Organic Chemistry - Video Tutorials & Practice Problems
On a tight schedule?
Get a 10 bullets summary of the topic
1
concept
Organic Chemistry & Stereochemistry
Video duration:
2m
Play a video:
Video transcript
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 60 2% 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 stereochemistry refers to the spatial three d arrangements of atoms and molecules, and that stereoisomers refers to the relationships of molecules that have the same exact atomic composition, but differ in their 3 d spatial arrangement. So let's take a look at our example below. And on the left here, what we have is maleic acid, where and on the right we have fumaric acid. Now, notice that maleic acid and fumaric 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 3 d arrangement of these 2 molecules is different. Maleic acid on the left has a cis configuration of the double bond, where the 2 bulky groups here are found on the same side of the double bond. That's what cis configuration refers to, recall. And fumaric acid on the right has a trans configuration where the 2 bulky groups are actually on opposite sides of the double bond. And recall 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 maleic acid and fumaric acid have the same chemical composition, but they differ in their 3 d spatial arrangements, that makes them stereo isomers. And so in our next video, we're going to, recap the difference between configurations and confirmations. So I'll see you guys in that video.
2
concept
Configurations & Conformations
Video duration:
4m
Play a video:
Video transcript
Now we've already said that stereochemistry refers to the 3 d spatial arrangement of an atom or molecule. But recall that stereochemistry that can only be changed by breaking and reforming bonds. Whereas confirmations are potentially flexible 3 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 4 distinct chemical groups bonded to it is called a chirality center. And you guys have learned in your previous courses that chirality centers can have up to, one of 2 possible configurations, an R configuration or an S configuration. And chiro the same chiral molecule that has opposite chiral configurations are called enantiomers. And enantiomers are non superimposable mirror images that can 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 realize 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 gonna do. And so our hands are non superimposable mirror images, and they are enantiomers. Now, down here in this example, we're gonna do 2 things. 1st, we're going to label the chiral configurations of these molecules, and then we're going to analyze the ethane conformations. And so in this video, I'm not going to go over how to determine the configuration of a chiral 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 only have 1 chiral center shown by the highlighted the chiral centers configuration, whereas the one on the right has an s configuration. And, the r, thalidomide here is used by doctors to treat morning sickness in pregnant women. Whereas if a doctor were to make a mistake and give a patient estheletamide instead of arthalidomide, 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 r to s configurations is by breaking and reforming, chemical bonds. Now over here, we have our ethane conformations. 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 of a molecule. And so if we have our ethane here with its hydrogens, it's almost as if we are looking right down the barrel of this molecule this way. And so notice that there are 2 possible confirmations. There's a staggered confirmation and an eclipsed confirmation. The staggered confirmation, notice that the hydrogens are very spread apart from one another. Whereas the eclipsed conformation, the hydrogens on opposite carbons are essentially overlapping in space. And so, this is why it's called an eclipsed confirmation. And so by, you don't need to break or reform bonds to change from a staggered confirmation to an eclipsed con conformation. And it is possible to convert between the 2, even though the staggered conformation is more stable and more 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.
3
concept
Organic Chemistry
Video duration:
3m
Play a video:
Video transcript
Here's your RNS naming cheat sheet. Remember there are 4 main rules to be able to figure out the configuration or RNS of any molecule. Let's go ahead and use these examples to apply these four rules. The first rule is that you assign priorities 1 through 4 on the chiral center by atomic mass. So what we would do is we would look at the atomic mass of each atom that's around the chiral center and try to figure out which one's heaviest, that would get 1. What we see is that for both of these, hydrogen is going to lose and it's going to get 4, 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 atom. Let's go next. So it says double bonds count twice, triple bonds count 3 times. So what I would do is, in this case this is what's going to break my tie, this is a single carbon, this gets counted as 2 carbons, and this gets counted as 3 carbons. So in my prioritization, this would now get 3, 2, and 1, and the same thing would apply to this one over here. K? If there is a tie still, then compare the next set of adjacent atoms. I don't need to do that because I don't have a tie anymore. But if I still had any ties, then I'd have to go to the next set of items and then compare those in a playoff system. Rule 2, if 4 is on the dash, number 4 is on the dash, then just simply trace paths from 1 to 2 to 3, and you're done. You can name your RNS. So are any of these already in the configuration of having a 4 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 4. And since it's counter clockwise, that means this is going to be an s. Great. So what about if your 4 is not on the dash, which is actually most of the time. Notice that for this molecule, 4 is on the wedge, not on the dash. We're going to have to swap whatever's on the dash with 4. 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, just sticking to one method and then just going with it. I like my method, but if you have a different way of doing it, that's fine. So what I would do is I would switch the 4 with with the dash. So my dash is 3, and my 4 is always going to be 4. I'm going to replace that with now a 4 here and a 3 here. You don't always, you would not always replace 43. I just happen to be replacing 3 in this case because that's the one on the dash. Great. Now I can trace my path from 1 to 2, from 2 to, from 2 to 3, from 3 to 1, I ignore 4 and it looks like an s, but because I start off with an s, I need to change it to an r. So this is an r. Okay. So now we just found our configurations for these 2 molecules using rules 1 through 3. And I just want to remind you of the last rule, which is that if you have multiple configurations on the same molecule as multiple chiral centers, then you should use locations and parenthesis to describe them. In this case, you can only use s, you you only need to use r because there's only one chiral center. But if there's more, then you should include the location of where that chiral center is, and you should put it all together in parenthesis. Alright? So that was your r and s naming summary. If you want more information on this topic, just click on the link below.
4
concept
Resonance
Video duration:
2m
Play a video:
Video transcript
So recall that resonance is the delocalization of electrons within a molecule. And all that means is that these electrons can shift within a molecule. And the delocalization of electrons creates an energy stabilizing effect that's, 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 delocalized, and then we have a pi bond or a double bond here on, the carbonyl group, which also has the ability for some electron delocalization. And so what you'll see is that these arrows here, recall, represent the movement of 2 electrons, and so we have the lone pair going, to this bond here, and we have the pi bond here or the the double bond electron density shifting up onto the oxygen. And so that creates an oxygen, atom here with a full negative charge, a, nitrogen atom with a full positive charge, and a, this 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 a full positive charge. And in fact, what it, a hybrid between these two resonance structures is is more appropriate in the best representation, and that's shown below. So you can see here in this representation, the oxygen 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 resonance happening in this molecule. And this concludes our lesson on resonance, 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.
5
Problem
Problem
A) Identify the chirality centers in the molecule.
B) Determine the configuration of each chirality center.
Video duration:
14m
Play a video:
Was this helpful?
6
concept
Fischer Projections
Video duration:
3m
Play a video:
Video transcript
So recall from your previous organic chemistry courses that Fischer projections are actually a type of molecular drawing style that are commonly used, especially to portray chiral compounds. Now, Fischer projections actually have 2 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 you, so 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 are popping out of the page at you. But in a typical Fisher projection, there all of the bonds are flat and so you don't see any wedges 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 to remember is that all of the vertical bonds, or the bonds that go up and down in a Fisher projection, they are always going into the page away from you. So they should really be presented as dashes. They should be presented as dashes even though they're normally not, and 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 are being presented as dashes that are 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. Alright. So if we take a look at our example for determining the r and s configuration of a Fisher projection, you have to keep in mind these 2, key features. And so we're going to, first look at the, the chiral carbon. So if we look at this carbon in the center here shown as this dot, it is a chiral carbon because it has 4 distinct chemical groups bonded to it. And we're gonna go through our normal RNS system for determining the RNS configuration. And so we'll go through our priorities. The nitrogen here gets priority number 1. This group up top gets priority 2, and the methyl gets priority 3. The hydrogen, of course, is going to get the lowest priority, which is priority number 4. And so then we go and we draw our arrows, So our arrows go from 1 to 2, from 2 to 3, and then from 3 back to 1. And this looks like a clock wise, 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 as wedges. So this horizontal bond here, the one that's connecting our 4th priority, is actually really popping out of the pages at us. So we can imagine it being a wedge. And so if the 4th priority is on a wedge, all you need to do is flip the configuration that it looks like. So this looks like, clockwise r configuration, but because the 4th 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 chirality center and the center is S. And so, we're going to get a little bit more practice determining the r and s configuration of Fisher projections in our next practice videos. So I'll see you guys there.