 So, by now you know that buttoin is more stable than but1in, right? This is because in but2in we have these hydrogen atoms. We in fact have 1, 2, 3 and 4, 5, 6. We have 6 hydrogen atoms that are attached at this alpha position that can undergo hyperconjugation with this double bond compared to only 2 hydrogen atoms in case of but1in, right? So therefore these pi electrons can get delocalized more in case of but2in making this molecule more stable, right? Now one of the ways in which we can actually test this experimentally is by checking for the heat of hydrogenation of but1in and but2in. So what does this heat of hydrogenation mean? Well, hydrogenation simply refers to the addition of hydrogen and it turns out that whenever we add gaseous hydrogen to alkene, it turns out that some amount of heat is always released. Now why does this happen exactly? We will come to this in a short while. But if you look at this heat, amount of heat that is released, this heat of hydrogenation, then you will see that these values are somewhat different, right? If you come and take a look at but1in, then the heat of hydrogenation of but1in is 30.3 kilojoules per mole minus of 30.3 kilojoules per mole. So this basically means that 30.3 kilojoules of heat is released. Then I add gaseous hydrogen to one mole of but1in. Now if you compare this with the heat of hydrogenation of but2in, both the cis and the trans form, let's not worry too much about the cis and trans right now. But if you look at the heat of hydrogenation of but2in, you can see that these values are slightly lower, right? It's around 28 kilojoules per mole. So how does this heat of hydrogenation relate to the stability of but1in and but2in? Let's find out in this video. Now whenever we add hydrogen to an alkene, let's say but1in, we actually get an alkene, in this case we are going to get butane. Now this reaction is actually really slow and we need to add some catalyst like some finely divided nickel to drive up the rate of this reaction. And we are going to talk about this in detail, how this reaction exactly works in a later video. But the bottom line is what actually happens is that these hydrogen molecules, this come and collide against this double bond. And this leads to the formation of new carbon hydrogen bonds along with the breaking of these carbon carbon pi bond and these hydrogen hydrogen bonds. So this leads to the formation of an alkene. So we have said that hydrogenation of alkenes is always exothermic, right? So how can we be so sure about that? Well, if you look at what's going on, you'll see that we are actually breaking this carbon carbon pi bond. So we are actually breaking one carbon carbon pi bond. And we are breaking these hydrogen hydrogen bonds, one hydrogen hydrogen sigma bond. So we are breaking one hydrogen hydrogen sigma bond. While on the other hand, we are forming two carbon hydrogen sigma bonds, right? So we are actually forming two carbon hydrogen sigma bonds. Now as you must be aware, it takes energy to break bonds but energy is released in the form of heat when new bonds get formed, right? You must also know that it's actually far easier to break pi bonds compared to breaking sigma bonds. So therefore, because we are breaking one pi bond and one sigma bond but forming two sigma bonds in its place and these carbon hydrogen sigma bonds are really strong. They're almost as strong as these hydrogen hydrogen bonds. So therefore, the amount of energy that's going to be released is going to be greater than the amount of energy required to break these bonds, right? So therefore, the change in energy, in chemistry we actually calculate these changes under constant pressure and we call it the change in enthalpy. The change in enthalpy is always going to be negative, right? This reaction is always going to be exothermic as it involves the breaking of these weak carbon-carbon pi bonds and the formation of really strong carbon-hydrogen sigma bonds. Now to understand these changes in enthalpies, let's simply call this the change in energies. A good way to picture these changes is in the form of an energy diagram. Now because this reaction is exothermic, energy is released during the course of the reaction. So this means that the energy of the products that are formed has to be lower compared to my starting material compared to the energy of my reactants, right? In other words, because change in energy is negative and change means final minus of initial. So the change in energy will be the energy of the products minus of energy of the reactants because this term is negative. So this clearly shows that the energy of the product has to be lower compared to the energy of the reactant, right? Now we can make sense of this data that I showed you earlier. These are experimentally determined values and this shows that the heat of hydrogenation of Butte 1 in is minus of 30.3 kilojoules per mole, right? So this basically means that the energy of my product which is Butte is lower than my starting point is lower than the energy of my reactants by 30.3 kilojoules, right? So this energy difference is going to be 30.3 kilojoules. Now if we turn our attention to the heat of hydrogenation of Butte, let's consider this trans Butte for reference. Its experimental heat of hydrogenation is only 27.6 kilojoules, right? Its only minus of 27.6 kilojoules per mole. So what happens when we hydrogenate Butte? Well if we add hydrogen to Butte, Butte 2 in, then even out here these hydrogens will get added to this double bond and we are going to get Butte, right? So even in Butte 2 in, we get the same product Butte as in case of Butte 1 in, but the experimental heat of hydrogenation this time turns out to be lower and its 27.6 kilojoules, right? So in case of Butte 2 in, the energy of the product which even out here is Butte, so it's still going to be at the same energy level, the energy of the product this time is only 27.6 kilojoules lower than the energy of the reactant, right? So this means that the energy of the reactant which is Butte 2 in has to be somewhere around here, right? So this energy diagram is going to look something like this and the difference between these two energies which is nothing but 30.3 minus of 27.6, so it's 2.7 kilojoules. This has to be the exact same energy difference even out here. So this shows that experimentally Butte 2 in is more stable, it has less energy compared to Butte 1 in, right? So what's going on? Why is the heat of hydrogenation lower in case of Butte 2 in compared to Butte 1 in? Well, the answer to this question as you must have guessed is hyperconjugation. Even though in both these cases we are breaking this carbon-carbon pi bond and replacing them with carbon hydrogen bonds, but because of hyperconjugation this pi bond in Butte 2 in is more stable, it's more delocalized. So therefore it's more stable and difficult to break, difficult to break. So therefore the amount of energy that's actually required to break the carbon-carbon pi bond out here is going to be higher compared to this one, right? So even though the amount of energy that's released is almost the same on forming this Butte in molecule, but the amount of energy that's required to break this bonds is going to be slightly higher in case of Butte 2 in, right? So therefore the total energy change, the total heat that's going to be liberated is going to be slightly lower in case of Butte in as slightly more energy is consumed during the bond breaking process of Butte 2 in compared to Butte 1 in. Now what about the differences in heat of hydrogenation between the cis and the transform of Butte 2 in? Now before you figure that out, let me give you an important hint. In cis-Butte in we have these alkyl groups that are placed closer to each other. So think of this as electron clouds. So these groups they can ripple, they can suffer repulsion thereby increasing the energy of the molecule and making it relatively less stable. On the other hand, because these groups are placed further away in transform, there is no such repulsion. So the energy of the transform is relatively lower, it's more stable compared to the cis form. Now that you have this information in hand, which amongst these two do you think will have a higher heat of hydrogenation? You can pause the video and think about this for a moment. Well, in both these cases, it doesn't matter if I take cis-Butte 2 in or trans-Butte 2 in, in both these cases on adding hydrogen, I am going to get Butte in. I am going to get Butte in both these cases. So therefore the energy of the product that is going to get formed is going to be the same. So if I look at my energy diagram, the energy of the product is going to be same in both the cases. But because cis is slightly higher in energy due to this repulsion between these alkyl groups compared to the transform, so therefore the heat of hydrogenation, the heat of hydrogenation in case of cis is going to be slightly higher compared to the heat of hydrogenation in the transform, right? In fact if we look at the data, let's get our data out. You can see that experimentally trans-Butte has a slightly lower heat of hydrogenation compared to cis-Butte, right? So therefore we can now go ahead and make a rule. We can say that if the product formed on the hydrogenation of two different alkenes is the same, then more stable the alkene, lower will be the heat of hydrogenation, right? So we can say that if the products formed are the same, then more stable my starting alkene, this means lower will be the heat of hydrogenation, lower will be the heat of hydrogenation, right?