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

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.

Concept #1: The number of unique β-carbons helps predict the number of possible products. 

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. 

For the following molecules, identify the number of unique products that could be obtained through elimination.

Example #1: Identify the number of unique products that could be obtained through elimination. 

Example #2: Identify the number of unique products that could be obtained through elimination. 

Example #3: Identify the number of unique products that could be obtained through elimination. 

Example #4: Identify the number of unique products that could be obtained through elimination.