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The World of Carbon

Allotropes of Carbon

At the beginning of the 19th century, compounds obtained from plants and animals were regarded as organic compounds.

Organic compounds that contain both hydrogen and carbon are called hydrocarbons. These are parent fundamental compounds.

Classification of hydrocarbons

Hydrocarbons are classified on the basis of their structure and bonding.

The given figure shows the classification of hydrocarbons.

Open chain hydrocarbons - An open chain is an arrangement of atoms that does not form a ring, i.e. it has two open ends. It may consist of straight or branched chain compounds.

Open chain hydrocarbons are again classified into two sub-groups.

(a) Saturated open chain hydrocarbons - The saturated (contains only single bonds) open chain hydrocarbons form a homologous series called the paraffin series or the alkane series.

Their formula can be summarised as CnH2n+ 2, where n is the number of carbon atoms in the molecule.

Examples- Methane (CH4), ethane (C2H6),propane (C3H8), Butane (C4H10)

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Alkanes are also known as 'paraffins'. The word ‘paraffin’, which is derived from a Latin word meaning "little activity", means that such compounds are very unreactive.

(b) Unsaturated open chain hydrocarbons - Unsaturated open chain hydrocarbons include the alkene or olefin series, the diene series and the alkyne series.

1. Alkene: The alkene series consists of chain hydrocarbons that contain a double bond between two carbon atoms. The general formula for the series is CnH2n, where n is the number of carbon atoms.

Examples: Ethene (CH2=CH2), Propene (CH3-CH=CH2), Butene (CH3-CH2-CH=CH2)

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Alkenes are also called olefins because they form oily liquids on reaction with chlorine gas.

2. Alkyne: Members of the alkyne series contain a triple bond between two carbon atoms in a molecule. Alkynes have the general formula CnH2n −2.

Examples: Ethyne (HC≡ CH), Propyne (CH3-C≡CH), 2-Butyne (CH3-C≡C-CH3)

Closed Chain Hydrocarbons - The hydrocarbons which contain a closed chain of carbon atoms are called cyclic hydrocarbons. These compounds contain carbon atoms joined in the form of a ring. They are further sub-divided into two sub-groups.

(a) Alicyclic Hydrocarbons - These cyclic compounds contain single bonds. These are closed chain or cyclic compounds but they behave more like aliphatic or open chain compounds in spite of their rings.Upon cyclisation, an alkane hydrocarbon chain would result in the formation of alicyclic compounds.

Examples:

(b) Aromatic Compounds - These organic compounds contain at least one Benzene ring. Aromatic compounds have alternate single and double bonds. The substitution of one or all hydrogen atoms in a Benzene ring can form aromatic compounds.

Examples:

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Aromatic compounds get their name from the fact that many of these compounds have a sweet scent.

Carbon is the versatile element present in food, clothes, medicines, papers, etc. In addition, all living structures are carbon based. The earth’s crust has only 0.02% carbon in the form of minerals and the atmosphere has 0.03% carbon dioxide. In spite of the small amount of carbon available in nature, the importance of carbon is immense.

Position of carbon in the periodic table

Carbon has an atomic number 6 and an atomic mass as 12 u. It contains 2 and 4 electrons in the K and L shell respectively. It belongs to the group IVA of the periodic table and has a valency of 4.

Occurrence: Carbon occurs in free as well as in combined state.

In free state, carbon occurs as diamond, graphite, and coal. Diamond and graphite are pure forms of carbon while coal is an impure form of carbon in which the percentage of carbon varies from 24% to 90%.

In combined state, carbon occurs in

  • bio-molecules such as cellulose, carbohydrates, fats, proteins, etc.

  • minerals in the form of carbonates

  • calcium carbonate (CaCO3) in lime, marble, and chalk

  • magnesium carbonate (MgCO3) in magnesite

  • calcium and magnesium carbonate (MgCO3.Ca…

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