Chiral examples 1. Created by Sal Khan.
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- Can I know the name of the last example molecule?(8 votes)
- At1:33, why is it not clear whether the two CH2s on either side of the CH are the same or not? Doesn't the C in the CH have to be bonded to Cl, H, and 2 Cs? I don't understand why Sal talked about looking at all 4 CH2s counterclockwise and clockwise from the CH before determining whether that C was a chiral carbon.(4 votes)
- Okay, I think I got it (check out the next video, Chiral Examples 2, at1:30)... If carbon is bonded to a group, that is what you look at, rather than the individual atoms to which the carbon is bonded. So by looking at the pseudo-butyl groups (those 4 CH2s), we can tell that the carbon we're checking for chirality is bonded to H, Cl, a butyl group, and another butyl group, not just H, Cl, C, and C.
A central C bonded to a hydrogen, propyl group, ethyl group, and pentyl group would be chiral, even though that central C appears to just be bonded to H, C, C, and C.(4 votes)
- The first example (at2:22), do we consider that the molecule is planar. Do we consider that if the molecule (any molecule) is shown like that to be planar and find if it's chiral or not..???
Also can we say that if a molecule has a chiral center then it is a chiral molecule... or it doesn't have to be?
- In general, if you have a small molecule with no sp3 centers in the molecule, the molecule is planar. But there are many exceptions. Here are a few common ones:
- Water is planar. The oxygen is sp3 but there are only 3 atoms so there's nothing to stick out of the HOH plane.
- Allene is not planar. See the wikipedia page and Sailor Astra's explanation below.
- Cis-hexatriene is not planar because of steric hindrance. This shows the "no sp3 centers" rule doesn't work for larger molecules.
The only way not to get caught by these "freak cases", is to draw out the orbital structure and the 3D structure (or at least imagine it).(4 votes)
- In the first example at around4:44, if you rotate from the CH-Cl 180 degrees to the left, wouldn't the two bottom CH2's then be on the top and the one CH2 on the bottom? So how is it supper impossible?(3 votes)
- That's what it looks like, doesn't it?This is one of the problems caused by drawing things free-hand. You have to remember that the ring is a regular pentagon, and he is really rotating the ring by only ²/₅ of a circle (144°), not half a circle (180°). Try it with a regular pentagon, and you will see that they really are superimposable.(5 votes)
- Can someone tell me if a C-atom can still be Chiral when he has 3 different groups and one is double-bonded?
For example: 1-bromo-1,2-dichloroethylene(3 votes)
- No; a C atom needs to have four different groups bonded to it to be chiral. In that example, the molecule will instead have E/Z isomerism across the double bond.(3 votes)
- At10:42, how come we cannot flip the molecule at 180 degrees and superimpose it on the other one if all the atoms are in the same plane?(2 votes)
- Keep in mind these molecules exist in 3D. The chiral carbons are sp3 hybridized and therefore have tetrahedral geometry. Sal drew them in 2D to make it easier to draw their mirror images which yes looks like we could just do a flip and superimpose them. However, if you draw them in 3D with the wedges or build a molecular model of the enantiomers you'll find they can't be superimposed. Hope that helps.(5 votes)
- Why doesn't the nitrogen in a tertiary amine with three unique substituents form a chiral center? Wouldn't the lone pair on the nitrogen be equivalent to the fourth unique group on a carbon atom?(3 votes)
- Good question – and the answer is yes tertiary amines are chiral, however due to "inversions" only certain types can be isolated as the pure enantiomer. For the ones that aren't fixed you almost instantaneously get a racemic mixture, which masks the chirality.
(Secondary amines are also chiral, but inversion occurs so rapidly that you can't isolate one enantiomer from the other.)
- how can we find the no of isomer a compound has?
thank u.(2 votes)
- I think you just have to practice till you get the vision, but here are some simple Unbranched Alkanes inc. the # of isomers
methane --> 1 isomers
ethane -----> 1 isomers
propane -----> 1 isomers
butane ------> 2 isomers
pentane -----> 3 isomers
hexane -----> 5 isomers
heptane ---> 9 isomers
octane ----> 18 isomers
- If a molecule has two chiral centers can it still be a chiral molecule?(3 votes)
- (S,S) may be the non superimposible mirror image of (R,R) and therefore its enantiomer.
Likewise (S,R) may be enantiomer of (R,S). Then (S,S) is a diastereomer of (S,R) and also of (R,S). And (R,R) is a diastereomer of (S,R) and also (R,S). All 4 molecules could be chiral or optically active. See http://en.wikipedia.org/wiki/Diastereomer(3 votes)
In the last video we learned a little bit about what a chiral molecule or what a chiral carbon or a chiral atom is. What I want to do in this video is go through a bunch of examples and see if we can identify if there are any chiral atoms and to also see if we're dealing with a chiral molecule. So let's look at our examples here. So here I have, what is this? This is chlorocyclopentane. So the first question is do we have any chiral atoms? And when we look at our definition that we thought of chiral atoms, it all comes from this notion of handedness and not being able to be superimposable on your mirror image, but we said that they're usually carbons bonded to four different groups. Let's see, do we have any carbons here bonded to four different groups? Well, all the CH2's, they're bonded to another CH2 and then two H'2. I could draw it like this: H and H. So they're bonded to two of the same group, so none of these CH2's are good candidates for being a chiral center or chiral carbon. They're both bonded to-- or all of them are bonded to two hydrogens and two other very similar-looking CH2 groups, although you have to look at the entire group that it's bonded to. But they're all definitely bonded to two hydrogens, so it's not four different groups. If we look at this CH right here, we could separate it out like this. We could separate the H out like this, and so since it's bonded to a hydrogen. This carbon is bonded to a chlorine, and then it's bonded to-- well, it's not clear when you look at it right from the get-go whether this group is different than this group. But if you go around, if you were to split it half-way like this, or maybe another better way to think about is if you were to go around this molecule in that direction, the counterclockwise direction, you would encounter a CH2 group, and then you encounter a CH2 group, and then you would encounter a third, and then you would encounter a fourth CH2 group, then you would come back to where you were before. So you would encounter four CH2's and then you'd come back to where you were before. If you go in this direction, what happens? You encounter one, two, three, four CH2's and you come back to where you were before. So all of this, this bottom group, depending on how far you want to extend it, and this top group, are really the same group. So this is not a chiral center or not a chiral carbon. It's not bonded to four different groups. And this is also not a chiral molecule, because it does not have a chiral center. And to see that it's not a chiral molecule-- let me see if I can backtrack this back to the way I wrote it right before. So you see that it's not a chiral molecule. There's a couple of ways you could think about it. The easiest way, or the way my brain likes to think about it, is just to think about its mirror image. Its mirror image will look like this. So if that's the mirror, you would have a chlorine. Then you have a CH, CH2, CH2, and you have a CH2, CH2, and then you complete your cyclopentane. Now, in this situation, is there any way to rotate this to get this over there? Well, if you just took this molecule right here and you just rotated it 180 degrees, what would it look like? Well, maybe a little over-- yeah, well, not quite 180 degrees, but if you were to rotate it so that the chlorine goes about that far, you would get this exact molecule. You would get something. It would look a little bit different. It would look like this. Let me see if I can do it justice. It would look like this. You would have a CH2. So let me let me do it up here where I have a little bit more space. If I were to rotate this about that far, I would get a CH. You get the chlorine and then you have your CH2, and then you have another CH2, CH2, and then you would have your CH2 up there. If you were to rotate this all the way around, or actually this is almost exactly 180 degrees, it would look like this. And the only difference between this and this is just how we drew this bond here. I could have easily, instead of drawing that bond like that, I could draw it facing up like that, and these are the exact same molecule. So this molecule is also not chiral. So let's go to this one over here. So what is this? This is a bromochlorofluoromethane, just to practice our naming a little bit. But it's very clear that we are bonded to four different groups. All of the different groups, or the atoms in this case that are bonded to this carbon, are different, so this carbon is a chiral center. And it should also be pretty clear that it is also a chiral molecule. If you were to take its mirror image, and this is very similar to the example we did in the first video on chirality, but its mirror image will look like this. You have the bromine on the right now. The hydrogen is still in back, and you have the fluorine above it. No matter how you try to rotate this thing, if you try to get the bromine all the way over there, all the way to that position, then the hydrogen would be in this position and the chlorine would be in that position. And no matter how you try to flip this around or rotate it or shift it, you will never be able superimpose this molecule on that molecule right there. So that is a chiral center and this is a chiral molecule. And there's a word for these two versions. We're going to go into the naming of them later on. It's a little bit more involved. We'll have a whole separate video on it. But these two versions of bromochlorofluoromethane, they sometimes have different chemical properties. And these are called enantiomers. And enantiomers are just the mirror images. Each enantiomer is a mirror image of each other, but they are stereoisomers. This is all just terminology. Stereoisomers. You're familiar with the word isomer, and isomer just means that you have the same atoms in your molecule. But then you have different types of isomers. You have constitutional isomers that say, OK, different things are connected to different things. Stereoisomers, the same things are all connected to the same things. You have a carbon connected to only a fluorine, a chlorine connected to the carbon, a hydrogen connected to the carbon, a bromine connected to the carbon. So all of the same things are connected to the same things, but they're a three-dimensional configuration. That's where we're dealing with the stereo part. Stereochemistry is the study of three-dimensional chemistry, as essentially understanding the actual three-dimensional structure of things. So stereoisomers mean that we have the same constituents, the same atoms. They have the same connections to each other. Bromine is still connected to carbon, which is still connected to hydrogen. That's all true over here. But their three-dimensional orientation is still different. And in this case where they are mirror images of each other, we call them enantiomers. And I should probably make one clarification. In the last few videos, I've been a little bit, you know, sometimes I'll say configuration and sometimes I use the word conformation. So sometimes I'll use the word configuration and sometimes I use the word conformation, and I actually should be a little bit more, or I should have been a little bit more exact about these. When you're talking about a configuration, you're actually talking about a different structure. To go from one configuration to another configuration, you would actually have to break bonds and kind of reassemble them. So these are different configurations. Because in order for them to be able to be the same thing, you would have to swap maybe the bromine and the hydrogen in there where they are relative to the carbon, so these are different configurations. Confirmations are really just different shapes or different orientations of the same molecule. So when we talked about cyclohexane being in a boat, so this is cyclohexane being in a boat conformation, or this is cyclohexane being in a chair conformation, it's the exact same molecule with the exact same connections. We didn't detach any bonds or reattach any bonds. They just flipped around a little bit. So these are two different conformations. These are two different configurations. To go from one configuration to another, you have to rearrange bonds. Now let's look at this molecule over here. Can we identify any stereocenters or chiral carbons or chiral atoms? And you have this carbon right here. Let's see, this carbon right here is bonded to a chlorine, a hydrogen, a bromine, and then another carbon. So this is bonded to four different things, so this is a chiral carbon. Sometimes they put a little asterisk there. If we look at this carbon right here you can-- well, it's bonded to a fluorine and another carbon, but it's bonded to two hydrogens, so it's not chiral. It has two of the same things that it's bonded to. You can even see a little axis of symmetry through it. If you look at that, you can kind of flip it over, and it's going to be the same thing. But this one right here, that is a chiral center. That is a chiral center, or chiral carbon, or chiral atom, or a symmetric carbon. You'll see it used in different ways. And because this molecule has got that chiral center, you'll see that if you were to take its mirror image, it would be an enantiomer. This is not superimposable on its mirror image. We could even try to draw it. And just so you know, you don't always have to do the mirror image on the right side. We can draw the mirror image on the left side. So if we want to draw its mirror image, it would look like this. You would have a fluorine, carbon, carbon, chlorine. You have your two hydrogens, and then you would have a hydrogen here, and then you would have your bromine here. No matter what you try to do, if you try to flip this around or whatever, you will never be able superimpose this on top of this, so these are enantiomers. These are both stereoisomers relative to each other. And either of these, regardless of which one you pick, are chiral molecules. I'm over the time that I normally want to go in the video, so in the next video I'll do even more.