Alkenes & Alkynes

Alkenes and Alkynes

Alkenes and alkynes are similar words...2 words off by a factor of one letter. Now they may seem similar, but are they in fact similar? Yes they are! Similarly to their names, their bonds are off by a factor of one; alkenes are double bonds, and alkynes are triple bonds. And they are also really similar to alkanes. Think of them as the three stooges.. or the three amigos! Whatever you choose to relate them to, just remember, alkanes -1 bond, alkenes – double bond, alkynes – triple bond.

Naming
   
    Alkenes

• General formula = CNH2N
• Naming is similar to alkanes
• Double bond placement is at lowest possible # and placed in front of parent name
• Find longest chain of carbon atoms and change the ending from the prefix –ane to –ene (ex. butane is changed to butene)
• Count the # of carbon atoms until you reach the double bond. Count from both sides of the chain to see which has the smallest #
• Write #’s of side groups and their names. **Side groups should be in alphabetical order!!**

Ex. Name this structure

             

                                        

                                                                         2-pentene

1. Count the longest chain of carbon atoms. In this case, it's 5, which is where we get the pentene.  
2. Count to see on which carbon atom the double bond is placed. So it's either on the 3rd or 2nd carbon atom, but we want the smallest #, so we'll go with the 2nd.

Sometimes, with double bonds you will get structures that are the same but have different geometry. These structures are called geometric isomers:

                               
                                          trans-2-butene                      cis-2-butene
   
In the example above, we have 2-butene. As you can see, they have the same structure, just the placement of the atoms is different.To distinguish the 2, we use "cis" and "trans".

• "cis" is used when the carbon atoms are above/below the double bonded centre of the structure
• "trans" is used when the carbon atoms go in a diagonal

When naming, add the "cis" or "trans" (which ever applies to the situation) to  the beginning of the name.


   Alkynes

• General formula = CNH2N - 2
• Ending changed from prefix -ane to -yne (ex. nonane is changed to nonyne)
• Double/triple bond placement is at lowest possible # and placed in front of parent name
• Count the # of carbon atoms until you reach the double bond. Count from both sides of the chain to see which has the smallest #
• Write #’s of side groups and their names. **Side groups should be in alphabetical order!!**

Ex. name the structure

                                   
                                                              heptyne

1. Count the longest chain of carbon atoms. In this case, 7, which gives us heptyne.
2. The smallest # of the placement of the triple bond is 1, but we don't need to write the 1 out, it is assumed it's 1 if there's no # before it

And that's it for alkenes and alkynes! Hopefully you understood and it wasn't too hard. Organic chemistry is confusing, but it just takes time and practice to get used to it.



 Extra help!

http://www.youtube.com/watch?v=6NygjuEFkIc

http://www.youtube.com/watch?v=oYoQpDtBLac