Ch. 8 - Elimination ReactionsWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

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.

 

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

Concept #1: The number of unique β-carbons in an anti-coplanar arrangement predicts the total number of products. 

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.

Time for some worked examples together. Who's ready?

Example #1: Identify if any of the following E2 mechanisms would not react to completion. Do not draw final products. 

Example #2: Identify if any of the following E2 mechanisms would not react to completion. Do not draw final products. 

Example #3: Identify if any of the following E2 mechanisms would not react to completion. Do not draw final products. 

Example #4: Identify if any of the following E2 mechanisms would not react to completion. Do not draw final products.