two topscore Ochem questions

[yahoogoogle]

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can anyone explain them to me? the first one is A and the second is D, thanks

http://download.yousendit.com/297BA8632B5044BC

http://download.yousendit.com/AB3B553F71984416

 

[Clubby]

The second one only shows one molecule, no idea whether it is a question or an answer.

The first one is pretty easy. In that one chair conformation, a BR and a Ch3 are equatorial. In A, they are both axial. In B, one is axial and one is equatorial. Doesn't work like that.

 

[yahoogoogle]

The second one only shows one molecule, no idea whether it is a question or an answer.

The first one is pretty easy. In that one chair conformation, a BR and a Ch3 are equatorial. In A, they are both axial. In B, one is axial and one is equatorial. Doesn't work like that.

thanks for the help. for the second one, the aswner is (2R,3R) which i thought should be all S.

 

[Flipper405]

thanks for the help. for the second one, the aswner is (2R,3R) which i thought should be all S.

I agree that both chirality centers are S.

 

[levyusupov]

for the second answer the answer is correct. it R,R stereochemistry.

if you breakdown the molecule to two parts and examine thetwo chiral centers separatly u will get two Rs.

 

[yahoogoogle]

for the second answer the answer is correct. it R,R stereochemistry.

if you breakdown the molecule to two parts and examine thetwo chiral centers separatly u will get two Rs.

what do you mean? the order of priority is halogens(1), methyl/ethyl(3) and hydrogens (4). could you care to elaborate?

 

[jdoc04]

for the second answer the answer is correct. it R,R stereochemistry.

if you breakdown the molecule to two parts and examine thetwo chiral centers separatly u will get two Rs.

 

[Flipper405]

I'm not seeing how you didn't "flip" the configuration in the second step because the H is pointing towards you. You have to assign R and S with the lowest priority substituent facing away. I also built a model of this, and both centers are clearly S to me. I can post some pictures of that if there's still some debate, but I'm pretty sure they're both S.

 

[yahoogoogle]

I'm not seeing how you didn't "flip" the configuration in the second step because the H is pointing towards you. You have to assign R and S with the lowest priority substituent facing away. I also built a model of this, and both centers are clearly S to me. I can post some pictures of that if there's still some debate, but I'm pretty sure they're both S.

i second that. I don't see how breaking a molecule can change S to R or vice versa

 

[Flipper405]

i second that. I don't see how breaking a molecule can change S to R or vice versa

Well, it can... I don't know the question because I don't have topscore. But if the molecule was broken, it could change the stereochemistry. The molecule shown is S,S though.

 

[jdoc04]

your right you flip it. it was late and I was on my 12th hr studying.