E1 Reaction - Online Tutor, Practice Problems & Exam Prep

The E1 elimination mechanism occurs when a weak nucleophile reacts with a highly substituted leaving group, similar to the SN1 mechanism. Both processes involve the formation of a carbocation as an intermediate, leading to a mixture of substitution and elimination products. The rate of E1 is unimolecular, dependent solely on the leaving group, not the nucleophile. Stereochemistry is not a concern in E1, as the carbocation can be attacked from any direction. This complexity can yield multiple products, complicating synthetic applications in organic chemistry.

Drawing the E1 Mechanism.

So now I want to talk about a mechanism that competes directly with the SN1 mechanism and that's called the E1 elimination mechanism. Alright. So if I were to sum up the entire mechanism into one sentence, what I would say is this. E1 occurs when a weak nucleophile reacts with an inaccessible leaving group. So let's just start right there. Okay? Do you guys remember another mechanism that we talked about that had similar conditions? Yeah, right? The SN1. Let's talk about the nucleophile part first. We said that SN1 is favored with what type of nucleophile? Strong or weak? Well, remember that strong favored the SN2 and E2. Okay? So weak is going to favor SN1 and E1. Why? Because remember that SN1 I'm sorry, remember that a weak nucleophile would not start the backside attack first. It would wait for something to happen. Okay? So remember that the weak nucleophile was just chilling and sitting back and waiting for something to happen.

Well, then what's the other condition? The other condition says that it's with an inaccessible leaving group. Inaccessible really is just another word for tertiary. Right? It's basically as many R groups as possible. Why was that good? Well, the reason that's good is because remember that tertiary alkyl halides would make really good carbocations. And remember that carbocation was the rate-determining step for SN1. Is this all kind of coming back to you guys? Well, it turns out that the same condition is also good for E1. E1 also likes to have an inaccessible leaving group. So if we have the same exact conditions that are good for both mechanisms, how do we know when one is going to happen or the other is going to happen? And it turns out we don't. So it turns out that these same exact conditions are also going to produce beta elimination in 2 steps. What that means is what we're going to find out is that SN1 and E1 always compete with each other and they're almost impossible to separate. What that means is that every time you see an SN1 reaction, remember I taught you SN1, E1 is also occurring and you really can't do much to stop it. And that's one of the messy things about these reactions is that they're often going to lead to a mixture of products and synthetically, that's not very useful for us. Okay? When I say synthetically, that's a word I'll use a lot more later as we get into Orgo 2, synthesis is a huge end goal of organic chemistry. Eventually, what you're going to be doing, get excited, you're going to be trying to synthesize a target compound. Your professor is just going to be like, hey, make this. And you're just going to have a list of reagents and you're going to have to put them together. Okay? And SN1 and E1 aren't very helpful for synthesis. The reason is because you get a mixture of products and that's bad because then you don't know exactly what you're getting at the end. Okay? So let's just go ahead and talk about this mechanism and see how you could get the SN1 or the E1. Alright?

So here I've got, as you'll see, a tertiary alkyl halide. And the first step is going to be the same, identical for both of these reactions. So what's that first step? Can you guys tell me? Is it backside attack? No. Okay. Why? Because my nucleophile isn't strong, so it can't initiate anything. Remember, it's not one of those arrows. It doesn't do that. So it's just going to wait around. What actually happens is that the leaving group leaves by itself. Remember that part? It's kind of weird but it dissociates. What that's going to do is it's going to give me an intermediate that looks like this. Single bond, H's are exactly the same, H H H. Okay. But now what I have is a carbocation. That carbocation came from the fact that the X just left by itself without getting anything in turn, so electrons are leaving. Okay? And then finally, see how I have R1 and R2? Those would still be here as well. So I'd have R1 and R2.

Cool? So it doesn't matter which one you put there because this is a carbocation, so you don't actually know what you would get. Okay? So now what? Well, it turns out that this carbocation is going to do what next? The nucleophile is going to react with it. Right? Cool. So my nucleophile would want to react with it and the first thing you would think is that the nucleophile has electrons to give. Remember, it doesn't have a negative charge but it does have a lone pair, so it could attack directly my carbocation. And it turns out that that is going to happen. That nucleophile is going to directly attack that carbocation and that's called the SN1 mechanism. So what we would do here is we would draw the SN1 product and the SN1 product would just be that I have now 2 R groups again. Okay. And I would have these H's are exactly the same. Okay. But then, I would get a nucleophile attached to wherever that carbon was. Okay?

Now, notice that I ...

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Here’s what students ask on this topic:

What is the E1 elimination mechanism in organic chemistry?

The E1 elimination mechanism is a two-step process where a weak nucleophile reacts with a highly substituted leaving group, leading to the formation of a carbocation intermediate. The first step involves the leaving group dissociating to form a carbocation. In the second step, a base removes a proton from a β-carbon, resulting in the formation of a double bond. This mechanism is unimolecular, meaning the rate-determining step depends solely on the concentration of the substrate, not the nucleophile. E1 reactions often compete with SN1 reactions, leading to a mixture of products.

How does the E1 mechanism differ from the E2 mechanism?

The E1 and E2 mechanisms differ primarily in their reaction steps and conditions. E1 is a two-step process involving the formation of a carbocation intermediate, favored by weak nucleophiles and highly substituted leaving groups. E2, on the other hand, is a one-step concerted mechanism where the base removes a proton as the leaving group departs, favored by strong bases and less substituted leaving groups. E1 reactions are unimolecular, while E2 reactions are bimolecular, meaning the rate of E2 depends on both the substrate and the base.

What factors influence the rate of an E1 reaction?

The rate of an E1 reaction is influenced by the stability of the carbocation intermediate and the nature of the leaving group. Highly substituted carbocations (tertiary) are more stable and thus favor E1 reactions. A good leaving group, which can easily dissociate, also increases the reaction rate. The nucleophile's strength does not affect the rate, as the rate-determining step is the formation of the carbocation, making the reaction unimolecular.

Why do E1 reactions often result in a mixture of products?

E1 reactions often result in a mixture of products because they compete with SN1 reactions under similar conditions. Both mechanisms involve the formation of a carbocation intermediate, which can either undergo nucleophilic substitution (SN1) or elimination (E1). Additionally, multiple β-hydrogens can be abstracted during the elimination step, leading to different alkenes. This complexity makes it challenging to obtain a single, pure product, complicating synthetic applications.

What is the role of the carbocation intermediate in an E1 reaction?

The carbocation intermediate plays a crucial role in the E1 reaction mechanism. It is formed in the first, rate-determining step when the leaving group dissociates from the substrate. The stability of this carbocation significantly influences the reaction rate, with more substituted carbocations being more stable. In the second step, a base removes a proton from a β-carbon, leading to the formation of a double bond and completing the elimination process. The carbocation's ability to be attacked from any direction also means stereochemistry is not a concern in E1 reactions.