III. Alkenes

A. Definition: Organic hydrocarbon molecules containing one double bonded carbon-carbon atom.

B. Rules for naming alkenes(prefix parent suffix)

1. The longest chain that contains the double bond is the parent.

        CH2- CH3  
       
| 7
8
  
  CH3   CH2- CH2    
 
\
/
5
 6    
   
C=
 C      
 
/
3
4 \
     
CH3-
 CH2   CH2- CH3    
1
2
          

 

2. The parent chain is given the same name as an alkane, but the alkene suffix (-ene) replaces the alkane suffix (-ane). The molecule above would be called an octene.

 

3. The carbon atoms are numbered consecutively from the end nearer the double bonded carbon-carbon atoms. The number of the first carbon atom with the double bond is used as a perfix to the parent name and is separated from it by a hyphen.

  CH3     CH2- CH2- CH3
   
\
     / 5 6 7
    C= C      
   
/
 3 4 \      
CH3- CH2     CH2- CH3  
1
 2          

trans-4-ethyl-3-methyl-3-heptene

 

4. If the compound is a cis or trans isomer with respect to the parent chain, the appropiate prefix followed by a hyphen is placed in front of the name.

 

                             
1
2
    
5
6
7
            
5
6
7
  
CH3 CH2-     CH2- CH2 CH3    
CH3   CH2- CH2 CH3  
    \
/
            
\
/
    
 
3
 C= C
4
          
3
C=
 C
4
  
    /
\
          
1
2
/
\
    
CH3     H        
CH3
 CH2-
H  
cis-3-methyl-3-heptene
       trans-3-methyl-3-heptene    

 

5. Alkyl groups and other substituent are named, and their positions on the parent chain are identified according to the numbering established by rule 3. Names and numbers are prefixed to the parent name.

             
CH3
CH3
   
 
\
/        
 
C=
 C        
 
/ 2
3
 \        
CH3  
CH2- CH3    
1   4 5    

cis-2,3-dimethyl-2-pentene

 

C. Additional rules involving multiple double bonds or cyclic ring structures.

1. If the compound contains more than one double bond, the location of each double bond is specified by a number. A prefix (di-, tri-, etc.) is written in front of -ene to indicate the number of double bonds.

CH2= CH- CH= CH- CH= CH- CH3
1 2 3 4 5 6 7

1,3,5-heptatriene

2. The ring of a cycloalkene is numbered to give the double bonded carbon atoms the numbers 1and 2. The direction of numbering is chosen so that the first substituent on the ring receives the lower number. The position of the double bond is not given because it is known to be between the C-1 and C-2 atoms (where the double bonds are located).

4-ethyl-5-methylcyclopentene