Rank each group of compounds in order of increasing heat of hydro...

Question

Rank each group of compounds in order of increasing heat of hydrogenation. (b)

Accepted Answer

Hi everyone and welcome back in today's video, we're looking at this problem which asks us to arrange the following Elkins according to their heat of hydrogenation from lowest to highest. We first of all have to recall that the heat of hydrogenation is inversely proportional to the stability of a specific alkaline. So we may say that the more stable the lower the energy of hydrogenation, we may just indicate that with the entropy sign. In other words, if an al keen is really stable, it's heat of hydrogenation will be low and if that al king is not stable, the heat of hydrogenation will be high. Now, what we want to do here, we want to essentially rank these compounds from lowest heat of hydrogenation to highest and that means we want to rank them from the most stable Up to the least table one. Now knowing this, we essentially understand that these two parameters, they are inversely proportional. So if we're going from lowest to highest heat, we're going from highest to lowest stability. That's how it works pretty much. And now, knowing that we are considering stability, we simply wish to recall three factors that affect the stability of al kin's. Number one would be the number of double bonds. Number two would be substitution. Number three is conjugation. The greater the number of double bonds, the greater the heat of hydrogenation. In other words, the ALC in is less stable, it definitely makes sense that it's harder to add three molecules of hydrogen to three double bonds than it is to add to one. Double bond substitution essentially increases the stability of an L. Keen. We already know that mono substituted Elkins are less stable than di tri and tetra substituted ones. And conjugation essentially implies that the double bonds are linked. There is no separation between them with sp three carbon atoms. For example, in particular, if you look at this, al keen is conjugated because there are no separations with sp three carbon atoms and the double bonds are adjacent to each other. Now let's consider everything. And let's start with thinking about the number of carbon carbon bonds. We may just label the number of carbon carbon double bonds. My apologies. So that would be two double bonds, let's say two D. Here we have three double bonds. So that would be three D. That would be two double bonds. We would say two D, two D, two double bonds, one double bonds and one double bond. Great. So first of all we understand that the number of double balance is the primary factor that we wish to consider. The second factor is substitution and the third one is conjugation, but we already have a bit of information to give us a good start. These two are two al kinds with one double bond. So they both will have the lowest heat of hydrogenation between the T. We simply have to think about the other effects. What about substitution? This is a di substituted Elkin. So let's say die and this is a tri substituted because there are two al kio substations here across the double bond and here we have three of them. This one this one and that one. So a try substitute. Al keen is more stable because stability increases with substitution. And we may say that the final structure is the first in our ranking because it has only one double bond and it it has the greatest substitution. So this structure Would have the lowest heat of hydrogenation because it's the most stable followed by the 2nd 1. So we may already label that. Okay, we got our first one and our second one. The second one is this one because it still has one double bond. All theater structures have two or three double bonds. So after the structure we get our next one with only one double bond. But because it's a less subsided alki and it's less stable and therefore has a greater heat of hydrogenation. So that's our second structure. Now we will think about the 3rd 1 and to think about the 3rd 1, we have to notice that the three other ones that we are considering now are those that have two double bonds. And now we need to think about both substitution and congregations. So let's do that. This one is tri substituted at this position and the disposition is thy substituted and it's also conjugated. So S A C. This Elkins is thy substitute at this position and I substituted at this position because we see two out kill substitutions at each double bond. And is it conjugated? Yes, because there's no separation between the double bonds is conjugated. Number two is a di substituted this position di substituted at this position but it's not conjugated. There's a separation here that's an sp three carbon atoms so not see not conjugated. So die die specifically we have di tri versus die, die and die die due to the greater substitution. This would be expected to be the most stable L king out of the three and also it's conjugated. So that's just perfect. It only validates that our answer is correct. We're going to choose our next structure to be this one because it's the most substituted one overall and it has conjugation involved. So we may stay that this is our next structure and now between the two that are di substituted at each position. We noticed that one of them is not conjugated while the other one is. So the first one will be the conjugated one because it's more stable. So we are going to draw our 19. And because it's conjugated and more stable it gets our position number four in the ranking followed by this one because it's di substituted at each double one but it's not conjugated, so it's less stable than the than the conjugated one and we are going to draw it as our next structure. So that would be ranked number five with a double bond at this position. And finally, we are just left with a with a trying right that has three double bound. So it requires the greatest heat of hydrogenation, meaning that will be our final structure in the ranking With two double bonds. Even though it's conjugated, right? There's no separation between these double bonds. We don't really worry about conjugation because it's the only structure that has three double bonds. So this is the primary factor in comparison of the heat of hydrogenation. We may now label our final answer. This is the rank. We have arranged our compounds in terms of an increasing heat of hydrogenation and the correct answer is this multiple choice question is B However, if you have any questions, feel free to leave them down below and thank you for watching this video.

Rank each group of compounds in order of increasing heat of hydrogenation. (b)

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