Organic Chemistry
/ Alkyne Nomenclature

The general molecular formula for alkanes is *C _{n}H*

(c) Alkynes

Hey everyone. So, for this question we're dealing with alkynes and if we have the general formula for a regular alkane it being CnH to the 2n plus 2. Now, we need to figure out the general formula for alkynes, okay? So what do you think is going to be the general molecular formula? Well, remember that alkanes are going to look something like this, right? this is just a regular bond line or a butane and we know there's going to be if it's four carbons there's going to be 10 hydrogen's, right? Just by looking at this formula, so now let's say we added a triple bond and kept it as four carbons, okay? So, here we have carbon 1, 2, 3 and 4, right? This is going to be your carbon one, carbon two, carbon three and carbon four, we know that our formula is going to be C4 but now how do we know the amount of nitrogens, do we have to count them? Well no, because you guys are going to remember that a triple bond will decrease the number of hydrogen's by four now when we have a triple bond, so you could expect that we have. Now we're counting our number of hydrogens, so let's erase these numbers, how many hydrogen's are we going to have all the way to the left at that carbon one? Well, just one, okay? what about at this carbon? so it's going to be no, no hydrogen's are, right? What about here? here's going to be two hydrogen's, right? 1, 2 and at top we know this is going to be a methyl group, so we should expect three hydrogens there, so in total we get six hydrogen's which is exactly our hydrogens have decreased by four from our alkane. So, our form is going to become C4H6, so what's the general formula for that? Well, it's going to be Cn, what do you guys think? Now, it's going to be minus 2. So, CnH2n minus 2, okay? So this is the general formula for your alkynes, so hopefully that makes sense and let me know if you have any questions.