In Section 10.8.1, you learned that alkenes react more quickly...

Question

In Section 10.8.1, you learned that alkenes react more quickly with electrophiles than do the corresponding alkynes (k_alkene/k_alkyne > 1). Explain why there is a greater disparity in the alkene versus alkyne reactivity in the addition of HBr as compared to the addition of Br₂ [The rate data are not real, but are meant to illustrate a real trend.]

Accepted Answer

Welcome back everybody. Our next problem says in general, alkenes are more reactive to electro files than their corresponding alkins. However, this reactivity difference is much greater and more evident in the addition of HBR as compared to the addition of BR two, explain why this is the case. And then we have a little quick diagram that says in terms of relative rate and it shows an alkene reacting with BR two. So diatomic bromine and we have our product of a vicinal di Halide. So bromine added to each of those carbons and it's now a single bond and this is greater than the reaction we're talking about the relative rate. So the relative rate is greater than the reaction of an alky VR two giving AC C double bond with BRO means on each carbon. And then we're shown down below the reaction of al alkene with H pr hydrogen bromide. And we've just added one bromine to one of the carbons and we see two greater the signs. So a much greater relative rate than the reaction of an alky with H Pr adding just one bromine to one of the carbons and going from a triple bond down to a double bond. So why do we see this difference in terms of the relative effect in the reaction rate being so much better with the reaction of H Pr versus BR two. Well, to understand that we need to look at the mechanisms of these reactions and what we will want to look at in particular is the rate determining step for each one. And how the difference between an alkene and alky affects that rate determining step. These editions both involve roaming being added, but they have two quite different mechanisms. So in terms of the reaction with BR two, we have the formation of a bridged intermediate and that bridge intermediate has a structure where we formed a triangular three member ring. So you have the two carbons and the bromine, each carbon having a bond to the bromine and a positive charge on the bromine. But in the case of the alkene, there's a single bond between the two carbons as part of this ring. In the case of the alky, there's still a double bond within this ring, you've only used one set of pi electrons to form that bridge intermediate. So the key factor here is ring strain. And when we think about ring stream, we want to think about bond angles and the bond angles are different here because you have different hybridization. So for the alkene, those carbons have an sp three, whereas the carbons in the al kine that now has a double bond are sp two hybridized. So the sp two hybridized carbon whose desired bond angles are 100 and 20 degrees is going to experience a lot more ring strain being constrained to that 60 degree angle in a triangular ring than the sp three hybridized carbons with their 109.5 degree preferred bond angle. So the ring strain is going to be greater in the bridge and intermediate form from the alky. So with the ring strain being greater, this is a less stable intermediate and a less stable intermediate. Of course, we recall will lead to lower reactivity and a slower reaction rate. So that's what's going on with the BR two reaction. Now, let's look at the HBR reaction. In this case, we have the formation of a carbo cion. You have your pi electrons from the carbon carbon double bond or triple bond, grabbing a proton from the HBR and then forming a Carbocaine with the addition of that proton. And the same thing of course happens with the alky one set of py electrons coming over and grabbing a proton. And in that product is called a vinyl carbohyd where you have a double bond and the positive charge is on one of the carbons in the double bond. Now, that can be a little confusing because we're used to thinking about perhaps resonance stabilized carbo cion, a lilic Carbocaine where you have the positive charge on a carbon adjacent to a double bond, that's a more stable arrangement because the resonance stabilizes them. In this case, the positive charges on one of the carbons in the double bond. And this is a less stable carbo Cron. And this can be a little bit tricky to understand why this is the case, but it also comes down to hybridization. So recall that when you have a Carbocaine, you have an empty orbital. And so in the case of the alkene or I'll heal carbo cat with two bonds to carbon atoms, one bond to a hydrogen atom is sp two hybridized. Whereas the carbons in our vinyl Carbocaine are sp hybridized. And this one's a little tricky to think through. But sp hybridized orbitals have more s character than sp two hybridized orbitals. And that means they're lower in energy than the sp two orbitals and being lowered in energy that makes the atom more electron. So in this case, you have a positive charge on a more electron atom. And that's why it's a less stable carbo clam. So we have a less stable intermediate as we had in our other uh example of the reaction with BR two. But we have one more factor here which is that these electrons have to participate in grabbing this proton in sharing their electrons participating in this electro pilic attack. And the pi electrons of alkenes are more loosely held than the pi electrons of all kinds. And again, this comes back to hybridization and the fact that the electrons in the sp hybridized orbitals are lower in energy, they're closer to the nucleus and the electrons in the sp two hybridized orbitals of an alkene. So when we look at this overall picture, the reaction with HBR has two factors making that reaction faster or the reactivity f or higher reactivity of an alkene versus an alky. Whereas in the reaction with BR two, we're just talking about the stability of the bridge intermediate, just the one factor. So that's why we have the reactivity difference being much greater and more evident in the addition of HBR as compared to the addition of BR two. So we have the factor of the pi electrons of the alkenes being more loosely held and the fact that the Carbocaine that forms is less stable in the case of all kinds than alkenes. So we've got these two factors working together causing a greater effect on the difference in reactivity than in the case of BR two where you're just talking about the stability that bridged intermediate. See you in the next video.

Key Concepts

Electrophilic Addition Reactions

Carbocation Stability

Sterics and Electronic Effects

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