in this video, we're going to do some organic chemistry review, so recall that organic chemistry focuses on the structures, properties and reactions of carbon containing compounds. And what's really interesting is that carbon makes up about 62% of the dry weight of the human body, which is a very large percentage for just one element. And that goes to show how important carbon is to life. And so recall that stereo chemistry refers to the spatial three D arrangements of atoms and molecules. And that stereo is a MERS refers to the relationships of molecules that have the same exact atomic composition but differ in their three d spatial arrangement. So let's take a look at our example below and on the left. Here, what we have is melodic acid, where and on the right, we have few mark acid. Now notice that Malaya acid and few mark acid have the same exact chemical and atomic composition. So if we count up the numbers and types of atoms, they would match now, notice that the three D arrangement of these two molecules is different. Malic acid on the left has a cyst configuration of the double bond where the two bulky groups here are found on the same side of the double bond. And that's what cysts configuration refers to recall and few mark acid on the right has a trans configuration where the two bulky groups are actually on opposite sides of the double bond and recalled that the double bond here prevents free rotation of the molecule. So this group over here cannot rotate over to the other side because the double bond prevents it. And so because Malaya kassid and few mark acid have the same a chemical composition. But they differ in their three D spatial arrangements that makes them stereo ice summers. And so in our next video, we're going toe recap the difference between configurations and confirmations, so I'll see you guys in that video.

now, we've already said that stereo chemistry refers to the three D spatial arrangement of an atom or molecule, but recall that stereo chemistry includes both a molecules configuration as well as possible confirmations. Now configurations are referring to a fixed three D arrangement or a somewhat permanent three D arrangement that can, on lee, be changed by breaking and reforming bonds. Whereas confirmations are potentially flexible, three D arrangements that are not permanent and can be changed without breaking or reforming bonds now also recall that a specific carbon atom within a molecule that has four distinct chemical groups bonded to it is called a Chire ality center. And you guys have learned in your previous courses that Kyra Ality centers can have upto one of two possible configurations and are configuration or an s configuration. And Cairo, the same Cairo molecule that has opposite Cairo Configurations are called an anti immerse and an anti immersed are non super imposible mirror images that could have different chemical properties. And so the classic example is our hands. So our hands are mirror images from one another, and when we try to superimpose them or overlap them, we realized that we cannot overlap them when they're facing in the same direction. Unless I were to take this thumb, rip it off and plug it in over here, which I'm not going to do. And so our hands air non super imposible mirror images and they are in the anti Immers. Now, down here in this example, we're going to do two things. First, we're going toe label the Cairo configurations of these molecules, and then we're going to analyze the ethane confirmations. And so in this video, I'm not gonna go over how to determine the configuration of a Cairo center. But that's something that you guys need to know. So if you don't remember how to do that, watch the videos that I'm going to link you guys to in the comments below. And so notice that these two molecules on Lee have one Cairo center shown by the highlighted carbons here, and the Cairo centers have opposite configurations. The one on the left has an R configuration, whereas the one on the right has an s configuration. And, uh, the are thalidomide here is used by doctors to treat morning sickness in pregnant woman, whereas if a doctor were to make a mistake and give a patient s thalidomide instead of Arthur Levitt letter meid. Then the baby could possibly have birth defects. And so you can see how even though these two molecules look very, very similar, they are completely different, and they can have different chemical properties that lead to different results. So the only way that you can change our toe s configurations is by breaking on reforming, uh, chemical bonds. Now, over here we have our ethane confirmations, and we're showing a specific type of molecular depiction known as a Newman projection. So recall that a Newman projection is like looking down the barrel off a molecule. And so if we have our ethane here with its hydrogen, it's almost as if we are looking right down the barrel of this molecule this way. And so notice that there are two possible confirmations. There's a staggered confirmation and an eclipse confirmation. The staggered confirmation notice that the hydrogen czar very spread apart from one another, whereas the clips confirmation the hydrogen on opposite carbons are essentially overlapping in space. And so this is why it's called an eclipsed confirmation. And so by, uh, you don't need to break a reform bonds to change from a staggered confirmation to an eclipse confirmation. And it is possible to convert between the two, even though the staggered confirmation is Mawr stable and mawr prevalent. And so this is a good summary of configurations and confirmations, and I'll see you guys in our next video when we talk about resonance.

Here's your RNs naming cheat sheet. Remember, there are four main rules to be able to figure out the configuration or R and s of any molecule. Let's go ahead and use these examples to apply these four rules. The first rule is that when you assign priorities one through four on the Cairo Center by Atomic Mass. So what we would do is we would look at the atomic mass of each Adam. It's around the Carl Center and try to figure out which ones heaviest that would get one. What we see is that for both of these, hydrogen is gonna lose and it's going to get four. But for the other ones, all I have is carbons, carbon, carbon and carbon. So I have a three way tie, so I can't use the atomic mass rule for this. Adam, let's go next. So it says double bonds count twice. Triple bonds count three times. So what I would do is in this case, this is what's gonna break my tie. This is a single carbon. This gets counted as two carbons and this gets counted as three carbons in my prioritization. This would now get three to and one. And the same thing would apply to this one over here. Okay, If there is a tie still then compare the next set of adjacent Adams. I don't need to do that because I don't have a tie anymore. But if I still had any ties, they don't have to go to the next set of atoms and then compare those in a playoff system. Rule two, if four is on the dash, number four's on the dash. Then just simply trace path from 1 to 2 to three and you're done. You can name your R and s. So are any of these already in the configuration of having four in the back or in the dash? Yes, this one right here. So that means I can already go ahead and go from 1 to 2 from 2 to 3. And from 3 to 1, you always ignore four. And since it's counter clockwise, that means this is gonna be an s. Great. So what about if you're four is not on the dash, which is actually most of the time? Notice that for this molecule four is on the wedge, not on the dash. We're gonna have to swap whatever's on the dash with four. So and by the way, not all professors do it this way. But this is the way that I do it. And it's helped thousands of students all over the country. So I would recommend, um, just sticking a one method and then just going with it. I like my method, but if you have a different way of do it, that's fine. So what I would do is I would switch the four with the dash. So my dash is three and my four is always gonna be four. I'm going to replace that with now a four here and a three here. You don't always You would not always replace four and three. I just happened to be replacing three in this case because that's the one on the dash. Great. Now I can trace my path from 1 to 2 from two toe from 2 to 3 from three toe one. I ignore four, and it looks like an s. But because I start off with an S, I need to change it to a are. So this is an r. Okay, so now we just found our configurations for these two molecules using rules one through three on. I just want to remind you of the last rule, which is that if you have multiple configurations on the same molecule has multiple carol centers, then you should use locations and parentheses to describe them. In this case, you can only use as you only need to use our because there's only one Carol center. But if there's more, then you should include the location of where that Carol Center is, and you should put it all together in parentheses. All right, so that was your RNs naming summary. If you want more information on this topic, just click on the link below.

so recall that resonance is the de localization of electrons within a molecule. And all that means is that these electrons can shift within a molecule, and the de localization of electrons creates an energy stabilizing effect that stabilizes a molecule. And so, if we take a look at our example below, what you'll notice is that we have a lone pair of electrons on this nitrogen that can be D localized. And then we have a pie bond or a double bond here on the Carbonnel group, which also has the ability for some electron de localization. And so what you'll see is that these arrows here recall represent the movement of two electrons. And so we have the lone pair going to this bond here, and we have the pie bond here or the double bond electron density shifting up onto the oxygen. And so that creates an oxygen. Um, Adam here, with a full negative charge, a nitrogen atom with a full positive charge and a this, uh, carbon and nitrogen bond here with a double bond character. And so recall that these transient states are not actually existing in a molecule. So there's never a moment where there's a full negative charge or for positive charge. And in fact, what, uh, hybrid between these two resident structures is more appropriate in the best representation, and that's shown below, so you can see here in this representation theocracy, Jin has a partial negative charge, which is different from a full negative charge. The nitrogen has a partial positive charge, and then we have a dotted line here from the oxygen to the nitrogen, showing that these bonds have double bond character to them. And so this is the best representation of the residence happening in this molecule, and this concludes our lesson on residents. And I'll link you guys to videos in the comments section if you guys need some more practice and I'll see you guys in our practice videos.

so recall from your previous organic chemistry courses that Fisher projections are actually a type of molecular drawing style that are commonly used especially to portray Cairo compounds. Now, Fisher projections actually have two very important key features that we always have to consider. The first key feature is that all of the horizontal bonds or the bonds that go from side to side are always popping out of the page at use. They're always coming out of the page at you, and even though they're normally not presented as wedges, they really should be presented as wedges. And so we can see that in our example below. And so what you can see is that the horizontal bonds, the one that go from side to side here they're being presented as wedges, that air popping out of the page at you. But in a typical Fisher projection, they're all of the bonds are flat and so you don't see any wedge is being presented. But you still have to remember that the horizontal bonds are still popping out of the page at you, so that's a very important key feature to remember now, the second important key feature toe remember is that all of the vertical bonds or the bonds that go up and down in official projection they are always going into the page away from you. So they should really be presented as dash is they should be presented as dashes, even though they're normally not. We can also see that in our example below. So you can see here that we have the vertical bonds, the ones that go up and down and they're being presented as dashes that air going into the page away from you. And so, in a typical Fisher projection, notice that again the vertical bonds are presented as just normal lines. But you have to remember that these vertical bonds here are actually going into the page away from you. All right, so if we take a look at our example for determining the R. N s configuration of a Fisher projection, you have to keep in mind these two, uh, key features. And so we're going to first look at the Cairo carbon. So if we look at this carbon in the center here showing that says dot it is a Cairo carbon because it has four distinct chemical groups bonded to it and we're gonna go through our normal R. N s system for determining the R. N s configuration. And so we'll go through our priorities. The nitrogen here gets priority number one. This group up top gets priority to, and the method gets Priority three. The hydrogen, of course, is going to get the lowest priority, which is priority number four. And so then we go and we draw our arrows. So our arrows go from 1 to 2 from 2 to and then from three back toe one. And this looks like a clockwise configuration and clockwise configuration would be an R configuration. But because this is a Fisher projection, we have to remember that the horizontal bonds, the ones that go side to side, really are popping out of the page at us even though they're not presented his wedges. So this horizontal bond here, the one that's connecting our fourth priority, is actually really popping out of the pages at us, so we can imagine it being ah, wedge. And so, if the fourth priority is on a wedge, all you need to do is flip the configuration that it looks like so this looks like, uh, clockwise are configuration. But because the fourth priority is popping out of the page at us, we have to flip the configuration, and it's actually an s configuration. So the configuration of this Kyra ality center in the center is s. And so we're going to get a little bit more practice determining the R. N s configuration of Fisher projections in our next practice videos, so I'll see you guys there.