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Synthesis using alkynes

Using a flowsheet showing different reactions of alkanes, alkenes, and alkynes to solve an organic synthesis problem. Created by Jay.

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Video transcript

Here's a flow sheet created by Doctor Schwartz, who was my organic chems professor. And what's nice about this flow sheet, is it shows you all of the reactions-- or most of the reactions-- that you studied in the first semester of organic chemistry. And it shows you how all of those reactions are connected to each other. And this is very, very helpful when you're trying to do a synthesis problem. When you're trying to synthesize one molecule from another molecule you have to know the reactions that connect those different functional groups together. And a flow sheet is one of the best ways of doing it, to show you all of those different connections here. And synthesis is what organic chemistry is all about. How do you synthesize this molecule from that molecule? How do you synthesize a molecule that's important to humanity, that's used in medicine to help to save lives or help to make everyone's life better in society? So synthesis is really what organic chemistry is all about. And it can be very difficult, which is why you'll see a lot of these problems on organic tests. And that's why did you need to do practice problems using flow sheets. Let's check out our problem here. All right? Synthesize the following molecules using only acetylene, methyl bromide, in any inorganic reagents or solvents. So we'll start with the two propanol molecule. And we'll think to ourselves OK, how do I make this alcohol from acetylene? And at first, it's not so obvious what to do. One approach is to think backwards. And this is called retrosynthesis. So I'm going to draw a retrosynthesis arrow here. So it looks like that. And I think well, OK, what can I make an alcohol from? So I can make an alcohol from an alkene. That's one of the reactions that we've seen this semester in organic chemistry. Like I draw an alkene like that. And I go back up to my flow sheet. And let's see if I can find that reaction. So I'm going from an alkene to an alcohol, right? So I look at my flow sheet here. And here is an alkene right here. And here I'm going from an alkene to an alcohol. So I'm using this arrow right here. So I have two choices of reagents, right? And I can add water and sulfuric acid, which would be a Markovnikov addition of the OH, right? Or I could do a hydroboration oxidation which would be an anti Markovnikov addition of the OH. So I look down here and say, OK, which one do I want? Which one do I want? I want a Markovnikov addition. I want to add the OH to the most substituted carbon like this. So it's going to be water and sulfuric acid for this transition. So water and sulfuric acid, like that. Now, I have an alkene. But I want to get back to acetylene. So I go back up here to my flow sheet. And I try to find acetylene on my flow sheet. So acetylene is all the way over here on the top right here. That's acetylene. And I have an alkene. So I need to think about retrosynthesis. I need to work backwards. OK, so what can I make an alkene from? I can make it from this alkyne. So I can make it from this alkyne right here. And how do I turn that alkyne into an alkene? Well, I can either use hydrogen gas and Lindlar palladium, actually, which would give me a cis product. Or I can use the sodium metal and ammonia which would give me the trans product here. So let's think about retro synthesis. All right, so I have an alkene. I'm going to make that alkene from an alkyne. And I need to think about what reagents do I want to use here? So would the alkaline look like? Well, if I'm going to add hydrogen across a triple bond I can just think about turning this molecule on the left into an alkyne. So all I have to do is go like that. This added a triple bond to the molecule. And I made it linear. Because that's, of course, what alkynes are. So how did I do that? Well, in this case you don't worry about Cis or trans. So it's pretty easy just to use hydrogen gas and then Lindlar palladium for this transition here. So this is my alkyne. And I want to get back to acetylene. So I draw my retrosythesis arrow here. And I think, OK, how can I make that molecule from acetylene like that? So I go back up to my flow sheet. All right, so how do I make terminal alkyne from acetylene? Well, I'm right here. I have my alkyne. And I want to make it from acetylene. So I'm going to do an alkylation reaction. And because I only need one alkyl group, I'm only going to do it one time. So I'll need a one-time alkylation reaction. And the first step you add is very strong base, sodium amide. In the second step you add a primary alkyl halide. So let's go ahead and draw that down here. So first step, I want to add the sodium amide like that. In my second step, I want to add an alkyl halide. So what alkyl halide do I want to add? Well, I want to add a methyl group onto my alkine. I want to add a methyl group. And up here it says I can use methyl bromide. So that will be the alkyl halide that I will use for this alkylation reaction. So it's CH3 BR like that. And so now we've done it, right? So we used retrosynthesis. And if you were to write this on a test you would probably write it in the reverse order here, right? So you would start with acetelyne. And then use the regular arrow. And the first step, add sodium amide. And then add methyl bromide for an alkylation reaction to put this alkyl group onto acetylene. Now you have a terminal alkyne which you can turn into an alkene by the addition of hydrogen gas and a poison catalyst, which stops the hydrogenation at the alkene form. And then you can use a Markovnikov addition of OH using water and sulfuric acid to add OH to your alkene. And then you're finally done. So that's the approach that you should take when you're doing a synthesis reaction. All right, so we have time to do one more here. Let's do the synthesis of the molecule on the right. OK, so you look at the molecule on the right. And you think to yourself, all right, so let's go ahead and redraw that molecule on the right down here. And we have some more room, OK? So here I have my molecule on the right. Oh, let's go ahead and use the yellow for our synthesis problems here. So this is my molecule. I have two bromines, trans from each other. And immediately that should make you think about a reaction we just did in the last video, right? Halogenation will add these guys on trans to an alkyne. So I can add I can add bromine. And my solvent would be carbon tetrachloride. So CCl4. We just did this in the last video. So watch the last video for halogenation of alkynes. And what would that give us? What would that give us for our alkyne here? So we have our alkyne. And then we'd have to have an ethyl group on the side. And that would be the reaction. So now we have an alkyne. And I need to make this. I need to make this alkyne acetylene. So I'm once again at this stage. So I'm once again right here. I have this alkyne. I want to make it from acetylene. So I'm going to do my alkylation again. So I'm going to add sodium amide in my first step and an alkyl halide in my second step. So let's go ahead and draw that. So let's go ahead and draw my retrosynthesis arrow here. So I'm trying to synthesize that alkyne from acetylene. So let's go ahead and draw acetylene in here. And I know I can do that in two steps. First step, add my sodium amide like that. And in my second step, I need to add an alkyl halide. What kind of alkyl halide do I need to add? I need to add an ethyl group onto acetylene. So something like ethyl bromide would work. So an ethyl bromide like that, and that would give me my product. And you might think, I'm done. I'm done with my synthesis. But in reality, you are not done. Because if you go back and you read the question, the question says, synthesize the following molecules using only acetylene or methyl bromide, not ethyl bromide. So you actually can't use ethyl bromide in terms of stopping right here. You need to figure out a way to make ethyl bromide. Ethyl bromide contains two carbons. So let's see if we can think about a way to make ethyl bromide from acetylene right here. So let's think to ourselves. OK, so this way works. Now I have to figure a way to make ethyl bromide from acetylene. So once again, we use retrosynthesis. So retrosynthesis here. And I think, OK, I have an alkyl halide. How do I make an alkyl halide from an alkyne? And once again, it's helpful to look at your flow sheet. So let's go back up and look at our flow sheet. So we want to make an alkyl halide from an alkyne. So let's find our alkyl halide. And it's all the way over here. So our alkyl halide is all the way over here. So if we're doing retrosynthesis, I want to get back to an alkyne. So I can make an alkyl halide from an alkene if I add a hydrogen halide, so HX, OK? So retrosynthesis, I'm going to make my alkyl halide from an alkene using a hydrogen halide here. So I'm going to go ahead and let's see, what am I going to write? I'm going to have us start with an alkene. So that's my alkene. It's just ethene or ethylene. And my halogen is bromine here. So my hydrogen halide would have to be HBr. So if I add HBr to ethylene I will make ethyl bromide. And I'm getting there, right? I have an alkene now. But I have to start with acetelyne. So how do I make an alkene from an alkyne? Once again, just go back to refresh everyone's memory on the flow sheet. So now I am right here on the flow sheet. I have my alkene. And I know how to make an alkene from an alkyne. All I have to do is hydrogenate it, using my poison catalyst, using my Lindlar palladium. So let's go ahead and draw that in. So I can make my alkene. I can make my alkene from an alkyne if I add hydrogen gas and my poisoned catalyst, Lindlar palladium here. And that'll be acetylene, actually, all right? So if I take acetylene here, and let's go ahead and check our synthesis. Do I meet all of my qualifications? And again, if I start with a acetelyne-- and let's just walk back through here-- and I add hydrogen gas and poison catalyst I'm going to hydrogenate my alkyne to for, my alkene right here. I take my alkene and I add HBr to it. And that's going to add H+ plus and Br- across my double bond to give me ethyl bromide as my product. And I take another molecule of acetylene and to that, I add sodium amide. And I add the ethyl bromide that I just created from acetylene to alkylate my alkyne. Put an ethyl group on there. And then finally, I do a halogenation reaction of alkynes to add on my bromines anti to each other like that. And so we're done. And so once again, if you write this on a test, it probably makes sense to write it the other way. But this is just a good way of thinking about it with the flow sheet. So let's go back up here, take one more look at the flow sheet. So I encourage you to use this flow sheet. Make your own. Do lots of practice problems. Do lots of synthesis practice problems. You can have your friends make them. And then you can solve their synthesis problems. You can make synthesis problems yourself. Again, this is a very important skill to master if you want to do well in organic chemistry.