All living organisms are composed of Carbon atoms or its compounds making it the most vital element. In other words we can call this element the building block of life. However, on death, a living organism loses Carbon 14 atoms, a slightly radioactive isotope but not Carbon 12, the stable form of Carbon. Our existence on earth is dependent on Carbon atoms. But in the earth’s atmosphere, the amount of Carbon is surprisingly very small. The total amount of Carbon in the air is 0.03 per cent – present as Carbon dioxide. The earth’s crust contains 0.02 per cent Carbon in the form of minerals, such as Carbonates and hydro-Carbonates. Hence, the total amount of Carbon on earth is 0.05percent. But don’t go by the meagre amount present in nature, the element, Carbon, is very versatile.
In nature, Carbon is present in many forms. Each is different from the other in its physical properties. Carbon is involved in the basic structure of all living organisms. Secondly, Earth also contains several allotropes of Carbon such as graphite, diamond and fullerenes. Thus, we have seen that despite the small amounts of Carbon present on earth, there are various forms in which Carbon is found – as solids, as gas and within the living cells of organisms. Carbon is also unique in other respects: in the way it combines with not only other elements but also its own atoms. Let’s discuss one of the most important organic compounds of carbon that is known as hydrocarbons.
Organic compounds entirely made up of carbon and hydrogen are called as hydrocarbons. This category includes saturated and unsaturated compounds as well as open, cyclic and benzenoid aromatic compounds. Hydrocarbons are classified as saturated and unsaturated based on the nature of the bond between carbon atoms.
Similarly depending upon the nature of chain/ring structure hydrocarbons are classified as chain, branched and cyclic hydrocarbons. In cyclic hydrocarbons there is special category of benzene and benzene ring containing hydrocarbons called as aromatic hydrocarbons. Hence the classification of hydrocarbons can be explained by the following chart.
Hydrocarbons that don't have a benzene ring in their structure are called as aliphatic compounds
. Aliphatic can be simply understand as non-aromatic. This is broad category contains many hydrocarbons like linear, branched, cyclic, saturated and unsaturated hydrocarbons. Butane a common constituent of LPG and octane a common constituent in petrol are some of the aliphatic hydrocarbons.
Aliphatic hydrocarbons whose carbon chain is closed end to end to form a ring structure is called as cyclic hydrocarbons. For example cyclobutane and cyclopropane are cyclic hydrocarbons. This category includes both saturated and unsaturated compounds like cyclobutene.
Aliphatic hydrocarbons where there is single bond between all the carbon atom are called as saturated hydrocarbons. They are called so because all the valence of carbon atom is completely filled in it. In the saturated hydrocarbons the carbon atom is sp3 hybridized.
The structure of saturated hydrocarbon is tetrahedral with respect to each carbon atom. They are less reactive and they cannot undergo addition reaction, where some atoms of groups are added to a compound. Methane, hexane, cyclopentane are examples for saturated hydrocarbons.
Aliphatic hydrocarbons where there is double/triple bond between carbon atoms are called as unsaturated hydrocarbons
. They are called as unsaturated
because the valence of carbon is not completely filled. Hence they can undergo addition reaction with atoms or groups to give saturated molecules. The carbon atom is sp2/sp hybridized in alkenes/alkynes. This category includes alkenes, alkynes. The structure of alkene is trigonal while the structure of alkyne is linear. Ethylene, isobutene, acetylene are examples of unsaturated hydrocarbons.
Benzene is a ring hydrocarbon made with three double bonds. But it is unusually stable and will not show any properties for three double bond. Hence the compounds containing benzene ring are classified separately as aromatic compounds.In aromatic hydrocarbons the root word 'aroma' means perfume. All the aromatic compounds have characteristic smell and many of them are used as perfumes.
Benzene, naphthalene are example for aromatic hydrocarbon.
Polyaromatic hydrocarbons are also otherwise called as polycyclic aromatic hydrocarbons. They are hydrocarbons made up of two or more benzene rings fused together without any other substituents. Naphthalene, anthracein are some examples of polyaromatic hydrocarbons.
They posses more threat to environment as a major pollutant in soil. They are obtained from petroleum by distillation.
The hydrocarbons are named in according to IUPAC convention. In the naming of hydrocarbons the following rules are used.
- The longest carbon chain is selected as root chain and others alkyl groups are substituent. For example the longest chain in the following hydrocarbon contains five carbon atoms and there are two methyl groups as substituent.
- The longest chain is numbered so that the substituent get the least number. Here the chain is numbered from left to right so that the substituent get the number 2.
- Then the hydrocarbon is named as Locant Number + Substituent + parent chain name. Here 2,2 is the locant indicating the position of substituent. Dimethyl is the substituent and pentane is the parent chain name with five carbon atom. Hence the compound name is 2,2 Dimethyl pentane.
- If a hydrocarbon contain unsaturated bond then the longest chain should contain the double/triple bond. Similarly on numbering the chain the unsaturated bond should have least number irrespective of the location of substituent. For example the following compound is named as 3-methyl 1-butene.
- Cyclic compounds are named by prefixing cyclo in before the name. For example the following compound is named as cyclobutane.
- Chlorinated hydrocarbons are a type of halogenated hydrocarbon in which a hydrogen is replaced by a halogen like chlorine. The reaction is free radical substitution reaction and chain reaction.
- Many commercially important compounds like chloroform, carbon tetrachloride, benzene hexachloride are examples for chlorinated hydrocarbons.
- Many important compounds like Grignard reagent, DDT can be prepared from chlorinated hydrocarbons.
Compounds having same molecular formula with different structures are called as isomers. In hydrocarbons the following isomerism will exist.
1. Chain Isomerism
Compounds having same molecular formula but difference in carbon chain pattern like linear/branched or called as chain isomers
Butane and 2-methyl propane
2. Position isomerism
Compounds having same molecular formula but difference in the position of functional group in the chain are called as position isomers
. This isomerism is shown by alkenesExample:
1-butene and 2-butene
3. Functional group isomers
Compounds having same molecular formula but difference in nature of functional group are called as functional group isomers
. For example alkenes are functional group isomers for cycloalkanes.
These are the hydrocarbons containing more than one benzene ring fused together. For example napthalene, anthracene
are polycyclic aromatic hydrocarbons. They are main pollutants in the environment and carcinogenic in nature.
There are many examples for the hydrocarbons in our daily life.
- Marsh gas is made up of methane with formula CH4.
- Liquefied petroleum gas (LPG) is made up of Butane and iso-butane.
- Petrol is made up of Octane.
- Like this other fossil fuels like diesel, crude oil are also hydrocarbons with higher chain length.