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Current time:0:00Total duration:10:03

Video transcript

on the Left we have one way to represent the ethane molecule the only problem with this drawing is it doesn't give us much information about what ethane looks like in three dimensions on the right is another drawing of the ethane molecule but this this drawing gives us more information this is a wedge and dash drawing remember a wedge has a bond coming out at you in space or bond coming out of the plane of the page a dash is a bond going away from you in space or a bond going into the plane of the page and finally if you just draw a straight line like this that means a bond in the plane of the page this wegend - drawing represents one conformation of ethane confirmations are different arrangements of atoms that result from bond rotation we know that there's free rotation around this carbon-carbon single bond and this wedge in - drawing right now represents represents what's called the staggered conformation of ethane but if we rotate about this carbon-carbon bond we're going to get different arrangements of the atoms and therefore we will get different confirmations this is this is easiest to see with a model set so up next I have a video we're at a model set of ethane and I rotate around the carbon-carbon bond so we can see different confirmations I'm going to make these hydrogen's green in the video so those green hydrogen's are attached to this carbon which will be the front carbon and then we have hydrogen's attached to the back carbon that I will make white so let's watch the video and look at the different confirmations of ethane here we have the staggered conformation of ethane looking at it from a wedge and - perspective if we rotate this a little bit we'll see the staggered conformation of ethane from a sawhorse perspective if we sight down the carbon-carbon bond we'll see the third way of looking at this molecule and this is called a Newman projection if I take one of these hydrogen's here and I rotate it so I'm keeping the back carbon stationary and I'm rotating the front carbon every time I rotate this that represents a different conformation of the ethane molecule so there are many many possible confirmations in theory I rotate back to the original position here so this is the Newman projection where we just were the other conformation that we really care about for ethane is what's called the eclipsed conformation so I just rotated the the green hydrogen so there in front or eclipsing the white hydrogen's in the back so this is the eclipsed conformation as a Newman projection I could turn this a little bit so we could see what an eclipsed conformation looks like as a sawhorse from a sawhorse view and then finally I can turn a little bit so we can see the eclipsed conformation from a wedge and dash perspective now that we've seen the video let's look at three ways to represent the staggered conformation of ethane and we'll start with the wedge and dash drawing so you could see that this hydrogen this carbon this carbon and this hydrogen are all in the same plane so they're all in the same plane here which is why they're all these bonds are drawn as straight lines in our wedge and dash drawing if we look at this carbon on the Left right we know that there is a hydrogen in green coming out at us in space a hydrogen in green going away from us in space and then this hydrogen which is in the plane of the page for the hydrogen's in white this hydrogen is in the plane of the page this one's coming out at us in space and then we have one going away from us in space and it's pretty easy to see that in the picture here so we're just going to match the drawings to the pictures our next way to represent the staggered conformation is what's called a sawhorse drawing so for this sawhorse drawing this carbon right here is this carbon and it has three hydrogen's in green attached to it so this is a hydrogen in green in green and in green so we also have this carbon-carbon bond so that's this bond here on our sawhorse drawing and then finally for this carbon in the back this is the one that has hydrogen's in white so these are the three hydrogen's in white if we look down that carbon-carbon bond if we look down this axis right here so if you put your eye Pageant putting your eye right here so here's the eye and you look down this axis you will see the Newman projection just like we saw in the video and so this dot this point here represents the front carbon that's this carbon right here so let me go ahead and write that down so this point represents the front carbon and the front carbon is bonded to our hydrogen's in green so this would be this would be a hydrogen in green so is this one and so is this one the front carbon blocks the carbon in the back you can't see the carbon in the back in this drawing but we know it's there and we have to represent it somehow so in a Newman projection it's this it's this circle it's a circle back here that represents the back carbon so this is the back carbon the circle let me write that down the circle is the back carbon even though we can't see it when you actually have the model in front of you and the back carbon is the one that has the hydrogen's in white so this should be a hydrogen in white so this one and so is this one so here you can see the hydrogen's in white back here and you know they're attached to a carbon which you can't see it because the front carbon is blocking it so let's talk a little bit more about Newman projections we can we can talk about the angle between between this hydrogen and white and this hydrogen in green so think about the angle between these it's 60 degrees so we have a 60-degree angle between these two hydrogen's so I could write it down here - so sixty degrees this angle is called the dihedral angle or the torsional angle so I'll write those down so you could call this the dihedral angle or you could also call it the torsional angle let me write that down here - so torsional angle or you could also call the torsion angle and this angle between the hydrogen's will be important when you're talking about confirmations here the angle is 60 degrees which means that these hydrogen's are not right on top of each other there's space between these hydrogen's on the Newman projection the green hydrogen's are staggered compared to the white height so if you go around you can see that there's space between all of these and that's why we call this the staggered conformation of ethane and finally let's look at the other important conformation of ethane which is the eclipsed conformation and we'll start with the wedge and dash drawing so we have this hydrogen this carbon this carbon and this hydrogen these are all in the same plane so I draw this line here and we can see we can see these bonds are all in the same plane up here on my wedge and dash drawing for this carbon on the Left we can see that we have a hydrogen in green coming out at us right so here's your hydrogen green coming out at you a hydrogen and green going away from you back here and then this hydrogen in the plane of the page for the hydrogen's in white this hydrogen is in the plane of the page all right this hydrogen is coming out at us in space and this hydrogen is going away from us in space for the sawhorse drawings let's move on to here this carbon is the front carbon so that's this carbon right here the one that's bonded to the three hydrogen's in green so let me highlight those so we have one hydrogen this one right here is going up so that's our hydrogen and green going up this hydrogen over here is going down a little bit to the right and this hydrogen's going down and to the left and then we have the carbon-carbon bond so let me draw that in here so here's our carbon-carbon bond I made it much longer up here just so just though the atoms wouldn't interfere with each other on our sawhorse drawing and then we get to the carbon in the back so here's the carbon in the back which is bonded to three hydrogen's and I made these the hydrogen's in white so here's one two and three so here are the hydrogen's and white on the picture finally we get to the Newman projection for the eclipsed conformation and remember the Newman projection is what we would see if we stare down the carbon-carbon bond so if we sight down this carbon-carbon bond if we put our eye right here on our sawhorse projection and we stare down the carbon-carbon bond we see this for our Newman projection right first we see this point here representing the front carbons at that point is the front carbon on our Newman projection and the front carbon is bonded to the hydrogen's in green so here's one two and three so up here one two and three so this would be the bonds this represents the bonds of that front carbon to those hydrogen's the back carbon is pretty difficult to see but we know that the circle represents the back carbon right the front carbon is in the way I'm not sure if you might be able to see a tiny bit of the back carbon here but the back carbon should be blocked by the front carbon the back carbon we know has our hydrogen's in white so here are the three hydrogen's and white so back here you can just barely see them barely see them poking out this time when you think about the angle between your hydrogen's this time your dihedral or your torsional angle is zero degrees right think about think about this hydrogen being perfectly upright in the hydrogen in the back being perfectly up writes the angle between those is zero so your dihedral angle is zero degrees here for the eclipsed conformation the green hydrogens the green hydrogens are eclipsing the white hydrogen so that's why we call this the eclipsed conformation