hello everybody my name is Iman welcome back to my YouTube channel today we're continuing our lecture on isomers where we left off is on configurational isomers now to talk about configurational isomers we want to start with chirality so objects that are not superimposable on their mirror images are called chiral objects and the opposite of this is something called a chirality now an example of chirality is your hands if you place both face down on the table and then you try to take your right hand and place it over your left you notice that they are not super imposable all right now how do we take that and think about it in regards to molecules and compounds well in terms of identification for organic chemistry here for chirality a chiral Center is a carbon with four different groups attached to it your first step should always be to identify the number of chiral centers in a molecule all right and again the way you identify that is by finding a carbon that has four different groups attached to it all right so once again an object is considered chiral if its Mirror Image cannot be superimposed on this original object this implies that the molecule lacks this internal plane of symmetry um so chiral Center is a carbon with four different groups on it this carbon will be asymmetrical is the is going to be an asymmetrical core of optical activity and it's going to be known as a chiral center now two molecules that have non-super-imposable mirror images of each other are called enantiomers molecules may be may also be related as diastereomers these molecules are chiral and share the same connectivity but are not mirror images of each other and this is because they differ at some but not all of their multiple chiral centers all right and we're gonna dive into Indian tumor enantiomers and diastem diastereomers as well but before we do that before we do that we Now understand what a chiral carbon is four different groups attached to a carbon let's learn how to identify whether that chiral Center gets a r or an S assignment and that requires for us to talk about R and S forms which is part of objective three so we're taking a pause from objective two and configurational isomers to go over R and S forms because they will play a really important role in US looking at molecules and trying to decide are they enantiomers are they diastereomers are they miso compounds we're going to learn what all that means and being able to look at two molecules and compare and determine whether their enantiumerous diastereomers or meso compounds requires us to be able to one identify chiral centers and then assign chiral centers r or S configuration and then comparing and contrasting based off of that all right so let's cover R and S configuration now that we can identify chiral centers carbons with four different groups attached to them let's learn how to assign find the configuration of the chiral centers now at every chiral Center you you will either assign r or S but what does that mean this is an attempt to relay information in the spatial arrangement of atoms at a chiral carbon Center the con Inglot pre-log rules will help us do just that in five steps the first step is to identify the four atoms directly attached to the chiral Center all right to that chiral carbon the second rule is to assign a priority to each atom based on its atomic number the highest atomic number receives Priority One and the lowest atomic number receives a priority of four this lowest atomic number priority is often a hydrogen atom now step three says if you have a tie if there's a tie if two atoms have the same atomic number then what you want to do is move away from the chiral Center and look at the look for the first point of difference all right one constructing list to compare remember that a double bond is treated as two separate single bonds when you are thinking and trying to determine priority between two groups that tie so if there's a tie move on to the next atoms attached until you can assign priority for all groups step 4 says rotate the molecule so that the fourth priority is on a dash remember dashes imply that that atom is going behind the plane of the page wedge means it's coming out of the plane of the page and then step five determine whether the sequence one two three follows a clockwise order if it does it's r or if it follows a counterclockwise order then it is assigned s all right now the question here all right the question is here is well how do we begin to visualize what this means all right how do we begin to visualize assigning priority for example and how do we begin to uh you know assign priority and then how do we begin to make sure that our fourth priority is always on a dash how do we rotate molecules correctly to do that and then determine the one two three one two three sequence so for example let's say we have a molecule like this all right we have a bromine group we have a hydrogen um we have a let's see a nitrogen here and it has two methyl groups attached to it and then that's it all right if we're looking at a molecule like that where is the chiral is there a chiral Center that's the first question the answer is yes here's a car here's a chiral Center this carbon is chiral because it has four different groups attached to it has a bromine it has a hydrogen it's attached to a nitrogen and then it's attached to this carbon as part of a methyl group all right so we've assigned we've found and identified a chiral carbon a chiral center now what we want to do is begin to assign priority to each atom based on its atomic number on their atomic numbers so bromine here is going to have the highest atomic number it gets Priority One it's followed by nitrogen for two carbon for three hydrogen for four cool now there's no ties here thankfully if there if there was carbons on both sides for example then you would move to the next um you would you would look to what the carbons attach to and then move out until you can identify a priority so let's stick to this example and then we'll look at what if it was a carbon and we had a tie in priority so this is what it is now bromine 1 nitrogen 2 carbon 3 Priority hydrogen 4 priority now we step four says to make sure that the fourth priority is on a dash good for us here it is on a dash so what do we do we ignore it and then we draw um from one to two to three all right and notice how that is clockwise all right that is moving clockwise so it gets assigned that chiral Center gets assigned r all right fantastic now let's pretend let's draw a different molecule all right let's draw something a little different um let's say this is let's do this um let's say that this is a chlorine atom this is hydrogen this is a methyl group right here and this is a carbon that's attached to two other carbons cool let's identify a chiral Center here's a chiral center right here this carbon's attached to four different groups a chlorine a hydrogen a methyl and then this group right here all right so cool that's a chiral Center that that little dot that's highlight that little carbon that's highlighted in green that's a chiral center now the second step is to assign priority well that's a chlorine that's a hydrogen this is a carbon right here it's attached to three hydrogens this is a carbon as well it's attached to a carbon over here a carbon over here and then there's also a hydrogen now let's assign priority chlorine obviously gets one now we have a tie for two carbons hydrogen obviously gets four anyways the tie here is between these two carbons which one gets two priority which one gets three so they're both carbon so what we want to do is make a list of what these carbons are attached to this carbon is attached to three hydrogens this carbon is attached to two carbons and one hydrogen all right carbon beats hydrogen all right and so this carbon right here gets priority two and this one gets priority three now thankfully again the hydrogen is on the dash so we ignore it and then we draw Arrow from one to two to three this is counterclockwise so this gets this chiral Center gets an S assignment now what if it's not so easy that the hydrogen is the fourth group is not on a dash how do we approach that how do we begin to visualize how to rotate a molecule appropriately to get a hydrogen on a dash well there's actually a little bit of a trick to this all right so there's three different ways that you can s that you can imagine how this could play out and we're gonna go over each of them and how to tackle that all right so we can identify chiral Center and we can you know assign r as configuration but for determining um you know clockwise or counterclockwise there are a couple of hints to keep in mind all right first is if your fourth group is on a dash then you are so lucky you don't have to if your fourth priority group is on a dash you're lucky you just ignore it goodbye and you draw your arrow from one to two to three and determine if that's clockwise it gets r or if it's counterclockwise it gets s assignment okay that one is the easy one what if your fourth group all right is not on a dash but it is right next to a dash all right look at this fourth group it's not on a dash but it's right next to a group that is on a dash the three priorities on a dash instead of the four but they're right next to each other what do we do what we do is we still figure out our one two two three sequence all right here's our one here's our two and here's our three all right we determine our one two three we assign it that's clockwise so it gets r and then we give it the opposite configuration we give it an S because our fourth group was never on a dash all right so that is the trick for that if you're if you're fourth group is net not on a dash but it's next to it you can do this all right here's another example same thing your fourth group is not on a dash but it's right next to a group that is so what you can do is still just draw your one two three that's counterclockwise that's that would be given S no you give it the opposite because that fourth group was never on a dash but it was near it all right so that's the trick for that the last thing is well if you if you have a fourth group and it's not on dash but it's right across from the dash so here's the fourth group here's the dash what do you do well you just do your one to two to three sequence all right and just give it that appropriate configuration so this is counterclockwise it gets s and you don't worry about it and this is because to get the fourth group to the dash when they're across from each other you're gonna do two rotations and they'll ultimately cancel each other out in that way all right so those are the three tricks I always use if I don't know how to properly rotate a molecule so this requires no 3D visualization on your part to appropriately rotate molecules which could be way faster on a timed exam like the MCAT all right so now we can take all this information we learned about chiral centers and RNs configuration to understand and identify stereoisomers but there is one more thing one more thing that I want to cover before we do that before we start talking about what enantiomers are what diastereomers are and what meso compounds are we said we know how to find a chiral Center that's a carbon with four different groups and we know how to do RNs configuration at a chiral center with those four different groups but what about all right what about carbon-carbon double bonds all right what about CIS and trans that's an important discussion for us to have all right so what how do we treat double Bonds in regards to configuration well this is where we start to talk about e and z forms so e and xenomenclature is used for compounds with poly substituted double bonds recall that simpler double bond containing compounds they can use this CIS and trans system all right but you can all you know you might also be given or asked to use the Enz designation all right so how do you do this well you identify where your double bond is so for example here's a double bond all right you figure out your highest priority groups all right so for looking at these um this is going to be these two are going to be your higher priority groups here the bromine and chlorine all right cool so you've determined your your two highest priority groups you've determined where your double bond is you're going to draw a line through it and you've determined your two highest priority groups now if your two highest priority groups are on the same side which they are based off of where you drew a line Acro through your double bond if they're on the same side which these two priority groups are on the same side all right this is given Z assignment all right but if we look here here's our carbon double bond our first our our first priority group is here and our second priority group here and what we notice is that they're on opposite sides if they're on opposite sides are two highest priority groups then this is given an e assignment all right e and then you could proceed with the name so this is how you do e and z designation of alkenes all right so this was also again a little jump into the third objective right we're kind of embending embedding the third objective as we have a what I think is a chronological order discussion on configurational isomers all right so now we have we understand R and S and now we understand what e and z is as it relates to CIS and trance all right fantastic so now we know how to identify chiral centers we know if we find a carbon with four different groups that's a chiral carbon and then we can designate it an r or S configuration based off of the rules we learned if we have double bonds all right remember we already know CIS and trans um but we're gonna we're gonna be focusing on the e and z designation here if your two highest priority groups are on the same side of the double bond then this gets a z-con for me a z assignment and if the two highest priority groups are on opposite sides of the double bond it gets an e assignment all right we'll get into more depth in relation to the importance of this really for the rest of the discussion we're going to be talking more about RNs configuration and how we can use this idea to Define enantiomer's diastereomers and miso compounds appropriately all right so now we can talk about enantiomers diastereomers and meso compounds all right so your first step is always to identify chiral centers and then assign them as r and s so let's begin talking about enantiomers enantiomers are non-superimposable mirror images they cannot appear identical simply by rotation now how can you tell if mental rotation is difficult for you right if you're looking at these two molecules and you're trying to tell if they are enantiomers or not and you can't mentally rotate it appropriately especially assigning and changing the wedges and dashes appropriately how can you tell if these two molecules are enantiomers or not well easy we're going to rely on R and S configuration assignments to help all right so here's how it works for enantiomers if you have only one chiral Center if you have one chiral Center and you're looking at two molecules and you're trying to determine if there are enantiomers first you find that one chiral Center for each of them and then you're gonna work and assign each one R and S Let's Pretend okay we're pretending here that this is r and this is s okay I'm just pretending here all right this one's R and this one's s they both just have one chiral Center and one of them is r and one of them is s because they are opposite all right one is r and one is S at the chiral Center then they are indeed enantiomers all right so that's if there is one chiral Center and you're always going to need at least one chiral Center to even say or begin to investigate if two molecules are chiral centers and if there's just one chiral Center then it becomes easy a sign the chiral Center for each of the molecules that you're comparing each of the two molecules are comparing if in one molecule that carbon Center is r and the other one is s then they are enantiomers because they have the opposite R and S assignment all right if you're looking at both of these and they're both are then they're just identical all right we're not talking about enantiomers now there could be two chiral centers right you can have two molecules that you're comparing and you're trying to determine if they're enantiomers and they both have two chiral centers all right in that case if at all chiral centers between the two molecules they have opposite R and S assignment all right then they are enantiary so let's pretend all right here's a molecule here's a chiral Center bromine chlorine nitrogen all right this one's on a wedge this one's on a dash and then you're comparing the same one here this one's on a wedge this one's on a dash I'm not going to assign them here I just want to draw them all right and this is a carbon if and this is a carbon and it has another chiral Center so let's do that this is attached to a carbon this is a hydrogen this is a fluorine all right so here we have two chiral centers let me draw them out appropriately all right I'm just drawing stuff so I can get the point across we're not going to actually investigate them I'm just trying to get the point across so let's do this as a wedge that's bromine Dash chlorine this one has a hydrogen and a fluorine so this these two molecules we're comparing them we want to know if they're enantiomers or not they have two chiral centers here and here here and here so there they have the two same chiral centers if we were to determine that at this chiral Center you know what let me change their colors all right this one's red and this one's red because they're the same and this one's blue and this one's blue if we decide if we were to figure out what the RNs assignment is here and we decided that this is r and we looked at the other molecule at the same chiral Center we determined this is s and then we did the same thing with the other chiral Center this one is um s and this one is r all right now because they have because the two chiral centers between the two molecules because all the chiral centers between the two molecules have opposite R and S assignment at those positions then they are enantiomers if they were both identical like RS for this one and then also RS for this one these are just going to be identical now when you have two chiral centers you also have to worry about diastereomers you also have to consider if they're diastereomers which has a different rule the rule that their opposites here is what gives it away that they're enantiomers all right so that's how you use R and S configuration to determine whether all right two molecules are enantiomers whether they have one chiral Center or two chiral centers or more all right fantastic now before we get to diastereomers and miso compounds I want to just scroll over here all right and side note on Indian tumors that's going to be important for the MCAT now we said enantiomers are superimposable mirror images and thus have opposite stereochemistry at every chiral carbon all right they have the same chemical and physical properties those enantiomers except for one thing they differ in regards to how they rotate plane polarized light all right they have the same chemical and physical properties except for rotation of plain polarizer light and reactions in a chiral environment now Optical activity refers to the ability of a molecule to rotate plane polarized light all right so you can have um a compound that rotates the plane polarized light to the right all right it can rotate the plane polarized light to the right or you can think of this as clockwise all right this is dextro rotario dextro Rotary all right so that's assigned D all right and it's usually labeled as a positive you can have the light be rotate to the left this is called counterclockwise this is levo rotatory all right and this is identified as L minus usually written or is just labeled as minus the direction of rotation can't be determined from the structure of of a molecule you have to actually determine it experimentally and so that is it is not related to the absolute configuration of the molecule now the amount of rotation depends on the number of molecules that a light wave encounters all right this depends on two factors the concentration of the optically active compound and the length of the tube through which the light passes chemists have set standard conditions of one gram per milliliter for concentration and one decimeter for length to compare the optical activities of different compounds now rotations measured at different compound concentrations and Tube lengths can be converted to a standardized specific rotation using this equation right here this is the specific rotation in degrees all right this is the observed rotation in degrees C is the concentration and L is the path length all right now if you were to have a mixture with both positive and minus enantiomers in equal concentrations they form what's called a racemic mixture in these Solutions the rotations cancel each other out and no Optical activity is observed all right fantastic so that was an important thing that I wanted to mention all right as a side note for enantiomers all right now let's also go ahead and Define diastereomers so diastereomers are non-superimposable and not mirror images so in other words non-identical stereoisomers now here's the key for thinking about diastereomers they're going to have the same molecular formula they're going to have the same connectivity and they're going to at least have um two chiral centers all right so unlike enantiomers all right going back to Indian tumors the requirements were of course that they have the same molecular formula and the same connectivity those are the two conditions of stereoisomers but enantiomers needed at least one chiral Center and then you assign RNs to each and compare positions if at all chiral centers between the two molecules they have opposite R and S assignment than they are enantiers if all are identical then the molecules are identical now when we think about diastereomers here's the key for that you have the same molecular formula you have the same connectivity you need at least two chiral centers and how you approach this is you're going to assign RNs to each all right and then compare positions so let's look at these drawings right here all right let's draw let's look at these drawings right here and then let's begin to understand enantiomers and diastereomers the role for Indian tumors was if they have opposite RNs configuration at each carbon Center chiral Center that you're comparing then they are enantiomers but with diastereomers if at some chiral centers that you are comparing between two molecules you have a mix of the same RNs assignment and opposite then they are diastereomers so let's let's try to do this all right I am going to hypothetically assign R and S configurations at these just so that we can get the point all right so this is R and let's say that this is also are for example okay so these are both are let's say that this is r and this is s let's say that both of these are s and then let's say that this is s and this is R okay now look let's look at this point in this molecule and then this chiral center right these are the same chiral carbons in both of these molecules this one is r and this one is s cool they're opposites at that chiral center now let's look at this chiral Center for these two molecules all right this one is r and this one is s they're also opposite at that chiral Center so both of these chiral centers we looked at in these two molecules have opposite R and S assignment this one had R this one had S at the same chiral carbon at this same carbon this one had R this one had s opposites opposites at both of those chiral centers that means these molecules are enantiomers all right cool let's look at these molecules now I'm going to circle them in blue we're just looking at them as as we choose all right let's look at these two if we look at this chiral carbon at both of these molecules one is s one is R that's opposite cool if we look at the second chiral Center oh this one's R this one's s at both chiral carbons they have opposite R and S assignment this one's s this one's R this one's R this one's s opposite at the same chiral position the same chiral carbon these two are also enantiomers all right so these are enanti tumors these are enantiomers cool so we understand enantiomers now if there is opposite R and S at the same chiral Center for all the chiral centers that you're looking at all of them have opposite R and S configuration however many your molecule has whether it's one two three or more all right then that defines Indian tumors now what about diastereomers all right so let's do some erasing all right let's do some erasing all right these are RR all right let's look at these these two all right let's compare them at this chiral carbon for both of these molecules they're assigned are they're the same cool if we look at the second chiral carbon in these molecules one is r and one is s so at one chiral Center they're the same add another chiral Center though they're the opposite this mix of same and opposite is what defines diastereomers so these two molecules if we were looking at them we would Define these to be diastereomers all right so that is the distinction between Indian tumors and diastereomers based off of the r and s assignments now one more note on diastereomers CIS trans isomers are a subtype of diastereomers in which groups differ in their position about that double bond about that immovable Bond so going back to our priority rules and alkene is z if the two if the high priority substituents are on the same side of the double bond and their e if they're all on opposite sides of the double bond all right so that's enantiomers that's diastereomers fantastic but wait there's one more thing you have to be wary of when you're looking at stereoisomers for molecules that have reflectional symmetry and that is miso compounds miso compounds are chiral compounds that have multiple chiral centers all right they have at least two chiral centers all right meso compounds have excuse me they have an internal plane of symmetry so they will be optically inactive because the two sides of the molecules cancel each other out now I'm going to show you how to identify miso compounds all right I'm going to show you how to identify miso compounds this is the rule if you have at least two chiral centers all right if you have at least two chiral centers and you have a internal plane of reflection plane of reflection all right if you have an internal plane of reflection and then you have also all right if you have if your molecule has reflectional Symmetry and at least two chiral centers then the pair of quote-unquote enantiomers with r and s assignment on one and then s and r on the other that you had thought was an enantiomer pair is actually a miso compound they are simply rotations of each other so for example look at this molecule it has two chiral centers it also has an internal plane of reflection now if you go ahead and assign these this might be R and this might be S and then this might be S and this might be R so you notice that they're opposites here and then they're opposites here and you might think enantiomers but because there is an internal plane of reflection and you're looking at two chiral centers these molecules are actually meso compounds they are the same molecule they're just simply rotated all right so that's the one catch all right you want to be careful because there's an internal plane of reflection so what you thought was a Indian tumor in this case is not all right so let's put all these rules together so that we have kind of a workflow for this all right if there is one chiral Center all right and they have opposite RNs configuration their eye enantiomers if they have the same then they're identical that's with one chiral Center if you have two chiral centers and they're opposite R and S configuration at every chiral Center then they're enantiomers if there are some opposite and some same RNs configurations then they're diastereomers and if they're all exactly the same then they're identical now the only thing is if there's reflectional symmetry double check for miso compounds all right fantastic now there's one more last thing that we want to cover and that's Fisher projections all right vertical so this is right here um a common depiction of of molecules you'll see in in um biochemistry all right sorry that took I had a little brain fart there I was like where do they see this biochemistry all right now if you were trying to take this and convert it to say Bond line all right take this Fisher projection convert it to bond line that's going to be an important skill to have all right it's going to be important to understand the workflow for this all right now here's how I do this here's my recommendation all right we're going to identify these as the ends and we're going to go ahead and draw this kind of structure right here kind of looks like a table top all right we're gonna put this on one end and put the other group on the other end all right top group on the left bottom group on the right cool then we're going to see how many carbons are in between those two groups three so we're going to draw one two three points all right one two three points cool then what are we gonna do we're gonna draw the groups that are attached to each carbon so this is going to be numbered our first carbon this is our second carbon and that's our third one two three all right now anything on the right gets assigned a dash so what's on the right at Carbon one and oh group so we draw it on a dash anything on the left gets a wedge so the hydrogen gets a wedge look at Carbon two what's on the right oh group alcohol group we draw it on a dash what's on the left hydrogen that goes on a wedge all right keep that rule all right third carbon what's on the right hydrogen group that goes on the dash what's on the left alcohol group that goes on the wedge cool now your ball line is not going to look like this it's going to look more like a zigzag of carbons right because that's the format for Bond line so we don't want to keep it like this we want to keep this part but then we want to bring it down here we want to take this group down here and then we can go back up and then it looks like a normal Bond line so what we're doing is we want to bring this group to a downward position all right to do this we're going to switch these two groups in their assignment of dashes and wedges so now what we draw all right is this alcohol group stays on a dash we're gonna forget the implicit hydrogens here in our bond line now we brought we took this group we brought it down and we switched the wedges and dashes when we did that all right so the alcohol group is now on a wedge instead of a dash all right over here we didn't flip anything so we keep it as is the alcohol groups on the wedge hydrogen on a dash but we don't draw our hydrogens and then we add our ch2oh group at the end and that's how you go from fissure projection to bond line and sometimes you know if Bond line is your preferred way of looking at things and identifying you know RNs configuration then this is the way to convert from Fischer projection to bond line in order to to do that everybody has their preference in regards to that you can still do it here all right you just look at it and um you can still do it in the Fischer projection my preference and what I recommend is convert it to bond line it's super easy and then go ahead and approach your RNs configuration from there all right so that's all I have for you all right that was configurational isomers so continuing objective two and then we also covered objective three embedded within each section all right let's cover the important points again we said configurational isomers can only be interchanged by breaking and reforming bonds all right we defined uh we learned how to do RNs configuration based off of five rules we also learned some quick trip tricks on how to do that if your fourth group is on a dash and when it isn't on a dash then we talked about um Enz forms as well so if an alkane is z and alkane is z if the highest priority substituents are on the same side of the double bond and then their e if they're on opposite sides all right and then we talked about enantumerous diastereomers and miso compounds and we learned how to distinguish whether two molecules we're comparing r enantiomers or diastereomers or even meso compounds based off of a couple rules that we have written right here all right and then we learned how to convert Fischer projection into Bond line so that we can go ahead and just do our normal RNs configuration as we have learned it all right with that we've covered our lecture on isomers now if you want more information and practice well we'll do a practice Problem video in the next lecture in this playlist but also I have an ochem1 playlist chapter 4 and chapter 5 for everything that we covered in this lecture and I do a lot of practice problems there so if you really need more of a refresher on this content all right be honest with yourself if you need it Go invest in that time watching those longer lectures those more in-depth lectures with even more problems as well so that you can definitely make sure you understand these concepts for the MCAT all right I'll see you in the next video where we do practice problems leave any comments concerns questions down below other than that good luck happy studying and have a beautiful beautiful day future doctors