Conformational analysis of butane

here we have the butane molecule and this is carbon one carbon two carbon three and finally carbon four and if we stare down the carbon two three bond so here I'm rotating the molecules so we stare down the carbon two three bonds this is a staggered conformation of butane and if we rotate if you rotate the front carbon to keep the back carbon stationary and rotate sixty degrees and we're going to get an Eclipse conformation so I left it a little bit off so you can still see the bonds in the back so from the eclipsed conformation of butane I rotate again and we get a staggered conformation rotate another 60 degrees here and we get an eclipsed conformation and I'm gonna turn to its side so we can see how close these methyl groups are in space if I rotate around you can see what the model set these hydrogen's actually hit so those hydrogen's are close enough where they hit and the model set and that's called steric hindrance or steric strain so back to the eclipsed conformation of butane if we rotate again that we get a staggered conformation we're going to rotate again to get another eclipsed again slightly a little bit off so you can still see the bonds in the back here and rotate one more time to get back to our staggered conformation here's an energy diagram showing the different confirmations we saw in the video and these pictures are just stills from the actual video we started with the staggered conformation of butane right here which has a certain potential energy we went from this staggered conformation to this Eclipse conformation right here by rotating 60 degrees it takes energy to go from the staggered conformation to this Eclipse conformation the eclipsed conformation is higher in energy the eclipsed conformation is less stable remember the higher the potential energy the less stable the conformation the lower the potential energy the more stable so the staggered is more stable than the eclipsed the energy difference between these two confirmations let me go ahead and draw a dashed line here so this energy difference between these two confirmations turns out to be approximately 16 kilojoules per mole so it takes energy to go from the staggered conformation to this eclipsed conformation from the eclipsed conformation we rotated 60 degrees and we got this staggered conformation notice this staggered conformation is a little higher in energy than our first staggered conformation so if I draw a line right here we can see there's an energy difference between our two staggered confirmations so the energy difference turns out to be approximately 3.8 kilojoules per mole going from this staggered conformation up here to this Eclipse conformation takes energy so if I draw a line here so indicating the bottom this energy difference is approximately 19 kilojoules per mole so approximately 19 kilojoules per mole and notice that this Eclipse conformation this eclipsed conformation is higher in energy let me change colors here this Eclipse conformation is higher in energy than this Eclipse conformation so if we draw a line right here we can see there is an energy difference of approximately 3 kilojoules per mole so this is the least stable conformation this conformation this Eclipse conformation has the highest potential energy from this Eclipse conformation we could go to this staggered so that's a decrease in potential energy notice this staggered has the same energy as this staggered conformation so they are degenerate going from this staggered conformation up here to this eclipsed or would take energy and notice this Eclipse conformation is the same and energy as this one over here so if I draw a line I draw a line you can see it's the same energy so these two are degenerate these two eclipsed confirmations are degenerate and finally going from this Eclipse conformation back down to our staggered conformation this is lower in energy now let's look at the confirmations in more detail and we'll start with this staggered conformation buting and let's go ahead and number the carbons if this carbon is number one so we called it in the video that carbon is attached to this carbon which is carbon number two we stare down the carbon to three bond to get our Newman projection and in the video you can't see carbon number three because carbon number two is in front of it but when you're drawing a Newman projection you represent the carbon and back in this case carbon number three with a circle so the circle here represents carbon number three and finally this would be carbon number four let's think about the dihedral angle between our two methyl groups so between this methyl group and this methyl group well that would be a hundred and eighty degrees so hopefully you can see there's a hundred and eighty degrees between our two groups so the dihedral angle is 180 degrees this conformation is called the anti conformation and the anti conformation is lowest in potential energy therefore the anti conformation is the most stable conformation for butane and that's because we take these bulky methyl groups and we put them as far away from each other as we possibly can and all of our bonds are staggered so if we think about these bonds here everything is staggered so that makes this the most stable conformation if we rotate 60 degrees from the anti conformation in the video I kept the back carbon stationary and I rotated the front carbon we would get this conformation and this is an eclipsed conformation so think about this bond eclipsing this bond and this hydrogen eclipsing this hydrogen I didn't draw them as being completely eclipsed just so we could actually see what's going on here and remember in some of the earlier videos we talked about the energy cost associated with a pair of eclipsed hydrogen's as four kilojoules per mole so this energy cost is four kilojoules per mole we also talked about the energy cost from a methyl group eclipsing a hydrogen so in the video on propane and this was approximately six kilojoules per mole so six kilojoules per mole for a methyl group eclipsing a hydrogen at the same situation we have down a hydrogen and a methyl group eclipsing each other so this should be an energy cost of six kilojoules per mole - if we add all those up so this would be 4 plus 6 is 10 plus another 6 is 16 we can see that's the energy difference between these two confirmations so 16 kilojoules per mole higher so this eclipsed conformation is higher in potential energy let's look at the other eclipsed conformation so that's the one over here this is the the highest in potential energy so this must be the least stable conformation for butane if we look here's we have a pair of hydrogen's eclipsing each other so that should be four kilojoules per mole we have another pair of hydrogen's eclipsing each other so that's another four kilojoules per mole and then we have two methyl groups eclipsing each other so think about think about this bond and this bond eclipsing each other and these methyl groups being right on top of each other so what's the energy cost associated with two methyl groups a methyl group eclipsing a methyl group we can figure that out because we know the total energy cost is 19 kilojoules per mole so this is what we don't know so I'll call that X so what is X well if we add everything up it should get 19 so 19 is the total and we have 4 and 4 and X so 4 plus 4 plus X is equal to 19 so obviously X is equal to 11 so 11 kilojoules 11 kilojoules per mole is the energy cost associated with a methyl group eclipsing another methyl group and so there's torsional strain there but there's also steric strain or steric hindrance which we saw in the video these two methyl groups the hydrogen's can actually get close enough to touch in the video when you're using a model set and that that is destabilizing so if you have increased steric hindrance that destabilizes your conformation and that's why this conformation this Eclipse conformation is the highest in potential energy it's the least stable so if I draw a line over here just remember that is this Eclipse conformation is even higher in potential energy than this Eclipse conformation because of these two methyl groups being so close together and finally let's look at our staggered conformation so our other staggered conformation is right here notice this staggered conformation is higher in energy than our anti so if we look at our methyl groups we think about the dihedral angle so if I think about the angle between these methyl groups here that is sixty degrees and we say we say that this is the gauche conformation so let me go ahead and write this as the gauche conformation and for the gauche conformation this is a little bit higher in energy than for the anti conformation and that's because these two methyl groups are closer together in space we don't really have to worry about torsional strain here but we do have to worry about steric hindrance so these hydrogen's on these methyl groups can get pretty close to each other in the gauche conformation that has a destabilizing effect therefore having a higher potential energy for this conformation so the gauche conformation wallet's staggered this is more stable than our eclipsed confirmations the gauche conformation is not as stable as the anti conformation because these methyl groups are relatively close together in space