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Alkenes

Alkenes are hydrocarbons that contain one or more carbon-carbon double bonds. Alkenes with only one double bond have the general formula CnH2n

Naming alkenes has the same format as naming alkanes, but be sure to change the -ane ending on the parent name to -ene ending. This shows that the compound is an alkene.

  1. Determine the longest continuous chain of carbons that have the double bond between two of its carbons. The parent chain must contain the double bond.

  2. Number the carbons in the chain so that the double bond would be between the carbons with the lowest designated number. Numbering could start either from the left end or right end of the chain. The location of the double bond, not the location of the branches are used for numbering the alkene.

  3. Identify the various branching groups attached to this continuous chain of carbons by name.

  4. The format is as follows: (location of branch)-(branch name)(parent chain)

    Example: 

       CH3CH2CH=CH2
   4  3  2  1
    1-butene
    		 
       CH3CH=CHCH3
   1  2  3 4
    2-butene
    		 
             CH3     CH3
         |       |
   CH3CH2CHCH2CH=CCH3
   7  6  5 4  3  21
    2,5-dimethyl-2-heptene



Geometric Isomers

In alkanes, rotational freedom is possible but for alkenes, due to the carbon-carbon double bond, it is impossible to rotate the bond without breaking it. Geometric isomers are isomers that differ from each other based on the position of the attached groups relative to the double bond. Two substituents that are on the same side of the double bond is called cis, meaning "same", two substituents that are on the same side of the double bond is called trans, meaning "across".