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here we have the Eclipse conformation of ethane and if I turn it so we sight down the carbon-carbon bond will see the Newman projection for the eclipsed conformation notice I have some hydrogen's in green here and that's just to help us visualize rotation around the carbon-carbon bond so I'm going to rotate the front carbon to keep the back carbon stationary so now we get the staggered conformation of ethane I'm going to keep rotating 60 degrees so I'm going to rotate again and now we have an eclipsed conformation I rotate again and now we have a staggered conformation of ethane I rotate another 60 degrees and we get an eclipsed conformation I rotate again and we get a staggered and you get the idea one more rotation and we get back to an eclipsed conformation here we have a graph of the potential energies of the confirmations that we just saw in the video so we started out right here with this eclipsed conformation of ethane so we are at this particular potential energy as we rotate to get to this staggered conformation you can see there's a decrease in potential energy so the staggered conformation has a lower potential energy than the eclipsed conformation well rotate again we get back up here to this Eclipse conformation notice that takes energy so it takes energy to go from this staggered conformation to this Eclipse conformation going from the eclipsed to the staggered right that's a decrease in energy going from the staggered up to this Eclipse that's an increase in energy going from the eclipsed down to the staggered again is decrease and finally back to the original Eclipse conformation would be an increase in energy notice that all of our eclipsed confirmations here have the same potential energy if I draw a line this is all the same potential energy whoops I didn't draw very nice line there but you get the idea therefore we say that these are degenerate in terms of energy same thing for the staggered conformation all of these staggered confirmations if I draw a line in here have the same potential energy so the staggered conformation Asians are lower in energy than the eclipse confirmations and actually the difference is 12 kilojoules per moles if I write that in here so 12 kilojoules per mole that's talking about the difference in energy between the Eclipse confirmations and the staggered confirmations lower in energy means more stable and the way the easy way of thinking about that is imagine these things as hills and if I have like a boulder or a rock or something down here at the bottom of the hill and I'm comparing that Boulder Iraq up here to a boulder at the top of the hill in physics you can set your potential energy equal to zero at the ground so let's say that this is ground level I say my potential energy U is equal to zero joules at this point and so the boulder at in this valley here let's say it's 10 joules right let's say this is 10 joules here at this point and then it would take energy to push this Boulder up this hill to this point and let's say the final let's say the final potential energy of the board at this point would be 22 joules right it takes energy in order to do that and this final position all right this final position is less stable and this is the higher potential energy so higher higher and potential energy means less stable lower in potential energy means more stable so why do we have a difference in energy between the staggered and the eclipsed conformation well this is called torsional strain so this difference in energy is 12 kilojoules per mole is called torsional strain and the source of torsional strain has been a topic of debate one of the current theories has to do with molecular orbital theory I'm going to go with one of the one of the older ones which talks about the electron pair repulsion and the electron pair repulsion Tsar greatest when the bonds are eclipsed so if you think about the electrons in this bond being close to the electrons in this bond and you have that same thing over here and over here so in space these electron pairs these bonding electron pairs are closer together in the eclipsed conformation than they are in the staggered if I go to here to the staggered you can see if you're thinking about these electron pairs they're relatively they're further away than they are in the eclipsed conformation and we know that electrons will repel so electron pair repulsion czar greatest when the bonds are eclipsed and therefore that's higher energy and the electron pairs are further away from each other when you're talking about staggered therefore more stable the total energy the total energy cost between the two confirmations is twelve kilojoules per mole and we have three pairs of eclipsed hydrogen's if I go back up to here right here's one pair of eclipsed hydrogen's there's another pair and here's another pair so if the total energy is twelve kilojoules per mole and I have three pairs of eclipsed hydrogen's we could say that the energy cost for each pair of eclipsed hydrogen's is four kilojoules per mole so this would be four kilojoules per mole for this pair of eclipsed hydrogen's four kilojoules per mole for this one and four kilojoules per mole for this one adding up for a total of twelve so our total energy cost is twelve and now we can think about two hydrogen's eclipsing each other as having an energy cost of four kilojoules per mole we've just seen that the staggered conformation of ethane is more stable than the Eclipse conformation of ethane and if you want to turn a staggered conformation into an eclipsed conformation you would need energy and at room temperature there's enough energy for the staggered conformation to turn into the eclipsed equilibrium is reached between the two confirmations and at room temperature approximately 99% of ethane molecules have an approximately staggered conformation whereas only about 1% have an eclipsed conformation again that's due to stability staggered is more stable than eclipsed