 So, hello guys. Good morning. Today in this session, we are going to understand a very important concept of organic chemistry and that is substitution reaction. Okay, so there are two types of reaction mainly we have. It's not like only two types, but two types of reactions only we have, which affects the, you know, which those type of reactions takes place in chemical, in organic chemistry. Okay, just a second. So, today in this session, we are going to see substitution reaction, right? It's a long chapter. Okay, it's a long topic also. There are so many things, you know, we need to understand, to understand these kinds of reactions. But since this is, you know, oriented to oriented towards KVBY, so we'll only discuss the important topics here. Okay, so we are going to see substitution, substitution reaction. Okay, so if you look at the name, if you look at the name, it with the name only you can understand that there is substitution taking place, right? But substitution of what? That's the question. Substitution is taking place here. So first of all, what is a substitution reaction? Okay, so basically, when you look at the reaction, any example if I show you, okay, so suppose if I take this example, we have a compound, a molecule CH3Cl, CH3Cl, and this reacts with a compound of iron called SH-, SH- you can consider similar to, similar to OH-. Okay, SH-, OH-. All these are similar, so you can consider that. Okay, so if you see this to reaction, so in this what happens, this SH- is able to replace Cl- from the reaction, okay, and forms thiol as the, okay, and forms thiol as the product. So the product here depends upon what nucleophile we are using here. Okay, so this reaction takes place and this converts into CH3SH and Cl- goes out. Cl- goes out. So if you look at this reaction, this SH-, it displaces the chlorine over here. Okay, and suppose I am taking some solvent here, H2O. This reaction takes place in this solvent. Basically, the purpose of solvent is what is the reaction of, the medium of reaction. This molecule and this iron present in the solution and the reaction takes place. Medium it is basically. So what happens here? This iron substitutes chlorine in this molecule, hence it is a substitution reaction. Hence it is a substitution reaction. Okay, so what all things are there? You see this compound where the reaction takes place? This compound we call it as substrate. The compound on which the reaction takes place this SH-, this attacks on to this substrate, this particle. So this is attacking particle, attacking particle and this negatively charged attacking particle is nucleophile. This is obviously the product we have and this that goes out here, this is the, this is the leaving group we have, leaving group because it leaves from this molecule, it goes out leaving group. What is this H2O? This H2O is the solvent. So basically there are four different types of term that we are using here. We are using substrate, we are using attacking particles, we are using solvent, we are using leaving group. All these terms affects the substitution reaction. Okay, like I said there are different, different types of substitution reaction we have, 7A types of substitution reaction we have. Okay, and all these reactions depends upon these four factors, substrates, attacking particle that is nucleophile, solvent and leaving group. Correct? So this is what we need to understand, to understand the substitution reaction. Okay, so overall if I write down this reaction, the reaction is in general expression if I write down the reaction is we have RL the substrate and we have a nucleophile represented by this. I have taken here negatively charged but it's not like nucleophiles always negatively charged. It can be neutral also with lone pair of electrons. So either negative charge or neutral, possible. Okay, this is one type of attacking particle. We have another type also possible in presence of solvent. Yes you, RNU, the nucleophile displaces the leaving group and leaving group. Overall the reaction is this, right? Substitution reaction. This is what it is happening. Okay, this attacks over here and this goes out like this. Okay, so the substitution reaction like I said there are many different types of substitution reaction we have. But for KVPY, we are going to focus on only two that is SN1 and SN2, SN1 and SN2. Only these two we are going to focus on. Okay, SN1 and SN2. But before going into this, let us discuss these four factors here, substrates, attacking particle, solvent and leaving group. Okay, in short we will discuss the brief discussion we will have. So first of all, we write down substrate. So what are substrates? Substrates are generally the molecule which takes part in the reaction like supposedly I will write down here CH3Br. Right, we can also take a tertiary alkyne halide. Secondary also we can take BR. Okay, we can take benzyl group. Suppose this with CH2X, there are many other examples of substrates. Basically the molecule which taking the part in the reaction. So it depends on many. Basically you should focus on this is 1 degree alkyne halide, 3 degree, this is 2 degree and this is benzyl. Benzyl halide. Now the second one is the leaving group is the leaving group. Leaving group is the one which leaves into the reaction. Like I have given you the example CL- and L- that is the leaving group. Okay, now if I, if you see this reaction, okay the general reaction that I have written already, the reaction is RL plus Nu nucleophile react in presence of solvent RNU plus L minus. Now if this negative charge on this leaving group is stable, okay, then it is a good leaving. If negative charge is stable, then it is good leaving group. Good leaving group. LG represents leaving group. Okay, so more tendency to go out, more will be the rate of reaction. Okay, and tendency will be more to go out when this negative charge is stable. How do we find out the stability of negative charge? For that we have various effects, electronic effects, right, inductive effect, mesomeric effect and resonance. With all those points we can understand the tendency of this leaving group. Okay, but few orders you should all, you should know that the order of leaving group is this. Like if you talk about halogens, okay, chlorine is the poor living group among halogens. Chlorine, chlorine, then bromine, iodine. Iodine is the best leaving group we have. Chlorine is also a good leaving group. Iodine, I minus is most stable here because the negative charge is spread over a large volume. That's why the order is this. Okay, now if I show you some example here, suppose we have CS3, C double bond O, O minus. Okay, CS3, S double bond O, double bond O, O minus. Okay, so this negative charge is more stable because of more resonance. You have resonance this side, you have resonance here also with this double bond, with this double bond and here we have resonance with this double bond. So because of this extra double bond O, the negative charge is more stable and hence the leaving group ability is this. The second one is a better leaving group. Okay, next one if you talk about OH minus and SH minus, again the science factor is dominating here. Right, so SH minus is more stable, a better leaving group. If we talk about CH3O minus and phenoxide ion O minus, here we have resonance, this negative resonance stabilized, a better leaving group. Okay, so like this we can compare the leaving group ability also of any group. Any neutral molecules are better leaving group than the charged one. This one also you must remember. Neutral molecules, molecules are better leaving group, leaving group than than the charged one. Okay, for example, if you are comparing OH minus and H2O, H2O is a better leaving group than OH minus. Okay, RO minus and then the conjugate base of it ROH is a better leaving group than this. Okay, pH O minus and pH OH is a better leaving group. Okay, RC double bond O minus RCOOH is a better leaving group. This you must remember for leaving group. Okay, next you see solvent, the third factor we have that is solvent. Solvent, we have two types of solvent. Why it is required? It is required for for the movement of particles, required for the movement of particles. Right, and when the particle moves, they collide and reaction takes, that's why it is important. Okay, now this solvent we have two different types of solvent. Okay, solvent we have two types, solvent we have two types. The first one is polar solvent and the second one is non-polar solvent, polar and non-polar solvent. Polar solvent again classified into two categories that is polar protic and polar uprotic. Non-polar solvent, example of non-polar solvent is we have CCL4, SO2, benzene, all these are non-polar solvent. Non-polar solvent, the dipole moment is zero for non-polar solvent. That is the property of this. You must have studied this in physics, dipole moment because of the vector quantity. Okay, polar solvent, the dipole moment mu is not equals to zero. Little bit I'll talk about this dipole moment just a second. Now what is polar uprotic? Polar uprotic solvents are solvents in which solvents in which hydrogen atom is attached with hydrogen atom is attached with electronegative element like oxygen, nitrogen, chlorine, etc. These are polar uprotic solvents. Example of this solvent is, example of this solvent is we have H2O. So basically all these discussions are not required. You just need to know what is polar uprotic solvent and what is polar uprotic. You just need to know the few examples of it. So polar protic solvents like H2O, we can have alcohol, oxygen-hydrogen bond, we can have phenol, oxygen-hydrogen bond, we can have acid, oxygen-hydrogen bond, we can have amine, nitrogen-hydrogen bond. Okay, many examples we have in this. Now the another thing that you need to memorize here, okay I'll discuss this later. Polar uprotic solvent is what? Polar uprotic solvent or the solvent in which there is no hydrogen and electronegative element for example CH3 H3O, CH3. This we call it as dimethyl sulfoxide, DMSO. You'll get this only written in the example. Dimethyl sulfoxide, ethers. Ethers are also polar uprotic solvents. Okay and there are many examples. Okay, these two are important. I'm just trying to make you understand the things for KVPY. Okay, I'm not going to do any stuff. Okay, this is the effect of solvents. Okay, now one thing you know that the nucleophilic substitution, the substitution reaction that we have and since we're taking nucleophile here, like I'll give you the example RL plus Nu gives RNU. So it is the nucleophile which substitutes here, which gets substituted here. This reaction is nucleophilic substitution reaction. The reaction is nucleophilic substitution reaction. You look at this example this one. I have written SN1 and SN2. So what is this SN1 represents here? Substitution reaction in the general term. It is more specific. This particle is nucleophile. This is nucleophile. This nucleophile substitutes here and eliminates the living group Cl. Hence, the reaction is nucleophilic substitution reaction. Right, so this stands for SN1 stands for first order, first order nucleophilic substitution reaction, nucleophilic substitution substitution reaction. Okay, what is this order? You don't focus on this now. Just let it be. It is an experimental quantity. You should know what this SN1 stands for. This stands for first order nucleophilic substitution reaction. Similarly, this one stands for second order nucleophilic substitution reaction. Like I told you already that solvent also has effect on these kinds of reactions. So when you take polar protic solvent, okay, this one polar protic, this one favors SN1 reaction. Polar protic solvent favors SN1 reaction. Okay, polar aprotic solvent, this one, it favors SN2 reaction. It favors SN2 reaction. And since this is the nature we have for these two solvents, that's why you should know some examples of polar protic and polar aprotic. Again, I am telling you this SN1 and SN2 reactions are very competitive reactions. In all the reactions, we cannot say that only SN1 is taking place. Right, so there will be some amount of SN2 also, but we'll talk about the one which is dominating, which gives the major product. Okay, so any reaction in which the SN, the substitution nucleophilic substitution reaction is taking place, there we have the tendency for both type of reaction, SN1 and SN2 both. Okay, SN1 and SN2 both. The one which gives the major product, according to that only we say that this reaction goes under this particular mechanism, SN1 or SN2. Okay, so this is the third factor we have that is solvent. Now the last one we have here, that is, that is nucleophile. Again guys, I'm telling you this is not that important for your exam. Okay, this part because there are so many things here that we should know, we have to study, but I'm just giving you a brief idea of it. If you get any questions on this in case, you could find out the possible answers there. Okay, so nucleophile like I said it is the attacking particle. What is a nucleophile? It is either neutral or negatively charged, negatively charged, attacking particle, attacking particle. Okay, if it is neutral, then it must have known where like for example we have water, neutral nucleophile, phenol, neutral nucleophile, alcohol also, neutral nucleophile. Look at this reaction here. Suppose if I write down this CS3O minus plus CH3Cl, it gives CS3O CS3 and Cl minus leaves out. So how does this reaction happen here so far? You see this is more electronegative. We have discussed this, eye effect. Chlorine is more electronegative. So this chlorine attracts the bond pair of electron towards its side and this becomes delta negative and this becomes delta positive. Now with this we're trying to understand a little bit about the reaction, how the reaction takes place. Now when this becomes delta positive, so negative charge on this oxygen, this attacks onto this carbon since it is delta positive. And when this attacks over here, the Cl minus takes this bond pair of electron and goes out as a leaving group and we get this. This is nucleophilic substitution reaction. Now when you talk about this reaction, that is CH3O minus plus H2O, so reversible reaction, it converts into CS3OH and OH minus goes out. So here this one is not a nucleophilic substitution reaction. This one is acid-base reaction because why it takes X plus from the solvent and forms this. So if you look at the property here, both ions are same but this one is behaving as a nucleophile and this one is behaving as a base here. So there is a difference in nucleophile and base. Nucleophiles are better electron donors. They can give electron to this. And base city is tendency to accept H plus. It has less tendency to give electron pair to this. This is the one thing. So these are the four factors we have here which affects the rate of these reactions. So let us understand what is SN1 and SN2 reaction. So first type of nucleophilic substitution reaction we have that is right down the heading SN1 reaction. So like I said, it is first order nucleophilic substitution reaction, first order nucleophilic substitution reaction. Okay, the general term is what if you look at the reaction, it is very important to understand this because both SN1 and SN2 looks like similar if you do not understand the mechanism of it. Like suppose if I take here RL and some nucleophile in presence of solvent, it gives RNU plus L minus. Now you will definitely observe when we start discussing SN2 here then also I will write down the same reaction. This reaction only I will write down. Right, so if you do not understand the mechanism, mechanism means step by step description of a reaction. If you do not understand it, you won't understand then what is the difference between SN1 and SN2. So let us first understand the various steps involved in this reaction, that is the mechanism. Okay, so what happens the RN leaving group bond that we have, it is delta positive and delta negative. We have discussed this when we have alkyl halide, RCL, right, previous example we have seen. So this carbon and halogen bonds or nucleophile bond, it dissociates into finally dissociate, it converts into R plus and L minus. This takes this bond pair of electron and forms. Right, now this step is the slowest step we have, slowest step and it is RDS, rate determining step. Okay, RDS stands for rate determining steps. Okay, rate determining steps. So there are various steps involved in any reaction. The steps which is the slowest step is called the rate determining step. Means if you find out the rate of this reaction, we will consider this reaction, not the other one. Always we consider the slowest step. All this information I am giving you, correct. Now in the next step what happens, the second step of the reaction, the R plus and on this R plus, the nucleophile that you are taking, this nucleophile, this nucleophile attacks onto this R plus and forms a bond with the carbon atom which has the positive charge, R10. So this is how the product forms. Okay, it is a two step reaction and the product forms this overall the reaction is the solution. Now one very important point here that this R plus, it is the carbocation it is forming, carbocation forms. More stable carbocation, more will be the rate of the reaction. Okay, more stable carbocation, more will be the rate of the reaction. Now you will see when this carbocation forms, it means the leaving group must be very good. Then only it will go out and forms a carbocation. Right, must be very good. So property of which favours this reaction is what? That first of all write down characteristics of this reaction. So first of all, the carbocation forms here or I will write down the rate of the reaction, rate of the reaction, I will write ROR. The rate of the reaction is directly proportional to the stability of carbocation, stability of carbocation. Like I said SN1, SN2 both are competitive reactions. Okay, if there is tendency of forming stable carbocation, then it will be SN1 reaction. Why not SN2 will discuss that. Okay, but rate of the reaction is directly proportional to the stability of carbocation. Okay, rate also it is directly proportional to the nature, good leaving group. If the leaving group is very good, it can easily form carbocation and hence SN1 reaction favours. It is also inversely proportional to the basic strength of the leaving group. Basic strength means electron donating power. Okay, how it can take H plus from this. Okay, if this is a good base, this will easily attack onto this arpeggio sample. Even it won't wait, this nucleophile won't wait for this carbocation to form. Once the partial charge developed, this will attack onto this. This will attack onto this and the leaving group. But since it is SN1 reaction, so we have to have this step also, which is a formation of carbocation. So we should take the nucleophile, the strength of the nucleophile in such a way, the basic strength of this, that it wait for the carbocation to form. If it is very strong, the basic strength is very high for this nucleophile, then this won't wait for this fully developed carbocation to form, for this carbocation and this will attack on the partial charge only like this, we have discussed in the previous cases. So the basic strength of nucleophile should be weak so that it will wait for the carbocation to form and then it will attack on this. So this is the reason we have why the basic strength is weak. Now one very important thing here that is the, since the carbocation is forming, so the second property is the rearrangement of carbocation also possible, rearrangement of carbocation possible. And we do this rearrangement and this rearrangement happens, rearrangement happens in order to get more stable carbocation, more stable carbocation. This rearrangement is possible with hydride shift, hydride shift, methyl shift, methyl shift, phenyl shift, phenyl shift, etc. We will have these two, which is important. SN1 reaction, I have already told you, what is that shift I'll discuss, hydride shift management, I'll discuss that. I have already discussed polar-protic solvent is required for this, this I would suggest you must memorize this. Polar-protic solvent is required for this purpose. Okay, order of the reaction, I have already told you it is 1, SN1 it is order of the reaction is 1. So these are the few properties of this SN1 reaction. Okay, few properties of SN1 reaction. Now you look at this example and with this example we'll try to understand the hydride and methyl shift. Suppose we have a compound, consider this one, CS3C, CH3H, CH3CO. This when reacts with H2O. So here what happens you see this is the substrate and this is the nucleophile. Oxygen has no unfair, so it is nucleophile. Now what happens in this you see that this carbon has delta positive charge because it is delta negative, nucleophile. Right, so in the first step what happens the Cl- goes out and we'll get a convocation here which is this. CH3C, CH3H, CH3CO. Plus Cl- the leaving first step is this. In the second step what happens the H2O nucleophile which is this H2O. Oxygen has no unfair, it is a nucleophile. This has the tendency to attack on to this right but this actually won't happen here. If you consider this okay you can write down one product but that would be the minor problem. First of all I'll write down the product here. This attacks on to this and this converts into CS3C, CS3H, CH, CH3 and here we have H2O. Since oxygen loses its lone pyrrolechron so it has one positive charge. Positive charge on oxygen is not stable so to stabilize this one of the hydrogen here loses its electron to this oxygen and comes out as the H+. Okay it gives this electron here and H+. So the product of this reaction would be CH3CH, CH3, then CHOH, CS3. So you are getting an alcohol from alkali. Now this nucleophile could be anything. It can be anything for example that we have seen. We can have any nucleophile over here. So depending upon the nucleophile you'll get the final product. For this case we are getting an alcohol. Okay but this won't be the final product of this reaction. Okay because the carbocation is forming here. This carbocation goes under rearrangement. Okay see it is not like the rearrangement is always possible. Okay the purpose is to get the more stable carbocation. Okay so for that we'll do this rearrangement. If we don't do it actually in the reaction it happens on its own. It's not like we allow them to go under rearrangement. Wherever it is possible it goes under rearrangement forms the most stable carbocation of that molecule and then the nucleophile attacks. Right so first we'll get the most stable carbocation here and then the nucleophile attacks onto the same carbocation like it attacks over here and then it gives the final product. So here what happens the rearrangement takes place and rearrangement gives you the more stable carbocation which is nothing but this. You see one of the hydrogen from this takes this electron pair and it's rearranged itself onto this carbon. This is hydride shift one to hydride shift. So it goes under hydride shift hydride shift and forms this. What does it form? You see this here. It forms CH3, CCH3, CH2, CH3 and this carbon which loses this hydride ion gets a positive charge. Now you see you can compare the stability of this carbocation and this carbocation here. This carbocation the first one is getting stabilized through hyper conjugation here and the number of alpha hydrogen here would be 3 plus 1 that is 4. Here the number of alpha hydrogen would be 3 plus 3 plus 2 that is 8. So obviously more alpha hydrogen this one is more stable. Right so this is 1 comma 2 hydride shift gives you more stable carbocation. If further by any rearrangement we can increase the stability of carbocation we'll do that. It's not like only one time the rearrangement is possible. We can have n number of times but the only purpose is in every step you will be getting the more stable carbocation. In every step you'll be getting the more stable carbocation and then only it goes on. Otherwise more stable to less stable it won't go. Less stable to more stable it's possible. Okay now we get this carbocation here now you see one more thing here since hydrogen gets shipped over here it is hydride shift. Suppose we do not have hydride here but we have methyl group like this then 1 to methyl shift. If you have phenyl then phenyl shift. Right in this case not possible but depending upon the atom or group attached to it we call it as hydride shift, methyl shift, phenyl shift. Right what is the purpose of this shifting? The purpose the only one purpose is in every step we should get the more stable carbocation. Right that is the purpose. Now you look at this product in this reaction so finally we get this carbocation as the most stable carbocation in the given example this carbocation. Now this is similar now as the previous case we have water behaves as nucleophile. So this will attack onto this carbon atom and it converts into CCH3, CH2, CH3, O, H, H positive charge on it and hence finally H plus comes out from this carbon atom to stabilize the oxygen this bond pair comes over here and it converts into CCH3, CCH3, O, H, CH2, CH3. So this product is the major product of the reaction. Why major? Because it forms from the most stable carbon atom, major product. The previous one this product is the minor product of the reaction because it forms from the less stable carbocation, the less stable carbocation. Okay so what is the overall reaction here? The question was this if I write down the overall reaction we have discussed the mechanism of it but overall the reaction would be this CH3, CH3, CH2, CH3, CL with H2O. Correct? That the two product here we get is one is major other one is minor. The major product is you can directly you know do this if you do some practice you'll get it easily. First of all the carbon with halogen will get a positive charge here right and then one to hydride shift will get a positive charge here. This positive charge gives you OH on this carbon atom and this positive charge gives you OH on this carbon. Okay the two possible product here we get is we have CH3, CCH3, O, H, CH2, CH3 plus the another product is CH3, CH3, CH3 and O. This one is major product and this one is minor. Okay so this is the overall reaction like this the reaction proceeds. Correct? So this is SN1 reaction. Always take care of one thing that in SN1 reaction will get carbocation first and then the nucleophile attacks on this and wherever the carbocation forms we always try to get the most stable carbocation. Okay one more property I forgot to write down here. The one more property you write down it is a two step reaction, two step reaction. Okay in the first step the no the carbocation forms and the second step the nucleophile attacks. Okay the rearrangement of carbocation all these things we consider in first step formation of carbocation is the first step. Okay guys I hope you have understood this okay there are like I said so many things that we should understand you must have some doubt also over here right but those things we could not do because of the time constraint I'm just trying to give you the basic idea of these reactions so that in that exam if it comes you can have an idea of that question and then you can give it a try at this. They are not going to ask you very tough questions. Few properties of leaving group and the polar protein solvent, polar protein solvent you should know the examples of these two based on that we can have the product in the reaction. Okay so thank you guys thank you so much next class we will start with Sn2 reaction okay we'll see Sn2 and then we'll see the various examples of both Sn1 and Sn2 and that is it for substitution. Okay thank you so much.