E2 - Recognizing Beta-Hydrogens

In order to predict E2 products, we’ll have to get good at recognizing how many different and eligible β-hydrogens exist.

Recognizing Different Beta-Carbons

Concept: The number of unique β-carbons helps predict the number of possible products. 

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Video Transcript

Now I want to dig deeper into one of the more important parts of elimination and that's the beta-hydrogen.
All right, so you guys might remember that elimination reactions basically pull off beta-hydrogens and then they make double bonds. If you think back to the definition of elimination what we remember is that – let's say that you have a single bond to a leaving group and a single bond to a hydrogen, what winds up happening is that these two sigma bonds get pulled off and turned into one pi bond. That's the whole process of elimination. That's actually the definition is that two sigmas turn into one pi.
So that's not bad. But the tricky part comes in with the beta-hydrogens because it turns out that rarely will you just have one beta-hydrogen that applies towards this rule. Many times you're going to have several beta-hydrogens that you have to choose from. On top of that, that complicates things more because if you choose a different beta-hydrogen to extract that might actually make a new product. What that means is that we're opening ourselves up for the possibility of multiple products.
In this page, what I want to do is just really practice how to determine if you're just going to get one product or if you have a possibility of up to three products. Usually, that's the maximum amount of products you can get, three, because that's the maximum amount of beta-carbons you can have, three.
Let's go ahead and talk about how to figure that out. The way you figure that out is by counting the number of non-equivalent beta-carbons. So remember that the beta-carbon is attached to the alpha-carbon. I'm going to go through all this again, so it's fine. If those beta-hydrogens have at least one H, that's going to be its own unique product. So for every beta-carbon that's unique, that has it's own H that can be taken off, that's going to represent one possible product.
Like I said, sometimes you're just going to only have one product. Only one of the beta-carbons will have a hydrogen on it. But other times you're going to get up to three products and that's what we're going to do now.
I want you guys to look at example (a) and try it yourself. Try to figure out exactly how many different products you can get from (a) by looking at the beta-carbons and seeing if they have hydrogens. And then I'll explain the entire question, how to do it. So go ahead and get started. 

Elimination reactions remove β-hydrogens to create double bonds.

  • The number of non-equivalent β-carbons with at least one -H determines the number of possible products.

Example: Identify the number of unique products that could be obtained through elimination. 

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Example: Identify the number of unique products that could be obtained through elimination. 

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Example: Identify the number of unique products that could be obtained through elimination. 

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Example: Identify the number of unique products that could be obtained through elimination. 

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E2: Anti-Coplanar Requirement

Now we know how to find β-hydrogens, but it turns out that E2 reactions require an anti-coplanar arrangement (also called anti-periplanar) in order for the orbitals to overlap and create a new pi bond.

Concept: The number of unique β-carbons in an anti-coplanar arrangement predicts the total number of products. 

4m
Video Transcript

All right guys, so now we're going to talk about a really important topic that only applies to the E2 mechanism and that's called the anti-coplanar requirement. As I told you guys already, E2 reactions are going to require an anti-coplanar arrangement between the leaving group and the beta-hydrogen in order to go to completion. And that's because the orbitals need to overlap in a certain way in order to make a new pi bond, which is that double bond that you get at the end. So that's the first thing we need to know.
Now, not only are we going to have to look at how many different beta-hydrogens we have, but now we're going to have to look at an extra level of complexity which is how many of these beta-hydrogens can be in the anti position or are in the anti position.
That means that we're going to require two steps to figure out the amount of products that we have. First, we're going to look at beta-hydrogens and then after we've figured out the number of beta-hydrogens, we're going to figure out are they anti-coplanar or not.
On top of that, there's one more thing you guys should know which is that when you have a leaving group and a beta-hydrogen on a cyclohexane, that's actually going to form a chair. Remember that cyclohexanes usually are in the chair conformation. When you're dealing with an elimination on a chair, instead of calling it anti-coplanar, we're actually going to call it a diaxial requirement. Instead off – this is the same thing as anti-coplanar.
Why is that? Why do I say coplanar? Why do I say diaxial? Because the only way that the leaving group and the beta-proton can be anti to each other is if they're on adjacent axial positions. The reason is because think about the equatorial positions. The axial positions go like this, the equatorial positions go like this.
Let me see. I'm doing this all wrong. But let's say that the axial positions are like this, the equatorial positions do this. That's not an anti arrangement, that's actually like a gauche or something like that.
So in order for the elimination to occur, you're going to need to rotate a chair to the axial position first even though that's the less stable position and that actually has something to do with it as well. Even though this is less stable, I need to rotate it like this in order to make my reaction happen because I need my groups to be anti, not gauche.
That looked like I was doing a really weird dance, so I hope you guys enjoyed that.
What we're going to do here is a really quick practice, not a lot of drawing. In fact, I don't want you to draw anything yet. All we're analyzing is would these E2 reactions happen or not. Notice that I have a strong nucleophile and I have either a secondary or a tertiary alkyl halide. Remember that I said secondaries and tertiaries can do an E2 because they have a bad back side, or not that great.
So I want you guys to figure out first of all how many beta-hydrogens you have. How many different beta-hydrogens would you have? And then once you figure that out determine would they be anti-coplanar or not in order to make the reaction occur. So this is two steps. First of all, do the same thing that we did for the beta-hydrogen exercise. Figure out how many different ones we have, but then on top of that, figure out how many of those are actually anti-coplanar and that's going to be the number of possible products for E2. All right, so go ahead and try it with the first one and then I'll explain it.

On a cyclohexane chair, the leaving group and β-hydrogen must be DIAXIAL to each other in order to fulfill the anti-coplanar requirement. 

Example: Identify if any of the following E2 mechanisms would not react to completion. Do not draw final products. 

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Example: Identify if any of the following E2 mechanisms would not react to completion. Do not draw final products. 

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Example: Identify if any of the following E2 mechanisms would not react to completion. Do not draw final products. 

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Example: Identify if any of the following E2 mechanisms would not react to completion. Do not draw final products. 

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Problem: Predict the product

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Problem: Predict the product

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Problem: Predict the product

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