A wayward chemist proposed the following mechanism for the add...

Question

A wayward chemist proposed the following mechanism for the addition of HBr to an alkene.

(a) Why is this mechanism unlikely?

(b) Compare the reaction coordinate diagrams for the actual mechanism studied in Section 8.3.1 and this alternate mechanism on the same graph.

Accepted Answer

Hi, everyone. Let's take a look at our next problem. It says an alternate mechanism proposed for the addition of HBR to an alkene is shown below. Number one, explain why this mechanism is highly unlikely. Number two, draw the energy diagrams for the actual mechanism and the alternate mechanism. So the alternate mechanism proposed, we see we have a cyclops and HVR and it shows the electrons from the cc double bond from that pi bond attacking the bromine atom. And then the electrons from the brh bond to going on to the hydrogen. In the next step, we have a Carbocaine intermediate, a positive charge on one of the carbons from the cc double bond with a bromine added to the other carbon. And we've produced a hydride ion from the last step. This hydron ion is attacking the positively charged carbon resulting in our product of bromo cycling. So the bromine and the hydrogen have been added. So first of all, why is this highly unlikely? Well, in step one, we have electrons attacking a bromine rather than a hydrogen. So when we look at HBR, the bromine has a partial negative charge, it's much more electron than hydrogen and much larger. So it is very unlikely that you would have a, you're trying to force electrons onto something that already has a partial negative charge instead of the hydrogen, which has a partial positive charge. Essentially, you're trying to force electrons onto an already electron dense atom. So that would be why this is highly unlikely. Now, we're supposed to draw an energy level diagram for this mechanism versus the actual mechanism. So first, let's look at the actual mechanism. So we draw our cyclopse and we'd expect our electrons to attack the hydrogen of our H pr with the electrons from that H pr bond. Then going on to the electron roaming instead, that leads to the intermediate with the carbo cion, we've added hydrogen onto the carbon and then our negative bromine atom now bromide. So br minus attacks that positively charge carbon and we form our Bromma product. So first, let's skip the energy level diagram for the actual mechanism. So we have our chart, we have energy on the left side, our energy level and then time on the bottom the reaction proceeding. So they're both going to start scroll up a little in the same place. We start down low and we'll have our initial intermediate as the electrons in the pi bond attack the hydrogen. So that'll be our initial rate determining step. So the highest sort of peak there. So we'll say rate determining for the first step. So there is our highest activation energy. Then we'll have a little dip for the Carbocaine intermediate, all that and then another little peak lower for the next transition state M and down an energy to R product. And I actually need to redraw that because I started my uh initial reactant too low and ended up with a product that was higher in energy than my reactant, which is not going to happen. So let's start up a little higher. Try that again, my peak for my activation energy coming up dip for my Carbocaine intermediate, second peak for my second transition state and then down to the bottom for my products, lower energy state. So I'll draw my little Carbocaine intermediate. You remember that's what that little dip is for. So this blue curve is the actual mechanism. Now, I'll draw on red, what I'd expect to see with my alternate mechanism will start in the same place. But our initial step will be much higher because we're trying to force the electrons onto this electron atom. It's going to take a lot of energy to do that. So starting at the same point, that first step will be much higher. I'll still have a dip for Carbocaine intermediate and recall when we're looking at the uh energy levels of our products along the way that that energy level is determined. So I'll put e by the bonds broken. So the energy you have to put in to break the bonds minus the bonds formed since energy is released and they form, we're starting off with the same bonds being broken. But in our alternate mechanism, we're forming a carbon bromine bond and this is going to be lower in energy then a carbon hydrogen bond. So we've started the same bonds broken, but we subtracted less energy. So our intermediate state will be a higher energy than our, of our alternate mechanism will be a higher energy than our original mechanism. So we still have this higher energy level of our intermediate Carbocaine intermediate. Then the activation energy for a second step will be slightly lower because our intermediates are less stable and more reactive. So it doesn't go up quite as high in terms of relative height there. But it's still the overall curve is higher than the actual mechanism and then down to the final product, which will be the same. So you can see the whole curve for the alternate mechanism is higher hm than the actual mechanism. So there you go, we have our explanation of why this alternate mechanism is highly unlikely. And our energy diagrams for the actual mechanism and the alternate mechanism showing that this alternate mechanism has higher energy in all the intermediate stages. Thus making it much less likely. See you in the next video.

A wayward chemist proposed the following mechanism for the addition of HBr to an alkene.

(a) Why is this mechanism unlikely?
(b) Compare the reaction coordinate diagrams for the actual mechanism studied in Section 8.3.1 and this alternate mechanism on the same graph.

Key Concepts

Electrophilic Addition Mechanism

Carbocation Stability

Reaction Coordinate Diagrams

Watch next