 Hello friends welcome back again friends in this session we are going to discuss about resonance energy ok. Sometimes you must have seen the question that they ask you to determine not exactly determine they ask you to compare the resonance energy of the given molecule ok in the question it is given. So what is resonance energy first of all ok. So resonance energy the definition you write down it is the energy difference between the energy difference between the resonance hybrid hybrid and it is the energy difference between the resonance hybrid and the most stable most stable resonance structure ok. We know what that the resonance hybrid has the least energy ok. This information we already have we have already discussed this that the resonance hybrid out of all the resonating structure it is the combination of all the resonating structure right. The resonance hybrid is the combination of all resonating structure and it has least energy among all the r s resonating structure ok. This is for sure ok. For example you see if I write down this C H 3 C double bond O O minus suppose the molecule is A right and in this obviously resonance is possible conjugated system right. So other resonating structure will be what C S 3 C single bond O minus double bond O this is suppose B right and for this if I draw the resonance hybrid. So these are what these are r s resonance hybrid is what C H 3 C double bond O sorry single bond O O and we have partial double bond character here like this delta negative and delta negative or we can also say minus half minus half charge will be there ok. So this is r h resonance hybrid or the real structure ok and this suppose I am assuming this as C right. So we know the fact here and the fact is the energy of energy of C right which is the resonance hybrid here it is less than the energy of A or B right because it is the real hybrid r h and this is the resonating structure r s ok. If I draw the energy profile here energy profile that could be something like this this is suppose the energy axis we have and A and B has the same energy A and B has the same energy equal resonance we have. So energy of A and B are same but energy of C is lesser than the energy of A and B right. So this difference here this difference is the resonance energy right. So resonance energy r e is equals to we can write energy of energy of A or B minus the energy of C this is the resonating structure ok. Now the point is this is the theory part how do we identify or compare the resonance energy in the given molecule ok. So comparison of resonance energy comparison of resonance energy first of all you see this is structure if it is more stable ok more stable structure will have lesser energy and hence the difference in energy will be more and hence the resonance energy will be more ok. So more stable structure will have the higher resonance energy ok. If you have more stable structure its resonance energy will be high ok. So first case that we are comparing here is in case of we have 6 7 cases possible. So first case I am discussing in case of aromatic non aromatic T aromatic system we know the order of stability is what aromatic then non aromatic then anti aromatic ok. So the same sequence will have for resonance energy the order of resonance energy r e will be equals to it will be maximum for aromatic compound then we have non aromatic compound the last we have anti aromatic. So if the compound given is aromatic anti aromatic or non aromatic so simply aromatic compounds will have the maximum resonance energy then non and then anti. For example you see these three compounds if you have to compare obviously you see this compound is aromatic this is non aromatic and this is anti aromatic ok. So this is non aromatic with 6 pi electron anti aromatic with 4 pi electron ok. So resonance energy will be maximum for aromatic compound and then we have non aromatic compound and then we have anti aromatic compounds ok. So if simply if you have the simple compounds which is aromatic non aromatic anti aromatic will compare the resonance energy as per their stability ok. But what happens if the two molecules are aromatic or anti aromatic in nature or even non aromatic for example you see second case I will take if compound is or non aromatic any one of these three if all the compounds given are aromatic or non aromatic or anti aromatic in this case what happens the resonance energy r e is directly proportional to the conjugation more conjugation more will be the resonance energy for example you see this is aromatic this one ok. You see this is aromatic in nature 2 pi electron this is also aromatic in nature 6 pi electron ok more conjugation more will be the resonance energy for example you see both are non aromatic ok this is also non aromatic and this is also non aromatic more conjugation more will be the resonance energy ok. Now third case the third case is compound shows equal resonance shows equal resonance has more resonance energy compound shows equal resonance has more resonance energy. Now the first thing is what is equal resonance equal resonance we have already discussed and we call it as equal contributors also the compound in which we will get equal contributors right remember when we were discussing the application of resonance we have classified resonating structure into two types equal and unequal contributors and then we have discussed also bond order to calculate right. So whenever the molecule shows equal resonance its resonance energy is more for example you see if I write down this one C H 3 C double bond O O minus and C H 2 C double bond O C H 2 C double bond O H will right ok. So this molecule shows equal resonance equal r s and hence the stability of this is more here right. For example you see O negative S double bond O O negative double bond O and S double bond O O negative O negative and a lone pair on this ok we will have a double bond here. This shows equal resonance stability or resonance energy will be ok. Very two common examples we have two very common examples that we will see now. Third one we have benzene and pyrrole obviously benzene has equal resonance more will be the energy ok. One example is this we have pyrrole and again pyrrole equal resonance we have here energy will be more ok. So in all these example we have equal r s here also we have equal r s. So three possibilities we have discussed when we have aromatic non aromatic or anti aromatic then we have either anti aromatic or non aromatic or aromatic then we have this right. Third is when the compound shows equal resonance then the compound which has equal resonance will have more resonance energy few examples we have seen ok. Now the next one we have here is cross conjugation or extended conjugation. Fourth case is cross and extended conjugation. I will explain this one with an example. Suppose we have these two molecules these two molecules ok. Now here you see this double bond is in resonance with this pi bond also and it is also in resonance with this pi bond also right. But with the both pi bond it cannot you know show resonance at the same time ok. So what happens either you can draw resonating heat structure here with this pi bond or this pi bond right. But here you see it is in conjugation 1 2 3 4 and 5. So this is what extended conjugation we have and this one either like this you can draw or this this way and this way because here you see the conjugation is not extending beyond this point ok. Here also conjugation is not extending beyond this point. So we have a sp3 hybrid as carbon in between right and these two are not in conjugation with each other. This pi bond is in conjugation with this this pi bond is in conjugation with this. These two are not in conjugation with each other. Here you see this two this two are in conjugation with each other means this pi bond pi electron is delocalized from this point to this point right. However this is also not completely delocalized ok because again we have one sp3 hybrid as carbon atom. So we this is we call it as extended conjugation and this one is cross either way we can go we cannot go both way ok. So in case of extended conjugation the resonance energy for extended conjugation will be more for from the resonance energy of cross conjugation ok. So the molecule 2 will have more resonance energy than the first molecule. Similar examples you see we can draw few more resonance energy here it will be more right one more example you see right. So here you see extended and cross this is the order of resonance energy. So in case of extended and cross conjugation the resonance energy is more for extended conjugation ok. Fifth case you see that is overlapping tendency overlapping tendency. What it means the difference in in the size of orbital orbital increases bond energy bond energy decreases and hence and hence the resonance energy R e decreases ok. So if you have suppose 2 p pi and 2 p pi bond. So 2 p pi 2 p pi bond gives more bond energy resonance energy than 2 p pi 3 p pi or 2 p pi 4 p pi or 2 p pi 5 p pi ok. For example you see ok. So here you see it is in resonance but here we have 2 p 2 p here we have 2 p 3 p 2 p 2 p 2 p 3 p 2 p 3 p 2 p 4 p right. So this has more resonance energy than this molecule because we have better overlapping than this again oxygen carbon carbon sulfur better overlapping here more will be the bond energy carbon chlorine carbon bromine carbon chlorine has 2 p 3 p and carbon bromine has 2 p 4 p overlapping resonance energy will be more in the case of chlorine ok. So this is we will compare here when the difference in orbitals are involved right 2 p 3 p 2 p 4 p like this. If you have 2 p 2 p orbital then we will compare on the base of electronegativity means when the size and the overlapping tendency is not there when the size of the orbital is exactly same right and in that case we will see electronegativity ok with that is the next case we have that we will discuss see why this happens here because you see the size of 2 p orbital is suppose this and 3 p orbital will be this 4 p orbital will be this 5 p orbital will be this right. You see the overlapping tendency is very less because of size difference here it is very difficult to overlap here or with this even with this or with this right. So because of this difference in size the overlapping is not easy not possible ok and that is why the bond energy is less and resonance energy is also less ok. The next one is electronegativity this will apply when the size of overlapping orbital orbital is same then we define resonance energy on the basis of electronegativity ok. So what we can say here less electronegative less electronegative more will be the tendency of more the tendency of of losing electron right. If the electronegativity of the atom is less right if the electronegativity of the atom is less it has more tendency to lose electron which also increases hence resonance energy is more right. Basically you have to see the atom with lower electronegativity for that the you know resonance energy will be more ok. But this we can only apply when the size of overlapping orbital is same it means we have either 2 p 2 p orbital is getting overlap or 3 p 3 p. We cannot compare this electronegativity thing we cannot take this you know factor into count when the size in orbit difference in orbital is there like here we do not assume that ok. So for example you see this two molecule right nitrogen being less electronegative resonance energy is more resonance energy is more for this less for the first one there is a mistake we have a double bond here then only the resonance is possible ok. This molecule OH NH 2 resonance energy energy you have to compare ok. So here you see we have 2 p 2 p overlap this is again 2 p 2 p overlap and this is 2 p 3 p overlap right 2 p 3 p overlap ok. So the size factor is dominating here right. So obviously here the resonance energy will be least third number is this this two same orbital we have will compare electronegativity nitrogen is less electronegativity electronegativity first and then OH is second. One more example we will see here we have 2 p 2 p 2 p 3 p. So obviously oxygen will have more resonance energy than sulfur. So the point is factor you see here if size difference is there we will consider that ok. But we have then size difference electronegativity difference also we have then here this one is the minimum these two will compare in the base of electronegativity ok. So electronegativity we only compare when we have equal size of the orbital right here we are considering the overlapping concept 2 p 2 p 2 p 3 p ok. This is the next factor we have whether we have one or two effects that actually affects the you know resonance energy of the molecule and the last one we have here is the fries rule fries rule ok. What is this rule that if the molecule has molecule has more number of benzenoid form resonance energy will be more. More number of benzenoid form gives you more resonance energy. For example you see we have anthracene this molecule is anthracene this molecule it is phenanthracene phenanthracene ok. So if you see this molecule here we have 2 benzenoid form but here we have 1 2 3 3 benzenoid form. So more benzenoid form more will be the resonance energy. So resonance energy of A is more than to that of B this is B ok. So on the basis of number of benzenoid form we can compare resonance energy. One more example we will see here compare these two ok. If you draw the resonating structure of this you will always get 2 benzenoid form. For this you will always get 2 benzenoid form. This one you see the resonating structure one of the resonating structure will be like this ok. So here we have 2 but here we have 1 benzenoid form right. So the actual number of benzenoid form of this molecule which is the will be between 1 and 2 but in this it is 100 percent sure that will always get 2 benzenoid form into this ok. Hence in this case the resonance energy of B is more than to that of A ok. So this is how we can compare resonance energy of various molecule depending on aromaticity non aromaticity and anti aromaticity ok. If this is the there this is this compounds are there then aromatic compounds are the will have the maximum resonance energy then non aromatic and then anti aromatic. If the molecules all are anti aromatic or non aromatic or aromatic then we will see the conjugation more conjugation gives you more resonance energy right. Next thing is what next thing we will discuss the number of we will discuss the overlapping concept ok. Size in the difference in overlapping is more then resonance energy is less ok. When the overlapping things are same when size of the atoms are same we will check electronegativity right. We can also check fries rule that is nothing but the number of benzenoid form ok. So like this we can compare the resonance energy of various molecule ok. So in this session we have discuss all possibility of resonance energy. Next session we are going to discuss the acidic and basic nature of the compounds. Thank you.