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Current time:0:00Total duration:13:37

Video transcript

let's get some practice drawing in Newman projections our goal is to look down the carbon to carbon 3 bond for this compound and in Part A our goal is to draw the most stable conformation so let's number our carbons this carbon we could say is number one this carbon is number two this carbon is number three and this carbon is number four if we're going to look down the c2 c3 bond that's this bond right here so we're going to put our I along this axis if I draw if I draw this out right here we're going to put our I right here and we're going to stare down the carbon two carbon three bonds we're going to draw what we see our goal is to find the most stable conformation and we know from earlier videos that would be a staggered conformation so we'll draw one staggered conformation and then we'll draw the rest and then we'll pick which one is the lowest energy which one is the most stable so here's carbon one and then we have carbon two with the hydrogen coming out at us and a methyl group going away from us in space attached to carbon three we have a methyl group coming out at us in space and a hydrogen going away from us in space and then we have carbon four our job is to stare down the carbon c2 c3 bond and draw our Newman projection so if I rotate it so we're staring down that c2 c3 bond we can see a Newman projection we can see a staggered conformation for our compound to get another staggered conformation I could rotate the front carbon and keep the back carbon stationary and that gives us another staggered conformation and I could do it again I could rotate the front carbon to get another possible staggered conformation in the video we stare down our c2 c3 bond so this bond right here we put our eye here and we looked at the possible staggered confirmations for our compound it's important to be able to draw the Newman projections without using a model set so let's go ahead and do that so for staring at carbon two so this is carbon two right here and our Newman projection that's where presented by a point so I draw a point here and then we'd have a hydrogen going up and to the right so if your eye is right here your hydrogen goes up into the right from this perspective so the hydrogen going up into the right we have a methyl group going up and to the left so a ch3 up and to the left and then finally we have another methyl group going straight down so a ch3 going down for carbon 3 remember carbon 3 is this one right here you can't see carbon 3 if your eye is right here carbon 2 is in the way but in our Newman projection we represent that with a circle so this circle represents carbon 3 and what's attached to carbon 3 there's a methyl group going straight up so again if your eyes hear this methyl group is up so we have a ch3 going up we have another ch3 going down and to the right so this is going down and to the right and then we have a hydrogen going down into the left so here is our Newman projection for a staggered conformation next we rotated the front carbon and held the back carbon stationary to get another staggered conformation so let's go ahead and do that let's hold the back carbon stationary we're going to take this methyl group we're going to rotate it all the way over to this position if we do that then this hydrogen rotates all the way over to this position and finally this methyl group would rotate over to this position the back carbon stationary so let's go ahead and draw the back carbon a circle and the ch3 is going to stay in the same spot this ch3 stays in the same spot and this hydrogen stays in the same spot so we rotated the front carbon does really matter if you rotate the front or the back carbon but let's go ahead and draw in our R groups here on our front carbon so here's C 2 and we moved the methyl group in magenta over to here so let me go ahead and draw in that methyl group that methyl group moved over to here next the hydrogen in move down here so I can draw the hydrogen and blue here my Newman projection and then finally the methyl group over here in red moved over to this position so the methyl group in red moved over to here and now we have another staggered conformation for our last staggered conformation we're going to do the same thing we're going to take the methyl group in magenta rotate it over to here and then that would cause the hydrogen right here to rotate over to this position and the methyl group in red to rotate over to this position so we keep the back carbon stationary so we draw that in as a circle again we put in our ch3 we put in our ch3 we put in our hydrogen those aren't moving and the methyl group in magenta is now down so this is the methyl group in magenta the hydrogen that moved over to this position and finally the methyl group in red moved over to this position so now we have our staggered confirmations we can double check ourselves by comparing these Newman projections to what we saw in the video so here are the staggered the staggered confirmations from these are stills from the video and you can see they match the Newman projections that we drew finally we need to choose the most stable out of these three so what is the most stable conformation well we know that when that when we have this situation let me go ahead and use let me use blue for this so when we have a methyl group when we have a methyl group and methyl group here all right there are sixty degrees apart this is a gauche interaction so we have a methyl methyl gauche interaction and from the video on on butane confirmations on butane we saw that a gauche interaction has 3.8 kilojoules per mole as an energy cost so we have 3.8 kilojoules per mole for this gauche interaction and we have another gauche interaction here so another 3.8 kilojoules per mole and another gauche interaction so there are three gauche interactions for this conformation for a total of 11.4 so the total would be eleven point four kilojoules per mole as an energy cost so three point eight times three what about this conformation well here's a gauche interaction here's a gauche interaction and here's a gauche interaction so again we have three gauche interactions so for this conformation the total energy cost is also eleven point four kilojoules per mole what about this conformation our first one well here's a gauche interaction so that's three point eight kilojoules per mole and here's a gauche interaction we have only two for this conformation so only two gauche interactions that would be a total of seven point six kilojoules per mole which is the lowest energy so this is the most stable conformation in Part B our goal is to draw the least stable conformation and we know that the least stable conformation is the one that's the highest in energy so let's go to the video where I take the model set and I go from the staggered conformation to an eclipsed conformation and then we look at all the possible eclipsed confirmations one of them is going to be the least stable here we start with our staggered conformation and if I rotate it a little bit we get an eclipsed conformation I left it a little bit off so you could still see the back bonds I rotate it again and get another eclipsed conformation now for this one if I turn it to the sides you can see these methyl groups are really close together in space and so that steric hindrance would destabilize this conformation so if we go back to the eclipsed conformation we rotate again we get to the final eclipsed conformation here are the pictures of the eclipsed confirmations from the video and our goal is to pick the least stable but just for practice let's try drawing all of them as Newman projections so we'll start with the one on the left here and we're staring at C 2 so that's C 2 which we represent by a point and then C 2 has a methyl group going up and I drew a little bit to the right just so we can see what's going on behind it and then we have a hydrogen going down into the right so there's a hydrogen bottom to c2 going down to the right and then over here is another methyl group so to the left is a ch3 for the back carbon even though we can't see it we represent it with a circle so the back carbon right there was a c3 what's bonded to c3 let me use a different color and hopefully you can see there's a methyl group back here bonded to c3 s let's draw that in in red and then there's another methyl group over here so this is a methyl group I'll draw that one in a ch3 and then we have a hydrogen down here so here is the hydrogen so that would be the Newman projection for this eclipsed conformation let's move on to the next picture so this eclipsed conformation so we're staring at c2 so that is our that's our dot here and then we can see there's a methyl group going up into the right so a little bit to the right so there's a ch3 going that way there's a ch3 going down so this ch3 going down and then we have a hydrogen going to the left so hydrogen going to the left here and then in the back right we have our carbon 3 and we held the back carbon stationary in the video so there's no change you can see there is a methyl group straight up here in the back so ch3 and then there's a methyl group going to the right back here so a ch3 and then there's a hydrogen going to the left so this is hydrogen here so here's the Newman projection for this conformation and to go from the one on the left to the one on the right we rotated the front carbon so let me go ahead and show which carbon is which so if you took if you took this carbon and wrote this methyl group I should say and rotated it over here that methyl group in magenta becomes this methyl group the hydrogen right here in blue gets rotated over to this position so that's this hydrogen and then finally this methyl group right here in green would get rotated over to this position so that's this methyl group alright for our last eclipsed conformation over here we're staring at c2 so let's draw that c2 here we have a hydrogen hydrogen going up a little bit to the right we have a methyl group going down so we draw in that ch3 and then I have a methyl group going in this direction so a ch3 here draw in the back carbon so there is c3 again we didn't change anything so again there's a methyl group going straight up so ch3 a methyl group going this way and then so that one right here and then a hydrogen going this way so here's our hydrogen if we we show going from this conformation to this conformation again we'll start with the one in magenta so this one in magenta was moved over to here so that's this ch3 the hydrogen in blue was rotated over to this position so that's this hydrogen and finally the methyl group in green it was rotated over to here and so now we have we have our Eclipse conformation and let's analyze them and determine which one is the highest in energy if we start with our first Newman projection so this one right here we have a methyl group eclipsing a hydrogen and we know from earlier videos that's an energy cost of six kilojoules per mole here we have another situation with a hydrogen and a methyl group eclipsing each other so that's six kilojoules per mole finally a methyl group eclipsing a methyl group which we know is 11 kilojoules per mole so that's a total of 23 kilojoules per mole the energy cost for this eclipsed conformation so let me go ahead and write that here that's 23 kilojoules per mole let's go the one on the right next because you can see it's the same thing we have a methyl group eclipsing a hydrogen so that's 6 we have another situation with the hydrogen and a methyl group eclipsing each other so that's 6 and we have a methyl group eclipsing a methyl group so that's 11 so this one's also 23 kilojoules per mole if we look at the one in the middle it's a little bit different we have a pair of hydrogen's eclipsing each other we've already seen that's four kilojoules per mole so that's four here we have a methyl group eclipsing a methyl which is 11 and then we have another methyl group eclipsing a methyl which is another 11 so what is 11 plus 11 plus 4 that's a total of 26 kilojoules per mole so this is the highest in energy this is the least stable conformation the one where the methyl groups are the closest together in space because these relatively bulky methyl groups these destabilize this conformation so you want to get the bulky groups as far away from each other as you possibly can