 If you get it, you win. Okay, stop talking. Okay, see, stop talking. Suppose we have one molecular formula, CT-H6O. This molecule, you see. Can you write down the structure? Can you write down the structure of this molecule? What is the structure of this? What is the structure of this one? Something's missing. So what is the cycle of the molecule? The total takes one of them is an O. Cyclic? Cyclic with three carbons and one O. Possible? No, what is the O we like outside the ring? I mean, it works. How many structures are possible? How many structures are possible? No, it can't be. No, it can't be. How many structures are possible? Harris, what does that mean? Keep it inside. What is the structure possible for this? So it has two, like Keto and Icy. Keto and Icy, how do you know that? How do you know that? C2H5CH2. As a TOU, we can calculate. Oh yeah. We can calculate TOU degree of unsaturations. What is TOU for this? It's a... It's a... You don't work even if you have an O. Yeah, we don't count that often. So it's worthless only for ring and double bonds? Yeah. It gives you the number of unsaturations actually. Ring is also kind of unsaturated. What is DOU? Katanya? Abhinav? Harris? No. You don't even want to think also. Okay. We have two different structures possible for this. Okay. If I talk about the acyclic structure. The one is we can write CS3, C double bond O and CS3. Right. You see the molecular formula is in C3 and C4. Another one, you can also write CS3, CS2, C double bond O. Okay. These two structures are possible. Even if I take this one, C2H6O. For this also two different structures possible. One is alcohol, CS3, CS2, OH. And other one is ether, which is CS3O, CS3. Correct. Right. So the point is these two molecules are entirely different molecules. One will have alcohol, sorry, ketone as a functional group. And this is anti-hyde as a functional group. Right. So obviously the two different molecules we have a different functional group. So their physical and chemical properties will also be different. Right. But their molecule formula is same. Right. So we can write this molecule as two different structures. One is ketone. Other one is anti-hyde. This molecule we can also write as two different molecules, which is alcohol and ether. So these kind of molecules are. So for the fossil, what's wrong with the cyclic thing? You can draw cyclic also. This is also fine. If you take this one. No. This is also fine. You can draw the possible structure is this. Okay. But I am talking about, I guess, with DOU it gives you one. So one gives you only two possible ways. So one ring we have to do is what? Ring is, one DOU means what? You can have one pi one. Or one ring. One pi one or one ring. In this one pi one, one pi one. This structure is possible. It's not wrong. So with oxygen, is DOU's formula the same one? Yes. You just keep oxygen as another carbon. Yes. Just you neglect oxygen. Just you take it as carbon, hydrogen, halogen, nitrogen. If oxygen is present, ignore that. It's not required. So what was the formula? C plus one minus H plus X minus one by two. DOC might have done it wrong. You are saying H is written in capital. H plus X minus one minus N. Minus N. Wait. H plus X minus N divided by two. Okay. This is the start of the deal. So in DOC, you have to give it a formula. So in the chemical formula, you have to give it a formula. I used to give it a formula. No, sir. Okay. This is the formula. For this one, if you calculate DOU, you will get zero. That's why I have drawn the structure where there is no pi bond present. One is alko, another one is either. Right? So this gives you an idea that whether we have double bond present or not in the molecule. Correct? So these molecules, which has the same molecular formula, but they are different. But they are, you know, physical and chemical properties are different. So these kind of molecules, we call it as isomers of each other. Right? Isomerism is what? Isomerism is a phenomenon. In which a given molecule can exist in two different structural units. Right? Two different structural formula. Correct? So these are the isomers of each other. Isomers, we always define in a pair. We cannot say this molecule is isomer. It's not possible. This is the isomer of this. This is the isomer of this or vice versa, we can say. Okay? Right on isomerism, it is a phenomenon. It is a phenomenon in which a given molecule, a given molecule can represent can represent into, can be represented into two or more, two or more structural unit, two or more structural unit in which their physical and chemical properties are different. In which their physical and chemical properties are different. Next slide. All these different structural units, all these different structural units are called isomers of each other. Example right now, these two examples are right now. Are called isomers of the classification. These two examples are right now. Two types of, two major classification we have in isomerism. One is structural, structural isomerism and other one is stereo isomerism. You have done this chapter in the school. Finished. No, isomerism, one of the things you have done is isomerism. You have done and confirmation? No. Structural isomerism for the classified into five or six different. The first one is chain isomerism. The second one is positional isomerism, functional isomerism, then we have metamerism. Next one we have ring chain and the last one are tautomers, tautomers. Tautomers is very important for competitive examiners. We leave whatever you see, tautomers is very important. All these, these are very basic. You have definitions, examples. Sometimes they ask me the question, these two molecules are, shows what kind of isomerism. Chain positional and also definition if you know you can find it. This is one part of structural or part of isomerism. Stereo isomerism again classified into categories. That is, configuration and confirmation. Configurational again we have two types, that is geometrical, geometrical and optical. Basically for exam point of view, tautomerism is very important, geometrical, optical, confirmation, all these are important. Optical isomerism is there in a chapter in 12th standard that is coordination compound. Geometrical, optical we have there also. So usually what they do, they mix the concept here and the coordination compound, the chapter will do in 12th standard. They mix the two concepts in the last question. Last year we JMA Equations were there. That is why this chapter is very important because this application is again there in 12th standard. So we will discuss all these first and then it is geometrical, optical and confirmation. See confirmation isomers are what? I will just give you a little bit of idea. Geometrical isomerism is because of unsaturation present in the molecule. For example, suppose if I draw this molecule C double bond C, Cs3H, Cs3H. What is the name of this compound? If I ask you write down the structure of but2n, write down this structure or you write down this structure. Both the structures are different. Why they are different in nature? Whatever distance we have here between the two methyl group is suppose we have L1 and here it is L2. So this L2 is greater than L1. Now because of this difference in the distance between the two methyl group, what happens that the steric hindrance if you consider here, it will be more than this because two bulky group if it is close more hindrance will be there. If it is far apart the hindrance will be less. So here the hindrance is less, here the hindrance is more. So stability factor if you consider this one is more stable because of better packing and less hindrance. Trans molecule will already have better packing. That's why they have more bending point also. All these properties we will see later. But this is geometrical isomerism. Geometrical isomers because of unsaturation double bond. Why these two molecules are different because we have hindered rotation over here. We cannot rotate C, if you rotate this carbon atom this way you will get this compound. But this rotation is not possible and why it is not possible? Because this pi bond forms because of lateral overlap. Suppose this is a sigma bond we have and pi bond forms like this. This overlap is pi bond. If you want to rotate one of the carbon atom you have to break this pi bond which is not possible. You cannot break the bond. So this molecule is different, this molecule is different. Because this rotation is hindered because of pi bond. But if you write this compound C H single bond C H, C H 3 H, C H 3 H. And this one is C H single bond C H, C H 3, C H 3 H. These two compounds are same or different? Same compound. However this C H is in this side but we have a single bond here. So rotation is not hindered. We can rotate all of the carbon atom. So these two compounds are exactly same compound because they have one single constant. So how do we know when there will be interference and then we will have a hydrogen bond form. Like in the first case with lesser distance. Can't we say that hydrogen might take an electron and give an electron. But that's only with electronegative element right? Hydrogen bond is possible when hydrogen atom is bonded with the electronegative element like chlorine, oxygen and nitrogen. Mainly these three. To some extent it's chlorine also it is possible. It's sulphur also it is possible. So here there is no hydrogen bond. And it's not like you take an electron. The condition you provide. So we always take an electron and give it. None of the compounds will be stable there. So it's not possible. Again, it's not possible. It's a story. I'm telling you that story. This one time we were watching a movie. First if you see the movie you'll understand 50% of them. So then you have this 60%, 70%. That's why for competitive exam. Organic chemistry you have to finish twice. That's when you have to do it. So first time you have to take it as a virtual story. You feel like you understand everything. But when you get the question, you get the answer. So it's not that easy. Because acceptance or the way of approach the subject. That is very different from the physical chemistry. So that is very important. So the point is it's not like hydrogen will take an electron and bond. We have solvent, temperature, reaction condition, everything is there. So this is geometrical isomerism because of double bond. Optical is the behavior of a molecule towards plane polarized light. What is plane polarized light? I'll discuss that in detail. But the molecule has a nature when the plane polarized passes through a molecule. It either rotates in clockwise or anti-clockwise. Suppose light is going through the molecule. It deviates either in this direction or in this direction. So when deviation is there, the molecule is said to be optically active. There are molecules through which if the light passes, it comes out without any deflection. So those molecules are what? Optically inactive. Optical activity is the behavior of a molecule towards TPL, plane polarized light. If the question you get in the exam is whether this molecule is optically active or not. You won't do this experiment. Why don't you go to your lab and perform the experiment and check whether deviation is there. It's not possible. So there is a definition. How to find out whether the given molecule is optically active or not. We have a different way. We will see how to do that. Conformational isomers is again the rotation of one carbon atom across carbon-carbon single bonds. When you rotate this, the relative distance of methyl-methyl group or hydrogen-hydrogen will change. Suppose we have two methyl groups present like this. Exactly opposite to each other. Now when you rotate this, the distance of the methyl group is changing. So it keeps on increasing and increasing and then decreases. So when this distance changes, this alters the property of the molecule also. So same molecule with different properties with this rotation. We call it as conformers of each other or conformational isomers. So all these three things are very important. Studio isomers is more important in this chapter. We will discuss all these things later. So first we will see this. Sir, so those top two compounds with the single bond and it rotates. Those are not the same compounds because they are conformational isomers. In this one, we have taken the same, this position and this position. If you see, their property will be same because the alkane, their dipole moment will be zero. Relative distance will also be same. When I do this in the detail, you will understand. CS3 will be the same. So when we do this in the detail, you will understand that compound is same. But when you change the position of CS3, their position will be different. Here I am not changing the position. I am just giving you a molecule. But in alkane, we can define conformational isomers. We can define conformational isomers in alkane. We have this example. Conformational isomers are ethane, propane, butane one is the most important one. We will get four different conformers over there that we will discuss.