 can we start? Let me know. So as we were discussing about what? We were discussing about the chiral carbon. And the next one more term I have written over here that is chiral carbon is one term and there is another term that is chirality. This chirality is the property of a molecule is the property of a molecule, chiral carbon and chirality has nothing to do with each other but they are completely different from each other. So now the point is coming back to this question when a little bit more information I let me give you when a molecule contains only one then it is it is optically active. When the molecule contains only one chiral carbon then it is optically active. The essence or essence of chiral carbon is not any criteria a molecule to show optical to show optical activity. How many of you understood these two points? How many of you understood this? I will just come back to this one again. If I ask you if I give you a molecule here suppose in the question in the exam a molecule is given a molecular formula is given. And you have to find out whether this molecule is optically active or not. For a molecule to show optical activity the molecule must rotate the plane polarized light. I am not discussing each and every point of optical isomerism here. I am giving you a little bit of idea. When we do this optical isomerism we will discuss all these things in detail. The point I am trying to make here it is what that suppose a molecule you have to find it out whether it is optically active or not. The definition for a molecule to show optical activity is that it must rotate the plane polarized light when it passes through it. So the point is when you have a plane polarized light what is plane polarized light you let it be we will not discuss it now. You just understand we have a light over here. It is polarized in a single plane that is what it means. So we will discuss this in optical isomerism. So when a plane polarized light passes through a given molecule there are three possibilities. One is what the light passes through as it is without any deflection. So when there is no deflection the molecule is said to be optically inactive. OI stands for optically inactive. But if you have a molecule and this molecule if you allow this plane polarized light to pass through PPL and if it rotates or deviates in anticlockwise or clockwise direction then the molecule is said to be optically active. What do you define optical active nature? It is the property of a molecule to deviate the plane polarized light from its original direction. That is the optically active molecule. Now since this is the experimental thing you have to do this experiment in the lab. We have a device called polarimeter. From that only we use this kind of experiment correct. So what polarimeter is required for this. So obviously if I give you one molecule if you get one molecule in the exam in the question paper and if they ask if they ask I will discuss that Ashutosh wait wait I will discuss that wait PPL. If they ask that you know whether this molecule given molecule is optically active or not then obviously in the examination hall you cannot perform this experiment correct. This is the experimental thing right. So we have another way to find out the optically active nature of the molecule and that nature is the presence or absence of symmetry plane of symmetry and all okay. And that property we call it as chirality. So chirality is the property of the molecule by which you can divide the molecule into two equal half right. You cannot divide the molecule into two equal half. If the molecule is given and if there is no plane of symmetry the molecule is said to be chiral molecule right. If the molecule is chiral it is optically active okay. And again chirality has nothing to do with chiral center presence or absence of chiral center. Did you understand this? PPL is what suppose when you have a light source correct. This light source actually reduces light in all direction light is going in all direction. We have a system that is there in exist in wait, wait, wait, I'll explain Preethi wait. So you have a light source and if you have a system called polarimeter. So in this polarimeter there is an arrangement. What arrangement is that I'll tell you. In that there is slit kind of arrangement is there okay. Thin slit we have here like this okay. Parallel thin slits are present like this. It is just like a window like when you see the window there is a rod there is a bar like this it is there right. So if suppose you are going to pass this light from this particular you know slit that you have here. The light which is parallel to this these slits will only pass through yes or no. It is a bit you know difficult to explain over here in online class. See one thing you can understand if one light which is like this parallel to this slit or this slit. These lights will pass through the slit right. One light which is in this direction like which is going like this in this direction. This light will collides with this bar that you have here slit and reflects back correct. So when you have a slit like this so like another side of this slit you have all the lights which is parallel to each other like this. You will get all the lights which passes through this slit will be parallel to each other like this fine right. This light we call it as what it is the plain polarized light because it is polarized along a single plane. I hope it is clear now correct. Tell me fast did you understand this what is PPL okay. Simple thing is what it is the light which is polarized in one single plane correct and for that we use a system here that is polarimeter. In polarimeter we have an arrangement of slits like this which allow all those lights which is parallel to this slit only. All other lights which is not parallel that will reflect back okay and the other side of the slit will get all lights which are parallel to each other this is plain polarized light. Now this will discuss this into the class okay optical isomism will discuss we are deviating from the topic okay little bit of information is required here that's what I'm giving it over here okay. So we'll discuss it in the class if don't waste your time over here. Now when this polarized light is allowed to pass through the molecule which you have to find out whether it is optically active or not then these two condition will be possible in this case it is optically active and in this case is optically inactive okay. Now the point is this experiment all these things we cannot do into the examination hall okay. So we have a alternate way to find out the molecule to be optically active or not right. So for that we have a property of molecule that is chirality. Chirality is the property of the molecule by which the molecule cannot be divided into two equal half okay. If the molecule cannot be divided into two equal half it means there is no plane of symmetry there is no symmetry present in the molecule. When there is no symmetry present then the molecule is optically active this is what we are going to do to solve the question. Understood this correct. What is chirality? Chirality is the property of a molecule by which a molecule cannot be divided into two equal halves correct. If the molecule is chiral and when I say molecule is chiral it means it has nothing to do with the chiral center remember I am repeating this point again and again all asymmetrical molecules are chiral you can see you can save her okay. The point is chirality is different chiral center is different chiral carbon is different okay both are not related at all okay. So chirality is the necessary condition necessary condition for a molecule to be optically active. Means if a given molecule you have to find out whether it is optically active or not you have to check whether the molecule is chiral or not. How do you check the molecule is chiral you must have to check we should not have any plane of symmetry into this. Yeah correct we do not have any plane of symmetry then only when there is no plane of symmetry then only the molecule is chiral if the molecule is chiral it is optically active it is clear chiral optically active and all yes or no tell me fast correct. Now coming back to the last you know the previous thing here what I have written here you see a molecule contains only one chiral carbon then it is optically active okay and just know what I said chiral carbon the necessary condition for a molecule to be optically active is chirality. Chirality is the necessary condition and chirality and chiral carbon are completely different from each other okay. So here you see the next line but the presence or absence of chiral carbon is not any criteria for a molecule to be optically active okay. So obviously what you have to do here that whenever a molecule you have to judge whether it is optically active or not we will check chirality we won't check chiral carbon but the fact here is what if the molecule has only one chiral carbon right listen to me very carefully if the molecule has only one chiral carbon present then it is optically active it has been observed right that all the molecules which contains only one chiral carbon it is optically active okay for this molecule if I ask you whether it is optically active or not okay by looking at the molecule since it contains two chiral carbon we cannot say it is optically active or not because the number of chiral carbon are two right but if the number of chiral carbon is one then we can definitely say that the molecule is optically active do you understand this Sukit does not like chiral molecules why so is it clear am I clear with this point that presence or absence of chiral carbon is not the condition the condition for a molecule to be optically active is chirality only but if the molecule has only one chiral carbon it is optically active right why have why it happens you see this car this carbon has one chiral carbon here this molecule right this carbon is the chiral carbon because it is sp3 hybridized and four different groups are attached we are coming back to the topic now did you understand did you understand this chirality chiral carbon and all can I explain this okay so now you see this molecule has one chiral carbon if it has one only one chiral carbon by any means if you try to find out any plane of symmetry in this it is not possible you don't have any plane of symmetry into this that's why it is optically active okay so to sum up all this the necessary and sufficient condition for a molecule to show optically activity is chirality right what is chirality it is the property of a molecule by which a molecule cannot be divided into two equal half right second point is this third point chirality has nothing to do with chiral center or chiral carbon third point is this right presence or absence of chiral carbon is not any criteria for a molecule to be optically active fourth point is this okay and the last point is what however chiral carbon is not any criteria but if the molecule contains only one chiral carbon it is optically active is it clear now all of you understood this okay anyways we'll discuss this again into optical isomerism okay so we'll let it be for here only now coming back to this stereochemistry of sn1 reaction this molecule is what this molecule is optically active why because it has only one chiral carbon present right now when this leaving group goes out this sp3 converts into sp2 right now when the nucleophile attacks onto this since it is a planet molecule so this nucleophile can attack from the front side or from the back side another way if I say OH-ion can attach onto this molecule from the same direction in which the X was present or from the opposite direction also both way possible okay so when the direction is same as the X was present over there then the configuration of the molecule will be same I'll write down into the next page you see you see the molecule we have here is this then we have phenyl here CH3 base we have OH- and here we get phenyl we have here and here we have CH3 positive charge onto this carbon atom right this is the formation of carbocation first step now under this carbocation the nucleophile suppose we have OH- will attack now the point is this OH- can attack from the top or from the bottom right two product are possible here one product is what when this is the first product we get here OH attached from the opposite side the two groups are same here pH and CH3 or the another possibility is what OH group attached from the same side from this of this leaving group so the another molecule will be COH phenyl CH3 okay now you see if you compare this molecule and this molecule the only difference is what instead of X we have OH okay so when the OH will attach from the same side of the leaving group it will retain its configuration this is the retention of configuration the configuration will be same as the molecule we have here when the OH group attached from the opposite side we say there is a inversion of configuration inversion of configuration the configuration will get reverse now here what happens you see again it is a no concept of optical isomerism that we will not discuss now it takes time okay so we'll get two product here one is inversion and other one is retention product both product will have opposite configuration opposite configuration means what that we define in terms of R and S what is R and S you don't ask me we will discuss this in optical isomerism okay but opposite configuration means what if one molecule can rotate the PPL I am randomly writing this okay rotate the PPL if one molecule can rotate PPL clockwise right then other molecule will rotate the PPL rotate PPL anti-clockwise you understand my point that is what the meaning of opposite configuration okay did you understand till here tell me first what is R and S I will explain that into optical isomerism because because to understand that you should know CIP rule priority order and all many things are there that's why I'm not going over there okay the point here you have to understand is what the SN1 reaction gives you two product one product will have the same configuration as the molecule we have and other product will have the different opposite configuration okay now when I say configuration means what the behavior towards the PPL will be same as the given molecule okay if the behavior is same then we say the retention of configuration is there when the behavior is opposite then we say that the inversion of configuration is there okay so if this molecule according to this if retention is anti-clockwise it means this molecule also rotate PPL rotate PPL in anti-clockwise direction okay so these two retention and this gives you inversion okay now the thing here it is what since this is a planar molecule sp2 hybridized so ideal condition if you say which is not true practically right but ideal condition if you take then the OH- since it is what it is planar so the possibility of this OH- to attack onto this carbocation is equally from the top and then from the bottom right the probability or the tendency for this OH- to attack onto this carbocation is equal from the top and from the bottom right so we will get in this case we will get equal amount of this and equal amount of this correct so suppose if this is product A if this is product B right so you will get equal moles of A and B equal moles of A and B so now you see you have a mixture because it is a mixture you have a molecule and in the mixture you will get these two finally so in the mixture if you have A plus B now in this if you allow this PPL to pass through then what happens when it comes out from this mixture will it deviate or not if you take this mixture which is equimolar mixture we have this equimolar mixture of A and B if you take and we allow this PPL to pass through then whether there is any deflection or deviation in this PPL we observe or not yes or no it should not