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. 

Explain in detail the differences between the mechanisms giving rise to the following two experimental results.
Draw the major product(s) for the following reaction in the box provided. Indicate stereochemistry where appropriate. When a racemic mixture is formed, you must draw both enantiomers and write RACEMIC. The mechanism of each reaction (SN2, E2, SN1, or E1) is written below the reaction arrow. Therefore draw your product accordingly.
Identify the statement as either True (A) False (B) The leaving group and the departing hydrogen in an E2 elimination reaction has to be on the same side.
Give the major product for the following E2 reaction.
Would this reaction proceed as written? Provide a brief rationale.    
Provide the major product for the following reaction.
Predict the major product(s). In the present case, the reaction needs to be performed at 85°C. In fact, at 70°C the less stable chair conformation can be accessed. 
Draw and label cis and trans-1-bromo-4-tert-butylcyclohexane. Explain which compound reacts faster (via the E2 mechanism) with sodium methoxide (NaOCH3).
One of the stereoisomers below undergoes E2 reactions in the presence of a strong base MUCH faster than the other in the presence of potassium tert-butoxide (KOt-Bu). (Selecting the correct isomer is not enough, your explanation should be sufficiently clear and complete that the reader/grader is left with no doubt that you understand.) a) Construct both the chair and its flipped conformer for the two stereoisomers below  b) Determine which stereoisomer will have the faster rate of reaction: a) or b)  c) Explain this observation mechanistically using full sentences making sure to answer the question of WHY one will undergo an E2 reaction faster than the other.
Predict the product of the reaction:
Menthyl chloride and neomenthyl chloride have the structures shown. One of these stereoisomers undergoes elimination on treatment with sodium ethoxide in ethanol much more readily than the other. Which reacts faster, menthyl chloride or neomenthyl  chloride? Why?
 Which compound will undergo E2 reactions faster? 
For the following transformation, use curved arrow notation (electron-pushing) to show the movement of electrons. Show all formal charges.
The following three reactions are similar, differing only in the configuration of the substrate. One of these reactions is very fast, another is very slow, and the other does not occur at all. Identify each reaction (X,Y, or Z) as fast, slow and no reaction, and provide two rationales for your choice. Provide the elimination product for the reactions that do occur. Finally, provide a rate law using short-hand acronyms (e.g., [RX], [Nu], [Base], etc.) for the species involved.Fast:                ___________Slow:               ___________No reaction:    ___________Rate law:         ___________   
Draw the predominant product expected for each of the following E2 reactions.
For the following reaction, provide a structural formula for the  major organic product(s). If no reaction occurs, write N.R. If the major product is a mix of stereoisomers, give a structural formula for each of the stereoisomers in the mixture. Also, write equal if equal amounts of stereoisomers are formed, or unequal if unequal amounts of stereoisomers are formed.
The syn-isomer under “E2” conditions yields the more substituted alkene whereas the anti-isomer yields the less-substituted alkene.1. Provide reagents for E2 elimination.2. Using “chair” depictions, rationalize the reaction outcomes.
Draw the structure of the major product in box 1. Complete the chair conformation of 1-bromo-2,4-dimethylcyclohexane in box 2 showing all hydrogens.
Identify possible product(s) of dehydrohalogenation of  trans–1–bromo–2–methylcyclohexane.
 Provide the major product for the following reaction. 
Provide the major product for the following compound.
Provide the full mechanism and draw the final product for the following E2 reaction. 
Provide the full mechanism and draw the final product for the following E2 reaction. 
What is the product of the following reaction?
Draw the structure of the product of this reaction. Use the wedge/hash bond tools to indicate stereochemistry. If there are alternative structures, draw the most stable one. If no reaction occurs, draw the organic starting material.
For the following dehydrohalogenation (E2) reaction, draw the major organic product(s), including stereochemistry.
Draw the main organic products of the reaction. Indicate the stereochemistry, including all hydrogen atoms, at each stereocenter. Omit byproducts such as salts or methanol.
Draw the structure of the product of this reaction Use the wedge/hash bond tools to indicate stereochemistry. If there are alternative structures, draw the most stable one. If no reaction occurs, draw the organic starting material.
Which of the following alkyl halides would afford the indicated product upon reaction with sodium ethoxide? 
Draw the product that will be formed in an E2 reaction, and indicate the stereochemistry