MORE ORGANIC CHEMISTRY: AlkENEs and AlkYNEs

Last post we learned about Alkanes, which were hydrocarbons that were interconnected by single bonds. Today, we will talk about Alkenes and Alkynes, which are hydrocarbons that have double bonds and triple bonds, respectively.

ALKENES
Alkenes are unsaturated hydrocarbons, meaning they form double or triple bonds. For alkenes, the double bond(s) can be located anywhere. HOWEVER, you always start counting the parent chain from the side that is CLOSEST to whichever double bond. In other words, the double bond always has the lowest number and is placed in front of the parent chain name.

Now here are some rules for Naming Alkenes:

• General formula: CnH2n
• Start from the element closest to the double bond.
• The standard naming rules for organic chem apply here.
• Replace the ending from "-ane" to "-ene".
• If there is more than one double bond, use the same greek prefixes before the "-ene" (eg. butadiene)

Here is an example: 2,3-dimethyl-2-pentene

Now a little something extra for Alkenes is that there is something called "cis & trans."
What is that you may ask? WELL LET ME TELL YOU FIRST. WHY YOU GOTTA ASK SO MANY QUESTIONS. SHEEESH.YOU IMPATIENT PERSON. (just kidding)
They are geometric isomers, which has the same formula but different geometry. There are different isomers, so that is why we give them different names like 'cis' and 'trans' based on their geometry.

Now from the image below, we can decide if a compound would be cis or trans.

• If the larger groups are both above (A,B) or below (D,E) the plane of the bond is  "cis". 
• If the larger groups are across the plane of the bond (A,E) or (B,D) then it is "trans".
• If there are 2 identical groups on either end of the double bond (A,D) or (B,E) then there are no geometric isomers => there are no need for cis or trans.
• Cis and trans are placed at the very beginning of the name

Get it? Got it? Okay! Moving on the alkynes (yay..)



but wait, let's do some exercises!
WOOHOO practice questions!
1) Draw 4-methyl-1,2-pentadiene
2) Draw trans-2-hexene
3) Name









Answers:
1)
2)






3) 3,4-diethylhexene

ALKYNES
Alkynes are also unsaturated hydrocarbons, forming triple bonds. The same rules of alkenes apply for alkynes.

Now here are some rules for Naming Alkynes:

• General formula: CnH2n-2
• Start from the element closest to the triple bond.
• The standard naming rules for organic chem apply here.
• Replace the ending from "-ane" to "-yne".

Here is an example: 2,3-dimethyl-2-pentyne

See? It's not that hard. All that's different is that there is a triple bond rather than a double bond!
Now lets test those smarticle skills of yours... Here are some practice questions:

1) Draw 4-ethyl-4-methyl-2-heptyne.
2) Draw 3,3,5-trimethyloctyne
3) Name

Answers:
1)







2)







3) 3-heptyne


And there you have it!
Enjoy organic chemistry .... evil smile*