orgo question

[ScottW3]

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I might be missing something but i can't figure out how they got the answer to this.

I think its A (the most substituted alkene), but topscore says B.

 

[weiy01]

yea, I got "A" also. My Topscore answer key also says "B." What does everyone else think?

 

[nhp3142]

It's not A because for elim to occur like this, Br and the adj H have to be anti peri-planar (ie: both axial pointing 180 degrees apart).
Neel.

 

[Lilith]

I remember seeing this question on Top Score test. Methoxide is a small base, so it should create the more substituted alkene, where as a bulkier base will create a less substituted alkene.

But like the nhp says, elimination requires the bromide and the hydrogen to be anti-periplaner, so the answer should be B.

 

[JavadiCavity]

nhp3142 is right. The H has to be anti-periplanar so that the p-orbitals are lined up. The H used in choice A is gauche, which won't work.

 

[ScottW3]

Thanks everyone. I see it now. How sneaky 👎