hey gang welcome back for another video here on joe chem okay gang this is going to be a short but i think important video in terms of conjugation and what i want to talk about is halogenating and specifically right we're only going to be doing this with bromine at the allelic position you're going to see that when you're talking about conjugated systems you know sometimes it will be advantageous or you need to actually complete a reaction where you will be sticking a halogen on the allylic position in certain scenarios and in fact we will only be able to do this with bromine but we need to use a different reaction than the free radical chain reaction that we learned at the beginning of ochem1 so if we take a look right this looks familiar this was one of the first reactions you most likely learned in organic chemistry period it's if you have an alkane and you have br2 light and heat you will we know the mechanism you create a bromine radical you create a carbon radical then you bring another br2 along you stick a bromine on we actually get to functionalize one of these carbons so you know without you know being new to conjugated systems you might think to yourself why does you know if it worked then it should work now right and so did i when i you know was in organic chemistry but in fact what happens is because we have this double bond and you know because we've done alkene chemistry what happens is because you have so much br2 up front right because bromine is liquid you'll probably have a lot of br2 molecules swimming around when you introduce your organic reactant what will happen is the bromination of a double bond you will in fact have the double bond grab a br you'll kick that off you'll create that cyclic bromonium ion if you remember that so oh that's kind of ugly long story short i won't go through the full mechanism but you end up having that addition across double bond bromine where you actually have you know just for fun we can remember that it's an anti-edition so you have one above one below that whole jazz so this what i'm trying to say is doesn't work the way we expect it to up top so how can we in the world functionalize allylic carbons in such a way by halogenating them right how do we do that sorry too many words how do we do it well what i'm going to do is i'm going to clean this up and i'm going to show you how we can do that with a new reagent and a new reaction the region is called nbs and you'll see that works just as well so again since the free radical halogenation is not an option for us to put a bromine uh greek to halogenate to brominate the allylic position what we can do is we can use a reagent called nbs and specifically you use nps and light and you may or may not have heat present so what does nbs stand for it's a crazy mouthful n-bromo succinamide you don't need to know how to say that if you can say nbs you're in good shape just in life and in organic chemistry so how does this work and this mechanism really isn't asked a whole bunch but i think asking the difference as to why you would use nbs for the allylic position versus free radical halogenation is a fair question so maybe be aware of the you know the details about what i'm about to say but just remember why you can't use the free radical halogenation in terms of the double bond being brominated versus the allylic position so what is so nbs looks like the following okay so really why nbs works so well for us in this situation is that we get a low concentration of br2 low enough to where we don't brominate the double bond it'll just you know give us br2 and then the bond will homolytically cleave when we get a bromine radical so it basically gives us little bits of br2 that then become radicals that then interact with the double bond so typically what you can do is you can sprinkle in some hbr or just something that will help uh do what i'm about to show you is that if you have some type of hbr or something like that you have kind of like an equilibrium and the equilibrium is very slight to where you have the bromine going with the bromine and the hydrogen going with the nitrogen so i know these are crazy arrows happening but basically we're having hydrogen with taking one of you know one electron from this bond and going with nitrogen bromine taking one electron going over with bromine so all that's really happening here is we're swapping the the nitrogen is getting a hydrogen instead of a bromine and then we make b art br br and once you've made this so again this happens in low concentrations so again this is not super important i think that you can reproduce these arrows but i think you know for your class i'm not in your class but if you can just realize that nbs reacts with something whether it be hbr or something to just give you little amounts of br2 then now erase this then it's business as usual literally the mechanism is so similar you have initiation where you have the low concentration of br2 homolytically cleaves and i will block let's box this don't get confused you have two bromine radicals then propagation one this is where we'll make our radical you bring in a bromine radical right then hydrogen gets plucked off by the bromine radical we create our now very stable allylic radical so we do make hvr as a byproduct which will right you remember will help us create more lower concentrations of br2 from the nbs then p2 you can run into another br2 right again in low concentration but still being produced but you also have more bromine radical being created just by actually doing the reaction so point being is that nbs is something you may see be as you start your organic chemistry 2 career in conjugate systems or maybe whatever you do conjugate systems in organic chemistry 2. and it comes down to the fact that we can't just use our regular free radical chain reaction skills we because of the fact that we you know because of what we know from alkanes or sorry alkenes this will you know whether it's cl2 uh and light or br2 light heat will you end up halogenating the double bond the way to get around that is using our new lovely friend nbs okay gang that does it for halogenating uh at the allylic position with bromine and nbs thank you so much for watching i hope you know if you're feeling liking and commenting or subscribing please do if anything i will see you all in the next video