 The next application of hyper conjugation is heat of hydrogenation heat of hydrogenation in short we will write it as H O H ok. So, what is heat of hydrogenation? It is the amount of energy energy released when when one mole when one mole of an alkene amount of energy released when one mole of an alkene goes under under hydrogenation ok. Why one mole we are using here? Because you see heat or energy heat of hydrogen energy what energy? Energy is a extensive property ok energy is an extensive property. So, whenever we define extensive property we have to define the amount also that we are taking. So, standard amount we always take is one mole only ok. So, when one mole of alkene will go under hydrogenation the amount of energy released is nothing but the heat of hydrogenation. For example, you see suppose this is the alkene we have and it is going under hydrogenation the product we get here is C single bond C H H plus del H right. So, energy releases in this process. So, this is the sense what it is an exothermic process. Why it is exothermic process? Because you see of there is one pi bond here right the pi bond between carbon carbon atoms ok and the sigma bond between hydrogen hydrogen atom ok sigma bond. So, here what happens one pi bond and one sigma bond breaks and this converts into two sigma bond carbon hydrogen sigma bond two sigma bond. So, what happens sigma bond is stronger than pi bond ok. So, from less is strong we are going towards the more strong. So, obviously here it is more stable. So, energy will release ok. So, since what happens one sigma and pi bond dissociates and forms two sigma bond which is a stronger bond hence energy releases ok. So, this is what the process we have here heat of hydrogenation. Now, you see one example if I take here plus H 2 plus H 2 the product will be what here it is butane right both the product is same due to in the energy releases here suppose it is del H A and here the energy releases is del H B. Why A? Because I have taken this molecule as A and this molecule as B ok. So, energy releases with a molecule A on hydrogenation is del H A energy releases with molecule B on hydrogenation is del H B. If I ask you A and B which one is more stable? Stability of A and B if you compare the stability of B is more than to that of A. Why B is more stable? Because more alpha hydrogen here you see the number of alpha hydrogen is 6 and here the number of alpha hydrogen is 2 more alpha hydrogen more will be the stability both reactants different reaction reactants gives different same product that is butane. So, if I write down the energy diagram here right since A is less stable. So, it is energy is more than B right B is more stable energy will be less and the product is suppose here we have the product exothermic reaction. So, this will have the lower least energy among the three right. So, product is same. So, the path of the reaction could be like this or it could be like this ok. Now, the enthalpy change in this reaction is what? The enthalpy change is nothing but this is for del H B right and enthalpy change for this will be this is del H A ok. So, from this graph what we can say that the mod of mod of del H A is more than to the mod of del H B del H B and which is more stable B is more stable. So, more stable product right sorry more stable reactant will give lesser value of enthalpy change ok. So, what we can say the stability from this data we can say the stability is inversely proportional to the enthalpy change del H which is nothing but the heat of hydrogenation ok. So, more stable compound gives you less energy less stable compound gives you more energy. How do we have assign stability by hyper conjugation by resonance by aromaticity any factor we can use, but one thing is very important and you know definitely true here is more stable compounds when go under hydrogenation gives lesser value of enthalpy change ok that is what we get from here ok. Now, on basis of this we will see or will you know compare some heat of hydrogenation of some compounds remember one thing always stability is inversely proportional to del H. Now, this is stability we can compare on the basis of hyper conjugation on the basis of resonance or any other electronic effects ok. If only hyper conjugation is there then we will compare through hyper conjugation if hyper conjugation resonance both are there will compare through resonance, resonance will be the dominating effect and more stable less will be the enthalpy change. So, whatever example we have done just before in you know hyper in the stability of alkene right the stability order if you reverse those order you will get the order of heat of hydrogenation ok because it is inversely proportional ok. We will see few examples now also, but whatever examples we have done just now in stability of alkene those examples also you can refer for heat of hydrogenation. The only thing you have to do is what write down the order of heat of hydrogenation for all those examples that we have done the reverse order of that this opposite order of that gives you the order of heat of hydrogenation ok. You look at this we will solve some questions on to this now ok. Now one more thing important here I forgot to tell you that this order this order we got for the molecule which has equal number of pi bonds ok. So, this you know this thing we can apply when the molecule has equal number of pi bonds then only we can take this as the reference ok. What happens when we have when the pi bonds are not equal in that case the heat of hydrogenation is directly proportional to the number of pi bonds because more number of pi bonds more will be the hydrogenation takes place more energy will release ok. So, this is the first case when molecule has equal number of pi bonds stability is inversely proportional to del h ok. When the molecule different number of pi bonds then the heat of hydrogenation delta h of hydrogenation is directly proportional to the number of pi bonds ok. So, like this we compare. So, both examples both parts are important we will see few examples on to this then you will understand how to compare the heat of hydrogenation of given molecules ok. Now I will write down few examples ok then we solve this ok. Now these examples you see we have to compare compare HOH heat of hydrogenation ok. Two molecules has different number of pi bonds more pi bonds more will be the heat of hydrogenation ok equal number of pi bonds ok then we will see what we will see the more stable compound will give you least which one is more stable you see this is in conjugation resonance possible here. Here the resonance is possible, but here the resonance is not possible we have here hyper conjugation ok resonances gives more stable product ok. So, this is more stable than this hence heat of hydrogenation of this product will be more ok. Now here you see we have resonance possible here and here again we have heat of sorry hyper conjugation order of heat of hydrogenation will be this ok. One exception we have here if you compare these three molecule ok. So, all the three molecules has different number of pi bonds. So, if you see the normal thing heat of hydrogenation number of pi bonds according to this if I write down this as a b and c ok a b and c then the order should be what c should have maximum then b and then c because it has the maximum number of pi bonds ok, but since this compound is aromatic and this aromaticity gives extra stability to this compound ok and that is why the order is not that we are assuming now ok. This two if you compare obviously greater number of pi bonds ok more will be the heat of hydrogenation ok. b should be more than a that is a true thing, but a and c if you compare however it is aromatic the order is found to be this. This is the order of h o h for this molecule this is an exception actually. So, you must remember this exception ok very important. So, like this we can compare heat of hydrogenation according to the number of pi bonds present in the molecule equal number of pi bonds then we will see what we will check the stability reverse of that gives you the order of heat of hydrogenation ok. If the equal number of pi bonds are not there then the heat of hydrogenation is directly proportional to the number of pi bonds ok and accordingly we will do all this example ok. Then go through this we will see the next application ok. The next application we are going to see here is heat of combustion that is h o c. So, what is heat of combustion? The amount of energy released when one mole of a substance is burned into air ok. So, here what happens the we can also say this is similar to heat of hydrogenation few just few things you have to keep in mind it is the amount of energy released right. So, if I write down the reaction C x h y any hydrocarbon plus x plus y by 4 O 2 gives x C O 2 plus y h 2 O 2 h 2 O this is the balanced reaction ok. Now, this is the combustion reaction what is combustion? It is the amount of energy released when one mole of a substance is burned into air it is the amount of energy released when one mole of a substance is burned into air ok exothermic process it is ok. Now, this substance can be anything it can be alkene alkene anything ok. So, the energy released here also the same thing we have energy released due to combustion is inversely proportional to the stability of the compound. Now, if the compound is alkene then again we will consider all those factor to assign a stability that is hyper conjugation or resonance anything right, but this is true when we have in case of equal number of carbon atoms number of carbon atoms. If the molecule has equal number of carbon atoms then the order will be this ok. What happens if the molecule has different number of carbon atoms carbon atoms the delta H of combustion is directly proportional to the number of carbon atoms right. Now, we see few examples here ok. So, for all these molecule we have to compare the heat of combustion ok. Now, you see this molecule first of all we will see what whether the carbon atoms is equal or not ok. So, we will see what 1 2 3 4 5 1 2 3 4 5 5 5 carbon atoms ok. Then we will check what stability equal number of carbon atoms we will check stability and stability is because of hyper conjugation here because that is only possible hyper conjugation we have 9 hydrogen and here we have only 2 3 5 hydrogen. So, stability of this is more right stability of stability of the first one is more and hence the heat of combustion of second one is more ok. Here you see number of alpha hydrogen if you see here it is more right, but here we do not compare stability because both molecule has different number of carbon atoms here we have 6, but here we have 8 different number of carbon atoms then the delta H is directly proportional to the number of carbon atoms order will be this different number of carbon atoms delta H will be directly proportional to the number of carbon atoms here the number of carbon atoms are same then we will check what stability ok. So, stability of this will be least stable why it is least stable because of maximum angle strain because of maximum angle strain. So, because of maximum angle angle strain least stable and hence the order of heat of combustion will be this ok. So, this is how we compare the heat of combustion of different molecule ok, but in this we have one more thing. So, the thing we have to keep in mind is you have to count first the number of carbon atoms if it is same then we will check stability and how do we check stability we can have the factor of hyper conjugation resonance anything reverse of that will be the order of heat of combustion. If you have different number of carbon atoms then more carbon atoms gives you the more number more heat of combustion ok. We can have one more possibility here and that possibility is what if the molecule contains double bond also and different number of carbon atoms ok. If the molecule contains double bond with different number of carbon atoms then stability will define with the help of what the number of carbon atoms there ok. So, the last point here these two cases we have discussed. So, third case if the molecule has double bond, but different number of carbon atoms. So, in case of double bond we can apply those hyper conjugation and all, but that is only possible when we have equal number of carbon atoms like you see this one this is the fourth molecule. The molecule has double bond, but different number of carbon atoms. So, stability will define according to the number of carbon atoms. So, that sorry the heat of hydrogenation molecule double bond, but different carbon atoms. So, in this case what we will write the delta H of combustion not hydrogenation delta H of combustion is directly proportional to the number of carbon atoms right. So, this is the case we have missed the number of carbon atom will dominate the double bond stability factor here. So, the order of delta H of combustion will be this ok. So, these are the three cases we have discussed to compare the del H of combustion ok. When the carbon has when the molecule has equal number of carbon atoms delta H is inversely proportional to stability. When the molecule has different number of carbon atoms delta H is directly proportional to the number of carbon atoms mainly it is true in case of alkene cyclo alkene ok. Third case is what when the molecule contains double bond, but different number of carbon atoms. So, in that case what we will write delta H of combustion is directly proportional to the number of carbon atoms and the order of the molecule will be this ok. So, this is it we have discussed so far hyper conjugation and how hyper conjugation is applicable in case of alkene and there are various application also we have discussed that is delta H of combustion stability of alkene and delta H of hydrogenation ok. Next class we are going to discuss about the hyper conjugation in carbocation and pre radical.