If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:32

Video transcript

let's get some more practice with newman projections so for this compound we're going to look down the c3 c4 bond and draw the most stable conformation so let's start by numbering our carbons this must be carbon one two three four five and six and if we look down the c3 c4 bond that's this bond right here we're going to put our I along this axis let me draw an I in here so we're going to stare down this way and draw what we see well I'll show you in a video in a minute what we would actually see but it's very important to be able to draw these Newman projections without the use of a model so let's start by thinking about what is attached to this carbon three here there's a methyl group coming out at us in space which means there must also be a hydrogen going away from us in space and what is attached to carbon four there must be a hydrogen coming out at us in space so hydrogen here and a hydrogen going away from us in space so now that we've drawn that in we can start to draw our Newman projection and we start with a point or a dot to represent carbon three so that point there is carbon three what is attached to carbon three well there is a methyl group that would be going down and to the right so if your eye is here you'll see a methyl group going down and to the right so let's draw in a ch3 down to the right like that and then we also have a hydrogen so this hydrogen will be going down into the left so let's draw in that hydrogen down and to the left next we would have a ch2 ch3 or an ethyl group and this would actually going straight up if we're looking at it from this perspective so this would be going straight up so let's write ch2 ch3 next when you think about carbon four so we're thinking about this carbon right here we wouldn't be able to see it because carbon three would be in the way but we know that carbon four is there and we represent carbon four with a circle on our Newman projection what is attached to Barban for well we know that we have a hydrogen going up into the right so let's draw that in so there's a hydrogen going up into the right attached to carbon four there's also there's also a hydrogen going up into the left attached to carbon four so let's draw that in on our Newman projection and then finally we would have a ch2 ch3 and this would be going it down so a ch2 ch3 going down so let's draw that in ch2 ch3 well it gets annoying to draw in all these CH 2 s and ch3 s so let's let's redraw this Newman projection let's say that ch2 ch3 we know that's an ethyl group so let's abbreviate that with ET and a methyl group let's just say that's M E and then we have our hydrogen right here and then for our back carbon we have a hydrogen here we have a hydrogen here and then we have an ethyl group going straight down so it's just a little bit easier to see this way and in the video I'm going to make an ethyl group red's you're going to see a red a red circle a red sphere for an ethyl group in the video and I'll make the methyl group blue so it's easier to show different confirmations if you just represent it by a sphere and you and you'll see what I mean and from the video we're going to figure out the most stable conformation we know that has to be a staggered conformation from earlier videos so we'll look at all the different staggered confirmations and we'll pick which one is the most stable here we have carbon one and then carbon two and then carbon three notice there's a methyl group coming out at us in space attached to carbon three then we have carbon four and we're going to stare down the carbon three four bonds let's rotate the molecule here let's stare down the C 3 C 4 bond and notice we have a staggered conformation up here we have an ethyl group on the right here we have a methyl group and then down here we have another ethyl group so hopefully you see the staggered conformation remember that red is an ethyl group so here's an ethyl group and here's an ethyl group and blue represents the methyl group it's just easier to work with the model set this way so we're going for staggered conformation so if I rotate the front carbon and keep the back carbon keep the back carbon stationary we get another staggered conformation if I rotate again then we get another staggered conformation hopefully you can see the Newman projection that we drew matches the picture from the video this is the same Newman projection I just made the ethnic groups red and the method group blue in the video we moved the front carbon we rotated the front carbon we held the back carbon stationary so this ethyl group in red would move over to this position the methyl group in blue would move over to this position and finally this hydrogen right here I'll make it green would move over to this position so that's this hydrogen in green let's go ahead and draw the next conformation the next staggered conformation if we held the back carbon stationary we can go ahead and draw in the back carbon and what's attached to the back carbon these hydrogen's and this ethyl group and next the ethyl group in red on the front carbon moved over to this position the methyl group in blue moved over to this position and the hydrogen in green moved over to this position so we can see that matches what we have we have for our picture down here we have our two ethyl groups are now goes to each other and then we have our methyl group over here in blue and our hydrogen in green let me highlight that the hydrogen in green is this hydrogen we can rotate one more time to get our last staggered conformation our ethyl group in red can rotate over here the methyl group in blue could rotate over here and that would mean the hydrogen and green has to rotate over to this position so the draw on the back carbon with the hydrogen's and then the ethyl group it doesn't matter if you rotate the front or the back carbon I chose to rotate the front carbon here and that would move our ethyl group over to this position so now our ethyl group is here the methyl group in blue would move up to here and the hydrogen in green would move over to here so hopefully we can see that let me highlight everything so here's an ethyl group alright so down here we can see our ethyl groups our methyl group in blue and finally our hydrogen in green so now we're finally able to choose the most stable conformation out of our three staggered ones here so we need to think about the gauche interactions that are present and we'll start with this conformation on the right here we have an ethyl ethyl gauche interaction and ethyl groups are pretty bulky so this gauche interaction would destabilize this conformation let's look at this conformation next we have an ethyl ethyl gauche interaction and we also have a methyl ethyl gauche interaction so we have to go SH interactions so that means that this conformation is even more unstable and finally we have this one right here we have only one gauche interaction and it's between an ethyl group and a methyl group and since this methyl group is not as bulky as another ethyl group that means this is the lowest energy conformation this is the most stable in Part B our goal is to draw the least stable conformation or the one highest in energy and that must be an eclipsed conformation so let's go to the video and let's start with the staggered and then go to an eclipsed conformation from that eclipsed conformation we'll look at the others and we'll choose the one that's the highest energy so here we have our staggered conformation if I rotate the front carbon we see one eclipsed conformation I can rotate again to get another eclipsed conformation so here's another one you can see this one has the two ethyl groups really close together I can rotate again to get our final eclipsed conformation here we have pictures of the three eclipsed confirmations from the video and a save time let's just analyze the pictures and then we'll draw the least stable conformation as a Newman projection let's start with the conformation on the right we can see we have an ethyl group eclipsing a hydrogen a methyl group eclipsing a hydrogen and a hydrogen eclipsing an ethyl group so we don't have any of the alkyl groups eclipsing each other so this one this one is definitely not the least stable we're looking for bulky groups interfering with each other so steric hindrance let's move over here to this conformation we have two hydrogen's eclipsing each other we have an ethyl group eclipsing a hydrogen and then we have a methyl group eclipsing an ethyl group so there's a source of some strain so that's going to increase an energy so this conformation is definitely higher in energy than this conformation but let's compare this one to our Center one for the center one we have two very bulky ethyl groups so an ethyl group eclipsing another ethyl group and that is very unstable right this increases the energy that's a lot of steric strain so this this is the least stable conformation these ethyl groups want to be as far away from each other as possible and here we put them very close together in space so let's go ahead and draw the Newman projection for this conformation and we start with this carbon so that carbon is represented by a point and attached to that carbon we have a methyl group going up into the left we have a hydrogen going up into the right and we have an ethyl group going down for the back carbon so here's the circle representing the back carbon we have a hydrogen going up into the left so there's our hydrogen we have another hydrogen back here so I'll draw that one and then finally we have an ethyl group going down so this is our least stable conformation for our compound drawn as an eclipsed and eclipsed conformation in a Newman projection