Alkene stability

- [Narrator] In an earlier video, we looked at the degree of substitution of alkenes, and that's going to help us when we're talking about alkene stability. So in general, more substituted alkenes are more stable than less substituted alkenes. So a di-substituted alkene is more stable than a mono-substituted. A tri-substituted is more stable than a di-substituted, and a tetra-substituted is the most stable of them all. So on the left we have a mono-substituted alkene. We have one alkyl group bonded to this carbon of our double bond. On the right is a di-substituted alkene. Now we have two alkyl groups and the di-substituted alkene is more stable than the mono-substituted alkene. To explain why, we need to go back to the idea of stability of carbocations that we saw in an earlier video. On the right we have a secondary carbocation. So this positively charged carbon is directly bonded to two alkyl groups, so this is a secondary carbocation. And the positively charged carbon is sp2 hybridized. So this carbon is sp2 hybridized, and so the geometry around it is planar. So let's go back to the picture on the left and we can see the geometry around that carbon is planar. And this positively charged sp2 hybridized carbon, just going to go ahead and mark this down here as being sp2 hybridized, should have an unhybridized p-orbital. On the picture here that's what the paddles are supposed to represent. This is our unhybridized p-orbital. And to stabilize this positive charge on this carbon, we have two methyl groups. So this methyl group and this methyl group are both electron-donating through an effect that is called hyper-conjugation. So here's one of the methyl groups on the left side, and notice the orientation of this methyl group. This carbon-hydrogen bond is able to donate electron density into the p-orbital on this sp2 hybridized carbon, and that stabilizes the carbocation. Same thing for this methyl group over here. It can donate some electron density into the p-orbital on this sp2 hybridized carbon, stabilizing the positive charge, and that's an effect called hyper-conjugation. This hydrogen here can't do anything because of the geometry so this bond doesn't have the right geometry to help stabilize the carbocation. So alkyl groups help to stabilize the positive charge on a carbocation. That's a similar idea with our alkenes. So here we have our alkene, and this carbon is sp2 hybridized, and so we have these alkyl groups, which we know are electron-donating, and we know that they can donate some electron density to this sp2 hybridized carbon. Sp2 hybridized carbons are more electronegative than sp3 hybridized carbons. So donating electron density can help stabilize this sp2 hybridized carbon, which stabilizes the overall alkene. So that's why we see more substituted alkenes being more stable than less substituted alkenes. Now let's do some examples. Let's rank these three alkenes in order of stability. So let's start by classifying them according to their degrees of substitutions. We'll start with the first alkene right here. And we look for the two carbons across our double bond. Sometimes it helps to draw in hydrogens. So once we've done that, it's clear that we have two alkyl groups bonded to this carbon. So this must be a di-substituted alkene. So let me write that down here. Let's move on to the middle one. So here are the two carbons across our double bond, and again I think it's often helpful to put in your hydrogens. So when you do that, it's clear you have only one alkyl group this time, and so this is a mono-substituted alkene. And then let's look at the one on the right. So here are the two carbons across our double bond, and the carbon on the left would have only one hydrogen here so that's one, two, three alkyl groups, so this is a tri-substituted alkene. We know that, in general, the more substituted alkenes are more stable than the less substituted alkenes. So out of these three, the most substituted would be the tri-substituted. So this is the most stable of these three. So next would be the di-substituted alkene. And finally the least stable one would be the mono-substituted alkene. So this one would be the least stable, and the tri-substituted would be the most stable. Next let's look at two isomers of each other. So we've talked about cis-2-butene and trans-2-butene. They're both di-substituted alkenes. So it turns out that trans-2-butene is the more stable of the two. So this one is more stable. And we can think about that in terms of steric hindrance. If we look at cis-2-butene, we have these methyl groups, relatively bulky, and they would sterically interfere with each other if they're on the same side of the double bond. So this steric hindrance destabilizes the cis-2-butene molecule. So we have increased steric hindrance decreasing the stability of cis-2-butene. For trans-2-butene, these methyl groups are on opposite sides of the ring, so they're far away from each other. That decreases the steric hindrance and that's the reason for why trans-2-butene is more stable.