hydrohalogenation that's going to be the topic of this lesson and this is going to be the first in a series of alkene addition reactions and for your standard hydrohalogenation you're going to add a hydrogen and a halogen across an alkene in this case that's typically either hcl hbr hi as your reagent therefore adding either an h and a cl h and br h and i across that alkene turns out it is markovnikov addition there is no stereoselectivity associated with it and it goes through a carbocation intermediate which means it is subject to carbocations in certain cases now there is one special case here with hbr this doesn't apply to hcl or hi this last one here but with hbr if you add a peroxide ror is our abbreviation here it will actually go through a totally different mechanism it will go anti-markovnikov this mechanism will involve radicals instead of a carbocation so no rearrangements and there's also no stereoselectivity with this version as well now this lesson is part of my brand new organic chemistry playlist i'll be releasing these lessons weekly throughout the 2020-21 school year so if you don't want to miss one subscribe to the channel click the bell notification you'll be notified every time i post a new lesson so the first alkene edition reaction we're going to look at is hydrohalogenation where we're going to add a hydrogen and a halogen across the alkene and you might recall we uh when we studied elimination reactions we did dehydro halogenation where you lost a hydrogen and a halogen and formed an alkene it's the exact opposite of hydrohalogenation here so elimination and addition exactly inverse reactions here so uh instead of forming an alkene we're going to be consuming an alkene and forming two new sigma bonds now with hydrohalogenation your agents are either hcl hbr hi and you can either add an h and a cl an h in a br or an h and an i and so in this case it also goes markovnikov so here's an example here the h ended up on the less substitute side the bromine ended up on the more substitute side so we've got our primary carbon here and our secondary carbon here the h1 on the primary the bromine one on the secondary that is markovnikov addition so and finally the stereoselectivity it turns out is none but the only time we would care is if we formed two chiral centers anyways and in this case where the bromine added has two identical methyl groups not four different groups not a chiral center and where the hydrogen added has three identical hydrogens not four different groups not a chiral center with no chiral centers formed we get just the one a chiral product all right so this is an example but we definitely want to take some time to look at the mechanism here so if we take a look in the first step of every mechanism we know the alkene is the nucleophile and whatever else you're adding is the electrophile in this case hbr is a strong acid you learn in gen chem it's also a strong electrophile and it's going to get attacked here specifically that partially positive hydrogen hydrogen can only have one bond so we're forming a new one here the old one here has to break and normally i wouldn't draw in this hydrogen but i'm going to draw it in here so we can kind of keep track that hydrogen again bonds on the left substitute side notice i didn't draw in the product up here so and i'll draw it on the first couple here but eventually i'm going to stop drawing these h's so because you're not going to draw them on your exam either cool but the h ads are the less substitute side and the more substitute side is now missing a bond you just have a pi bond now he doesn't only three bonds and he's a carbocation and then we also form this bromide ion here cool and that's step one of the mechanism and because we formed a carbocation here you typically will have to check for rearrangements now it turns out there's only going to be three of the reactions for all the alkene reactions that form a carbocation and therefore they're the only three that are subject rearrangements but this is indeed one of them whether you use hcl hbrhi you're gonna have to check to see if there's rearrangements well we've got a secondary carbocation you generally only have a chance of rearranging to one of the adjacent carbons which are both primary in this case and therefore they would not be more stable carbocations and there's no favorable rearrangement for this reaction cool now this is not rocket science here so if you can identify who's electron rich and who's electron poor it will help you immensely in memorizing some of these mechanisms and in this case with a negative formal charge bromine is definitely electron rich and that makes him a likely candidate to be a nucleophile so in our carbocation with a positive formal charge definitely electron poor makes him a likely candidate to be an electrophile in a typical nucleophilic attack mechanistic step the nucleophile attacks the electrophile i.e the nucleophile attaches to the electrophile and so this bromine is just attaching to that carbon right here which looks like our final product cool and again we added the h on the left substitute side the bromine on the more substitute side and that was markovnikov addition also we didn't form any chiral centers and so we just get that one a chiral product now it turns out we have a second option with hydro halogenation so here we have hcl hbr hi works exactly the same way but for specifically hbr and hbr only you have another option with hpr you can mix it with what's called a peroxide and ror is our abbreviation for a peroxide where those r's could simply be hydrogen which would be hydrogen peroxide hoh is h2o2 or they could just be some sort of more commonly make this an organic peroxide make them like methyl groups or benzene rings or something like this so but oftentimes more commonly you'll just see it written generically just like this and i like to think that hbr and roar is the name of the reagent so we'll see why that's convenient in just a second and in this specific reaction we're going to add an h and a bromine however it's not going to go markovnikov now it's going to go anti markovnikov and so i like to think of hbr and roar the roar scares hbr into going anti-markovnikov so just my stupid little way of remembering it there's also no stereo selectivity in this reaction either now the mechanism of this one we are not going to cover just yet so mechanistically on this one it goes through a radical intermediates not carbocation so totally different mechanism so and we'll study those in the radical chapter a little bit later so but suffice it to say it does go anti-markovnikov but not because we're violating the rules of chemistry or trying to get the less stable carbocation or anything like that we're not even going through a carbocation totally different mechanism so but if we predict some products here if we did the exact same reaction but added some peroxide with our hbr now all of a sudden the bromine ends up on the less substituted carbon the hydrogen would have added to the more substitute carbon and there's our product so neither one of the carbons that became sp3 hybridized are chiral centers neither of them have four different groups and so we get just the one a chiral product all right so so far we haven't formed any chiral centers and it makes this really easy don't have to show any stereochemistry get one a chiral product life is good so i want to give you an example where we actually do form a chiral center and so let's do one more here and this time around i'll use hcl could have used hcl hbrhi so still going markovnikov then normal hydrohalogenation so first step we're going to go grab an h the bond to chlorine breaks and we're going to form a carbocation here and i highly recommend we just draw those carbocations out just to keep track so the h ends up on the left substitute side of the alkene over here not drawn in so and the more substitute side is going to be the carbocation in this case it's secondary the two adjacent carbons one's primary one secondary neither one is better this one's equally stable but has to get better to be a favorable rearrangement for one of the adjacent carbons so no favorable rearrangement here and that's where chlorine ultimately is going to attach and if we take a look we'll see something different here where the h added is not a chiral center there's not four different groups but where the chlorine added does indeed have four different groups a chlorine a hydrogen that's not drawn in a methyl and the carbon of an ethyl four different groups that's a chiral center and this is not sufficient answer so with a chiral center it could exist as r or s and if you form one chiral center no matter which alkene reaction you're looking at all of them will always form r and s and so you could represent this one of a couple of different ways so one and the most common way is just to draw both different versions out one where the chlorine is a wedge and one where the chlorines are dash so draw both r and s so although some professors will also do this and we are kind of the kings of laziness in organic chemistry and the fewer things we can draw the better and so sometimes we don't even show the stereochemistry on that chiral center but we draw plus minus just to show that you can get both the positive and the minus enantiomer another way of saying you get both the r and the s so cool but more commonly you'll see professors do it the top way so and less commonly the bottom way i will tell you that every professor will accept the top not every professor will accept the bottom so you're better off just drawing out both enantiomers in this case now if you found this lesson helpful consider giving me a like and a share a couple of the best things you do to help promote the channel and if you've got questions feel free to leave them in the comment section below if you're looking for the 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