9. Alkenes and Alkynes
Not all alkenes were created equal. Like carbocations, alkenes are stabilized through a phenomenon called hyperconjugation.
Hyperconjugation allows adjacent -R groups (mostly C-C and C-H σ-bonds) to create shared molecular orbitals with π-bonds, stabilizing the bond.
Understanding trends of alkene stability.
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So it turns out that all kinds are stabilized by a force called hyper conjugation. Okay, so let's go ahead and write this down hyper conjugation and as you guys have already learned or will learn hyper conjugation is a force that also stabilizes carbon cattlemen's. So in fact, when we talk about all kinds many times, we'll talk about carbon Catherine's hand in hand because it's the same exact force that's stabilizing both of them. So how does this work? Basically what it means is that in a double bond, I have a pi bond that's being formed by two overlapping p orbital's Okay, that pi bond can be stabilized by having sigma bonds that are close by share electrons with it. Okay, and that's exactly what happens in hyper conjugation. So, as you'll notice what I have here is a double bond overlapping on the top of the bottom and then what I have is an adjacent sigma bond right here between the carbon and the age. And it turns out that the more groups that I have, the more hydrogen I have overlapping their bonds with the bonds from the with the pi bond, the more stable that pi bond is gonna be because these electrons are gonna be able to share and basically donate a little bit of a little bit of their density to the double bond. Alright, so what that means is that the more our groups that I have around my double bond, the more stable it's going to be all right. And that's exactly going to be the trend that we use to determine the most stable alkaline. So basically, since this phenomenon of hyper conjugation is only possible with our groups, the more substance to the alkaline, the more stable it is. Okay. And that leads us to the following trend. The following trends of alkaline stability just has to do with how many are groups can I pile around a double bond? So as you can see, the best kind of double bond possible would be called tetra substituted. Okay. Why? Because it's just four groups? That means I have the maximum number of our groups right around my double bond as I start taking groups away and replacing them with H. That's going to reduce the amount of hyper conjugation that can stabilize my double bond. So as you can see, tri substituted would be next, then die, then mono. And then finally the worst. So sad face over here is un substituted because un substituted can't hyper congregate at all. There's nothing that can donate electrons to that pi bond. So it's pretty much just gonna be unstable. Okay, now, it turns out that for the for the purposes of di substituted, there's actually a few different ways that we can order those r groups. Let's go ahead and look into that more basically, when you have a di substituted double bond, you have options. It's not like you're just gonna have one type of di substituted, you could have them where both our groups are facing the same side of the double bond, that would be sis you could also orient them so that both sides are facing opposite sides of the trans. And then finally you could orient it so that both of our groups are on the same corner of the double bond actually coming off the same carbon. And this is a word called gem which stands for the word germinal. Okay, just you guys know gem Inal is a position word, it's actually a word that will use more in order one or go to later. But all it means is that I have two things coming off the same carbon. The way that I like to think about it is gem Inal is like the word gemini and gemini means twins. Right? So it's like you have two things coming off the same carbon. These are groups are like twins, they're both coming off the same carbon. Okay, so for whatever reason it is um germinal is going to be most stable, then, trans is going to be more stable than than trans is gonna be the second most stable and then sis is gonna be the least stable. Now the between cis and trans is really easy to understand because cece these groups are kind of interfering with each other, they're in each other's space whereas trans, they're facing opposite to each other so they're more stable, there's more room to breathe now why gemini is more stable than trans. I'm not exactly sure but it's just something that you guys can memorize and you guys can know it for your test. Alright, So here's a really easy question. Just following up on what we just talked about, We have four al kinds here, go ahead and try to rank them in order and stability. And then when you're done, I'll go ahead and answer it.
So basically, the more –R groups attached to the double bond, the more stable the double bond will be.
Specifically, when it comes to di-substituted bonds, the order os stability is gem > trans > cis.
- trans > cis due to steric hindrance. Groups have less freedom of movement in the cis position.
- gem > trans due to better hyperconjugation. The full explanation for this trend is beyond the level of this course. Just memorize it.
Rank the following alkenes in order of lowest to highest heat of combustion.
Heat of Combustion
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all right. So the first place we needed to start was we need to figure out what is lowest to highest heat of combustion. Okay, so remember earlier when we talk about heat of combustion, what heat of combustion has to do with is it's a measure of energy. So if you have a high heat of combustion, Okay, So high entropy basically, then what that means is that you have high energy, okay? And if you have high energy, that means you have low stability. Okay, So if it's saying here in order of lowest to highest heat of combustion, that means I want to start with lowest, which is gonna be the most stable and end with the highest, which is gonna be the least stable. Does that make sense? So even before this question began, there was a little bit of thinking that you needed to do. Now that we have that figured out, let's go ahead and order these guys. So we know that there's definitely gonna be a winner here. And that winner is gonna be number one. Why is that? Because if we identify the types of substituted dull bonds, we have this one is Try substituted. Okay? Because my double bond doesn't count, and it has three branches coming off of it. 123 So, three yellow things. Let's just call it like that Really easy. Okay? Two is gonna be mono substituted. Why? Because if I circle adult bond, not including that I only have one branch coming off of it. So one yellow area. So that would be mono. We know that's not gonna be very good. This one would be Di substituted. Because I have two branches. 12 die. And then this one would also be diet. Oops, I did that wrong. This one would also be di substituted because I have two branches, one here and one here. So now all I have to do is have to figure out which of these is the most stable. Which of these is the least I know the most stable is going to be number one, because that's try substituted. And I also know that the least stable is gonna be number two. Because that one is mono substituted. The hard part comes between numbers three and four. Because both of these air di substituted. So how do I tell which one's better. The one that's better is gonna be the one that is general, and the one that's worse is gonna be the one that assists. Because for the cyst one, both of my groups are facing the same side of double bonds. That's really bad for the gem inal they're facing on the same carbon, which is actually good. So what that means is that three is gonna be more stable than four. But still four is gonna be more stable than two because four is actually di substituted and two is only model substituted. So in that case, this is better than dice. Attitude is better than just mono substituted. Alright, guys, that's a really basic concept. This is an easy question on your exam. Most likely, you will get a question like this. So this should be free points for you guys. All right, hope that made sense. Let's go ahead and move on to the next topic
Additional resources for Alkene Stability
PRACTICE PROBLEMS AND ACTIVITIES (7)
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