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Current time:0:00Total duration:7:31

Video transcript

here we have a model of the propane molecule and if we stare down this carbon-carbon bond here we will see a Newman projection and this is a staggered conformation if I rotate about that carbon-carbon bond that we're sighting down we get another conformation this is the eclipsed conformation and I'm leaving it a little bit off so you can actually see the bonds in the back there so that's an approx and eclipsed conformation I can rotate again and get a staggered conformation and if I keep rotating here the next one would be an eclipsed conformation so again I'm leaving the bonds slightly off to the side so you can actually see the ones in the back I rotate again to get a staggered conformation and the next one would of course be eclipsed so let's look at that so the eclipsed conformation and then finally one more time to get back to our staggered conformation of propane so there's your staggered conformation here we have the energy diagram for the confirmations we saw in the video and make sure you've seen the conformational analysis of ethane video before you watch this one on the y-axis we have potential energy so as you increase in the y-axis you're increasing in potential energy and we started with the staggered conformation of propane and we rotated it 60 degrees we held the back carbon stationary and I rotated the front carbon 60 degrees to give us this conformation which is the eclipsed conformation of propane notice the difference in potential energies between these two confirmations the staggered conformation has a lower potential energy and the eclipsed conformation has a higher potential energy remember the lower the potential energy the more stable the conformation so the staggered conformation is more stable than the eclipsed conformation so it takes energy to go from the staggered conformation to the eclipsed conformation and the analogy that I used in the earlier video was a bolder so if you have a bolder at the bottom of the hill and you're trying to push the boulder up the hill to the top here it takes energy to do that at the top of the hill the boulder is less stable so higher the potential energy less stable lower the potential energy more stable so our staggered conformation is more stable than our eclipsed as we rotate and we go from this eclipsed conformation to this staggered conformation that would be a decrease in potential energy going from this staggered conformation to this Eclipse would be an increase in potential energy going from the eclipsed to this staggered would be a decrease and you see the pattern going from staggered up to this eclipsed would take energy and then going from the eclipsed down to this staggered is a decrease in the potential energy all of our eclipsed confirmations have the same value for the potential energy they are degenerate in terms of energy same thing for the staggered confirmations these all have the same potential energy value so there's a difference in potential energy between the eclipsed confirmations and the staggered confirmations and that difference in energy turns out to be 14 kilojoules per mole so we're talking about the energy difference between the eclipsed and the staggered conformation we know there's an energy difference of 14 kilojoules per mole between the staggered conformation of propane and the eclipsed conformation and that's called the torsional strain let's go ahead and draw a Newman projection for each one of these confirmations so just as practice let's start with the staggered conformation and we'll start with this carbon in front here which is represented by a point so I'll draw on a point here what is bonded to that carbon well there is a ch3 group a methyl group up here so let's draw a line straight up and draw in a ch3 and there's a hydrogen going to the right and a hydrogen going to the left so there's my hydrogen going to the right and there's my hydrogen going to the left we know there's a carbon behind this carbon that I marked with a point here we just can't see it because the front carbon is eclipsing the back carbon but we know that these hydrogen's in the back here are attached to that back carbon so we represent the back carbon with a circle when we're doing Newman projections so that's supposed to represent the back carbon and then we would have a hydrogen coming out to the right like that so that's this hydrogen a hydrogen coming out to the left that's this hydrogen and a hydrogen coming straight down so that would be this hydrogen so there's your staggered conformation for propane next let's draw the eclipsed conformation as a Newman projection so a little bit harder but let's start with this carbon again so this is the one this is the front carbon so this is represented with a point right here and then we would have a ch3 a methyl group going off to the right so let's draw that in so we have a ch3 going off to the right we have a hydrogen going down and I'm gonna draw this a little bit off-center so instead of drawing it straight down I draw it a little bit off to the left just like did in the picture here to make it easier to see the bonds in the back so there's a hydrogen going down a little bit to the left and then we have a hydrogen going going in this direction so let me go ahead and draw that in here so here's a hydrogen next let's think about the back carbon we can't see it but because this front carbon here is eclipsing the back carbon but we know that the back carbon has three hydrogen's attached to it all right this one this one and this one which we can just barely see so let's add those in on our Newman projection so the back carbon is represented by a circle here and let's start with this hydrogen right back here that would be going in this direction so it's being eclipsed by the methyl group but we draw it a little bit off to the side so we can still see it's there next let's do this hydrogen so it's going down pretty much straight down so we'll draw that in there and then finally this hydrogen over here so this hydrogen we could represent it like that so now we have Newman projections for the staggered conformation and for the Eclipse conformation let's go back to that 14 kilojoules per mole that torsional strain so let me write that in here so 14 kilojoules per mole in the video on confirmations of ethane we already know that each pair of eclipsed hydrogen's has an energy cost of 4 kilojoules per mole so this pair of eclipsed hydrogen's all right that's four kilojoules per mole as an energy cost right here same with this one so this one's four kilojoules per mole so now we can figure out the energy cost associated with a methyl group eclipsing a hydrogen because we know the total should add up to equal 14 so four plus four plus what is equal to 14 obviously the answer is six all right so this must be six kilojoules six kilojoules per mole so six plus four plus 4 gives us our total torsional strain of 14 kilojoules per mole so now we know that the energy cost of a methyl group eclipsing a hydrogen must be six kilojoules per mole