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Alkene Compounds

December 10, 2012

Alkenes are hydrocarbons that contain a carbon –carbon double bond (c=c) in their molecule. General formula for Alkene compounds is CnH2n (n= number of carbon atom). Alkene compounds contain two hydrogen atoms less than alkanes and are thus designated as unsaturated hydrocarbons. As a class Alkene compounds are commonly known as Olefines.

Ethylene is the first member of the series. Propylene is the second member of the series. Alkene compounds occur free in nature. They are, however, produced in large amount by cracking of petroleum.

The carbon –carbon double bond in alkenes is made up of one σ bond and one π bond.

Alkenes are more reactive than Alkanes. This is due to the availability of the more exposed π electrons.

Nomenclature of Alkene Compounds

There are two ways of naming Alkene compounds:

  1. Common system. The Common systems of the first four members are derived from those of the corresponding alkanes by changing the ending -ane to –ylene.

CH2=CH2                   Ethylene (ethane-ane+ylene)

CH2=CH-CH3            propylene

  1. IUPAC system. The IUPAC names of alkenes are derived from those of the corresponding Alkanes by chaning the ending –ane to –ene.

CH2=CH2            Ethane (  Ethane-ane +ene)

CH2=CH-CH3      Propene

The IUPAC names of higher alkenes are obtained by applying the following rules:

  • The name of a hydrocarbon is based on the parent alkene having the longest carbon chain of which double bond is a part.
  • The chain 1 numbered from the end near the double bond and its position is indicated by the numbers of carbon atom at which the double bond originates.
  • The name of the parent alkene with the position number of the double bond is written first and then the names of other substituents prefixed to it.
  • When there are two or three double bonds in a molecule, the ending –ane of the corresponding alkane is replaced by –adiene or –atriene to get the name of the hydrocarbon.

Properties of Alkene Compounds

The first three alkenes (ethene, propene and butane) are gases at room temperature next 14 members are liquids, and alkenes with higher carbons are solids.

They are colourless,  odourless except ethane, which has a rather pleassnt odour.

Alkenes are more reactive than alkanes this is due to the π electrons of a double bond are located much farther from the carbon nuclei and are thus less firmly bound to them.

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