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Current time:0:00Total duration:11:42

Video transcript

to hydrogenate and alkene you need hydrogen gas and a metal catalyst something like platinum or palladium or nickel and there are many others but these are the ones most commonly used so what happens is those two hydrogen's from the hydrogen gas are added across your double bond and they're added on the same side of where the double bond used to be so it's a syn addition let's take a look at why this is a syn addition of hydrogen's so we have our metal catalyst over here so let's go ahead and draw our flat metal catalyst and these metals are chosen because they ads or pi drogyn really well which means that if you bubble hydrogen gas through right the hydrogen is going to be absorbed to the surface of that metal catalyst like that and then your alkene comes along and your alkene is also flat right the portion of the molecule that contains the double bond alright so these two carbons is carbon this carbon is sp2 hybridized which means that the stereochemistry around those two carbons that it's going to be flat so this portion molecule is flat so you have one thing that's flat approaching something else that's flat so the only way those hydrogen's can add are to add them onto the same side right so if this carbon and this carbon if you add this hydrogen to the carbon left and add this hydrogen to the carbon on the right and then you go ahead and you draw the rest of the bonds right this would now be a wedge and then a dash and then this would be a wedge and a dash you can see those two hydrogen's have added onto the same side so this these two hydrogen's are these two hydrogen's for our sin addition notice we're also changing from sp2 hybridization to sp3 hybridization over here on the right so we have to think about stereochemistry for this reaction for your products as well so let's let's take a look at an actual reaction here and let's see if we can follow along so if this if this was my reaction right I went to hydrogenate to this alkene so I would add some hydrogen gas and I could you know choose whichever metal catalyst I wanted to I would add two hydrogen's on the same side right so I could add two hydrogen's on the same side so just like I did up there so we would get now everything changes from sp2 hybridization to SP 3 so we have wedges and dashes to worry about and usually you wouldn't see it drawn like this that's too much work quite frankly it would be much easier just to say oh well all I have to do is take away the double bond and there's my product so for some of these reactions are very very very simple just take away the double bond and you'll end up with your alkane like product let's take a look at oxidation states for this reaction so I'm going to I'm going to redraw this reaction and this time I'm going to draw in my atoms and I'm also going to draw in my electrons here in a second so I'm just just drawing out all the atoms here so I have all these methyl groups to worry about and then I have electrons in these bonds right each one of these bonds consists of two electrons so I'm going to go ahead and put in all of my electrons here like that now let's assign oxidation states to those two carbons of that formed our double bond right so let's look at let's look at oxidation state for the top carbon remember when you're doing oxidation states you're worried about electronegativity so oxidation states are all about electronegativity so go back and watch the earlier video on oxidation states so we have here comparing the electronegativities of carbon and carbon well obviously they're the exact same so in the struggle for these electrons it gets divided right each one of these carbons is going to get one of these electrons so that's the case for all of these right here so that carbon has four electrons around it it's the exact same thing for this carbon right this carbon has four electrons around it to assign an oxidation state we take the number of valence electrons that atom usually has which carbon normally has four of course and from that we subtract the number of electrons we we we just drew around it for our dot structures without before each one of those carbons has four so each of these carbons has an oxidation state of zero let's look at the product and let's see if we can assign some oxidation states for the product right so our product over here on the right we had carbon and we had some methyl groups bonded to that carbon we added on hydrogen right so each one of these carbons got a hydrogen added on to it and let's go ahead and fill in our electrons in these bonds all right so once again each bond consists of two electrons like that and now we have a single bond between our carbons and let's assign some oxidation states so once again we know that the two carbons have the same same electronegativity right so the tug-of-war for these two electrons right here it's a tie alright so it's a tie it's a tie what about carbon versus hydrogen carbon is actually more electronegative than hydrogen so in the war over the two electrons in the carbon hydrogen bond carbon wins because it's a little bit more electronegative so we're going to assign this extra electron here to carbon and then again carbon versus carbon so that carbon gets that electron as well same thing down here alright so it's a tie at the tie Carbon beats hydrogen and over here it's a tie so in the dot structure on the right the oxidation states the normal number of valence electrons before from that we subtract the number of electrons in our in our in our picture here which would be five electrons each one of these carbons has five electrons around it so it gained electron and it's a four minus five will give us a negative one so the oxidation states of these two carbons is negative one and we can look at our original oxidation state states of being zero went from zero to negative one that's a decrease in the oxidation state right a decrease in the oxidation state means reduction so this is a reduction reaction so the alkyne is reduced by the addition of these two hydrogen's and you'll see other definitions for oxidation states you'll you'll see a gain in hydrogen's is is is reduction that's another definition that's often found in organic chemistry textbooks and while that's true it's to me it makes more sense to go ahead and assign your oxidation states and watch the oxidation states change as you add those hydrogen's as your molecule gains hydrogen's so this is a reduction let's look at the stereochemistry of the hydrogenation reaction so let's do an example involving stereochemistry so let's say your alkene let's do that ring again wasn't a very good one so let's let's say your alkene looked something like this and you're going to react that with hydrogen and with platinum all right well your first thought might be okay this is simple all I have to do is take away that double bond and I'm done well sometimes that's true but in this case we actually formed two new chirality centers right so this top carbon here is a chirality Center and this bottom carbon here is also a chirality Center so sometimes it's not quite that simple we need to think about the syn addition of those hydrogen's right when you think about the possible products that would result so we're going to get let's see we're going to get two products here let's look at the one the left well one possibility is I could add those two hydrogen's on this on the same side as a wedge right so I have one hydrogen as a wedge the other hydrogen is a wedge that's our syn addition and that means that at this top carbon here this ethyl group must be going away from me so there's my ethyl group going away from me and down here at the bottom carbon the methyl group must be going away from me so that's one possible product the other possibility instead of having my two hydrogen's add as wedges I can have my two hydrogen's add as dashes so there's a hydrogen and then here's a dash and there's a hydrogen so at this top carbon here now my ethyl group is coming out at me at this bottom carbon now now my methyl group is coming out at me like this so I have I have two possibilities right and if I if I look at these two products all right I can see that they are enantiomers they are mirror images of each other so these two these two would be my enantiomers and these would be the products of my reaction so be very careful when thinking about syn additions here let's do one more example of a hydrogenation reaction let's do a let's do a bridged bicyclic compound so let's look at let's look at a famous bridged bicyclic comp let's see if we can draw it here so I'm going to there's my bridge and then I'm going to go like this and then I'm going to draw that back carbon a little bit off like that and my double bond is going to go right here and then this is going to be a methyl group and then on up here going to be two methyl groups like that so this is this is alpha pining alright this is alpha pining right here found in turpentine and you can see there's an alkene on this so if I took this alpha pining molecule and I wanted to hydrogenate it so I use I could use palladium and charcoal play diem in carbon and and if I think about what happens in this mechanism right I know that my metal catalyst there my palladium is going to be flat like that and so when it has those hydrogen's right when the Palladium adsorbs those hydrogen's right it's going to add those two hydrogen's to my double bond think about this guy over here think about the Alpha pining as molecules like a spaceship right and the spaceship is is approaching the docking station so the spaceship is slowly going down right the spaceship is going to approach the docking station and there's only one way the spaceship can approach the docking station and that is the way in which we have drawn it right here it could not flip upside down and approach it from the top because of the steric hindrance of these methyl groups right so this is that this is the way that it approaches and this part of the molecule your alkene is the flat part right so it's easiest for the molecule to approach in this way the spaceship analogy always helps my students so there's only one product for this reaction and let's see if we can draw it here so there's one product for this reaction and let's see what it would look like it would look something like this so we have our two methyl groups right here so the hydrogen's are going to ad from below right so you can see the hydrogen's are going to ad from below so this hydrogen let's say it adds right here that's going to push this methyl group up all right so it's going to push that methyl group up so that I'll group gets pushed up when that hydrogen adds right down here and then this other hydrogen is going to add to the opposite sign right this other hydras going to add to this one right here and so we can show the addition of that hydrogen so there's my syn addition of these two hydrogen's and there was something else in that carbon it was another hydrogen so another hydrogen got pushed up right here as well so that that is your only product the only product of this reaction the hydrogenation reaction is very sensitive to steric conditions