Orgo question help please!!

[nyr201]

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on destroyer (2010 ed) # 103...can someone explain these rules to me abt elimination on a cyclohexane and why the leaving group and H have to both be diaxial, also when in a bond line formula is the black wedge axial and dashed wedge equatorial or vice versa? thanks in advance

 

[quillow]

If you have a subscription to Chad's videos, he describes it pretty well. The leaving group and H have to be diaxial because that's the mechanism for E2. For the wedge/dash, it helps to draw it out. But basically, the leaving group has to be axial (it won't work if it's equatorial). If the leaving group is axial pointing up, the H would have to be axial pointing down... so the leaving group would be wedge and the H would be dashed. Or you could have the leaving group be dashed and H be wedge (as long as they are opposite). I hope that makes sense. Someone correct me if I'm wrong but I actually just watched Chad's video on this and I think that's how it goes.

 

[nyr201]

If you have a subscription to Chad's videos, he describes it pretty well. The leaving group and H have to be diaxial because that's the mechanism for E2. For the wedge/dash, it helps to draw it out. But basically, the leaving group has to be axial (it won't work if it's equatorial). If the leaving group is axial pointing up, the H would have to be axial pointing down... so the leaving group would be wedge and the H would be dashed. Or you could have the leaving group be dashed and H be wedge (as long as they are opposite). I hope that makes sense. Someone correct me if I'm wrong but I actually just watched Chad's video on this and I think that's how it goes.

sweet thanks i must have missed that part or forgot ill go rewatch 😀

 

[gigawatt]

The two substituents that are eliminated have to be anti-periplanar because their p-orbitals have to align 180 degrees apart to form the new pi-bond. In a chair conformation, it's easy to see why 180 degrees apart means both have to be axial. In a dash/wedge diagram, all you look for is them being trans to the plane of the ring.

 

[UCB05]

As for the second part of your question, you can't directly relate equatorial/axial to dash/wedge, since cyclohexane can freely flip its ring, which will change the equatorial/axial position of a substituent on the ring. However, when you have more than one substituent on the ring, you can make a distinction since their relative positions do not change. Here's how it goes:

1,2: axial-axial = trans, equatorial-equatorial = trans; axial-equatorial = cis, equatorial-axial = cis
1,4: axial-axial = trans, equatorial-equatorial = trans, axial-equatorial = cis, equatorial-axial = cis
1,6 (same as 1,2): axial-axial = trans, equatorial-equatorial = trans, axial-equatorial = cis, equatorial-axial = cis

1,3: axial-axial = cis, equatorial-equatorial = cis; axial-equatorial = trans, equatorial-axial = trans
1,5: axial-axial = cis, equatorial-equatorial = cis; axial-equatorial = trans, equatorial-axial = trans


basically, 1,2 and 1,4 axial-axial point away from each other so they're trans. 1,3 and 1,5 axial-axial point the same direction so they're cis. Ring flip changes axial to equatorial and vice versa, but doesn't change the cis/trans spatial orientation, so 1,2 and 1,4 eq-eq is also trans and 1,3 and 1,5 equ-equ are cis. Swap on of the substituents from axial to equatorial changes the stereochemistry from cis to trans or vice versa.

 

[nyr201]

As for the second part of your question, you can't directly relate equatorial/axial to dash/wedge, since cyclohexane can freely flip its ring, which will change the equatorial/axial position of a substituent on the ring. However, when you have more than one substituent on the ring, you can make a distinction since their relative positions do not change. Here's how it goes:

1,2: axial-axial = trans, equatorial-equatorial = trans; axial-equatorial = cis, equatorial-axial = cis
1,4: axial-axial = trans, equatorial-equatorial = trans, axial-equatorial = cis, equatorial-axial = cis
1,6 (same as 1,2): axial-axial = trans, equatorial-equatorial = trans, axial-equatorial = cis, equatorial-axial = cis

1,3: axial-axial = cis, equatorial-equatorial = cis; axial-equatorial = trans, equatorial-axial = trans
1,5: axial-axial = cis, equatorial-equatorial = cis; axial-equatorial = trans, equatorial-axial = trans


basically, 1,2 and 1,4 axial-axial point away from each other so they're trans. 1,3 and 1,5 axial-axial point the same direction so they're cis. Ring flip changes axial to equatorial and vice versa, but doesn't change the cis/trans spatial orientation, so 1,2 and 1,4 eq-eq is also trans and 1,3 and 1,5 equ-equ are cis. Swap on of the substituents from axial to equatorial changes the stereochemistry from cis to trans or vice versa.

wow that is excellent, thanks a lot