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Current time:0:00Total duration:7:20

Halogenation and ozonolysis of alkynes

Video transcript

let's look at two more reactions of alkynes and we'll start with the halogenation of alkynes so I start with my alkyne over here and I add to my alkyne 1 molar equivalent of a halogen right so x2 my solvent is going to be carbon tetrachloride and I'm going to add those two halogen atoms across my triple bond in an anti addition right so those two halogens end up on opposite sides from each other's this is an anti addition of my halogens like that now I could add two molar equivalents of my halogen and if that happens each of these carbons is going to end up with two bonds two halogens like that for my product now the mechanism of the halogenation of alkynes is not completely understood so because of that we're just going to move on to 2/1 practice examples instead of showing the mechanism alright so let's look at let's look at an alkyne so we'll go ahead and draw an alkyne over here so here I have my my carbon a triple bonded to another carbon I'm going to put I'm going to put a methyl group on one side here and let's go and write a ch3 in here and I'll put an ethyl group on this side so ch2 ch3 so to that alkyne I'm going to add I'm going to add bromine and I'm going to use carbon tetrachloride as my solvent and I'm going to say 1 molar equivalent of my bromine is added so when you do your stoichiometry just 1 molar equivalent like that so I'm going to add my my to bromine zon anti to each other right so let's go ahead and show my triple bond became a double bond all right and my two bro means we're going to add on anti to each other so they're going to add on opposite sides of my double bond like that and I still have a methyl group over here on the connector to the carbon on the left so go ahead and put in my methyl group like that and the carbon on the right still has an ethyl group attached to it so ch2 ch3 so that would be the result of the halogenation of this alkyne let's take a look at one more reaction of alkynes let's look at those analysis all right so the OHS analysis of alkynes let me go ahead and write ozone alysus here and we've seen this reaction before similar reaction when we when we did when we did this with alkenes right so this time we're going to do with alkynes let's take a look at an alkyne all right so there's an alkyne I'm going to say it's an internal alkyne meaning the triple bond is found in the interior the molecule it's not on the end of the molecule so let's let's look at those analysis of internal alkynes first and when you're doing ozonolysis you're all you're adding ozone to the molecule in the first step so we went in a very very detailed mechanism for for the ozonolysis of alkenes and you can go back and watch that video for this video we're not going to go through any kind of mechanism we're just going to go for the products so we add ozone in the first step and in the second step we're going to add water and what this does is this this Cleaves your triple bond and it gives you carboxylic acids as your product okay so two of them so let's go ahead and draw those two carboxylic acids and then we'll try to point out where everything comes from so here's a here's one of the carboxylic acids and then here is going to be the carboxylic acid it's going to result on the right side so they'll make this R prime over here so let's let's go ahead and point out which carbons are one here so let's let's show that this carbon over here is bonded to an R group so that's this carbon bonded to an R group and then over here on the right this carbon is the one bonded to an R prime right this carbon is the one bonded to an R prime so you you cleave your triple bond right you break your triple bond and you're going to create two separate molecules from this okay so so we get carboxylic acids from this reaction let's look at ozonolysis of terminal alkynes now so let's instead of the triple bond being in the interior of the molecule now the triple bond is on the end of the molecule so that makes this a hydrogen right here all right so let's let's add once again ozone in my first step and water in my second step and on the left side at the left side is going to give us the same product as before all right so let's go ahead and identify that in blue this kharbut in this our groove we saw before that's going to give us a carboxylic acid like that so let's go ahead and draw that again so it's the same that portion of the molecule gives us the same product as before alright so we get a carboxylic acid and once again the carbons this is carbon and blue and that that our group in blue those are the same ones on the left side of your reaction now on the right side of my reaction just going to put that hydrogen in there on the right side of the reaction the terminal alkyne portion you're actually going to get carbon dioxides ok so now you only have only one carbon to think about right this is the only carbon you have right here so you're going to get co2 out of it alright so let's go ahead and draw co2 as your product like that remember it's a linear molecule and so this this reaction was used decades ago for structured structure determination right if if a terminal alkyne was present and you reacted it with ozone and you would you then you get some carbon dioxide in and you can also analyze the molecule by the carboxylic acids that you get right so you could for example you could see you know how many our group you know how many carbons are in your R group and then that would give you an idea about the structure and and and all those kinds of things so this used to be a very important reaction in in organic chemistry now with all the spectroscopy stuff that that that organic chemistry has this reaction isn't really used as much anymore so let's look at look at one one example of of an ozonolysis reaction so let's look at this one right here like that so we're going to take that terminal alkyne we're going to add ozone to it in the first step and then we're going to add water to it in the second step and let's see how many carbons we're dealing with here sometimes that's what confuses students right so there's one carbon two carbon three carbons and on this side is our fourth carbon and that carbon the far right is bonded to a hydrogen making this a terminal alkyne so I have four carbons to worry about and when this undergoes alkyne cleavage all right it's going to it's going to cleave the molecule here all right it's going to break that triple bond and so you're going to get two products you're going to get one product with three carbons all right over here on the left and then one with one carbon on the right so the product with three carbons on the left is going to be a carboxylic acid so all you have to do is draw a three carbon carboxylic acid so let's go ahead and do that so here is our three carbon carboxylic acid that is one of our products and then terminal alkynes will give us co2 which takes care of the other carbon on the right over here so we're going to get co2 as the other product of this reaction so that that sums up all of our reactions of alkynes in the next video we're going to take a look at some synthesis problems using all all the stuff that we've learned in the first semester of organic chemistry and especially including some of the alkyne reactions