 The third type you write down, the third type of conjugated system is pi sigma n free radical, pi sigma n free radical, one electron right, one dot, one single electron. Example of this is C H 2 double bond C H single bond C H 2 and free radical. So, in this we draw the resonating is like this. So, this pi electron, one electron comes over here it is in half arrow, because one homolysis it is, one electron is taken up by this carbon, another electron is taken up by this carbon. And this free radical free radical combines forms a double bond and the structure is C H 2 radical C H double bond C H 2, correct. How many hours will you have? Free radical is any carbon atom, which has three bond pair and one unpaired element. Unpaired electron, so it is number of electron is what 7, 7 electron, but it is electron deficient, but cannot behave in Lewis acid, because more than 8 electron is not possible. If you have, if you remove this electron also, it becomes carbon atom, right. If you have one electron radical add one more electron it becomes a carbon atom. So, it is an intermediate which forms during the process of a reaction. Generally free radical forms in those reactions where we have we use sunlight, etch in it, ok. So, in carbon-carbon bonds homolysis takes place. Homolysis means when you have a carbon-carbon bond it has two electron on it, homolysis and heterolysis two types of bond cleavage we have, right. So, one heterolysis is when the two bond pair is taken up by only one carbon atom, ok. C negative C positive like this, which is in this case not possible because one of the atom must be more electronegative. Like suppose we have carbon and chlorine bond, carbon-halogen bond, chlorine is more electronegative. This bond pair is taken up by this chlorine and it forms C plus the carbocation and C L minus, ok. So, this is heterolysis. Now, when you break this bond homolysis, one electron is taken up by this carbon one by this carbon one. This forms a radical in this carbon-carbon radical and it is possible when you use sunlight generally, ok. So, free radical is what free radical is it is an intermediate which has three bond pair and one unpaired electron paramagnetic energy. But at type 4 we have pi sigma pi this is type 4, ok. Carbonate pi bond C H 2 double bond C H single bond C H double bond C H 2 its R S is what this pi electron comes from here and this pi electron on this carbon. So, they get C H 2 positive charge C H double bond C H 2 C H 2 negative. This is the R S. In comparison to this, no if you compare the stability of this two, right. If you compare the stability of this two, this one is more stable, more stable 2 pi bonds. But while reaction it would not give reactions with this, the addition takes place with this carbon and that carbon. We will discuss that in alkene chapter, ok. Reaction of H X on alkene, ok. It gives conjugated tie-ins it is. What is the name of this compound? 1 3 butadiene tie-in 1 3 butadiene, ok. So, 1 3 butadiene gives you 1 4 addition not 1 2 addition. The reason we will discuss in alkene, ok. This is the type 4 system here. What about this one? In this do we have resonance possible? Do we have as possible in this? Sir, that was not possible. It is not possible. Why? It is possible. But I see my pile. So, how do you know something is playing? All carbon atom are sp2 hybridized. So, is it now one? No. So, sp2 means? SP3. Sp3 is the same, this diagram is planar but you will be in the same plane. Carbon 3 bond is the same. Sp3 is like this, but in the same plane the diagonal planar. So, it is planar. Sir, type 4. Type 4, yes. Type 4, yes. Sir, should we form a timeline which has the same number? Where? Is it that the other structure also should have the same number? In this it is not possible. But if you have further conjugation of the slide, then also it is possible, of course. Sir, this is also still resonant. It is resonating. See resonating is such that you can draw till conjugation is there. Like I affect me what we say, it is distance dependent effect because sigma electrons are involved. Here distance factor is not there, but it requires only conjugation. So in the molecule we can draw a resonating structure till conjugation is there. Like here you see this part is not in conjugation, sp3 hybridized. So we can draw a resonating structure of this one from this carbon to this carbon. This is not involved in this. So this resonating structure is what? We can draw it. We will get a double bond here, negative charge here and positive charge. For this one if you draw this pi electron comes over here and this pi electron jumps onto this oxygen. This is C H single bond C O minus H double bond and positive charge. Can we draw this way? This pi electron comes over here and this pi electron comes over here. Sir, how do you know which way? That is what I am asking. Can we draw this way? This comes over here, this comes over here. You can draw. Draw a name here. Sir, even in the second row. Yes, that will be a different resonating structure. But that is the least. It is not stable at all. Because you see oxygen is more electronegative. So it has tendency to attract electron. That is why the best way to draw the R is shifting of electron towards oxygen. Not away from oxygen. Sir, we cannot have positive charge or electronegative at all. That is what you see. You can draw any structure. Whether it is stable or not, that is a different event. But whether you can draw it or not, you can draw it. But usually what we say, usually we will draw the structure and we shift the electron towards the more electronegative. Sir, second one sir. You can have more, right? Double bond, single bond, positive. That is the type one that you can draw. Where? No, no. See what you can do. You can do one thing. This comes over here. And this comes over here. This gives you the same. Sir, sir, can you? Sir, can you? Sir, can you? Sir, can you? Sir, can you? Sir, can you? Sir, can you? Sir, can you? Sir, can you? Sir, can you? Sir, can you? So, let us draw, see. See, what you can do, you can shift this here. So, we will have a double bond here. And this is positive charge? This positive negatives make a bond. Axis carbon with positive negative charges has never come stable. carbon, carbon, double bond. You see this? Carmen, carbon, double bond. If you want to change this structure into this positive, negative like this. For this you have to break this pi bond, correct? And to break this pi bond you have to provide a lot of energy, right? So this is the high energy molecule. That you can do is the same thing. That you cannot start different. If you draw, you have to draw. Shifting of electron, you can do. Molecule won't do. Molecule though, automatic they are shifting of electron take. We are trying to study the structure of benzene by shifting the electron. But when benzene exist, the electron is already located in its own way. So when you draw, if you want to draw, you can draw it this way, reverse. But what molecule will do? Oxygen takes electron. So it goes this way. You can draw but molecule won't do. That's why that is the structure. Okay, understood this. Now apart from these four, there are two more conjugated system possible which is not that important. I'll just write down the type. Type five and type six you write down. Type five we have a vacant orbit P, sigma bond and a lone pair. Vacant orbital, sigma and lone pair. And type six we have a lone pair, P orbital lone pair, sigma and vacant P. Vacant P and lone pair. One example we have already done, P of three. It's lone pair in P orbital. Lone pair in P orbital, sigma bond, vacant P. Vacant P, sigma bond, lone pair. Example is this. Boron has vacant P orbital, we know this. Vacant P and fluorine has lone pair on it. So back bonding is possible. Back bonding is nothing but resonance. Each one of us. No, it's not. It has double bond characteristics. It has pi bond characteristics. Which forms when the sigma bond has been formed. After that the central atom, if it has vacant orbital and the other atom has the lone pair and if size is comparable, then this has tendency to do an electron. That is back bonding. Sigma bond form on it and back, tendency to do an electron is back bonding. Okay. But that BCL three that bonding is not that strong because chlorine may have three P orbital. And here we have two P. Two P three P overlap is not that great. For bromine also it is even lesser is stable. For BF three the back bonding is strongest. Why is this different from vacant P? Like this is vacant P. Vacant P and lone pair. Vacant P and lone pair. Vacant P the example is this. We have CH2 lone pair on it. Single bond CL. Chlorine is the element of third period, right? So it has vacant orbital. Vacant D orbital. So in this the resonance is possible. CH2 double bond CL negative here we have positive. Not that important but this gets more. All these are the six different types of conjugated system where the resonance is possible.