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Isomerism in Alkenes

 

 

Isomerism in Alkenes

Alkenes, like the alkanes can show structural isomerism. This is possible due to changes in the position of the double bond or in the position of their substituents. E.g.,

  1. CH3 - CH2 - CH = CH2
          1 -butene (C4H8) 

and CH3 - CH = CH - CH3
    
2- butene (C4H8)
   

 2. 
              3- methyl -1-butene (C5H10)
 

and
           2- methyl-1-butene (C5H10)
   

Geometrical isomerism

Alkenes can also show geometrical isomerism. This occurs due to the restriction to rotation about the C = C bond (the double bond is rigid).

Hence, the two groups attached to each carbon in the double bond are held rigidly in a specific orientation in space. Therefore, geometrical isomers have different orientations in space (each orientation appears distinct, and thus shows difference in both physical and chemical properties from the other), but have the same molecular formula and structure.

When the same groups (or priority groups) are on the same side of the double bond, the geometrical isomer is regarded as a cis. Example,

  
     Cis Butene (the methyl groups are on the same side)
 

       

When they are on opposite sides, the isomer is a Trans. Example, 

  
                                Trans-butene  

       

Further examples:

  
  1-chloro-1-propene
                      2-pentene

       

Note:

* Geometrical isomers (cis and trans) are different from each other in both physical and chemical properties. This is due to the difference in their orientation in space, which is due to the rigidity of the double bond.

* The Trans isomers are usually more stable than their Cis counterpart.

* Not all alkenes show geometrical isomerism. Geometrical isomerism is not possible if two priority groups are attached to one carbon in the double bond.

* For the fact that alkenes show both structural and geometrical isomerism, a particular alkene has more isomers than it corresponding alkane.

Examples of compounds which can not show geometrical isomerism:

  
      2-methylbut-2-ene                 2-methylprop-1-ene             2,3-dimethylbut-2-ene

       

  
           But-1-ene                           2-chloro-2-butene

       

Related:

Alkenes
Polymerization Reactions of Alkenes

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