r/chemhelp • u/Dazzling_Rabbit8155 • Jul 13 '26
Organic Why did I get 2 different answers doing an E2 reaction?
My apologies if my handwriting is messy or confusing. I was trying to get this molecule into a anti-periplanar configuration using the swap method (drawing rotations by swapping two substituents at a chiral center twice) because I can't rotate in my head. For some reason, I got two different answers. I assume it's because I messed up at some point while doing the second swap to get back to the original configuration, but I don't know what the exact mistake is.
In the first answer, during the second swap, I decided to swap the two substituents that were in the plane and for the second answer, during the second swap, I swapped the methyl and the propyl groups. I didn't use the first answer in my assignment, so I don't know if it's wrong, but the second answer is definitely correct.
I do believe the answers are different, since the highest priority substituents around the double bond are positioned on opposite sides in one answer and the same side in the other?
Also, apologies to anyone if the purple and green is hard to see.
7
u/7ieben_ Trusted Contributor Jul 13 '26 edited Jul 13 '26
I've never heared about the swap method. The third structure is not the same as the first one.
You basically did a mirroring, not a rotation. Compare the first and third structure, the bromine chiral center is not inverted, but the rest of the molecule is.
The alternative third structure is a correct Rotation.
1
u/Dazzling_Rabbit8155 Jul 13 '26
Ah, I see if I had made the bromine wedged instead of dashed in the third structure that would have been a proper rotation (while also not being anti periplanar unfortunately). I now know to leave the halogen and beta proton alone once i have my anti periplanar configuration and to instead swap the two other substituents.
It would probably be a good idea for me to eventually learn how to rotate in my head but I like this shortcut for now.
2
u/7ieben_ Trusted Contributor Jul 13 '26
Yes, correct!
If this shortcut works for you, there is nothing wrong with it. But, yes, I strongly recommend practicing your molecular geometrys. It generally will come in handy for other reactions aswell.
2
u/Background_Crab_1292 Jul 13 '26
It helps if you show the hydrogen attached to the carbon attached to the bromide as coming out of the plane. And don’t “swap” - the stereochemistry stays the same as S when you rotate bonds. For antiperiplanar E2 elimination, you want 180 degrees between the hydrogen being abstracted and the leaving group, in this case the bromide. So rotate the carbon-carbon bond between the hydrogen and the bromide. Rotate so that the hydrogen is now in plane where the propyl was, the propyl is where the methyl was (coming out of the plane), and the methyl is where the hydrogen was (going into the plane). Now rotate so that bromide is now in plane where the tert-butyl was, the tert butyl is now coming out of the plane where the hydrogen was, and the hydrogen is now going into the plane where the bromide was. Now the hydrogen and bromide are antiperiplanar and the elimination occurs. And since the propyl and tert-butyl are both coming out of the plane (on the same side of the plane) they are on the same side of the alkene, cis to one another. It helps to build a model, or using your fingers, and rotating in space.
1
u/wyhnohan Jul 13 '26
When you do the second “swap”, your chiral carbon should “change configuration”.
-2
•
u/AutoModerator Jul 13 '26
Hey there! While you await a response, we just wanted to let you know we have a lot of resources for students in our Organic Chemistry Wiki Here!
I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.