E2 - Mechanism Introduction

Time to discuss the most complicated mechanism of the bunch, E2. It’s an awesome reaction, but there are a few extra details we’re gonna have to keep track of!

Concept: Drawing the E2 Mechanism.

Video Transcript

So now I want to talk about a mechanism that competes directly with the SN2 substitution and that's the E2 elimination. So if I were to sum the entire reaction up in one sentence what I would say is this I would say, an E2 reaction happens when a strong nucleophile reacts with an inaccessible leaving group. I'm just going to stop right there for a second.
So if you remember back from what we learned about SN2, there's a little bit of a similarity there. Can you guys tell me what condition is similar to the SN2? The nucleophile. Remember that in an SN2 mechanism, you needed a strong nucleophile to start that back side attack. Same thing with E2, we also want a strong nucleophile.
Where the difference comes in is the leaving group. Remember that if you had a very accessible leaving group what would happen? Back side attack. So remember that back side attack was favored when you have this very accessible leaving group or very accessible back side.
Well for E2, we prefer an inaccessible leaving group. What that means is that these molecules are going to be generally bad at doing a back side attack, so they're going to prefer to do something else instead.
So what is that other thing? What they're going to do is beta-elimination, so elimination of a beta-proton. We're going to talk about that in a second. All in one step. So let's go ahead and get started.
Let's just start the mechanism off and you guys tell me where you think the first arrow's going to come from. Maybe you don't know where it goes, but at least you should be able to tell me where it starts. That's right. It's going to start at the negatively charged nucleophile just like it did for SN2 because this is a strong nucleophile, so it's going to initiate the contact first.
I'm coming over here and you'll notice that I have this nucleophile that wants to do – it sees this alkyl halide. There is a very strong dipole there. There's a partial positive right here. And this nucleophile wants nothing more than to give its electrons directly to that positive charge. So actually don't draw what I just drew yet, I'm just trying to guide you guys through the process.
The nucleophile wants to donate its electrons to that positive but there's a problem. The problem is that if you'll notice, count that carbon up, what you're going to notice is that this is actually a tertiary alkyl halide. Do you guys remember what I said about tertiary alkyl halides? Do they have a really good back side? No. They have a terrible back side. In fact, it's impossible to get through. Just can not get anywhere close.
So now this nucleophile is frustrated. It's like, “Well, I'm a strong nucleophile. I want to do back side attack, but I can't. So what am I going to do?” Well, instead it says instead of acting like a nucleophile and donating my electrons, maybe I can act more like a base. The way that bases act is that they are proton acceptors.
So it's saying, you know what, it's too difficult to do this backside attack, so instead let me just pull off a proton and by pulling off the proton, maybe I can donate my electrons that way. So we're going to go ahead and erase this arrow and that's not actually going to be what happens.
What happens is we're going to look for a beta-hydrogen that we can take off with my nucleophile as a base. So how to find beta-protons, just to remind you guys, would be that this is my alpha-carbon. The alpha-carbon is the one that's directly attached to my alkyl halide and then a beta-carbon is any carbon that's directly attached to the alpha. So this would be beta. This would be beta. And this would be beta. All of those are beta-carbons because they are carbons directly attached to the alpha.
And then any hydrogen that's directly attached to a beta-carbon is considered a beta-hydrogen. So what that means is that I have three beta-hydrogens right here and we might have other beta-hydrogens on those R groups, but the R groups are general so I don't know how many H's there are or not. So the only ones that I'm given here are these three. Does that make sense? So those would all be beta-hydrogens because they're hydrogens directly coming off of the beta-proton, I mean coming off of the beta-carbon.
So like I said, we're going to pull off a beta-hydrogen instead. Let's go ahead and draw that the nucleophile attacks that hydrogen right there.
Now is that hydrogen happy with that mechanism? Can I just leave it there? The answer is no. I cannot just leave it there because hydrogen only wants to have one bond and now it has two. So if I make that bond I'm going to have to break a bond. And this is the interesting part. We're going to take the electrons from the bond from the carbon to the hydrogen. We're going to give those electrons to that single bond. Basically, the bond in between the alpha and the beta is going to get a double bond. So alpha double bonded now to beta-carbon.
So now I have a double bond there. Is that the last arrow? Actually, it can't be because this alpha-carbon had four bonds already and now by making a double bond it would have five bonds. So if I'm going to make that bond, then I'm going to have to break another bond and the easiest bond to break is the one for the leaving group because remember the leaving group is going to be stable after it takes off, relatively stable.
Let's go ahead and draw our transition state. What our transition state is going to look like is like this. I'm going to draw everything that didn't change in the reaction with a solid line. So what that means is that I would have an H in the front and an H in the back that nothing ever changed. I would also have an R in the front and an R in the back that never changed. Those are the things that during the course of the reaction, nothing's happening to them.
But what is changing is that a bond is being broken and destroyed at the same time between the H and between the leaving group. So the reason I drew it with partial bonds is because this is a one-step reaction. So what that means is everything is happening at the same time. The H bond is being broken, the double bond is being made and the leaving group is leaving all at the same time. Awesome.
Now there are too many bonds here, so there'd be a negative charge distributed throughout. I'm just going to write the negative on the outside that just shows that the entire thing is negatively charged because we have one too many bonds.
And now what I want to do is show you one more unique thing about the E2 mechanism in particular. This only has to do with E2. What it is is that if you were to take a Newman projection of this transition state. Now I know it's been a really long time since we talked about Newmans, so try to unbury that information. I know you had already buried it or whatever. So try to think about a Newman projection. Remember that that was a way to visualize single bonds.
So here's my eyeball and if I were looking down the center of that bond, what would I see? Well, what I would see is that on the top I have two H's. And what do I have coming off the bottom? The bottom what I would have is a partial bond to an H. Then what would I get in the back? What I would get in the back is I have those two R groups with single bonds, so R-R, but then on the front I would have a partial bond to my halogen.
So there you have it, that's what the transition state is actually going to look like. And the unique thing about E2 is that the transition state will always look like this. It's always going to have that conformation where my X and my H are as far apart from each other as possible.
Do you guys remember what that conformation is called? Remember that's 180 degrees apart and 180 degrees apart equals anti. So it turns out that whenever you have an E2 elimination because of what's favored, the way that it's favored, it's only going to react once the X and the H are perfectly anti to each other or 180 degrees apart. And later on, I'm going to teach you guys how that's really important in predicting products.
But just to let you guys know, these two H's that I drew at the top would not have actually been able to react unless they rotated down into the anti position. So really even though I said that I had three beta-hydrogens, in this reaction, I only had one that was in the proper position to be eliminated. All right? Cool. Don't let that get you too confused because, like I said, we're going to have an entire section dedicated to this one thing about the anti.
So then what would the product look like? Well, we know that the H gets removed. So I'm just going to chop it off, even though this isn't part of the transition state, but we're going to chop off the H. We also know that the X gets removed. So what you get left is just a double bond in the middle with H's and R's on both sides. So that's what my product would look like. It would just be a double bond with two H's on one side and two R's on the other. That is an elimination reaction.
What I just did was I took two sigma bonds, this was a sigma and this was a sigma, I destroyed those bonds and I made a new pi bond. And that's the definition of elimination. You take two sigmas and you make one pi. Awesome.
Other things that I would get are just my leaving group and then my nucleophile with an H on it. Easy. 

Summary: A negatively charged nucleophile reacts with an inaccessible leaving group to produce beta-elimination in one-step.

Properties of E2 reactions:

  • Nucleophile =  Strong
  • Leaving Group =  Substituted
  • Reaction coordinate = Transition State
  • Reaction = Concerted
  • Rate =  Bimolecular
  • Rate =  k[Nu][RX]
  • Stereochemistry = Anti-Coplanar

Example: Rank the following alkyl halides in order of reactivity toward E2 reaction.