Class 12 Chemistry (India)
Introduction to chirality (handedness), and how chirality is related to the groups bonded to a central carbon. Created by Sal Khan.
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- You take the mirror image of the left hand, and it looks like the right hand. As he demonstrated, you turn the left hand 180 degrees so that the thumb is on the left side. How does that not superimpose the right hand? On the right hand, the thumb is on the left side, and so is the left hand flipped 180 degrees? Same thing for the CFCH3Br molecule. If you flip the 180 CFCH3Br molecule 180 degrees, shouldn't the Br be in place of the CH3 in the original molecule, instead of the H? Thanks(48 votes)
- 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.(107 votes)
- At minute1:12, Sal establishes that opposite hands are enantiomers of one another, but he says they are not "superimposable". Technically you can superimpose anything onto anything. I think the phase should be "not superposable", which means the hands do not coincide exactly when they are superimposed.(6 votes)
- I believe you have missed the point, or he didn't elaborate enough:
Non-superimposable merely means "not the same in 3D".
the point is not to just stack something on another. that is pointless
the idea is that you can not overlay your left hand on your hand (it's mirror image) with thumbs overlapping (superposing) thumbs and or pinkies with pinkies (and no you can't turn one hand upside-down and put them together like you're praying). The fact that right and left handed gloves exist proves this. They will never be the same in 3-D. That is what non-superimposable means. And that is why chirality matters. (Enzymes are the gloves).(22 votes)
- Out of curiosity, what is the name of the molecule he's drawing at4:10?(2 votes)
- what is stereochemistry?(3 votes)
- Stereochemistry is the study of the arrangements of atoms in space. It includes such topics as cis/tans isomerism, chair/boat conformations, and optical isomers — anything that involves the 3-D arrangement of atoms in space.(6 votes)
- Surely it is super imposable on its mirror at the beginning if you rotate it in the 3rd dimension? or is this just a model that only works in 2D?(2 votes)
- Not at all. Your right hand is a mirror image of your left hand. But no matter what you do in three dimensions, you're not going to fit a left-handed glove on your right hand or vice versa and have it fit. That is what superimposable means...are they identical? Of course not, otherwise gloves would be sold like most socks.(8 votes)
- does this mean that if there are no chiral atoms present, the molecule is not chiral ?(2 votes)
- No. Both responses are incorrect.
1. There are chiral molecules that have no chiral centers. Look up "allenes"
2. This is what I believe the second contributor meant. Cis and trans double bonds are sterogenic centers in that two different compounds are possible that are stereoisomers (diasteromers). However, that is not sufficient. You must have two cumulated alkenes (c=c bonds next to each other) with at least 2 substituents to have "axial" chirality.
There is also helical chirality. The DNA double helix is chiral. It is a right handed helix.
That is another topic however.(7 votes)
- Are all stereoisomers optically active?(3 votes)
- While all chiral molecules are optically active, not all stereoisomers are chiral molecules. When a stereoisomer is achiral/not optically-active, it is called a meso-compound. Definitely check out the video on meso-compounds for more information!(2 votes)
- And is chiral the same as optically active and achiral the same as optically inactive?(3 votes)
- To put it simply, chiral molecules are optically active and achiral molecules are not. I would not use the terms interchangeably, however, because enantiomers (chiral molecules) are not the only type of optically active compounds.(2 votes)
- I actually cannot understand why will the molecule be not superimposble (at5:36).(2 votes)
- He is arguing, correctly, that the images are NOT superimposable because the carbon is chiral.(3 votes)
- Is flipping on either axes allowed for superimposition? If yes, then the hands and the compound Sal made here, are superimposable.(2 votes)
- You can do whatever rotations you want but the point is for something to be chiral it cannot be superimposed on its mirror image.
Your hands cannot be superimposed on one another, if you line your palms the same way (as in put one hand over the other, palms down) the fingers are reversed, if you line your fingers up then one palm is facing down and one facing up. Do you agree with this?
The same is the case for the molecule. If you line any 2 of the groups up (Br, F, CH3, H) on both molecules then the other two will not line up. The molecules are chiral.
This image might help you see it better: https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/3369/2018/06/19202942/enantiomer2.png (taken from: https://courses.lumenlearning.com/suny-potsdam-organicchemistry/chapter/4-1-chirality/)(3 votes)
If I were to draw a hand, and let me just draw a hand really fast, so I'll draw a left hand. It looks something like that. That is a left hand. Now, if I were to take its mirror image, let's say that this is a mirror right there, and I want to take its mirror image, and I'll draw the mirror image in green. So its mirror image would look something like this. Not exact, but you get the idea. The mirror image of a left hand looks a lot like a right hand. Now, no matter how I try to shift or rotate this hand like this, I might try to maybe rotate it 180 degrees, so that the thumb is on the other side like this image right here. But no matter what I do, I will never be able to make this thing look like that thing. I can shift it and rotate it, it'll just never happen. I will never be able to superimpose the blue hand on top of this green hand. When I say superimpose, literally put it exactly on top of the green hand. So whenever something is not superimposable on its mirror image-- let me write this down-- we call it chiral. So this hand drawing right here is an example of a chiral object. Or I guess the hand is an example of a chiral object. This is not superimposable on its mirror image. And it makes sense that it's called chiral because the word chiral comes from the Greek word for hand. And this definition of not being able to be superimposable on its mirror image, this applies whether you're dealing with chemistry, or mathematics, or I guess, just hands in general. So if we extend this definition to chemistry, because that's what we're talking about, there's two concepts here. There are chiral molecules, and then there are chiral centers or chiral-- well, I call them chiral atoms. They tend to be carbon atoms, so sometimes they call them chiral carbons. So you have these chiral atoms. Now, chiral molecules are literally molecules that are not superimposable on their mirror image. I'm not going to write the whole thing. You know, not superimposable-- I'll just write the whole thing. Not superimposable on mirror image. Now, for chiral atoms, this is essentially true, but when you look for chiral atoms within a molecule, the best way to spot them is to recognize that these generally, or maybe I should say usually, are carbons, especially when we're dealing in organic chemistry, but they could be phosphoruses or sulfurs, but usually are carbons bonded to four different groups. And I want to emphasize groups, not just four different atoms. And to kind of highlight a molecule that contains a chiral atom or chiral carbon, we can just think of one. So let's say that I have a carbon right here, and I'm going to set this up so this is actually a chiral atom, that the carbon specific is a chiral atom, but it's partly a chiral molecule. And then we'll see examples that one or both of these are true. Let's say it's bonded to a methyl group. From that bond, it kind of pops out of the page. Let's say there's a bromine over here. Let's say behind it, there is a hydrogen, and then above it, we have a fluorine. Now if I were to take the mirror image of this thing right here, we have your carbon in the center-- I want to do it in that same blue. You have the carbon in the center and then you have the fluorine above the carbon. You have your bromine now going in this direction. You have this methyl group. It's still popping out of the page, but it's now going to the right instead of to the left, So CH3. And then you have the hydrogen still in the back. These are mirror images, if you view this as kind of the mirror and you can see on both sides of the mirror. Now, why is this chiral? Well, it's a little bit of a visualization challenge, but no matter how you try to rotate this thing right here, you will never make it exactly like this thing. You might try to rotate it around like that and try to get the methyl group over here, to get it over there. So let's try to do that. If we try to get the methyl group over there, what's going to happen to the other groups? Well, then the hydrogen group is going-- or the hydrogen, I should say. The hydrogen atom is going to move there and the bromine is going to move there. So this would be superimposable if this was a hydrogen and this was a bromine, but it's not. You can imagine, the hydrogen and bromine are switched. And you could flip it and do whatever else you want or try to rotate it in any direction, but you're not going to be able to superimpose them. So this molecule right here is a chiral molecule, and this carbon is a chiral center, so this carbon is a chiral carbon, sometimes called an asymmetric carbon or a chiral center. Sometimes you'll hear something called a stereocenter. A stereocenter is a more general term for any point in a molecule that is asymmetric relative to the different groups that it is joined to. But all of these, especially when you're in kind of in introductory organic chemistry class, tends to be a carbon bonded to four different groups. And I want to to stress that it's not four different atoms. You could have had a methyl group here and a propyl group here, and the carbon would still be bonded directly to a carbon in either case, but that would still be a chiral carbon, and this would still actually be a chiral molecule. In the next video, we'll do a bunch of examples. We'll look at molecules, try to identify the chiral carbons, and then try to figure out whether the molecule itself is--