 Hello friends, good morning. Welcome back to Centrum Academy. Today in this session, we are going to discuss about resonance. In the previous session, we have discussed what is you know, GOC, what is inductive effect and what are the applications of that. We have seen different, there are two main applications of inductive effect, which is nothing but the stability of intermediates. And the second thing is what? Acidity and basicity. So today in this session, we are going to discuss the second type of electronic effects, which is nothing but resonance. In this resonance only, we will discuss mesomeric effect, which we also call it as M effect or R effect, resonance effect. So what is resonance? First of all the question is, resonance is what? The delocalization of pi electrons or lone pairs. So the resonance, if I write down in one single line, resonance is the delocalization pi electrons or lone pair, pi electrons or lone pair of electrons. You also call it as the method of representation. It will also, we can also write it as, resonance is the method of representation of one molecule or ion into more than one structural units. And what is the difference in all these structural units? Let us write here, in which the position of pi electrons, lone pair and lone pair are different. The different is structural units that we draw here for a given molecule or ion. In all those structural units, the position of pi electrons and lone pair are different. That is the difference we have here in all these structures. And all these structural units, we give it a specific name. So these structural units, we call it as a resonance structure or we also call it as canonical structure. Resonance structure in short, because we will use this term everywhere in this particular session. So from now onwards, resonance structure will write it as RS. So whenever I write RS, it means I am referring to resonance structure. So for example, if I write down few examples here and then we will see some more detail, we will do some more detail discussion of this resonance thing. So what is the meaning of this thing that I have written over here? Method of representation of one molecule or ion into more than one structural units. So to understand this, let us take an example. So the first example I am taking here is this one, CH3 C double bond O, O minus. Now you see the another structure of this, which is the resonating structure, we can draw like this. What happens? This lone pair on electrons comes over here and this pi electron will jump onto this oxygen. So the another form of this, we can write it as CH3 CO minus double bond O. This is the another structure. One more structure if I write here, that is suppose benzene. Now in this, the structure of benzene is this, we have alternate pi bond present. Now if I number these carbon atoms, suppose 1, 2, 3, 4, 5 and 6. So in this, if I write down the another structure of benzene ring, which is nothing but this, where the position is same 1, 2, 3, 4, 5, 6. And here the double bond is present here, this is the double bond present. So these two are also resonating structure of each other. What is the difference in these two structure? We have double bond present between 1 and 1st and 2nd carbon atom and here we have single bond between 1st and 2nd carbon atom. So these two are the resonating structure. We can also take one more example of carbonate ion. Carbonate ion is what? C double bond O, O minus. So we can draw three resonating structure of this. In first one what happens? This electron pairs comes over here, this pi electron will go on to this oxygen atom. So carbon O minus double bond O, single bond O minus. Another structure what we can draw? That this pi electron comes, lone pair comes over here and this pi electron will go on to this oxygen atom. C single bond O minus double bond O, single bond O minus. All these are resonating structures. So like this we draw the resonating structure. So these are the few examples. You see if you analyze all these structures one by one, the position of lone pair and pi electron is different here. Here suppose if I write down, this is the first oxygen and this is the second oxygen. So we have double bond between carbon and first oxygen. Here we have single bond. So position of pi electron is different. Here the position of pi electron between first oxygen and carbon. Here between second oxygen and carbon. The position of pi electron is different, resonating structure. Position of double bond between first and second, third and fourth, fifth and sixth. Here we have two and three, four and five, six and one. So again the position of pi electron is different. If it is first oxygen, first, second and third, this is also first, second and third. So again here also if you see the position of pi electron is different in all these structures. So all these structures we call it as resonating structure. Now there are a few characteristics like how do we know that the given molecule will exhibit resonance and resonance possible in the given molecule. There are a few characteristics that you have to keep in mind. You have to keep in mind and only you can understand the resonating structure that like the resonating structure, resonance is possible or not in the given molecule. So we will come to this example again. First let me write what all the characteristics of resonance we have. So I will write down characteristics. First characteristics, the molecule or ion, whatever it is, it must be planar or I will write down system. System must be planar, conjugated. We will discuss what is this conjugated system after this actually. Second characteristics is what? Second characteristics, the position of atom is not changing. So I will write down the position of atoms, sigma electrons are fixed. The position of pi bonds and lone pair are inter convertible. Third point is what? The position of pi bonds and lone pair are inter convertible. Third point is what? The position of atoms is not changing. So the next point is what? Sigma electron has nothing to do with resonance. So sigma electron does not take part in resonance. Next one, all resonating structures, all resonating structure, R s, all resonating structure contributes the real structure and that is why we also call it as contributors. Next point is what? The resonating structure which is R s does not explain all properties of the molecule or ion. So all the resonating structure that you are drawing here, this does not explain the properties, all the properties of the molecule or ion. So these are the characteristics of resonance. So there are 2, 3 things that I have used over here. One is planet that you already know, the molecule must be in the same plane and it must be conjugated. Resonance is only possible when the system is conjugated in planet, otherwise it is not possible. Second thing that I have used here is this contributors, R s contributes to the real structure. R s does not explain all the properties of the molecule. So what is the meaning of this one? That all R s contributes to the real structure. What is the meaning of this? You see if I take this example, this one and this one, so it has few properties and this molecule has few properties and this is not the actual structure of this ion. Acetate ion it is, C s 3 C o minus, this is also acetylate ion. But none of these molecule represents the actual structure of the acetate ion. So what is the actual structure first of all? To understand the actual structure, I will take the example of this benzene ring. You must have seen that the benzene ring has the structure of benzene ring given in the book is like this. We have a ring and there is a ring within it. So what is the meaning of this ring that we have here? It represents the electron plug. So if you actually see the structure of this benzene ring, so in this ring what happens? We have an electron, suppose we have a cyclohexane like this here. So we have a ring above this ring, we have pi electron like this and we have pi electron like this also. So the electron cloud is present above this ring also and below this ring also. So this is the actual structure. If I draw the ring here, it is three dimensional structure you have to analyze this. We have an electron cloud over this ring and we have an electron cloud here also. All these things are the electron cloud. This is also an electron cloud. So the point is if I talk about the bond between first and second carbon, here it is double bond and here it is single bond. So which one of these structure you will take? So that is the question because we know these two structures are not the structure. So what we can say that this electron cloud is, all this pi electron cloud is distributed over this entire molecule. Like this means what? This pi electron is delocalized here. It is continuously moving from one carbon atom to another carbon atom. You see if this pi electron comes over here, we will get a negative sign here. This pi electron goes here. Here we have negative, here we have positive. So this negative positive forms a pi bond. Similarly, we have a pi bond here and here and we will get this structure. So none of this structure or the real structure, depending on the delocalization of pi electron will draw the hybrid structure of the molecule. The hybrid structure of this molecule is nothing but this structure which actually represents the delocalization of pi electron over the molecule. So in this case what happens? All this pi electron, 6 pi electron, 2 4 6, 6 pi electrons are equally distributed over the entire molecule like this and this continuously moving over the entire molecule like this. Here also it will like this. So none of this structure is the actual structure. The actual structure is nothing but this or this. If I ask you what is the bond order of carbon, first and second carbon, the bond order between these two carbon is what? It is between 2 and 1, the average of that. So it will be 1.5. So the carbon-carbon bond has bond order which is equals to 1.5. It neither has single bond nor has double bond. This is because of resonance. So all these structures, we call it as contributors. So both of these resonating structures contributes to the real structure. The real structure of benzene ring is this, not this one. Now the question is why do we use this structure in representing the reaction of benzene ring? Because for reactions we require static electrons. We cannot show that delocalized electron in the reaction. So to represent the reaction, we use resonating structure not the real structure. And all these resonating structures are contributors. Why contributors? Because they contributes towards the real structure of the molecule. Suppose one molecule has three resonating structures here. All these resonating structures will contribute to the real structures. And with this no combination will get the real molecule which has a different properties. So the real molecule will take few properties of this, this and this. So the real molecules will take property from each of its resonating structure. It is possible that all resonating structures will contributes equally to the resonance hybrid or the real structure. The real structure we also call it as resonance hybrid. So both are the same thing. So it is possible that all resonating structures contributes equally to the real hybrid or it is also possible that few structures will contribute more and few resonating structure will contribute less. So in case of more contributing structures we call those structures major contributors. And the resonating structures which contributes lesser that those structures we call it as minor contributors. So the contributors are also two types major or minor. It depends on the stability of all those resonating structure. More stable resonating structure will contribute more to the resonance hybrid less stable will contribute less. That is the point. So we will see that stability part also of the resonating structure. Now for this example you see if you have to draw the resonating structure of this molecule that will be this C H 3 C O O. And since the pi electron is de-localized from 1 to 2 first to second oxygen via this carbon. So it is de-localized like this. We have a pi electron like this. So here we have partial negative charge. Here we have partial negative charge. This also you can draw the de-localization of pi electron like this. So like this we draw the real structure of the molecule. So we will come to this part again this contributors part which one is major contributor or minor contributors. And you have to memorize this. The major contributors are those contributors which is more stable and those contributors will contribute more into the resonating hybrid. So we will come to this part again. Now the next thing we will see first that where this resonance is possible. What are the condition? What is the conjugated system? Like I said in conjugated system only resonance is possible. So in this how do we identify the structure is conjugated. That is we are going to discuss now.