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Alkenes: Nomenclature And Isomerism

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Alkenes: Nomenclature And Isomerism - Lesson Summary

An unsaturated hydrocarbon with a double bond present between the carbon atoms is called an alkene. They are also called as olefins. One of the bond in double bond of alkene is strong sigma bond and other bond is weak pi-bond.



A sigma bond is formed by head-on overlapping between two sp 2 hybrid orbitals and pi-bond is formed by side -ways overlapping between two pure 2p orbitals of two carbon atoms.


 
Alkenes has two hydrogen atoms lesser than corresponding alkanes. Hence the general molecular formula of alkene is C nH 2n.

Due to presence of delocalized pi-bonded electrons, double bond in alkenes surfs as a source of electrons for electron seeking reagents. Thus presence of weaker double bond makes alkenes more reactive than the corresponding alkanes.

Nomenclature of Alkenes:
IUPAC name of alkenes: According IUPAC the name of alkene divided into two parts namely root word and suffix.


 
Ex: IUPAC name of the molecule



According to IUPAC alkenes are named by replacing ‘ane’ in alkanes with ‘ene’.
Ex:

For all the alkenes which contain more than three carbon atoms, the carbon atom with double will be given least possible number.
Ex:



An alkene with an alkyl group as substituent is number in a such a way that carbon atom with double bond will get least number.
Ex:


Isomerism in Alkenes:
In higher alkenes molecules can be arranged in more than one way. They can show two types of isomerism, namely structural isomerism and stereo isomerism.
Ex: A molecule of butane (C 4H 8) can be arranged in following ways.



These are called structural isomers.
Structure-1 and Structure- 3, structure- 2 and Structure -3 are said chain isomers.
Structure-1 and structure-2 are positional isomers.

In addition to these structures there exist other possible structures for butane which are called as geometrical isomers.

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