When alkanes contain double or triple bonds, that totally changes the way we name the molecule.
Concept: How to name alkenes and alkynes2m
Let's go ahead and add a few more functional groups, so now we're going to talk about aklenes, alkynes, and alcohols.
So first let's start off with the pi bonds, with the double bonds and the triple bonds. Alkenes and alkynes are going to be named by adding a modifier. So this is going to be the very first time that we're using a modifier in an IUPAC name, so I'm going to teach you guys how to do this.
What that means is that instead of using the ending -ane, like we always use. It stands for alkane. Instead of using that ending we're going to use, we're going to modify it to either be -ene if it's an alkene or -yne if it's an alkyne. That's why we call it a modifier. It modifies the root name. So now all of the sudden, a hexane becomes a hexyne if it has a triple bond in it. Does that make sense? Cool.
So then let's talk about some important rules here. Alkenes and alkynes are actually going to receive priority in numbering alkanes. This is also going to be the very first time that now when it comes to numbering the chain, we are going to give priority to this group. What that means is that I don't care about the closest substituent, all I care about now is how do I give my double bond or my triple bond the lowest number possible. Isn't that interesting? So now I'm just saying how can I make that number as low as possible. That's all I care about.
Then, finally, this can get tricky because these double bonds go over two carbons, so sometimes it can be tricky what's the location. Is it the first carbon or the second carbon? The location is always assigned to the first double-bonded carbon or triple-bonded carbon.
So what I want to do here is I want to go ahead and name this compound. I want you guys to go ahead and try it first with the rules and then I'll go ahead and I'll give you the name.
Alkenes and alkynes are named as modifiers, meaning we add suffix modifiers to the root chain:
Double and triple bonds get priority, so try to give them the smallest number possible.
Example: Name the following compound3m