 Just one minute. Yeah, you see optical isomerism is what again the same basic thing is same same molecule same molecular formula, but different behavior. But behavior if you talk about here. What we have observed that there are some compounds. Okay. There are some compounds through which through which if the light passes through. Okay. Okay. So when the light passes through it is then it is a phenomenon that we observed. Right. So first you try to understand what is optical isomerism. Okay. So what happens if a light passes through a given compound. Right. Then we observe what that this light get deflected like this, either in clockwise direction or in anti clockwise direction. Or it is also possible that this light passes through without any deflection like this. That is also possible. Okay, without any deflection. So when we have this observation, this one or this one, then the compound is said to be optically active. Okay, this deflection shows this deflection in the light. Deflection shows optical activity of the compound, optical activity of the compound. Yes. This yellow when there is no deflection, it means the compound is said to be optically inactive. So first of all, this observation we get or we found this observation with some of the compounds. And this phenomenon, the deflection in the light is called the optical activity of the compound that we are using this light that we are using here. This light is plain polarized light. I'll tell you what is this plain polarized light. This is plain polarized light. So the deflection in the plain polarized light when it passes through a given compound. Then the given compound, it's said to be an optically active compound. So it is the optical isomerism is nothing but it is the behavior towards the plain polarized light of a given compound. Understood. Okay, so plain polarized light is what you can understand by the name itself that the light is polarized along a along a given plane like parallel lights we have like this. Okay, this is the light we use parallel lights we use along a given plane. Because suppose if you have a light source, correct. So this light source radiates lights in all direction. Right this in all direction the light we have. Right. So suppose directly if you allow this light to pass through the organic compound. Right. So obviously we have light in all direction over here. So when the light comes out from this, whether it deflects or not we cannot say because here also we have the light in all direction. So with this light we cannot understand or we cannot conclude that the light is getting deflected. That's why what we do we convert this light into plain polarized light. Okay, and then this plain polarized light is allowed to pass through a given organic compound. If you observe some deflection either clockwise or anticlockwise, then the molecule is said to be optically active. If there is no deflection, then the molecule is not optically active. Did you understand the logic here the concept did you get it. Tell me, guys. Yes. No no no it's not chirality. It's not chirality. I understand what I said. Yeah, I'll explain. See when you have a compound suppose I have this compound, and I'll ask you whether this compound is optically active or not. And what you will do you'll take plain polarized light PPL and you allow this PPL to pass through this given compound. If there is deflection in plain polarized light, then the compound is said to be optically active. Got it. So what we have done why first of all what is plain polarized light, and why they're using this why not we are using normal light here, because you're observing this deflection. So we should have light in all in in one single direction, then only we can say okay it was along x. Now it is going on into XY direction. So there's deflection. But if we have light in all direction, then we cannot observe whether there is any deflection or not. That's why from a normal light source, right from a given light source, we have to convert this into a plain polarized light. How do we do that one simple method I'll tell you what is that, but that is why we are using that is the reason we are using plain polarized light here. Okay, so what happens and don't draw this I'll just give you what you need to draw. Actually what happens. We have a light source. And from this light source, it travels light emits in all direction, electromagnetic radiation in all possible direction light goes. Okay, now this light source from this the light we allow to pass through a Nicole prism. Nicole prism is what I'll tell you. It is a Nicole prism. When this passes through a Nicole prism, this light which is there in all direction. Now it is polarized along a single plane like this. This we call it plain polarized light PPL in short. Now this plain polarized light, we allow to pass through an organic compound which we need to figure out whether it is optically active or not. If we observe some deflection in plain polarized light, either clockwise or anticlockwise. If you observe this deflection. Then it is, then the compound is said to be optically active. Is it clear. This is the phenomenon here, and this entire setup is done. The entire thing is done in a device called polarimeter. So we have this device in which we have a space we have an inlet there where we can place the compound, and then from one side we can inject the light source from the light source we can inject the light. And then we have a Nicole prism which convert this into PPL, then all these things happen. Nicole prism is what you can understand this. It's like a, you know, you must have observed the bar, you know, fixed in the window like this. The iron rod or any bar you must have seen it fixed in the window like this. Yeah. Is it. So in Nicole prism also we have ultra fine slit like this. Okay, parallel ultra fine slits. Okay. Suppose a light is not parallel to the slit that we have over here that won't process through the Nicole prism. I suppose we have a, I'm just just trying to make you understand we have a slit like this suppose ultra fine slit we have like this. Now all those light which are parallel to this can pass through. If one light is going like this in the in a direction perpendicular to this. Like this suppose the light is going it will collide with this and reflect back if any other direction collide with this and lift it back only that light which is parallel to this slit like this one. This can passes through the slit. Did you understand this. So in Nicole prism. Long in the sense can show long in the sense the length of the Nicole prism. No that is not required like whatever the amount of light goes that is more than enough the diameter is such that the light passes through and then whatever light it passes through this. It just enough to detect whether the compound is optically active or not. Right. So the thing is this ultra fine slits through this only the light passes through and we have many ultra fine slits like this it is present. So all those lights which passes through the Nicole prism. It is along a same plane that we call it as plain polarized light. Did you understand this entire setup. Yes. Now this is just for your understanding that what is optical activity often compound. Obviously when you get the question in the exam you won't have all these things like it's not like the compound you will take and you'll go to the lab and perform the experiment and say whether it is optically active or not. This is that you cannot do. So it is just for your understanding, theoretically how do we get to know that the given compound is optically active or not. That is what we have to understand. Right. The phenomenon is this optical activity. So let's go down optical isomerism. Right down the compound having similar physical and chemical properties. The compound having similar physical and chemical properties. The compound having similar physical and chemical properties. It differs only in the but differs only in the behavior but differs only in the behavior. Towards the plane polarized light are called optical isomers and the phenomenon is optical isomerism. Compounds having similar physical and chemical properties but differs only in the behavior but differs only in the behavior towards plane polarized light are called optical isomers and the phenomenon is optical isomerism. Clear. All of you have written. Yes. Okay. Now you'll see if the plane polarized light PPL rotates clockwise that is right side clockwise. Then it is said to be Dextro Rotatory Dextro Rotatory Dextro is the word which names the right side. Okay. So Dextro Rotatory represented by a small D small D or plus plus means also Dextro Rotatory. Plus. If the PPL rotates rotates anticlockwise that is towards the left. It is Levo Rotatory that is L or minus. Okay. This you must remember D and L. Obviously this is experimental thing. Okay we cannot say for a given compound it is Dextro or Levo till we do not perform that experiment. Are you getting me. Okay. Any compound it is given D or L. We cannot find out this logically or theoretically we cannot find out whether it is D or L. To find out D or L we have to perform that experiment. Suppose you have a compound AB and you will ask me so what is the optical behavior of this Dextro or Levo. Then you have to check this either you give this compound perform this in the lab and then check whether it is D or L. Or you have to refer some book in that book some book the data is given D L like that. Then only we can. So it is the experimental thing theoretically we cannot find out. Is it clear. Okay. And hence they won't ask you this whether it is Dextro or Levo understood. Now, so the experimental thing we have discussed like what is optical activity. No explanation of that you know what happens when it goes to clockwise anticlockwise why we take PPL what is Nicole what it does everything we have discussed but like I said. This information you must have but you know all these are experimental thing. They can ask you some theoretical questions on this right but this won't help you deciding a given compound is optically active or not. Right now to understand whether a given compound is optically active what we have to do right so before going into that we need to understand few terms. First of all, okay, now this is important right from this only you will get questions most of the terms that we are going to use the first term you write down Karen center. What is kind of center definition right down and sp3 hybridized atom and sp3 hybridized atom, having, having all four atoms or groups different is said to be Carol center. If carbon is the atom, then it is called Carol carbon, then it is called Carol carbon any doubt in this see the basic difference between stereo central Carol center is what stereo center was can be sp2 or sp3 but Carol center is only sp3 very important. We have an exception in this exception is what tertiary a mind is optically inactive. Once again, don't write this once again guys write on one note before this exception. Write down the note. If the molecule has if the molecule has only one Carol atom, if the molecule has only one Carol atom, it is optically, it is said to be optically active. If the molecule has only one Carol atom, it is said to be optically active. So, listen to me very, very carefully here. I am not saying that presence of one Carol center is the condition for optical activity. Are you getting me. I'm not saying this I'm not saying the condition of optical activities one Carol center should be present is not the condition condition for optical activities something else will see that. Okay, but we have observed when the molecule has only one Carol center, it is optically active. Once again, we'll talk about it once again. Let me first explain the concept. So what I was talking about condition of like of optical activity is not the presence of one Carol center. Right, but it is observed that only one Carol center if it is present only one, then it is optically active if more than one is there, then we cannot say we have to see other things in that case. We'll just keep that in mind. Then you write down this exception, tertiary amine tertiary amine is optically inactive this is the exception we have tertiary amine is optically inactive. If you look at this tertiary amine like nitrogen has three different alkyl group present R1, R2 and R3. And obviously we know it has one lone pair also. So if you look at the Carol behavior of this just now I said, if only one Carol atom is there it is optically active. So if you look at this molecule nitrogen is the Carol nitrogen here. It is sp3 hybridized, right. Having four different groups three different alkyl groups and one lone pair. So that's not the logic that I said that if one is present optically active, but this is an exception it is not an optically active. It is because of amine inversion. Amine inversion we'll talk about it later, but you consider this as this as an exception. Okay, for you know optical activity does not show optical activity. Okay, you must keep this in mind. Okay, what is tertiary acid? What is the formula prakul? Tertiary acid, tell me. And what is your doubt? I'll write down, wait. That's what I'm talking about. You're talking about meso that's what you've written meso there. Yeah, meso tertiary acid is not optically active meso compounds are not optically active. Because we have a plane that's what I'm telling I haven't discussed the concept like how we get to know that given compound is optically active or not. Okay, so you have asked this question a bit early, like we haven't done this concept, but you must take care of one thing that meso compounds are optically inactive, because of plane of symmetry present into this answer to your question is plane of symmetry is there. That's why it is optically inactive. Then you have so many questions again, why plane of symmetry? What is the other things we have. So that's what I said, we'll come to this one by one. Okay, let me first, you know, explain, like what is the condition for optical activity we have then you will get to know. Right, so have some patience, let me finish this you will understand. No, obviously not today we don't have that much time but yes once we finish all these things you will understand why meso compounds are not optically active. Okay, so this is one thing kind of center we have discussed sp3 hybridized atom with four different atoms or group attached to it. Okay. Second term we have that is chirality. Have you heard this term chirality. Have you heard this term chirality. Okay. What is the difference between chirality and chiral. See if you think that chiral center and chirality are the same thing, you are absolutely wrong. Okay, then name is very much similar chiral chiral center and all like that. But chirality is has nothing to do with chiral center. Okay, so this term chirality right down first of all has nothing to do with to do with chiral center, chiral center. This is the chirality is the necessary condition necessary condition for a molecule for a molecule to be optically active optically active. So if the molecule is chiral, it is optically active we can say it's not like color center is present. Hence it is optically active. If the molecule is chiral, it is optically active so condition is chirality it's not chiral center but again I'm adding one more line here. If only one carol center is present, then the molecule is said to be carol and hence optically active. So most of the time in 80 to 90% of the cases, you can solve the questions by checking how many carol centers are present. Right, one only one optically active. That's the one thing. The point is what is chirality right down when the molecule cannot be divided when the molecule cannot be divided into two equal half equal half. It is said to be carol. It is said to be carol means there should not be any kind of symmetry. No symmetry should be there. If symmetry is there across that symmetry we can divide the molecule into two half. Right, so condition for a molecule to be optically active that there is no symmetry present symmetry means we can talk about plane of symmetry. We can talk about center of symmetry and other things also we will discuss little bit of plane of symmetry and center of symmetry also will discuss that. Okay, so a given molecule, if you think of a imaginary plane or imaginary line across which you can divide the molecule into two half, then the molecule is said to be optically inactive. Right, carol molecules, next line, carol molecules, non superimposable, non superimposable to its mirror image, non superimposable to its mirror image. Next slide right down, I'll explain all these things okay terms I understand the new new terms we have here so you need to understand all these terms here. So superimposability, impossibility is the 3d phenomenon is a 3d phenomenon. If two molecules, if two molecules are superimposable are superimposable, superimposable. If a single molecule, it looks like a single molecule looks like a single molecule when placed over each other when placed over each other. Superimposability is what suppose you have a one, you have one molecule and another molecule if you place one over other. If you cannot identify that there are two molecules present it looks like exactly similar, exactly similar than the molecule is said to be superimposable. Correct. Hands are not superimposable. Right, if you place the hand hands are not superimposable to each other. Right. Okay, so when we can divide the molecule into two equal halves, right, two equal halves then obviously when you add the two like two molecules which is divided. If you place one over each other, they look like one molecule one single molecule because they are identical, right, both are identical actually. So it is what canal molecules, since there is no plane of symmetry, hence it is non superimposable. Like I'll give you one example of superimposability, then you will understand that what we are talking about here. Did you copy this all of you. Okay, one last example we'll see for today. Suppose we have a molecule say this one, we have done the portion for your school, optical activity or optical isomerism, your school portion is covered. Okay, all these are comparative things. So suppose we have a molecule here CH3 OH OH CH3 H and H. What we are doing we are taking mirror image of this we have we place a mirror like this here. So it's mirror image would be like this. This two are superimposable. Okay, you can also understand the reason here you see this is a mess or compound because we we have a plane of symmetry over here. Like this if you cut the molecule to half. We have a plane of symmetry and this kind of compound we call it as meso compound. He has asked, that's why I told this here. And since we can divide the molecule into two equal half, we have a plane of symmetry, it is optically inactive. Don't write this meso and all, we'll discuss that later. But this is a mirrored image of this. Now what happens, this molecule and this molecule are superimposable. Superimposability we can understand, we can rotate the molecule, we have to place this molecule over this. By any means, rotation is also possible. Rotation possible means what? You can rotate the molecule in the same plane. Like you can rotate the molecule like this. Like you are, no, like the car guy's steering. Okay, you can rotate it like car guy's steering, correct? But out of the plane rotation is not allowed. Means you cannot flip it. Flipping is not allowed. Are you getting me? This is not possible. Could you respond guys, all of you? Yes, flipping is not allowed. Like car steering, you can rotate the molecule to check whether the molecule is superimposable or not, right? So what we do here, you rotate it by 180 degree. This CS3 will come over here. This OH will be here. This H will be here. This OH will be here. This OH will be here. And H will be on this side. And when you, after rotating 180 degree clockwise, you place this on the other molecule on its mirror image, it is exactly same. Yes or no, right? It will just come over each other. Like OH will come over OH. H will come over H. CS3 will come over CS3, correct? So this is superimposability of the molecule. Did you understand superimposability? Clear? Right? So if it is an optically active compound, its mirror image is non-superimposable. The reason is because we do not have any plane of symmetry into that. I'll give you one example. You see, carbon case, suppose I bought CS3 here. I'll take here OH. I'll take here H. I'll take here CL. Here's an example. Okay? This molecule, you see, it is optically active molecule. Why optically active? Because there's only one caron, when only one caron, sorry, carbon, which is carol here. SP3 hybridized carbon. It is a carol carbon here because it has four different groups attached to it. Simple. Now what happens if you have only one carol carbon with four different groups attached to it? We cannot find any kind of plane of symmetry. You think of any symmetry, you won't get it. There's no plane of symmetry in this molecule. And it is true with all the molecules which has only one carol center. That's why you can also check presence or absence of carol center. If it is only one, yes, optically active. If it is more than one, then we do not know. We cannot say. We need to check the other thing. Then in that case, you need to check whether the molecule is carol or not. Tell me, this carbon is carol or not? This carbon is carol or not? Yes. Yes, it is carol. Because it is, first of all, SP3, one, two, three, and fourth group is this bigger one. This carbon is carol or not? Yes, it is also carol. So this is two carol center. Two carol center is still it is optically inactive because we have plane of symmetry over there. Point I'm trying to make, what happens here? This carol is carol. Suppose it rotates a plane polarized light clockwise 15 degree. Randomly, I am saying clockwise 15 degree. This is Ulta. It will rotate the plane polarized light clockwise, anti-clockwise 15 degree. So overall, when the PPL comes out, we do not observe any deflection into that. So its counter here, its counter here. Yes. So that is what the thing is. I know we have done a lot of things today. Like it is a kind of topic that we need to analyze, we need to think, we need to go slow to understand all these things. Again, like next class also will take up from this only. I will discuss all these things again. Superimposability, carol, carbon and all. But for today's session, you must keep this in mind that the condition for optical activity is the molecule should be carol. Carol molecule has nothing to do with the carol presence or absence of carol center in the molecule. Carol center is different. Carolity of the molecule is different. Carolity is associated with the symmetry thing, plane of symmetry, center of symmetry. Carol center is nothing but it is an sp3 hybridized atom with four different atoms or groups attached to it. Is it clear? Yes. So tomorrow, obviously you have bio paper. After finishing tomorrow exam, NCRT, you must go through the superimposability, basically optical activity part. From NCRT, you must read. Otherwise, you'll get confused here, right? Not today, let it be. After finishing tomorrow's session, must read NCRT optical activity. They have taken the example of hand while the hands are superimposable or not, right? So we cannot rotate, like we cannot flip. We can rotate in the same plane, but we cannot flip. Out of the plane rotation is not allowed. So when you go through that portions of optical activity given in NCRT, you will have a fair bit of idea that what we have discussed today, okay? And then again, to the next class, we can take up from this. Without also, we can discuss. Yes, any doubt? Clear? Till here? Tell me, guys, clear? Okay, fine. Okay, thank you so much. See you in the next class. Next Tuesday, we do not have any class. Next to next Tuesday, we'll have class, okay? The normal schedule. Yeah, thank you, bye.