Ochem question

[SafetyDog]

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Hey guys, have a quick question.

Read the explanation, but it flipped the bromine so it's plane was on the page. This also moved all the other molecules to wedges and stuff and I just couldn't follow it...

Anyways, I put B because the methyl group being on the dash means that the hydrogen is on the wedge, which I thought was 180 degrees from the bromine and thus anti-periplanar. (Does this only work on cyclic rings such as cyclohexane?)

Can anyone help me visualize this besides changing the plane of everything unless that's absolutely what I have to do?

 

[Xo1991]

well you appear to know that it's between B and C. So then you need to redraw the structures so that the Halide and Halogen are on the plane of the page. This will allow you to visualize it the best. At that point you'll be able to tell how the substituents will situate themselves on the double bond after elimination. Remember that you can rotate about a single bond freely, and in this sort of problem you'd probably have to draw out each structure to determine which one works.

 

[denttal]

This is tricky! In E2 you always need the H you are removing and the leaving group in anti-periplanar positioning. However, when it is a simple alkane, the antiperplanar position is actually the leaving group and the H you are removing on the same plane. (180 degrees from each other). You are correct that if it is an E2 rxn on a cyclic structure then the anti-periplanar would be the H on a wedged and the leaving group on a dash (or vice versa).

So basically C is correct because the H you are removing is on the dash along with the Bromine. Keeping the stereochemistry of the rest of the subs will give you the Z product.

Since there is no option for like "2 or more of the above are correct" you know c is the correct answer, so you don't need to worry about rotating B.

However, if you wanted to check on B, you would need to rotate B in order to get the H and the Br anti-periplanar. If you work it out, you will end up getting the E product and not the Z. Let me know if you need any more help!

 

[SafetyDog]

Alright, thanks guys.

Think with your wording and the explanation on Bootcamp, I can visualize it a bit better now.