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Current time:0:00Total duration:11:49

Video transcript

here's a flow she created by dr. Schwartz who was my organic chemistry professor and what's nice about this flow sheet is it shows you all of the reactions or most of the reactions that you've studied in the first semester of organic chemistry and it shows you how all 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 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 and help to save lives and or help to make everyone's life better in society so synthesis is really the is really what organic chemistry is all about and it can be very difficult which is which is why you'll see a lot of these problems on organic tests and that's why I need to do practice problems using flow sheets so let's check out let's check out our problem here all right synthesize the following molecules using only acetylene methyl bromide and any inorganic reagents or solvents so we'll start with it we'll start with the 2-propanol molecule and we'll think to ourselves okay 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 retro synthesis so I'm gonna draw a retrosynthesis arrow here so so it looks like that I think well okay how do I make what can I make an alcohol from right 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 flowsheet and I can see let's see if I can find that reaction so I'm going from I'm going from an alkene to an alcohol right so I look at my flowsheet here and here is an alkene right here and here I'm going from an alkene to an alcohol and alright so I'm using this arrow right here so I have two choices of reagents right now I can add water and sulfuric acid which would be a markovnikov addition of the O H right or I could do a hydro operation oxidation which wouldn't be an anti markovnikov addition of the O H so I look down here and say okay which one do I want which one do I want I want a markovnikov addition I want to add the O H to the most substituted carbon like this so it's going to be it's going to be water and sulfuric acid alright for this transition so water and sulfuric acid like that now I have an alkene alright 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 right so acetylene is all the way over here on the top right here that's acetylene and I have an alkyne so I need to think about retrosynthesis I need to work backwards okay so what can I make an alkene from I can make it from this alkyne alright so I can make it from this alkyne right here and how do I turn that alkyne into an alkyne well I can eat either use hydrogen gas and lindlar palladium right rule would you give me a syst product or I can use the the sodium metal and ammonia which would give me the trans product here so let's think about retrosynthesis all right so I have an alkene I'm going to make that alkyne from an alkyne and I need to think about what reagents do I want to use here so what would the alkyne look like well if I'm gonna add hydrogen across the triple bond right I can just think about turning this molecule on the left into an alkyne so all I have to do is go like that alright just added a triple bond to the molecule I made it linear because that's of course what alkynes are so how did I do that well in this case you know you don't worry about sister trans so it's it's pretty easy just to use you know hydrogen gas and then lindlar palladium for this transition here so so now I have this is my alkyne alright and I want to get back to acetylene so I draw my retro so this is arrow here and I think okay how can I make that molecule from acetylene like that so I go back up to my flowsheet all right so how do I make how do I make that terminal alkyne from acetylene so I'm aligned right here I have my I have my alkyne and I want to make it from acetylene so I'm gonna do an alkylation react all right and because I only need one alkyl group I'm only gonna do it one time so I need a one time alkylation reaction and the first step you add is very strong base sodium amide the second step you add a primary alkyl halide so let's go ahead and write draw that down here so first step right I want to add first step I want to add sodium am I like that and my second step I want to add a an alkyl halide so what alkyl halide do I want to add well I want to add a methyl group onto my alkyne I want to add a methyl group and up here it says I can use methyl bromide all right so that will be the alkyl halide that i will use for this alkylation reaction so CH 3 BR like that and so now we've done it alright so we used retrosynthesis and if you were to write this on a test you would you probably write it in the reverse order here right so you would start with acetylene alright and then use the regular arrow in the first step add sodium amide and then at methyl bromide for an alkylation reaction to put this alkyl group on to 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 markovna competition of Oh H using water and sulfuric acid to add Oh H to your alkene and then you're finally done so that's that's the approach that you should take when you're doing a synthesis reaction alright so we have time to do one more here let's do the synthesis of the molecule on the right okay so you look at the molecule on the right and you think to yourself alright so let's go ahead and redraw that molecule won't learn right down here we have some more room ok so here I have my molecule on the right let's go ahead and use yellow for our synthesis problems here so this is my this is my molecule I have two bromines trans from each other and immediately that should 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 alright 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 all right 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 this side and and that would the reaction so now we have an alkyne and I need to make this I need to make this alkyne from acetylene all right so I'm once again at this stage all right so I'm once again right here I have this alkyne I want to make it from acetylene so I'm gonna do my alkylation again all right so I'm going to add sodium amide my first step in an alkyl halide in my second step so let's go ahead and draw that so in 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 all right and 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 on to acetylene so I need ethyl something like ethyl bromide would work alright so 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 all right the question says all right 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 let's think to ourselves okay so this way works now I have to figure wait to make ethyl bromide from from acetylene so once again we use a retrosynthesis all right so retrosynthesis here and i think okay 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 all right so we want to make an alkyl halide from an alkyne so let's let's find let's find our our alkyl halide and it's all the way over here all right so our alkyl halide is all the way over here so for doing retrosynthesis I want to get back to an alkyne so I can make an alkyl halide from an alkyne if I add a hydrogen halide so HX okay so retrosynthesis I'm going to I'm going to make my alkyl halide from an alkyne using hydrogen halide here so I'm going to go ahead and and and let's see what am I going to write I'm going to I'm gonna have a start with an alkyne alright so that's my alkyne it's just ethene or ethylene and my halogen is 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 I have an alkene now but I have to I have to start with acetylene so how do I make how do I make an alkyne from an alkyne once again just go back to refresh everyone's memory on the flow sheet alright so now I am right here on the flow sheet right I have my alkyne and I know how to make an alkyne from an alkyne alright and all I have to do is hydrogenate it right using my poison catalyst using my lindlar palladium so let's go ahead and draw that in alright so I can make my alkyne I can make my alkyne from from an alkyne if I add a hydrogen gas and my poisoned catalyst lindlar palladium here and and that'll be acetylene actually alright so if I if I take acetylene here alright and let's let's go ahead and check our synthesis right do I meet all my qualifications and again if I start with acetylene and let's just walk back through here and I add hydrogen gas and poison catalysts I'm going to hydrogenate my alkyne to form my alkyne right here I take my alkyne and i add hbr to it and that's going to add H+ and br- across my double bond to give me a thil 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 alkynes to add on my bro means anti to each other like that and so we're done and so once again if you're writing this on a test you know you probably make sense to write it the other way but this is 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 alright so I encourage you to to use this fluid flow sheet make your own write do lots of practice problems do lots of synthesis practice problems you can have your friends make them and then you could and then 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