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Current time:0:00Total duration:10:09

Video transcript

let's look at the hydrohalogenation reaction of alkynes we've seen this reaction before with alkenes in there there are some similarities and some differences so let's let's look and see what happens here we start with our alkyne our triple bond and we add our hydrogen halide and we add either 1 or 2 molar equivalence of our hydrogen halide if we add one equivalent of our hydrogen halide the hydrogen and the halogen are going to add from opposite sides and the the halogen is going to add to the most substituted carbon so this halogen here is going to add to the most substituted carbon which in terms of regiochemistry alright so go ahead and write here regio chemistry that was called markovnikov's rule alright so markovnikov's rule alright we're going to add the halogen to the most substituted carbon in the reaction so with 2 molar equivalents you're going to end up with two halogens on the same carbon like that so let's let's take a look at one of the one of the proposed mechanisms for the hydrohalogenation amount of alkynes and and it closely parallels the hydrohalogenation of alkenes and this isn't considered to be the perfect mechanism for alkynes we're going to start with that just just just to show why this is markovnikov's in terms of regiochemistry so let's say this was the mechanism we start with our alkyne and we have our hydrogen halide so our hydrogen halide like that and when we did this mechanism for alkenes we said the first thing that happens was all right with these electrons in this bond we're going to form a bond with this proton here and then these electrons are going to kick off on to your halogen like that so let's see what we can do so let's say let's say the hydrogen adds on to the right side alright so now there's only a double bond between my two carbons and hydrogen added on to the to the right carbon like that and so now this right carbon has a bond going down like this and now this carbon on the left has only three bonds to it alright so that gives it a plus 1 formal charge like that and then our halogen right it's going to have a negative 1 formal charge so our halogen would act as our nucleophile right and our positively charged carbon right here would act as our electrophile and so you would get the halogen adding on to that carbon and since a carbo cation was involved in the mechanism that's the reason why markovnikov's rule is seen right because you want to form the most stable carbo cation possible so the more substituted carbocation is the more stable one so this this really isn't quite the the correct mechanism and the reason why it's not quite the correct mechanism is because if this was the right mechanism right the rate of this reaction would be would be would depend on the concentration of two molecules by molecular so if we were to write the rate law for this for this mechanism right we would say okay I expect the rate of reaction for hydrogenation of alkynes we have a rate constant in there to be proportional to write the concentration of your alkyne and then the concentration of your hydrogen halide two things because that's what we started in this mechanism right we had our hydrogen halide right here we had our alkyne and that is not observed experimentally so this mechanism is is not quite right so this this mechanism is not quite what happens but it is kind of important to think about you know this carbon right as being the positively charged one and markovna comments rule applying so it helps to think about this mechanism but this mechanism isn't quite right because this is not the experimental rate law the experimental rate law turns out to be second order with respect to your hydrogen halide and third order overall all right so if you put a superscript two there it's it's actually dependent upon two molecules of your hydrogen halide and one molecule of your alkyne so since it's actually third order overall we need to come up with a different mechanism all right so let's let's look at the mechanism that is currently proposed as the mechanism for the hydrogenation of alkynes we start with our alkyne here and we need two molecules of our hydrogen halide so here's one molecule of our hydrogen halide like that and let's go ahead and draw the other one down here okay so let's put put in those lone pairs of electrons like that so in in in this mechanism all these all of the all three of these molecules are reacting at the same time so as as these electrons in here from the bond between the hydrogen and the halogen right the halogen is going to take those electrons and they're going to move and form a new bond with this carbon at the same time that's happening these two electrons are going to form a bond with this proton and then these electrons are going to kick off onto your halogen like that so if we were to draw the transition state right of all this stuff happening at the same time so go ahead and put in our brackets here like that okay so what's going to happen well we're going to have carbon double bonded to another carbon like that and then we're going to have we're gonna have a lot of partial bonds in here all right so let's use let's use a different color for partial bonds we have our hydrogen and our halogen and then we have our hydrogen and our halogen up here and then let's use let's use blue for partial bonds right so so this this halogen is forming a bond with this carbon here at the same time the bond this bond is is breaking in here like that and this this we started with the triple bonds right but that triple bond is is is leaving right to form a bond over here with this proton like that at the same time this protons bond is breaking with this halogen like that so so this is all one one giant transition state and if you think about what's happening right the reason why I wanted to show this mechanism up here is to show you right if this happened you know stepwise then this carbon the left will get your full positive charge right let me just highlight that this carbon left gets your full positive full positive charge this one right here and so if we think about what's happening in this mechanism down here it's not quite the same thing but since the bond is leaving this carbon on the Left right the triple bond is leaving this carbon on the Left this is the one that's going to get some partial carbo cation a character so it's going to be partially positive and it's hard to see in this mechanism how this carbon could be partially positive but up here it's easy to see because the triple bond is is breaking the carbon on the left and it's going over here to the carbon on the right so so that's the reason why it's important to thing about this mechanism this gives you your partial carbo cation character which explains the markovnikov's regiochemistry alright so and now we're pretty much done right once those electrons finish moving alright we're going to get let's go back to our yellow color here we're going to get our alkene alright so we're going to form our alkyne like that and let's see we've formed a bond with our halogen down here and then we form the bond up here with that proton and so that is going to be the product of our of our mechanism here like that so let's let's look at a couple reactions all right so let's let's look at the hydrogenation of alkynes here so we'll start with an alkyne so go ahead and draw it like that and remember you want to make it linear around your alkyne since they are linear so in the first reaction we'll just add 1 molar equivalent of our hydrogen halide like that so I know I'm going to add on hydrogen and a halogen across my triple bond I know that's going to form a double bond alright so the first thing I'm going to do is count many carbons I have let's see 1 2 3 4 and 5 so I know that I'm going to turn going to turn a a five carbon alkyne into a five carbon alkyne so let's go ahead and write draw our five carbon alkyne so 1 2 3 4 & 5 right and the alkyne is going to be between these first two carbons over here on the right like that so so that's part of my products but now I have to figure out okay which which one of these two carbons do I add my halogen to right do I add it to this carbon over here on the left side of my triple bond or I do I add that halogen over here to the carbon on the right side of my triple bond and to do that even think about markovnikov's rule right so if I add it to the right one that would give me a primary carbo cation I mean I mean everything about which which one will give you the most stable carbo cation and the most stable carbo cation would be your secondary one over here on the left and so that is the one that your halogen is going to add and your halogen is going to add over here on the left side so again watch watch those earlier videos for much more in terms of detail about carbo cations and stability and and you know markovnikov's rule so let's do let's do the addition of two equivalents of hydrogen halide or just make it in excess here right so in excess so you have to think about the fact that okay so once again I'm starting with five carbons so I'm going to get five carbons for my product here alright and go back go back up here to our original original reaction right you can see if you have two molar equivalents you're going to end up adding your two halogens right to the to the same carbon and I probably should have drawn this a little bit differently on the general reaction it also is going to exhibit markovnikov's for regiochemistry so when you're trying to figure out which carbon that is think about which one be the most stable carbo cation again so so once again we have two choices the carbon on the left side of the carbon the right side of the carbon on the left side gives us more stable right so we're going to add on two equivalents of our halogen here so we actually form a dot eight dihalide as our product so so that's that's hydrohalogenation of alkynes with one or two molar equivalents of your hydrogen halide