Is Your Molecule Set up For An E2 Elimination?

Here is your organic chemistry reaction question for today- E2 elimination

Alright my good friends…let’s take a look at the following example:

And what do you think the answer will be? A…or…B?

I’ll give you a few minutes to think about it. Take your time.

Start by looking at the relationship between your hydrogen and halogen

The first thing you should do with any E2 reaction? Check to see if you have a “trans” relationship between your hydrogen and halogen.

Let’s review our example…

What should you specifically look for?

You can see that our hydrogen and halogen are cis to each other. To clarify – we are looking for both groups to be any of the following:

  • 1 dash and 1 wedge (trans)
  • In the plane of the carbon chain (trans)

Now with these handy dandy points…we can see that we do NOT have this relationship. We have cis (both are dash).

Wrong relationship? No problem…Just rotate!

All we need to do is rotate. This will put everything in the correct orientation.

And now….simply eliminate!

And the answer is…

There are 2 ways you can check to see if you wrote the correct structure (before elimination):

  • Look at the relationship of the atoms in the plane of the carbon chain
  • Look at the relationship of the other dash/wedge combo

In our case..

  • The atoms in the carbon chain (ethyl and hydrogen) are trans to each other.
  • The 2 methyl groups are trans (dash/wedge combo)

Keep up the awesome work! 


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