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By far the most numerous and important compounds that carbon forms are those with hydrogen. Hydrocarbons as they are known are the most important of organic compounds. Some of the hydrocarbons occurring in nature are very simple, while some are very complex. They are therefore categorized into two.
Compounds of carbon and hydrogen whose adjacent carbon atoms contain only one (carbon-carbon) bond are known as saturated hydrocarbons. Their carbon-hydrogen bonds are also single covalent bonds. They are called saturated compounds because all the four bonds of carbon are fully utilised and no more hydrogen or other atoms can attach to it. Thus, they can undergo only substitution reactions. They are also representative of open-chain aliphatic hydrocarbons. These saturated hydrocarbons are called as alkanes.
Compounds of carbon and hydrogen that contain one double covalent bond between carbon atoms (carbon=carbon) or a triple covalent bond between carbon atoms ( ) are called unsaturated hydrocarbons. In these molecules, since all the bonds of carbon are not fully utilised by hydrogen atoms, more of these can be attached to them. Thus, they undergo addition reactions (add on hydrogen) as they have two or more hydrogen atoms less than the saturated hydrocarbons (alkanes).
Unsaturated hydrocarbons can be divided into 'alkenes' and 'alkynes' depending on the presence of double or triple bonds respectively.
| These organic compounds contain single carbon-carbon covalent bond.
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These organic compounds contain at least one double or triple covalent bond. 
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| Due to the presence of all single covalent bonds, these compounds are less reactive.
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Due to the presence of double and triple bonds, these compounds are more reactive.
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Saturated compounds undergo substitution reactions.
Example:
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Unsaturated compounds under go addition reactions.
Example:
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| The number of hydrogen atoms is more when compared to its corresponding unsaturated hydrocarbon.
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The number of hydrogen atoms is less when compared to its corresponding unsaturated hydrocarbon.
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The properties of tetravalency and catenation allow the formation stable chains of carbon atoms having different chain lengths and structures. The chains of carbon atoms may be linear or branched (open) or cyclic (closed) rings, sheets and three-dimensional lattices. For example,
The compounds can be branched when the carbon atoms are more than three. Some of the examples are:
| Methane
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CH4
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1
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| Ethane
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C2H6
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2
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| Propane
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C3H8
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3
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| Butane
Branched:
Iso-butane
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C4H10
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4
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| Pentane
Branched:
Iso-pentane
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C5H12
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5
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The branched chains give rise to a different structure to the parent alkane and are named differently. For example, in pentane (C5H12) one can see a straight chain (normal pentane), branched chain (Iso-pentane) and ring of cyclopentane (C5H10).
Thus when carbon having different chain lengths and structures combines with different elements it leads to the formation of a large number of compounds.
The unique feature of the carbon-carbon bonding has also led to the formation of compounds that can have the same molecular formula, but different structures. This phenomenon of different structural formula of the same molecule, giving rise to different properties of compounds, is called Isomerism. In the above illustrations pentane and iso-pentane display isomerism. Such compounds with the same molecular formula are called isomers of one another. Another common instance of isomerism is butane, where there are following two possible structures for the same molecular formula C4H10.
It is quite evident that carbon displays a great propensity in forming a large variety of carbon and hydrogen compounds. But carbon can also form bonds with other elements in a hydrocarbon chain, when one or more hydrogen is replaced by an element like oxygen, nitrogen, sulphur etc. such that the valency of carbon remains satisfied. When an atom or a group of atoms forms a bond with the carbon atom in the chain or ring of an organic compound, while showing some characteristic properties of their own, they are termed as a functional group. The property of the whole organic molecule is then due to this functional group. In such compounds, the element replacing hydrogen is referred to as a heteroatom. These heteroatoms confer specific properties to the compound, regardless of the length and nature of the carbon chain and form the functional group. Thus a functional group is the site of chemical reaction in an organic compound and all compounds containing a particular functional group undergo similar reactions.
For example, in alcohols like methanol and ethanol, -OH is the functional group and in acids like ethanoic acid, -COOH is the functional group. The -NH2 functional group possesses basic character. The functional group present in the following molecules is encircled. Free valency or valencies of the group are shown by the single line. The functional group is attached to the carbon chain through this valency by replacing one hydrogen atom or atoms.
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