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Current time:0:00Total duration:10:04

Video transcript

this video we're going to take a look at two ways to reduce alkynes the first way is is a reaction we've seen before this is the hydrogenation reaction entourage ination reaction and we saw before when we hydrogenated alkenes to form alkanes here we're going to hydrogenate an alkyne to form an alkene and to do a hydrogenation reaction we need some hydrogen gas right so some h2 right here and then a metal catalyst right so we're going to use little our palladium which is a special type of catalyst it will it will catalyze the reduction of the alkyne on the left the alkyne on the right however the reduction of the alkene to the alkane down here is slow so so slowly we can stop it if our goal is to just make an alkene all right so this this reaction will form a Cisalpine and it was a syn addition of our hydrogen's right so we're getting it the two hydrogen's adding on to the same side and this has to do with the mechanism of a hydrogenation reaction so you can check out the earlier video on hydrogenation of alkenes to see more details so lindlar palladium a poison catalyst it will reduce an alkyne to an alkene it will produce a sustained alright so that's how to make a Sisyphean let's take a look at the how to make a trans alkene so how do we reduce an alkyne to make a to make a trans alkene so here is our alkyne so we have our triple bond like that and we're going to add sodium metal we're going to add sodium metal and we're also going to add liquid ammonia like that so we're going to form a trans alkene alright so I'm going to put this time my two hydrogen's are going to be on opposite sides of each other all right so this is formation of a trans alkene like that and it does this by an anti addition of hydrogen's right so these are adding from opposite sides like that let's take a look at the mechanism to form a trans alkene so I start with my alkyne so go ahead and put in my carbons there and put our group on the left side and I'll make this an R prime group to distinguish it from the our group over there so we start with sodium right which we know begin group wanna has one valence electron like that and in the first step of the mechanism this the sodium atom is going to donate its valence electron to the alkyne so when we're showing the movement of one electron we use a half headed arrow right so I'm going to show this electron moving over here but it's only one electron so I'm going to do a half adder like that not a full headed arrow so one of these one of these one of these bonds here's between the carbons is going to break and one of the electrons is going to move over here to this carbon like that and one of the electrons is going to move over to the carbon on the left so let's go ahead and draw the result of all those electrons moving around right so we have an R group here and we had a triple bond right but now we only have a double bond between our two carbons and then we have our prime over here so the carbon on the right picked up an electron from sodium and it also picked up an electron from the breaking of that one bond there so now it has two electrons around it like that which gives us a negative 1 formal charge on this carbon so it's a carbon ion it's an anion here the carbon on the Left picked up one electron for the breaking of that bond like that so that that's a radical that's the thing we haven't talked about before so we actually form what's called a a radical anion here so let's go ahead and write that this is a radical a radical anion so radical because it's an unpaired electron there and then it also has a carb anion in the same molecule like that so we have these electrons that are pretty close together at least how I've drawn them right so we know that electrons are all negatively charged so all these electrons are going to repel each other so this isn't the most stable way for for this molecule to you to have as in terms of a conformation right these electrons going to repel and they're going to want to try to they're going to want to try to be as far away from each other as they possibly can so what's going to happen is we have our two carbons right here and let's say that these two electrons stay over here on this side right this one electrons going to go over to the opposite side they're going to get they're going to try to get as far away from each other as they possibly can and same thing with these are groups here right so this our group right is going to try to get as far away from this r-prime group as it possibly can right so this this trans conformation is is the more stable one so this is our this is our negatively charged carb anion right here so in the next step of the mechanism right we are ammonia is present so let's go ahead and draw an ammonia molecule floating around like that so here is our ammonia molecule and we the carb anion is going to act as a base and it's going to take a proton from the ammonia molecule right so this lone pair of electrons is going to form a new bond with this proton and these electrons are going to kick off on to the nitrogen so let's go ahead and draw the result of that acid-base reaction right so now we have our two carbons right with an R group right here R prime right here and now this carbon on the right is bonded to a proton to bond to a hydrogen like that and then we still have our radical down here so there's a there's one electron on that carbon as well alright so the next step of our mechanism well there's an there's plenty of sodium present all right so here's a sodium atom with one valence electron the sodium is going to donate this electron to this carbon right so we should use a half headed arrow to show the movement of one electron so if that sodium atom donates that one valence electron to that carbon let's go ahead and draw the results of that alright so we have two carbons double bonded an R group over here a hydrogen and an R Prime and this carbon had one electron around it it just picked up one more from a sodium atom so it's like that which would give it a negative 1 formal charge alright so this carbon has a negative 1 formal charge so let's go ahead and draw that negative one formal charge it's a carb anion and once again ammonia is floating around so let's go ahead and draw ammonia right here so nh3 like that and the same thing is going to happen as it before right the negative charge is going to grab a proton and it's going to act as a base and these electrons going to kick off onto the nitrogen here and so we we protonate our carbon ion and we have completed our mechanism because now we have our two our groups right across from each other and we added on two hydrogen's across from each other as well like that so we formed a trans alkene alright so that's that's the mechanism to form a trans alkene let's let's look at a few examples alright so let's start with let's start with let's start with this alkene right here okay so carbon triple bonded to another carbon and we'll put a methyl group on each side like that okay so let's do let's do a few different reactions with the same with the same substrate here so our first reaction will just be a normal hydrogenation with hydrogen gas and let's use platinum as our catalyst so this is this is not a poison catalyst this is a normal catalyst so what's going to happen is first you're going to reduce the alkyne to an alkene and then since there's no way of stopping it it's going to reduce the alkyne to an alkane so this is going to reduce the alkyne all the way to two an alkane so if we go back up here to the beginning remember we said that poison catalyst will stop at the alkyne but if it's not if it's not a poison catalyst because it's just going to hydrogenate your alkyne to an alkane down here so so this reaction is going to produce an alkane let's go ahead and draw the product alright so we know that there are four carbons in my starting material so there's going to be four carbons alright when I'm done here so alright these two these two carbons in the center here are going to turn into CH 2 s alright and then on either side we still have our CH 3 s so this is going to form so it's going to form butane as the product all right let's this time let's let's use a hydrogen gas and let's use a lindlar palladium here so lindlar palladium this is our poisoned catalyst right so it's going to reduce our alkyne to an alkyne and then it's going to stop and we're good to think what kind of alkyne will you get you will get a C you will get a cyst salqeen right so if we draw our two our two hydrogen's adding on to the same side all right so now we have our methyl groups going like that so our methyl groups will be going like this and this would be this would be our product a ASIS alkyne all right let's do one more all right same starting material all right so this one right here except this time we're going to add sodium and so we're going to use ammonia as our solvent and remember this will reduce our alkyne to an alkyne but it will form a trans alkene as your product so when you're drawing your product down here all right you want to make sure that your two hydrogen's are trans to each other all right so how they add on the mechanism and then your two methyl groups would also be on the opposite side like that so so look very closely as to as to what you are reacting things with right is it a normal hydrogenation reaction is it a hydrogenation reaction with a with a poison catalyst which would form a Cisalpine or is its reduction with with sodium and ammonia which will give you a trans alkene