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

Question

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

(a) hexa-1,2-diene; hexa-1,3,5-triene; hexa-1,3-diene; hexa-1,4-diene; hexa-1,5-diene; hexa-2,4-diene

Accepted Answer

Hey everyone and welcome back in today's video. We are going to solve this problem which states arrange the following al canes according to their heat of hydrogenation from lowest to highest And before we look at this problem, let's recall the basic principles of heat of hydrogenation. Heat of hydrogenation is the amount of heat needed to add a hydrogen molecule across a double bond. So let's understand that this is inversely proportional to the stability of an AL king. We may label that as the more stable the AL king, the lower the heat of hydrogenation, we will just simply add an entropy sign here to represent that, simply speaking. If an ALC in is really stable, it's just easier for us to add that molecule of hydrogen. So the more stable the ALC in the lower the heat of hydrogenation, they're inversely proportional. On the other hand, we also want to think about several other aspects. What is the stability in general? So regarding stability, we want to make sure that we take into account the number of double bonds or basically the the number of CC double bonds. That will be our first factor Number two, we would think about substitution And number three, we would think about conjugation. The greater the number of those double bonds, the greater the energy of hydrogenation, it makes sense that it takes more energy to add hydrogen to three double bonds as opposed to one double bond. On the other hand, substitution increases stability. So if a double bond is more substituted than the other, then it essentially implies that such a double bond double bond would be more stable. Congregation essentially implies that the double bonds are linked to each other and they are adjacent to each other, such as this one, they're basically forming a link because they are not separated by any sp three carbon atoms, we have an sp two carbon chain. So now let's put put all of that together, number one, the number of carbon carbon, double bonds. And we understand that in this problem, if we want to start from the lowest heat of hydrogenation, we want to make sure that the first structure that we draw is the most stable and the last one will be the least stable. Keeping that in mind factor number one is the most important factor. The number of double bonds, the most the most stable Elkin would have the least number of the double bonds. In other words, we notice that there's one double bond over here, 2323 and one. So we have only to Elkins one and six that have one double bond, but now we're trying to understand which one of the two is more stable and the answer is that structure number six is more stable. So we may label it as our number one in our ranking. So let's just add one over here. So we understand that this will be the first structure. It's more stable than the first one because the first one is a mono substituted alK in while this one is a di substituted substitution increases the stability, the more substituted elkin gets the more stable it does. And because this structure has to alcohol substitute wants is more stable than the one that has only one alcohol substitution. So that's number one. And we can immediately tell that this would be structure too because it still has one double bomb. Now we move on to structures having two double bonds. These are the two structures that have two double bonds, structures two and 4. And essentially when we are comparing them, we noticed that regarding substitution they are equally substituted. There are two substations here and two here, there are two substations over here and two over here, we may say that they are equally substituted. They have the same number of double bonds. What about conjugation? We notice that this is a conjugated ALc in because there is no separation between double bonds. While here we have a separation, this is an sp three carbon. So we may now easily say that this would be structure three because it's more stable while this would be structured for because it's less stable than the previous one. And finally thinking about the remaining two structures, both of them have three double bonds, so they would be the least stable, but the main difference between them is not only substitution or actually let's double check that this has one substitute. Print at the first double bond, we have one substitution over here, so they're pretty much equal. This is the I substituted, this one is di substituted and this one also as mono substituted just as the left side of the chain. So we were right there differently substituted. So that's one of the factors that we wish to take into account, which would conclude that the third structure is more stable than the fifth one. Since it has two double bonds that are di substituted while this one has only one double bond that is di substituted. However, in addition to that structure, number three is also conjugated because the double bonds are adjacent to each other. Structure. # five is not conjugated. It's just partially conjugated because there's a separation over here. So due to being more substituted and conjugated, this would be our next structure in our ranking. And we can say that this structure gets ranked number five and this structure gets number six because it contains the greatest number of double bonds and it's not conjugated. Now, we mean simply state that based on our rank, we can just read to our structures starting from the most stable. Let's count how many carbon atoms we have. That's 1234567. We are going to draw all the change that we need for this problem And we have a double bond at position number three. So that's our first one. It's the most stable elkin followed by the second structure which has that double bond position number one. Then we have our structure number three with 2 double bonds. Structure # four again see double bonds but not conjugated. Structure number five with 3 double bonds conjugated And finally structure number six with 3 double bonds. And basically we can say that it's not conjugated and also less substituted than this out this trying actually, now we got our ranking, we are ready to label this is our final answer. We have all the Elkins ranked in sums of their heat of hydrogenation and we may conclude that the correct answer to this multiple choice question as C. However, if you have any questions, don't hesitate to leave them down below in the comment section and thank you for watching.

Rank each group of compounds in order of increasing heat of hydrogenation.
(a) hexa-1,2-diene; hexa-1,3,5-triene; hexa-1,3-diene; hexa-1,4-diene; hexa-1,5-diene; hexa-2,4-diene

Key Concepts

Heat of Hydrogenation

Alkene Stability and Substitution

Conjugation and Resonance

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