Chiral examples 2
Chiral examples 2. Created by Sal Khan.
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- I'm sort of confused on chirality, I understand the mirror image part, but if you were to flip the molecule (imaging taking your left hand and flipping it so your thumbs are together), then wouldn't it be chiral?(32 votes)
- No, borrowing from Jason a few videos back, "The hands are not superimposable. Imagine those two hands he drew have the palms facing away from you. If you were to rotate the left hand 180 degrees, yes it would have the same silhouette, but it would be palm up, whereas the right hand is still palm down. So no, they are not superimposable. A palm up left hand is not the same as a palm down right hand." -Jason Kanzler(132 votes)
- Is the first molecule an alcohol? I'm just wondering because it appears to be in the form of R-OH(6 votes)
- Yep! That molecule's 2-hexanol, otherwise called, Hexan-2-ol.(5 votes)
- The last molecule example confused me, because he said that the last molecule is chiral and then when he draw the mirror image he said the molecule wasn´t chiral?(4 votes)
- Remember that there is a difference between chiral ATOMS and chiral MOLECULES. Look again at the carbon atoms that Sal identified as chiral atoms. There are four different substituents/groups attached to those atoms, which is why those are both chiral atoms. But, he shows that when the molecule is rotated, it is still the same molecule. That's why even with the chiral ATOMS, the molecule itself is achiral. You'll see in later videos that this is what is called a meso compound - molecules that have both chiral atoms and a plane of symmetry. A plane of symmetry renders a molecule achiral. Let me know if anything's unclear!(8 votes)
- Well, If a CH is connected to a both CH2 and a F don't both CH2s make a axis of symmetry?(5 votes)
- If I'm understanding you correctly, yes the CH2's are symmetrical to each other in relation to the CH. However, since there are also two fluorines on only one side of the CH, that makes it NOT symmetrical. So the CH should be a chiral center.(4 votes)
- Whoa whoa whoa: At ~6:30in this video Sal says that going different ways in this cyclo molecule will net us different results, thus making the respective carbons (from the starting points) chiral carbons. However, earlier in the video (at ~1:40) he said "we're not looking at the atoms, we're looking at the groups...". He then went on to say (at1:40), that the CH2/CH2/CH2/CH3 chain could be visualized as C4H9, citing this is (visually) considered a different group (rather than assuming it's just another CH2).
I'm assuming that the example given at6:30is a chiral carbon, because we hit groups earlier, or later, depending on which way we go, thus making mirror images impossible. However, is it the entire chain we're looking at (which would be written the same (i think): C5H8CF2) or is the fact that we're hitting the same chain, but different groups earlier/later within that chain, the determinant factor when naming a Carbon a chiral or not?
- Let’s number the C atoms of 1,3-difluorocyclopentane counterclockwise starting with the C atom at the 3-o’clock position. Then C1 and C3 are definitely chiral centres. The video, however, is a bit misleading. Despite what the video says, we are looking at atoms, not groups.
With 2 chiral centres, you can have up to 2² = 4 isomers:
1. Both C-F bonds as wedges
2. Both C-F bonds as dashed lines
3. One C-F bond as a wedge; the other C-F bond as a dashed line
4. One C-F bond as a dashed line; the other C-F bond as a wedge
Structures 1 and 2 are identical, but Structures 3 and 4 are a pair of enantiomers. The video deals with Structure 1 (or 2). Let’s use Structure 1, because it puts the H atoms as dashed lines (“behind”).
We can assign priorities to the groups directly attached to C1: H, F, C2, and C5.
F is obviously #1; H is obviously #4. We now have to assign priorities to C2 and C5.
The atoms attached to C2 are C, H, H.
The atoms attached to C5 are C, H, H.
There is no difference. To decide between C2 and C5, we must go one atom further out.
From C2, C3 has F, C, H.
From C5, C4 has C, H, H.
C2 is therefore #2, and C5 is #3.
If you assign these priorities to the groups, the sequence 1 → 2 → 3 goes in a counterclockwise direction (S).
Note that we are not looking at the entire chain. We are going out from the chiral centre along a chain one atom at a time until we come to a first point of difference. Then we stop, even if we haven’t yet come to the end of the chain.
Note: Structures 1 and 2 have a plane of symmetry. It is perpendicular to the paper and passes between C2 and the mid-point of the C4-C5 bond. The molecule has two chiral centres, but it is achiral because it has an internal plane of symmetry.
Hope this helps.(6 votes)
- So the first one is an enantiomer?(3 votes)
- Yes.1:27Sal states that this is a chiral molecule then @3:03he states that the mirror images are not superimposable. This is the definition of an enantiomer.
- Isn't 1-3 difluorocyclopentane achiral?(2 votes)
- cis-1,3-difluorocyclopentane is achiral, but trans-1,3-difluorocyclopentane is chiral.(4 votes)
- At6:08, when Sal tries to figure out if the carbon is a chiral centre, how does he know he needs to break the ring at the CH (the one bonded to F)?(2 votes)
- Sal isn't breaking the ring ... he is just looking along the ring starting at that carbon to see whether the ring is the same in both directions.
When he says "break it out" @5:33what he really means is something like "draw it in more detail".(3 votes)
- so the chirality depends upon the group like methyl or butyl and not on the same carbon atoms?(2 votes)
- Chirality depends on having four different groups on the same carbon atom. For example, if you has a methyl group, a butyl group, a hydrogen atom, and a chlorine atom on the same carbon atom, that carbon atom would be chiral,(2 votes)
- Should molecules be superimposible in 3d or just 2d model that we often happen to draw?
And also please let me know rules for choosing position of mirror if any (like can we take mirror in-between the structure of a molecule or is it that it always needs to be at point outside molecule).?(2 votes)
- It is always 3D when we consider whether two molecules are superimposable. That is why we use dashed arrows and wedges when drawing molecules in 2D. It is the best we can do, on paper, to show the 3D structure.
The mirror should be outside the molecule. The only exception is that a mirror inside the molecule is useful for determining whether the molecule is meso. Meso means that a compound has an internal symmetry plane such that one half of the molecule is a reflection of the other half. This is easier to understand in a picture - https://goo.gl/images/TG4L3Q. This molecule has two chiral centres, but the top half is a reflection of the bottom half. For this reason, despite the chiral centres, this molecule is optically inactive. If you place the mirror outside of this molecule, then you would see that the mirror image is superimposable on the original structure.(2 votes)
Let's do a few more examples of seeing if either an atom or an entire molecule is chiral. So here I have a molecule. Let's see if we can identify any chiral centers, or any chiral atoms, or asymmetric carbons, all words for the same thing, although I guess you could have chiral centers that aren't necessarily carbon, but it tends to be carbon most of the time, especially in Organic Chemistry class. So if we look here, the one that kind of jumps out, this carbon right here is bonded to three hydrogens and another carbon. So this is obviously not going to be a chiral center. It's bonded to three of the same group. These three guys are all bonded to two hydrogens each, so they're all bonded to two of the same group, so they can't be chiral centers. This carbon right here is bonded to three hydrogens, once again, three of the same group, not going to be a chiral center. This one looks interesting. It looks like it could be a good candidate it for a chiral center, or a chiral carbon, or an asymmetric carbon. Over here on the left, it's bonded to a methyl group, so this is a methyl group, and here on the right, it's bonded to a butyl group. Over here, it's bonded to an OH, and then over here, it's bonded to an H, so this is definitely a chiral carbon. We could put a little asterisk here. That's how they often denote that this is a chiral carbon. And if this doesn't make sense to you, because you might say, hey, Sal, look, this carbon is bonded to two other carbons. Isn't that the same thing? But the point here is that we're not looking at what atoms it's directly bonded to. We're looking at the groups that it's bonded to. In this case, this hydrogen is a group and an atom. Over here, it's an entire group. It's an entire butyl group. We have four carbons here. We only have one over here. Another way to think about it, we could have drawn this molecule like this. We could have had a carbon in the center, and maybe this methyl group is popping out like this. You have your CH3, and then you would have this hydrogen coming out maybe in the plane, and then behind it, you would have the butyl group. So kind of the back leg of the tripod, you'd have a butyl, and what is that? That's C4H9, right? That's six plus-- C4H9, so it's C4H9 in the back, and then above it, you have your OH. Above it, in a different color, you have your OH. And when you look at it like this, it looks just like the other chiral carbons that we had identified in actually the last video. It looks very similar to something like this. And when you take its mirror image, this is the same molecule. Here, I kind of made it a little bit more three dimensional, but if you take the mirror image of either one, you're going to find that no matter how you try to rotate it or shift it, you won't be able to superimpose it on its mirror image for the same reasons as the other ones. And I challenge you to, if you can. So this is a chiral carbon. This is a chiral center, we could say. Or we could even call it an asymmetric carbon. It could be considered a stereocenter or a stereogenic center. All of those are valid things to call this carbon right there. And this is also a chiral molecule. Now let's look at this blue example right here. And if we wanted to name it, just so we get a little bit of review, we could start at this fluorine right there: one, two, three, four, five. This is what? This is 1, 3-difluorocyclopentane. So that was a nice review of naming. But let's think about whether we have any chiral centers here and whether the molecule as a whole is chiral. So the immediate ones that we can kind of dismiss-- and actually let me get rid of this numbering because I don't want you think that there are somehow three hydrogens there. That was the number three hydrogen, number three carbon, number two carbon, and so on so forth, But let me get rid of them now that we've named the molecule. I don't want to confuse how many hydrogens we have at any of these points. So let's look at the carbons. Well, we could immediately dismiss that carbon, that carbon, and that carbon, because each of those are bonded to two hydrogens. If we wanted to break it out, they would look like this. So they're bonded to carbons, carbons, and then they're bonded to hydrogens. Now, these might be different groups. These might be different types of alkane groups that it's bonded to, so that doesn't necessarily throw it out of the running. But these two, the two hydrogens that it's bonded to, are definitely the same atom, the same group. We have an axis of symmetry through that atom, so it cannot be a stereogenic center. It cannot be an asymmetric carbon. It cannot be a chiral center or a chiral atom, so we can knock those guys out of the running. But this guy and that guy seem pretty interesting. Because if we were to break it out a little bit, you could break it out like that and break it out like, so writing a CH and actually show the bond to the hydrogen. And this guy is bonded to one hydrogen, one fluorine. And then if we were to work our way around the cycle, and these cyclic molecules are a little bit-- it's sometimes a little tricky to identify whether you're bonded to the same group or different groups. But actually, let me not make it too messy while we try to figure this out. To figure out whether it's bonded to the same group, let's kind of take a walk around the cycle, around the cyclopentane ring. If we go this way, if we go on a counter-- we'll do it in different color. If we go in a counterclockwise direction from the carbon in question, we're going to hit a CH2. Then we're going to hit a CH. So we're going to hit a CH2, then we're going to hit a CH. If we go this way, we're going to hit a CH2 and then we're going to hit another CH2. So this guy is fundamentally-- this bond is bonded to a different group than that bond up there is. It's also bonded to a hydrogen, also bonded to a fluorine. So this is bonded to four different groups, so this is a chiral carbon, so that is a chiral center. Now, the exact same argument can be made for this carbon right here. You can make that exact same argument, that, look, if you were to walk counterclockwise from this, you'd hit a CH2, then a CH2. If you were to go clockwise, you'd have CH2, then a CH, which happens to be connected to a fluorine. So you're actually going to see something different, depending whether you're going down into that group or into that group. And then it's also bonded to a hydrogen and a fluorine, four different groups. This is also a chiral center. Another way to think about it, and it's actually interesting to compare it to this molecule up here, which was not chiral and did not have a chiral center, this molecule up here-- let me draw it a little different to make it a little bit more clear. So this one, I could draw it like this. If you have the chlorine like that, over here, we thought about this as a potential chiral center, and it's kind of playing the same role as in that example down here, but you see over here, this is not a chiral center because there's actually an axis of symmetry for this molecule that goes through that carbon. So you can actually just draw an axis of symmetry that goes exactly through that carbon. The way I drew it, it's not completely neat, but you can see that that is the reflection of that, if I were to draw the bonds actually a little bit more symmetric. Over here, if we try to do the exact same thing, if we try to draw an axis of symmetry over here, if we try to draw an axis of symmetry, we can make that bond to the fluorine go through our axis of symmetry,, we'll see that that still is not the reflection of this because we have a fluorine up here. We don't have a fluorine over here. Now, we can do the same thing with this end. If you try to do an axis of symmetry, fluorine up there, no fluorine over here. So each of these are definitely chiral centers, while this carbon up here was not a chiral center. Now, the next question is, well, this thing's got two chiral centers, two chiral carbons. It's probably a chiral molecule. Everything else we've seen so far, if you had a chiral center, you had a chiral molecule. But let's take its mirror image. To take its mirror image, let me clear out some real estate over here, So let me clear out this. Let me clear it out. So what's the mirror image going to look like? Let me draw first the mirror. So the mirror image, you're going to have a fluorine over there. Then you're going to bond to a carbon, which is also bonded to a hydrogen. That's going to bond to a CH2. That's going to bond to a CH. That's the mirror image of that, which bonds to a fluorine. That's the mirror image of that. And then you go down. This is the mirror image of CH2 here. This is the mirror image of this. You connect them. Now, these are mirror images of each other. But they are also the exact same molecule. I could just literally move this guy over to the right, and it would be superimposed. They are exactly the same. So even though we have two chiral atoms, two chiral carbons, the molecule as a whole is not chiral. It is a non-chiral molecule.