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

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. 

3m

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

3m

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

4m

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

3m

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.

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. 

2m

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

2m

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

4m

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

4m

Problem: Predict the product

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Problem: Predict the product

2m

Problem: Predict the product

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