hello class what we're going to do now is we're going to take the molecule ethane and we're going to look at its Newman projection right as Newman projection would look like this where we have all hydrogens so that's our Newman and so what we're going to do is I'm going to take well first off let's see here so what we're going to do first is when we look at this Newman projection if I look at these two hydrogens right here this one on the front carbon and this one on the back carbon right there what we have is an angle between those two groups right there there's an angle that angle right there is 60 degrees and that angle is called a dihedral angle let's make sure we spell this right here so we have a dihedral and it's this dihedral angle that we are going to track in this little graph here on the x-axis we're tracking the dihedral angles this dihedral angle is also called the torsional torsional angle let's see I think I misspelled that one I did let's see here yeah okay so we have dihedral angle and torsional angle they're they're one and the same and that is looking at groups between you can ask what is the dihedral angle between any groups okay you could a very well have looked at the angle between this guy and that guy as long as you're looking at a group in the front carbon attached to the front carbon and one in the back carbon so what would the dihedral angle be between that those two that would be 180 degrees right so when we are looking at our ethane molecule what we're going to do so I can show you the different energies here amongst all these groups here it's just easier to erase it all real quick and I like that so what we are going to do is I'm going to color code these hydrogens here and we're going to track these two no no no I want to color code it I'll put them in pink okay so what we're going to do is we're going to I'm just double checking okay so what we're going to do is we're going to keep the back carbon the circle constant it's not going to rotate the back carbon is just going to stay there what we're going to do is take the front carbon and rotate the front carbon and so we're going to be moving this this pink hydrogen to different spots okay so what we have here is the Newman right here you can see the dihedral angle between this pink hydrogen in the front and the pink hydrogen in the back is 180 degrees so that goes right here and so just to save time I'm not going to draw in all the hydrogens I'm just going to draw in the ones of significance that's a dihedral angle so these right here these four those are hydrogens I'm just not going to draw them in just for ease okay so we have a dihedral angle of 180 degrees however this confirmation right here is really really important because every single dihedral angle every single one in this confirmation is 60 degrees all right so I know that might be sound confusing but let me re-state it and see if we can make sense of it what is the dihedral angle between the two pink hydrogens that is 180 degrees okay but now for a moment just ignore that these are pink hydrogens just think they're of them all as green hydrogens when all the groups have a dihedral angle between their adjacent groups when that angle right there is 60 degrees we call that a staggered conformation so this guy right here is considered staggered all right so that guy right there is a staggered conformation because all the dihedral angles are at 60 degrees now let's take the front carbon and just move it the front carbon and rotate it clockwise 60 degrees to my right what's going to happen here well I can definitely say that the back carbons are going to remain constant so that would mean that the pink hydrogen right there just stays all right and then if I take the front carbon and rotate it 60 degrees to the right what do I have here now this is the tricky part here because we're going to have it like this and like that maybe I should have drawn the front carbon first the pro let's try that make that look a little bit better here okay so we have this this and that and the pink hydrogen is there and then in the back those are all constant those did not move okay now when we look at this confirmation right now what is the angle between the two pink hydrogens okay that is 120 degrees that's what this number right here is showing that angle the hydro angle between the two pink hydrogens 120. but when you look at the dihedral angles of of them all so if you look at the dihedral angle between that group just in between there and in between there that's really not uh an angle because this hydrogen pink hydrogen is directly behind this hydrogen in the front so this has a Dye heat all the groups in this molecule have a dihedral angle of zero and when you have dihedral angles for every single group on that molecule we call that a eclipsed we will use blue here so this is a eclipsed e c five so this is a eclipse confirmation this is a staggered conformation and the confirmations that are the most important to us in ethane are the staggered and the eclipse because those are the two extremes between the conformations here we have it low energy high energy so what have we learned staggered conformations are lower in energy eclipsed are the highest in energy so we can continue just doing this process all the way through this uh analysis here so I'm going to pause the video and just draw out all the confirmations and what you're going to do is just track that pink hydrogen in the front and watch where it goes okay okay so now just look at where the pink hydrogen is going so we talked about this one and this one already so starting from this Eclipse confirmation I just take the front carbon and rotate it 60 degrees and now look at the dihedor angle between the two pink hydrogens it's 60 degrees but then when you look at all the substituents or all the hydrogens on the Newman projection they're all at 60 degrees so that makes this guy a staggered conformation eclipsed you can see that the dihedral angle between the two pink hydrogens is zero but they all the dihedral angles are zero so that is eclipsed staggered eclipsed staggered now the energy difference here between the staggered and the eclipse so I'm just going to draw a dotted line here and look at that energy gap right there that energy difference is 12 kilojoules per mole and so that's not a major energy difference and so there's enough energy at room temperature for the molecules to be rotating constantly they're constantly spinning going between just going through this whole process just fast fast fast fast and so these molecules here they're not stationary they're not stagnant but they're spinning constantly spinning and so we have a okay so so you know the energy difference that's important to know now what we need to figure out as Y is there an energy difference why is the staggered more stable lower in energy than the eclipsed and there's some debate on what that's all about but let's let's look at it this way so I'm going to draw a larger a Newman here we'll do it in this corner we'll see if we can get this to work here so why is the let's look at it see here hi so we have a hydrogen here right and then we have a hydrogen there we'll we'll draw it in the staggered conformation here why is this so why is this lower in energy well let's just think about it for a moment here in the fact that this carbon this hydrogen there's two electrons between those two atoms right and electrons aren't a line they're an orbital and so you could represent it more like an electron cloud okay and so we have the electron cloud right there on that front carbon hydrogen but then we also have the electron cloud of this guy right here okay between that carbon and in the staggered conformation we can get these two electron clouds the farthest apart as possible because those are electrons right there right they've negatively charged here and so if you get two electron clouds really really close together there's going to be repulsion and there's going to be an increase in energy so you could envision now if I redraw this and draw in the eclipsed confirmation okay there's the eclipse and if I'm representing that hydrogen and that hydrogen there they both have electron clouds we'll do that one in blue for the front carbon we'll do it in pink for the other one you can definitely see that there's going to be conflict there's going to be some repulsion all right now that in the literature they also discuss favorable interactions that when you are in the staggered confirmation you can have favorable interaction with anti-bonding orbitals but I'm not going to go into that too much as long as you understand the two principles that I just shared with you and so what we have here is when they are eclipsed there's repulsion makes it higher in energy and when that's in the staggered there's less repulsion lower in energy all right now we're going to take a look at the confirmations of butane and what we need to understand about butane is we are going to take a look and we're going to take it our I and we're going to take a look down Carbon 2 in the carbon 3 Bond and look at the different conformations and look at the energies of those different conformations and they're going to be this energy diagram is going to look a whole lot different than ethane so when we take and draw the Newman projection here as drawn what can we see and carbon 2 is going to be our front carbon we're going to have a methyl group there so this methyl group right there is that one and then oh let me draw my hydrogens then I have my two hydrogens right there and then in the back carbon 3 is the circle so that means the methyl is right there and then the two hydrogens right there so what I want you to do as I'm going to draw out the energy diagram and show you all the Newman projections what you're going to look and keep track of is the methyl groups where are they with respect to one another right now you can see that the dihedral angle between those two methyl groups is 180 degrees and so you can see that this confirmation is going to be my starting point and then as what I'm going to do is I'm going to take the front carbon and rotate it 60 degrees at a time and take you through this step by step so what we can say at this part point is when you look at every single dihedral angle in this Newman projection what is the dihedral angles they're 60 degrees so we could call this guy a staggered confirmation okay we could call it staggered but I'm going to introduce a new name to describe this because now we are focusing on the two largest groups that are on the front carbon or the the largest group on the front carbon and the largest group on the back carbon and tracking their relationship so when the two largest groups which in this case is what a methyl and a methyl when they are a 180 degrees have a dihedral angle of 180 degrees we call this conformation anti so we have a anti-conformation here because those two largest groups are 180 degrees from one another right so I'm going to erase this and draw out the energy diagram and then we're going to look at every single confirmation so here's the energy diagram and you can see that this is different now because what we have is we have a low here so 1 2 3 4. we have four different energy levels that we are going to look at now and I already talked about this lowest energy one and that is when the methyl groups are 180 degree have a dihedral angle of 180 degrees and we call that anti and that marker is not going to work let's try this one that is a anti-confirmation and we can also say that it's staggered and why can't we say that it's staggered it's because when you look at all the dihedral angles they're all 60 degrees to one another now please note that I'm just taking a shortcut here all these right here are hydrogens so I'm just not going to write that all throughout this this demonstration here okay so we have a name for the lowest energy the two largest groups when they're 180 degrees they're anti and when all that the hydro angles are 60 degrees we'll run another that is staggered now here we have an eclipsed confirmation all right so that is we would just call that an eclipsed because everything's the diet all the dihedral angles are zero but then we come to this guy right here we can look at it and see all the dihedral angles are 60 degrees so we could say hey this guy's technically staggered all right so we'll write it down here technically staggered but remember we are focusing our attention on these groups right there the two largest groups when the two largest groups have a dihedral angle of 60 degrees okay that has a really unique unique name and it's called the Gauss let's oh foreign that's the Gauss confirmation and you can see that the Gauss confirmation right here is a little bit lower in energy than the eclipsed all right but then when we rotate another 60 degrees we can see the dihedral angle between the two methyl groups right here are zero degrees all the dihedral angles are zero degrees so we can call this eclipsed technically right all the bonds are Eclipse eclipsing each other but this guy right here you see it has the highest energy and why would you expect this to have the highest energy because the two largest groups are next to each other right so just think about it this way if you had two hydrogens that are eclipsed to one another the two hydrogens are very very tiny so they're going to Eclipse they're going to be high in energy but if you replace those two hydrogens with methyl groups like this a watch I'll expand it out methyl groups are really really bulky and so because if I just represented by circles you can see that they're much larger than a hydrogen they take up more space and because they take up more space they're going to clash with one another all their electron clouds are really really close to one another going to cause some problems so this is going to be the most the highest in energy confirmation because it's eclipsed which is high in energy and when the two largest groups have a dihedral angle of zero that is just bad bad bad as we continue to rotate the front carbon you can see now from this second half on we're just going to repeat we're going to then get the two groups 60 the dihedral angle of 60 degrees so that means this guy right here is in the gauche conformation this is eclipsed and then we go back down to the lowest energy form where it's the anti-staggered conformation okay so now I want to talk a little bit more about uh this guy and basically go over some vocabulary and some explanation as to what's going on here why this is so much higher in energy okay okay when we look at these two confirmations and looking at their corresponding Newman projections I've highlighted the anti-confirmation which is the lowest energy and the highest in energy due to being eclipsed and the two largest groups having a dihedral angle of zero now to explain these energy differences we have two words we have torsional strain and steric string no torsional strain like I explained before has to do with the electrons within the bonds so what do we have here when we look at just some random um well I already explained torsional string okay so I don't I don't need to read talk about that again I already talked about torsional string but what butane now introduces is what steric string and steric strain is a little bit different because it has to do with just atoms getting in the way so if we take a look at butane in this conformation we have these two methyl groups that we had to to look at okay and methyl groups are pretty large because they have these three hydrogen atoms that just take up space okay and I could represent that methyl group just like with this big circle here it just takes up a lot of space right now you can see there are far away as possible right and that's a good thing because we cannot have atoms so crowned together and in the same place if there's not space for all the atoms to fit it just doesn't go in there very well but then when you take a look at when it's in this conformation hydrogen hydrogen hydrogen so all right they're a whole lot more bulky or sorry they're now very close to one another and I not I didn't draw this very well I'm going to draw in a way that I can exaggerate this okay all right so what what are we seeing here that these groups of atoms are in the same space and when they're really really crowded in the same space we call that steric string so the reason why this Eclipse confirmation is the highest in energy is because it has the maximum amount of torsional strain because every bond is eclipsed and then it has the maximum amount of steric strain because you have the two largest groups as close as possible to one another and that is just going to say no that's going to increase the energy all right so you got to remember those uh those words and the differences what in which they mean steric is just atoms getting in the way torsional is the electron clouds in the bonds getting really close to one another all right and you can see the torsional strain a whole lot better in this Newman because you have an electron cloud there the electron cloud there is just getting really really close to one another okay let's see here the next topic that is very important is something called chair confirmations and this is particularly looking at cyclohexane the six-membered alkane cyclohexanes what we're going to do is take the way we've been drawing this like when we have cyclohexane here we will draw wedges and we could draw our dashes like this right but what we're going to now do is draw these molecules in a chair conformation and in order to do that let's start with the most simple one right here okay and that it looks like this and I'm going to draw it really quickly and then I'm going to show you step by step of how I do it okay so we'll just clean that up a little bit there so what we have here is six carbons you can number them one two three four five six and you can see that it has six carbons okay so this representation and this representation uh say the same thing one difference though is in this structure here if we draw a wedge and a Dash Okay like that like this what is going to happen is on the chair we only have lines no wedges or dashes so like on carbon 3 here we would have a bond coming straight up and then another bond that looks like that all right and then if we had a bond on carbon two we would have one that would come straight down like so do you see how I put a gap between the two right there and then you would have something that looks like that so there's no wedges and dashes on the chair conformations but what we're going to learn shortly depending on if it's pointing up or pointing down will correlate back to if it's a wedge or a dash on this molecule okay so let's let me show you step wise how I draw my chairs okay don't worry about these groups or these substituents don't worry about that let's just worry about drawing the chair all right step by step how is that done so we will draw a reference here this is what we're what we're drawing is the chair what you want to do is you're going to take two parallel lines add a slant you draw one line and then you're going to draw another line parallel a little bit down but I go basically right in the center and I go down and then I go and make a parallel line of the same length so if that's four inches that's four inches okay and then I just make a triangle to connect them both down boom and then you can see this one right here goes up down like that and so what you want to see happen is you're going to have three parallel lines that's parallel that's parallel right there and we'll then you can see there okay they have the same angle here if your chair doesn't have three parallel lines then uh it's not drawn right all right so what if I could I have drawn a chair like this okay so this blue one how did I start so this blue when I started like this like that what if I drew it at a different angle what if I drew it this way like that so I didn't make that connection there very well did I like so both are correct all right yeah you can see that they they look a little different but they're both correct and both are going to be needed what's going to happen is you're going to have to be able to draw it like this and like this and we will talk about that as we progress through this lecture here but that's how you do a chair so this chair that I drew here looks a whole lot better than these two right here right these two I went a little fast okay so it takes practice lots and lots in practice but it's very very important that you get these structures drawn correctly because when you are given an exam you're going to have to draw these structures and if they're not drawn appropriately then it makes it very difficult for the graders to know what you're doing all right so there's a lot more that we are going to say about these chair confirmations that we are going to start looking into right now okay now if we have a chair week like I said there's not going to be any wedges or dashes we could have a bond straight up okay so you look at every Apex every point and it's either there's a bond that's going to be straight up or straight down and so you can see these on the ends are really easy to draw all right these two carbons that are on the inside of the chair you see how it's pointing up so I go up this one's pointing down so I draw it down just like that okay so those bonds when they're up like that directly up or directly down in that fashion there okay well let's see here so those bonds right there in green are are going to be called axial so those are axial bonds right now I'm going to draw another chair just so it doesn't start getting really messy and then we're going to combine everything okay so if I drop myself another chair here let's do it here like so and then when you look at the same carbons here and if you you can draw a what's called an equatorial Bond and an equatorial bond is when you look at this carbon here and you want to draw the equatorial Bond you go you skip this Bond and then you go to the next one and so this next Bond right here is the same angle you want to draw the equatorial Bond you see that there and then how would this equatorial Bond look on this carbon skip the bond go to the next one so it wants to be like that that angle now if I do these two ends skip it this one okay so that one has to be like that right there let's see here like that and then this one skip boom would have to look like that and those are called equatorial bonds now let's draw a chair conformation with all well wait I forgot to do the interior equatorial okay and so this one would be that's an equatorial Bond and then this one right here let's see here up down equatorial would be we Skip and then like that so we skip the first button and then we look at this one and say okay that has to be at that angle like that all right so I'm trying to get this Bond angle to match this Bond angle right there and the equatorial bonds right here go outside of the Ring they're not going to be like these axial ones where they go through the ring okay so those are the two different types of bonds that are going to be on a cyclohexane ring now when we combine them all right you're going to see that it gets a little bit messy but if we start out simple let's just start out simple for a second all right clean that up make sure the bonds are good what we can have happen here to keep it simple here that guy is axial and then this guy is going to be equatorial all right and so you can see that this is going to get a little messy when we put it all together so those in Orange are our equatorial bonds and now in green I will put in the axial bonds all right so you can see that it's going to get really really complicated or not complicated just messy because if this is my molecule here our cyclohexane what are on all these axial and equatorial bonds they're just hydrogens right so you just put in hydrogens for every single one here like that so that's the chair confirmation of cyclohexane but when it's cyclohexane you typically don't draw in all the axial and equatorial bonds because it just gets really really messy right and so we're organic chemists and we like to make things easy or make it more simple so that's what it what you're going to do but when we start adding substituents to the cyclohexane ring then we start drawing in our hydrogens and the things that are important so for example if I threw in a methyl group right here ch3 and I'm calling that carbon one and that's carbon one then we would draw in our methyl group and then we could draw in as well the dashed hydrogen now when we take a look at this cyclohexane ring right here it is mono substituted we have added one substituent the methyl group okay and so when we look at this chair right here when I point to the methyl group is the axial or equatorial you hopefully everyone say hey that's axial that's equatorial all right but there's this idea called a ring flip and a ring flip is an equilibrium process here okay and what's going to happen is this carbon right here is going to flip up all right and this carbon right here is going to flip down so this ring flip that I'm referring to is not taking let's say it's not taking the cyclohexane ring and if I represent the cyclohexane ring as like a pie so it's just a circular pie what I'm not doing is taking that pie and flipping it upside down that is not a ring flip of what I'm talking about here a ring flip when it comes to cyclohexane is when you look at these puckers you see how these two bonds come like this and there's this Pucker a ring flip is making that pucker go like this okay so if we number these carbons so we can track what's going on here if I call that carbon one two three four five and six what we're going to do is a ring flip which is basically changing the Pucker and so what you have to do to do a ring flip is you're going to take the way that you drew this ring or this chair sorry and the way I started this chair was I drew these two bonds right here very first okay you have to do the exact opposite for the ring flip so that means I have to come in at this angle like this okay not perfectly parallel and then like so so what I've done is a ring flip so the way I drew this one I have to do the opposite on this one and look at what happens this carbon one right here puckers up so that number one carbon is now right there if that puckered up then that carbon 6 had to pucker down and so that would be carbon six so one two three four five okay see that so carbon one puckers up carbon four puckers down and that's what we see one is there four is there once you figure out where carbon one goes you just number in the same order that you did you went one two three and numbered it now something to note that these this numbering scheme is not used to name the molecule okay I'm just using these numbers to keep track of where the carbons are so you can see them I'm not using it for IUPAC purposes so now what has happened is the methyl group here in this ring structure okay is axial but when you do a ring flip everything that's axial goes equatorial so I see that this methyl group is attached to carbon four it's still going to be attached to carbon 4 but the question is and I'm running out of space is it going to be axial or equatorial so let me redraw this ring a little higher up so we can see it okay so there I just read through it and so if this methyl is axial on this ring when we do a ring flip it's going to have to be equatorial on the other ring so that means this methyl right here is this methyl here you can't really see it because it's cutting off the board but that's our methyl and then this hydrogen is equatorial so that means it has to flip to the axial and so there is the hydrogen do you see that maybe I should write it this one's equatorial when we do a reef ring flip it now turns into the axial and that is very very important to be able to see so in summary for this little part here everything that's axial on this ring is going to be equatorial on this ring when you flip it that is very important to know now why is why should you care about this because look at what's going on here it's cut it's different confirmations here one is more stable than the other and so we need to understand the energetics here and which one is going to be better for or which one's going to be lower in energy right so let's take a look at the energetics here so let's draw ourselves a monosubstituted cyclohexane ring and I'm going to put a methyl group there now we want to do a rain flip this would probably be a good spot for you to pause the video and draw the ring flip and see what you come up with all right so let's draw this one let's actually change this guy to let's put it right here okay just so we can look at a different version here now when we do a ring flip I started with these two bonds right here so now I have to they're slanting this way so now I have to draw them in the opposite slant like that and that's like so sometimes the chairs just don't come out of the way that you envision so you can number this however you want as long as you are consistent so if that's carbon one what's going to happen when we do a ring flip that's going to go up that's going to come down so we have carbon six here one two three four and five okay so what is the answer here what does our product look like well carbon one is there carbon one is there this methyl is axial so we have to draw it equatorial over here and when we draw equatorial what do we have here we have a hydrogen now coming down there because what do we have here we have a hydrogen there which is equatorial so now it's axial so our methyl group has to match get the bond this Bond it needs to be parallel with that guy and it's not the best but almost okay but we have to we have to understand that this is equatorial so I'm drawing it slanted okay so that's in the equatorial position here you know something this may not uh suit my purposes the answer is still the same but you may not see it as well so I'm going to redo it really quickly here I'm going to put the methyl group back over here and when we do the ring flip we would have this like that and so our methyl group would be right there equatorial okay so I'm just doing this because it's going to be easier to see so not that the answer is going to be different the answer is the same for doing it up here or down here okay so this one is axial this one's equatorial at equilibrium which one is favored kind of like it's let's see the numbers here this one is favored by 95 this is five percent why because when you have large groups large groups are more stable in the equatorial position so hear me out write this down whenever you have a cyclohexane ring and you have large groups you want them in the equatorial position they're more stable so it begs the question why is this one uh unstable or at the least stable between the two and it has to do with this concept called one three dite axial let's see here axial interactions and that has to do to the fact that when you look at these carbons right here that are pointing up like so and then this one we have a hydrogen right there what happens is if you look at those hydrogens and those groups of atoms as a ball of electrons and then this methyl is pretty bulky right so we can represent that guy like this you see how big that guy is because it's so big in the ACT well it's big and when it's in the actual position it's going to interact with the other axial groups and that's going to increase its energy and it's not a stabilizing effect it's a destabilizing effect when you have the bulky group or the large group in the equatorial position you do not have the one three-diaxial interactions and so it is more stable lower in energy okay so looking at the time here what we will then do in class is we will practice these chairs and you're going to be introduced to looking at disubstituted and tri-substituted cyclohexane rings and you're going to be able you're going to have to be able to take a ring that looks like something that looks like this okay and we could put a chlorine there and a bromine there or whatever group we want and then you have to draw the appropriate chair and then you're going to have to say after you draw the two chairs which one is the most stable one and you're going to use this principle right here the 1 3 diaxial interactions to figure that out okay so with that that's where we will end this video for today and if you have any questions just let me know