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Current time:0:00Total duration:7:08

Video transcript

before we get into the nomenclature of alkynes let's review a few the things we learn in the video on SP hybridization so we learned what when a triple bond is present in a molecule right these carbon atoms right here are SP hybridized and we talked about how the geometry around our SP hybridized atoms must be linear right so this bond angle right here is 180 degrees which makes this portion of the molecule with the triple bond and it's linear like that and in that triple bond we know that one of those bonds is a sigma bond and two of those bonds are PI bonds okay so if you don't remember that stuff you need to go back and watch the video on SP hybridization so this particular alkyne right here has alkyl groups on either side of my triple bond so it's a die substituted alkyne and the more common term you'll see for that would be an internal alkyne so let's go ahead and write that so it's an internal alkyne which just means that the triple bond is found in the interior of the molecule and enter the chain that the triple bond is not found on the end so that's an internal alkyne which is which is different from a terminal alkyne so let's go ahead and draw a terminal alkyne and this is just where your triple bond is at the end of your carbon chain alright so if I put a hydrogen on one side now it's only mono substituted and the triple bond is on the end or that or the terminal of my alkyne so this is a terminal alkyne like this okay so it's important to distinguish between these because in future videos we'll see how things like terminal alkynes have special properties all right so let's let's talk about the nomenclature of alkynes now and let's start with the simplest alkyne a two carbon alkyne so a triple bond between two carbons like that and let's just clear up that triple bond there a little bit so this is our triple bonds between our two carbons like that and I have hydrogen's on either side so when we talked about two carbons for nomenclature our route was F and since we're dealing with alkynes right ending is going to be why any here so I stick the root together with the ending right so I take s and I add the I into it so it's called ethyne so a thon B the IU pack name for this molecule that's that's probably not the name that's used most frequently this is this is the most famous alkyne also called acetylene ice you've heard of acetylene torches before and you can do a very cool demonstration called underwater fireworks where you use acetylene in there so this is this is acetylene which is the common name or ethyne which is more the IU PAC name and this is the simplest alkyne all right so let's name another one here let's let's get an actual chain carbon chain in here and let's see how to name this one like that well the first thing you notice is the straight line that that that surrounds our triple bond there and remember that's because it's a linear it's a that part that portion of the molecule is linear because those carbons are SP hybridized so when you're drawing when you're drawing Lewis dot structures you'll notice that almost everyone will draw that portion of the molecule straight to better reflect the actual geometry of the molecule alright so I need to figure out how many carbons are in this alkyne right so I find my longest carbon chain including my triple bond so let's see here is one carbon two carbons 3 carbon 4 and 5 so a five carbon alkyne so remember your route for five carbons would be pent alright so be pentane if it was an alkane was its alkyne be pent on so this is pen time so I have that much so far next I need to figure out how to number my chain alright so I could number it from the left or I could number from the right my goal is to give my triple bond the lowest number possible so it's a lot like it's a lot like double bonds here so I want to give my triple bond the lowest number possible and that means of course I need to start from the right here so if I start from the right and I say this is carbon one then my triple bond starts at carbon 2 and then this would be carbon three carbon four and carbon five like that so the triple bond starts at carbon - so all I have to do is put a two in here and say - pen tine so it's in that respect it's just like naming alkenes here all right so that's - pen time let's do another one all right so one that's a little bit more complicated than that so let's let's look at this molecule here so there's my triple bond and then I have let's see I'll put in a chain and some methyl groups there and so this is my molecule all right so once again find find the longest carbon chain that includes your triple bond so that would be one two three four five six and seven so seven carbons would be HEPT right so it's hep teen alright so let's go ahead and write hep dine in here so this molecule would be hep tine like that and let's go ahead and number it's right I want a number to give my triple bond the highest priority here so in this case in this case if I start from the left or start from the right there'd be a two for either that methyl group or for the start of my triple bond the triple bond is going to win you want to give the triple bond the highest priority and that's the reason you're going to start numbering from the left okay so you're going to go one two three four five six and seven like that and so now we can see that our triple bond starts at carbon two alright so I'm going to write two heptane here like that and what else do I have well at carbon five and a carbon six I have methyl groups alright so be five six dimethyl so I'll go ahead and put that in here five six dimethyl - hep dine like that and this is this is a kind of the old-school way of naming it right so old I you pack recommendations would say to name it this way and maybe most organic chemistry professors will state wolf will still name this molecule this way but there's a a newer way that that I you pack recommends that you name this molecule so it would still be five six dimethyl except now you would go ahead and write HEPT and then you would put the two in between the endings they'd be HEPT - I like that so the first way is more old-school the second way is more the the new way of doing it either way is perfectly acceptable just make sure to do the one that your professor prefers