 So first thing we are going to understand is structural isomerism and first part to write down chain isomerism in a structure. Two or more isomers, definition of chain isomers right now, two or more isomers which has different number of carbon atom, which has different number of carbon atom in the parent chain. Different number of carbon atom in the parent chain are said to be chain isomers of each other. Two or more isomers which has different number of carbon atom present in the parent chain are said to be chain isomers of each other. Molecular formula must be same that is the condition right. We can define isomers with same molecular formula. For example, you see if I write down N-pentane, N-pentane is what? CS3, CS2, CS2, CS3. If I write down this compound, CS3, CS2, CS3, CS3, CS3, CS3, CS3 and CS3. Can you tell me the IOPSE name of this one? This is normal-pentane, N-pentane. I will take the name of this one, two methyl butane right. How many carbon atoms are there in the parent chain? One, two, three, four. How many carbon atoms are here? How many carbon atoms are here? This is two methyl butane. And this is two comma two. Thyme-ethyl-propane. Part new-pentane. You see all these three compounds, the number of carbon atom in the parent chain is different. What is the molecular formula for this one? C5S2L. The molecular formula of this one? You see all these molecules have same molecular formula but their structure is different. One has five carbons in the parent chain, four and three. All these are chain isomers of each other. We can say one and two are chain isomers, two and three are chain isomers, one and three are chain isomers. All these are chain isomers of each other. Why? Because they have different number of carbon atoms in the parent chain. Similarly, butane and isoputane also chain isomers of each other. Number of carbon atoms in the parent chain is one, two and three. Here we have one, two, three and four. So the number of carbon atoms in the parent chain is different. With the same molecular formula, the molecules are said to be chain isomers of each other. Let's write down position isomers. Two or more isomers which has different position of functional group, different position of functional group, locates or multiple bonds are said to be position isomers of each other. In all position isomers, the number of carbon atoms in the parent chain must be same. In all position isomers, the number of carbon atoms in the parent chain must be same. What is the name of this compound? We made two all. What is the difference here? The position of functional group is the first carbon. Here we have a second carbon. Number of carbon atoms in the parent chain is four. Number of carbon atoms in the parent chain is four. So we have four carbon atoms, butane one all, butane two all. The position of functional group is different. These two are position isomers of each other. See this example. These two are position isomers of each other. Beauty and beauty one in double bond composition is different, locates the position you can see here. If you write down chlorine at this carbon and chlorine at this carbon. These two are also position isomers of each other. What about this one? They are not isomers because molecule formula itself is not same. But when you place this CS3 here, then the earth is just a position isomers. So all these are position isomers. Number of carbon atoms in the parent chain must be same. One more point to write down here. The monobelant functional group. Write down monobelant functional group like COOH, carboxylic acid. Monobelant functional group like COOH, nitride, CN, aldehyde, acid halide. Does not show position isomerism. Monobelant functional group like carboxylic acid, nitride, aldehyde, nitride, aldehyde, acid halide. In all these functional groups, we always start numbering from the carbon atom of the functional group. So these functional groups will always get first position. Positioning always is the first thing. That position won't change. That's why they don't show position isomers. What about this two compound isomers? What kind of isomers we have? Suppose we have CH3, CH2, CH2, CN, CH3, CH, CH3, CN. What kind of isomers? Chain isomers. What is the name of this compound? No, they say it's a compound. The carbon inside it won't count, right? CN, CN won't count. No, we count in the polyfunctional group that we don't count. Monobelant functional group will count. Butane nitrile it is. First question here. Nitrile if it is there, we are bound to start the numbering from this carbon atom. So here also we have to start from this carbon atom. That's why you see the position is not possible because the position of the functional group is what? It's always one, right? But the number of carbon atom in the principal chain is what different? These are what? These are chain isomers. If the number of carbon atom in the parent chain is different, then that won't be the position like this. Similarly, this example you see. What about this one? What is the name of the second one? This has the chain name. It's 2-carboxic acid. No, why would he say that? The carboxic acid is the same type of adenuoy gas. What is the name of that? From the carbon gas. So I see it's a two carboxic acid. One percent group will count in the parent chain, no? Also I thought you see one percent group will count in the parent chain, no? No, I see it's a carboxic acid. Two carboxic acid is the same type of adenuoy gas. That's why you have more than one chain. One condition is better. Not only that, if you don't visit them they'll get you the adenuoy gas. that condition is, if the COH present on the ring, common glitches are all bruised, if it is not bruised, then it will not work, we have to count in the parent chain, we have to start numbering from this carbon, 1, 2, 3, so propanoic acid, second position we have methyl, 2 methyl propanoic acid, what about this one, 1, 2, 3, propanoic acid, so obviously the number of carbon and the parent chain is different, can't be position isomers, so these two are chain isomers. So, all these functional groups you see, we are bound to give first position to these functional groups, hence they want to show position isomers. So, what about 2 COH groups on a ring? So, tag are also equal. So, but then you have to specify, right? 1, 2. No sir, what is its 5 ring? 5 ring, position is the numbering you have to do accordingly, so that you will get the least possible you know. Sir, but then you can get different numbers for their different positions, right? Like what if there are 2 carbons apart or only 1 carbon apart or on the same carbon. No, no, no, you see, what you are talking about, when this carboxylic acid, we are talking about acyclic compound, ring, that's why ring will consider in ring chain isomers, for that we have one different criteria, okay? Sir, but here we consider the C L ring. Where? Now, in this one spine you see here, if you write down COH here, right? Or COH here? These two are position isomers of ring, why? Because this is not present in the ring, we have to give first position to the carbonyl atom, here also since the parent chain is 6 5 member carbonyl atom, 5 member carbonyl atom, that's why it is position isomers. But in this thing, we have to start from this carbonyl, we have to start from this carbonyl, but they won't super visualize, okay? No, sir, but Sir, I thought you said that COH can never form a positional isomer, but where it is? No, that means, that means if COH discarbonates them, if you consider in the parent chain, and that we have to do if it is acyclic, no, not acyclic. Okay, third type you write on functional isomers, functional isomers. Two or more isomers, two or more isomers, two or more isomers, which has different functional group are called functional isomers, are called functional isomers, okay? For example, alcohol and ether, we'll write down the example of alcohol and ether, alcohol and ether can show functional isomers. The example is ethanol and dimethyl ether, the first one that you took, CS3, CS2, OH, and CS3, OCS3, alcohol and ether, functional isomers, okay? Again you write down alcohol and phenol, alcohol and phenol functional isomers of each other. Alcohol, you see it is, we have a ring, CS2, OH, this compound and OH here, CS3. What is the name of this compound? The first one, it's 1, ethylene methanol, because this is the functional group attached with it, so this becomes the variant chain, right? And this is substituent, phenyl and this is methanol, so one phenyl or simply phenyl methanol, okay? This one is methyl phenol, right? Methyl phenol, you see the molecular formula is same. This one is the derivative of alkyl derivative of phenol and this one is the, what we can say, alcohol, right? OH on the ring is phenol, OH with alkyl group is alcohol, right? So this is alcohol, this phenol, derivative of phenol can show functional isomers of each other. It is methyl phenol or we can say 1, 2, 3, 4 methyl phenol, paramethyl phenol also we can say, okay? But ortho-paramethane, we don't write it in, aldehyde and ketone, for example, you see CS3, CS2, C double bond OH and aldehyde and ketone, CS3, C double bond OH, CS3. Again, functional isomers, functional group is different. Amines are also functional isomers of each other. Amines, CS3, CS2, CS2, NH2, it is 1 degree of mine, right? RNH2. What about this one? CS3, CS2, N, CS3, H, it is 1 degree or primary amine, it is 2 degree or secondary amine, you can check the molecular formula is same. Or if I write CS3, N, CS3, CS3, tertiary amine, all these three compounds are functional isomers of each other, okay? So, primary, secondary, tertiary, amines are functional isomers of each other.