Ochem question - antiperiplanar

[Moshi123]

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Can anyone please explain how wedged Ph became dashed Ph and H got the bond?

Thanks!!

 

[TobiasHarris]

this is an E2 reaction. The beta Hydrogen must be on the opposite face so that you can carry out the mechanism. In this case, the leaving group is on the plane. Therefore, you have to rearrange the groups attached to the Beta carbon so that the hydrogen is on the opposite side of the leaving group. Once u rearrange, the Hydrogen would be on the plane, the Ph group would be on the dash, and the methyl group on the wedge. Once you carry out the concerted rxn, it would lead to and double bond, and one Ph would be on a wedge, and the other on the dash. Therefore, this is represented by the E double bond (which represents the Ph groups being on opposite faces)

 

[Moshi123]

Thanks for your quick answer!! It makes sense now.
I have another question.. if you could answer this one as well.

What are "groups with pi-bonds"? How do you know if they have pi-bonds?

What is the Endo position (pointing down)?
When do they have bonds pointing up?

Thanks!!

 

[TobiasHarris]

lol that endo question was a little weird ( i have bootcamp too)...to be honest, i have no idea why it is that answer and not the other one with the carbonyls pointing up...my guessed explanation would be that those two substituents are obviously going to be on the same side (there is no reason for them to be on opposite faces). And im guessing that they would point down because it would avoid strain with that bridge which is pointing up lol

 

[Psalm104:33]

That's exactly what endo is, it's favorable because it would avoid strain torsion. I know it that way endo=dashed=down... I think if the bridge was on a different position then endo wouldn't be favorable.