.
- If only a closed ring is written, without any C or H, it represents that carbon is situated at each corner and the remaining valencies are satisfied with hydrogens.
Unsaturated Hydrocarbons-Alkenes and Alkynes
Hydrocarbons in which the two carbon atoms are connected by a double bond or a triple bond is called an unsaturated hydrocarbon. Hydrocarbons with double bond are called alkenes and with triple-bonded carbon atoms are known as alkynes (cycloalkynes), if a ring is present). Alkenes are also known as olefins. The general formulae of alkenes, cycloalkenes, alkynes and cycloalkynes is CnH2n, CnH2n-2 and CnH2n-4, respectively. Unsaturated compounds with three carbon atoms are given in the table.