hello everybody my name is Iman welcome back to my YouTube channel today we're going to talk about isomers and in this lecture we're going to cover three big objectives first we're going to talk about structural or constitutional isomers then we're going to move into conformational and configurational isomers because our second objective is all about stereoisomers and then objective three which is going to be sort of embedded within objective two as we cover it we're going to discuss relative and absolute configurations this means we're going to cover e and z forms as well as R and S forms and finally talk about Fisher projections now to kind of set this stage an important way that we distinguish between molecules is by identifying isomers of the same compound those that have the same molecular formula but different structure so keep in mind that isomerism describes a relationship just as there must be at least two children to have siblings two molecules can be isomers to each other but no molecule can be an isomer by itself all right throughout this chapter we're going to learn how to identify these relationships and describe the similarities and differences between isomers and the hope is to cover this content so we can understand this flow chart we start with the same molecular formula alright so that means the same kind of composition the same amount of elements and then what we want to ask is well do these two molecules that have the same molecular formula do they have the same connectivity if the answer is no then we're talking about constitutional isomers and if the answer is yes they have both the same molecular formula and the same connectivity then what we're talking about is stereoisomers and stereoisomers can be divided into configurational isomers and conformational isomers and we'll talk about both and of course under this topic of stereoisomers it's going to be important for us to understand e and z forms as well as RNs forms because understanding these two different forms will allow us to distinguish different types of for example configurational isomers all right so let's go ahead and get started first objective is structural isomers also known as constitutional isomers structural isomers are the least similar of all the isomers in fact the only thing that constitutional isomers share is their molecular formula meaning that their molecular weights must be the same aside from this similarity structural isomers are widely varied with different chemical and physical properties so if we look at this figure right here these are all the different constitutional isomers of pentane all right pentane is just five carbons all right and about 12 hydrogens C5 h12 that is pentane now we can notice here this is one structure that has this molecular formula C5 h12 here's another molecule that has the same molecular formula c5h12 notice how there is different connectivity in comparison to this first molecule called pentane This is isopentane notice the difference in arrangement of the carbons and the hydrogens and then here also a c5h12 molecule but this is neopentane and you notice again all right between the three they all have completely different Arrangements of the five carbons and 12 hydrogens the only thing they have in common is their molecular formula and hence their molecular weights they are completely different molecules and these molecules are different from each other in terms of chemo chemical and physical prop properties all right they will just have the same molecular formula but completely different connectivity and it's really as simple as that all right structural isomers are constitutional isomers are just that now what gets more complicated is when we talk about stereoisomers and I want to go back to this flow chart right here all right if you have this if you have two molecules and they have the same molecular formula all right the next thing you want to ask is do the compounds have the same connectivity if the answer is no then you have structural or constitutional isomers we just covered that now if the answer to this these both of these questions is yes yes you have the same molecular formula and yes these compounds have the same connectivity then what we're looking at are stereoisomers all right and the next question you want to ask is well can you convert into in between these two molecules just by rotation about a bond if the answer is yes then you're talking about con conformational isomers and if the answer is no then what you're talking about is configurational isomers we want to talk about each of these all right we're going to start with covering conformational isomers all right conformational isomers differ by a rotation around a single bond in comparison configurational isomers can only be interchanged by breaking and reforming bonds all right we're forming we're we're focusing right now on conformational isomers of all the isomers conformational isomers are going to be the most similar conformational isomers are in fact the same molecule only at different points in their natural rotation around a single Bond now while double bonds hold molecules in specific positions as we'll see with CIS and trans isomers later single bonds are free to rotate conformational isomers they arise from the fact that varying degrees of rotation around single bonds can create different levels of strain these conformations are easy to see when the molecule is depicted in a form called Newman projections all right Newman projections now Newman projections is a way in which we can visualize molecules along a line that's extending through a carbon-carbon bond axis so to restate conformational isomers they differ by rotation around a single Bond and Newman projections are the way to see that so it becomes obvious that the next thing we need to learn is how to draw Newman projections how to look and read and understand Newman projections and this is going to help us better observe this spatial arrangement of atoms in molecule in a molecule as rotation occurs around a carbon-carbon single Bond okay so it's going to be important for us to start mastering going from Bond line to Newman and vice versa and then knowing that we can play with our Newman projection and get a couple of confirmations by rotating that single bond with the ability to draw different confirmations of a molecule then we can start to analyze and identify lowest and highest energy conformations and then begin to label things as gauche anti Eclipse but that's for later on now one thing I want to note before we start this all right we are going to talk about conformational isomers we're going to talk about Newman projections um if you want more details all right if you want a ton of more practice problems regarding this let me just point you in the direction of that for more practice problems even after we cover this isomers like tried point you to my OK one playlist chapter four is going to be all about conformational isomers it's going to be all about Newman projections and I do a lot of more practice problems that are different there if you need it if you want more thorough of a review all right and if you've never taken ochem you should start there with my ochem specific playlist before you hop in here in the MCAT playlist while we do cover everything it'll become way easier to understand if you already have that Foundation of going through the ochem classes all right this is also the reason why I assume that you are familiar with Bond line and that what that is what we've been talking about thus far in this playlist so just a warning I should say all right so first thing we're going to learn to go from Bond line to Newman and let me make a very important note here Newman projections will look a little different depending on where you are looking at all right where are you looking at which carbon carbon single Bond are you looking at and in which direction you will be told in which direction to observe a single Bond and draw a Newman projection more often than not so let's look at this example we're going to learn how to go from Bond line to Newman here is our molecule all right we're looking at this carbon right here we're looking at it in this direction so we're staring at directly this carbon all right so we're going to call this carbon one all right and the carbon right behind it right here we're going to call that carbon two we're just setting the stage up all right now for this technique the carbon I am looking at directly I'm going to draw as a DOT all right and the carbon right behind it I'm going to draw as an open circle so I'm going to redraw this whole molecule to display that closed Circle for the carbon I'm looking at directly carbon one and the carbon right behind it I'm going to draw as an open circle this is not scientifically accurate do not ever draw this and say this is Bond line or Newman okay we're just doing this as a step to break this down so it becomes a lot easier to draw all right so the first carbon that we are looking at based off of what the problem tells us all right we're looking at this carbon one all right we draw it as a DOT the carbon right behind it we draw as a full circle open circle then what I'm going to do is I'm going to look at each site independently I'm going to look at Carbon one first all right I'm going to draw that part I'm going to draw the dot to signify carbon one all right what I notice is pointing down is this methyl group so I'm going to draw a methyl group pointing down and this chlorine is is drawn up and of course there's also a hydrogen that is implicit in the bond line but it's there all right and it's pointing up all right so this methyl group is pointing down and I'm going to draw the chlorine and the hydrogen up all right fantastic that is that first part that is carbon number one cool I'm going to leave that alone I drew it independently then I'm going to look at the second carbon right here that we drew as an open circle I'm going to draw it as an open circle again all right I have this methyl group pointing up so I'm going to draw it up all right this bromine is pointing down so I'm going to draw pointing down and I kind of want to draw this upward Y and then this inverse Y which is why I chose to draw the hydrogen up in this first one and I'm going to draw the hydrogen pointing down here all right so we have this upright y down right Y and what I'm going to do now is I'm going to take those two and I'm going to over lay them on top of each other all right in the layering one goes on top two goes at the bottom and we lay here and so just like that we drew the Newman projection for this molecule we have that first carbon it's attached to a chlorine and a hydrogen that are pointing up with this methyl group pointing down and then we have that back carbon open circle we have that other methyl group pointing up and then we have bromine and hydrogen pointing down and that is how we go from Bond line to Newman all right now again for this first carbon we saw what was attached and then we drew that out and then we looked at the second carbon behind it and we went ahead and Drew that now something that you'll encounter is most of the time when you're given this molecule you'll have most of the groups already drawn out as well as the implicit hydrogen all right and you'll see them as in wedges and dashes then that usually also helps you indicate what points up and what points down wedge means it's pointing out of the page Dash means it's going into the page and that helps you determine that oh for this site we have chlorine and hydrogen pointing up in the methyl group pointing down and for site two we have the bromine and hydrogen pointing down methyl group pointing up most of the time the drawings for Bond line will actually also sometimes explicitly show the hydrogens to help you point in the direction all right fantastic so that's how we go from Bond line to Newman also very good practice to learn how to go from Newman to bond line all right here is a Newman projection all right we want to draw it in bond line what we're going to do first is we're going to unravel the two layers we have this front layer where we have our Center carbon all right it has a oh alcohol group pointing up in a hydrogen pointing up and an ethyl group pointing down so we're going to go ahead and we're going to draw that independently all right oh group hydrogen up ethyl group down cool then we're going to ignore that and then we're going to look at the back here's our back carbon all right it's going to be attached to that to that front carbon but we're not going to worry about that yet we're going to draw the open Carbon we have a methyl group pointing up and then a hydrogen and chlorine group pointing down so we're going to draw those exactly like that then we're going to connect these two carbons all right and then we're gonna clean this up all right so this pretty much gives us the skeleton and that we just clean it up because those hydrogens don't need to be explicitly drawn in the bond line so we have this oh group we have this front carbon this back carbon and then there's this chlorine and then we have a methyl group pointing up and an ethyl group pointing down at each of their respective carbons and that's how you go from Newman to bond line unravel the two layers all right connect with a bond draw appropriately and hide the hydrogens all right so now we know and understand how to go from Bond line to Newman and from Newman to bond line next we also want to learn about conformational analysis here we want to remember that just like us groups of atoms all right also like having their personal space and we can use this idea although a little bit of reduction is to build an intuition for identifying high and low energy Newman projections and we're going to start here we have a bond line of butane all right let's draw the Newman projection okay if you're looking at this particular carbon right here you have your front carbon you have your back carbon all right this carbon has a methyl group pointing up and two hydrogens down all right or we could have one hydrogen up one hydrogen down either or all right I'm going to draw my methyl up and a hydrogen up and then one down I usually like to draw my front carbon in an upright y if nothing else is indicated to me all right and then the back carbon has a methyl pointing down all right and a hydrogen pointing down and one up all right so we draw that out all right overlay them and we have this Newman projection right here all right and notice how to this carbon we have a methyl group pointing up and then to this carbon we have a methyl group pointing down and notice how they are aligned nicely like that as well in the Newman projection cool so we have a bond line of butane this is the Newman projection for that and we now have this Newman projection it's going to help us better observe that single bond this specific single Bond right here I'm going to highlight in green all right it's going to help us understand that single Bond and how we can rotate things about it now if we take that initial Newman projection we just drew for butane all right we can begin to talk about things all right if we take the initial Lumen projection and then rotate this back carbon by 60 degrees and draw it all right if we if we take this initial one right here all right and we take this bad carbon here's the back carbon this is what's attached to the black carbon and we rotate it by 60 degrees all right and draw this is what we get number two and then if we do the same thing again take that back carbon rotate all the groups by 60 degrees all right we get this third Newman projection and then if we rotate Again by another 60 degrees we get this fourth so just by rotating that bad carbon 60 degrees 60 degrees another 60 degrees and another final 60 degrees we get Newman projection one two and three and four all right all we're doing is rotating groups around this single bond which you can do right single bonds are flexible and you can rotate all right so what we have here are drawings of starting with the initial Newman projection and then rotating that back group 60 degrees over and over again and then drawing it and now we get several confirmations of butane that have different groups near or on top of each other and then we can begin to classify what we would call these kinds of conformations all right we can begin to classify them with proper names so looking at that first one we drew all right nothing no none of these groups are on top of each other this is called an anti-conformation the methyl groups are the furthest apart these methyl groups are the biggest groups here everything else is a hydrogen and they're the furthest apart all right they're like about 180 degrees away from each other all right um 60 60 actually if you want them to be on top of each other even further all right so they're the furthest apart from each other here in this this is called an anti-confirmation there is not going to be any torsional strain or steric hindrance to increase the overall energy for this conformation for confirmation two here this is called an eclipsed conformation because there are groups that are on top of each other now all right when you rotate 60 now you have this methyl hydrogen interacting here and here and you have these two hydrogens also all right now there are hydrogen hydrogen Eclipse interactions that's going to result in a little bit of what's called torsional strain this results in an increase in potential energy actually the energy cost of this hydrogen hydrogen Eclipse interaction is about four kilojoules per mole all right we also have two methyl hydrogen Eclipse interactions this is going to result in some torsional strain and the energy cost of this interaction is actually about six kilojoules per mole all right now we have two of these all right so we have six kilojoules per mole for this one and six kilojoules per mole for this one and then four over here so we have nearly 16 kilojoules per mole of of energy cost from the torsional and uh from from the torsional strains um from these eclipsed interactions all right so just based off of that comparing one and two this is going to be a higher energy confirmation than this one this is lower in energy there's the groups are not so close to each other that they are you know getting into each other's Bubbles and there's an energy cost penalty for that all right then if we look at confirmation three this is a Gauss confirmation the methyl groups are near each other and they participate in what's called a Gauss interaction but they're not on top of each other so it's not an eclipsed interaction um now because they're near each other there is a bit of a steric interaction with the two big groups like methyls being that close to each other there is a slight energy cost for these two methyls being near each other but now on top and so it's only just about 3.8 kilojoules per mole all right so now comparing one two and three two is still the higher energy conformation second to that is three with only 3.8 kilojoules per mole of an energy cost and then one is the lower energy of the three now if we look at four again this is an eclipsed interaction again but now we have this methyl methyl Eclipse interaction and this has a really big energy cost in comparison to the other stuff we've seen this is 11 kilojoules per mole and we still have these two hydrogen hydrogens which is four and four so we have eight plus eleven this one is 19 kilojoules per mole this is the highest energy conformation that we've seen yet all right and so we can plot this actually we can plot confirmation one this is right here this is confirmation two higher in energy three is higher than one but less than two and then confirmation four is the highest energy now if you continue rotating it you'll eventually go back through all the confirmations again if you keep doing the 60 degree rotation all right fantastic now some important things to note some important definitions to make ourselves familiar with here um staggered conformation is the name for the confirmation with the lowest energy you might also hear anti-confirmation now dihedral angle what is the definition of this this is the difference between the two largest groups so it's the angle between the two largest groups on either side of the projection that's what the dihedral angle is torsional strain is results from the repulsion of electrons forming the bonds of two adjacent atoms torsional strain increases with the number of eclipsing hydrogens in a molecule and this of course has to essentially do with unfavorable molecular orbitals and anti-molecular orbitals overlapping all right and here again I just show you what we talked about in terms of higher energy lower energy conformations just listed here for eclipsed the two groups overlap each other right there's zero degrees of dihedral angle between the two methyls in an eclipse conformation in in a Gauss conformation the angle between these two methyls is 60 degrees all right these groups they are near each other but they don't like they're not on top of each other they are 60 degrees away from each other and then anti is one the um degree the dihedral angle is 180 degrees all right that means the two largest groups anti the two largest groups are opposite to each other that's going to be the lowest energy eclipse is going to be the highest energy fantastic now our last topic for this section for conformational isomers is talking about the confirmation of cyclohexanes so here is a cyclohexane in bond line all right now we're going to learn how to draw it in chair conformation but let's let's build this up a little bit here all right in the The Strain in cyclic molecules it comes from the angle strain that's sometimes created by stretching or compressing angles from their normal size or from torsional strain from eclipsing conformation and even sometimes from non-bonded strain from interactions between substituents attached to adjacent carbons now cyclic molecules will usually adopt non-planar shapes to minimize the The Strain and by twisting into a chair the angle strain and the torsional strain are now effectively zero so in the chair conformation at each carbon all right this is this is a pretty energy favorable way of drawing a cyclohexane is through the chair conformation here what we see all right is that in the chair conformation at each carbon at each carbon in the chair conformation there are two positions for substituents that that we can attach substituents we have this axial position that is either directly up or directly down and then we have these axial positions all right that are slanted up or slanted down all right substituents attached to the cyclohexane can be classified as axial either sticking up or down from the plane of the molecule or equatorial in the plane of the molecule and if you were more artistic than me you'd know that really you want to draw this at an angle these these sides at an angle mine are a little too straight so you want to be a little better at drawing these chair conformations Than Me by tilting them a little more on the sides nevertheless I'm not an artist so do forgive me um but you have these two positions axial and equatorial now the axial positions are going to follow the directions of the corner so here is the direction the corners pointing up so the axial position is directly up if we look at the next carbon that corner is pointing down so the axial will be directly down now the equatorial positions are going to be still remember in plane of the molecule they're going to be the opposite direction so if we're looking at this position right here the corner points up so the axial position is directly up that means the equatorial is going to be slanted down if we look at the next corner it's points down so the axial is straight down the equatorial will be slightly slanted up all right and then you can do that with all the corners in your cyclohexane and chair conformation all right like I've drawn around here now equatorial positions are going to be the best for bulky groups because it's more stable that way there are not going to be any um what we call one three diaxial interaction so if you notice for example if we look at this chair conformation right here we have a straight up axial position that let's call that position one and let's just call this position two and this position three at position three we have another axial position here um what can happen is if you have like two big bulky groups here there can be a one three diaxial interaction that could have an energy cost and make this conformation a little less unfavorable so equatorial positions are the best for bulky groups because it will reduce that energy cost now in cyclohexane molecules with multiple substituents the largest substituents will try to usually take the equatorial position to minimize strain something important that we should know how to do our ring flips um so you can go this is if you draw your initial uh chair conformation like this a ring flip would just take that initial position and and flip it over all right so during a chair flip axial components become equatorial and vice versa however components pointing up remain up in components pointing down remain down all right now also this is maybe a little TMI um this is very not common to be tested on but also noticing that you can take your chair conformation you can draw it in a Newman projection all right and this is what it would look like in a Newman projection remembering this base structure for the chair conformation also makes it easier to draw for example if we had um a group right here like a methyl group it'd be really easy if we identify whatever number we want this to be one if this was one then we would just replace this hydrogen here with a methyl group and that would be the Newman projection of this new chair conformation I just drew that has a methyl substituent all right so that is confirmation that is conformational isomers all right that was a lot of information let me quickly summarize it and we'll end the lecture here in the next video we'll go into configurational isomers so what we've talked about is that conformational isomers differ by rotation around a single Bond all right a single Bond we saw staggered conformations they have groups that are 60 degrees apart as seen in the Newman projections in anti-staggered molecules the two largest groups are going to be 180 degrees apart and the strain is minimized in that whereas in Gauss staggered molecules the two larger groups are 60 degrees apart all right and then in Eclipse conformation the largest groups are zero degrees apart Eclipse confirmations have groups that are directly on top or in front of each other as seen in the Newman projection and in totally Eclipse confirmations those two largest groups are directly in front of each other like we see here this was exactly confirmation four now we drew confirmation to this was also eclipsed but we didn't have the two largest groups on top of each other all right so this is just an eclipse confirmation this we can refer to as totally eclipsed conformation now something else that we want we did was we talked about um The Strain in cyclic molecules it comes from angle strain torsional strain and non-bonded strain and so cyclic molecules will usually adopt non-planar shapes to minimize this strain and so cyclohexanes take on this chair conformation substituents attach to cyclohexane can be classified as axial sticking up or down directly from the plane of the molecule or equatorial in the plane of the molecule axial substituents create more non-bonded strain all right in cyclohexanes with multiple substituents that largest substituent will usually take the equatorial position to minimize strain and with that we've covered objective one and two in the next video we'll cover objective too further talking about configurational isomers and then objective three let me know if you have any questions comments concerns down below other than that good luck happy studying and have a beautiful beautiful day future doctors