Destroyer OC #85/#72

[Dusk]

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In the 2011 version, the question is #85, the 2010 version it's listed as #72..

This seems like a pretty easy question, but I just don't get it.🙁

So it asks through the reagents Br2/Ch2Br2 , which compound produces two diastereomers.

Now the answer is choice C, a 3-ethylcyclopentene (If I named it right >_<)

But why the heck is that the answer. The answer shows that it gives more than one chiral center, thus diastereomers are possible. And it gives an example of a diastereomer. But if I assign the R and S on the compound I get an R,R and the other an S,S.
Wouldn't those be enantiomers? Am I just missing something obvious?

Any help would be very much appreciated and would ease this confused mind.

 

[PooyaH]

You have them backwards. Compounds with R,R or S,S chiral centers are diastereomers.

 

[rmm30]

Could you phrase the question any differently. It seems like your asking if you react Br2 with dibromomethane then you somehow get a cyclic, unhalogenated alkene???

 

[rmm30]

You have them backwards. Compounds with R,R or S,S chiral centers are diastereomers.

i think he/she is saying that one compound is R,R and the other is S,S. Which would make them enantiomers; having opposite config at every stereocenter

 

[PooyaH]

i think he/she is saying that one compound is R,R and the other is S,S. Which would make them enantiomers; having opposite config at every stereocenter

Oh, yeah with what you're saying then they should be enanteomers.

 

[Dusk]

Could you phrase the question any differently. It seems like your asking if you react Br2 with dibromomethane then you somehow get a cyclic, unhalogenated alkene???

sorry, you react this cyclic compound http://webbook.nist.gov/cgi/cbook.cgi?ID=C694359&Mask=8
with Br2/CH2Br2 and it says you get 3-ethyl-1,2-dibromocyclopentane diastereomers... I feel like they would be enantiomers. Given one compound is R,R the other is S,S. I might have assigned them wrong, but I don't think so.

 

[Dusk]

Also, is it always true that if you have more than one chiral center, diastereomers are possible? I just don't see how. 🙄

 

[rmm30]

i think the rxn is Bromine addition w inert solvent across a dbl bond. 2 chiral cnters are generated so you will have mark / anti addition with 2 products; enantiomers. One of which has R,R and the other S,S.

Seems like you are right unless we are both missing something.

 

[rmm30]

Also, is it always true that if you have more than one chiral center, diastereomers are possible? I just don't see how. 🙄

If you have n chiral centers, then you can have n^2 possible stereoisomers. So if you have 2 chiral centers eg. then you can have 4 potential isomers. I say potential bc you can have meso. The thing to remember is that with enantiomers EVERY stereocenter must be inverted. The consequence of that is that there can only be ONE enantiomer for any given molecule. The other stereoisomers are diastereomers. In fact, ANY stereoisomer that is not and enantiomer is a diastereomer.

 

[Dusk]

i think the rxn is Bromine addition w inert solvent across a dbl bond. 2 chiral cnters are generated so you will have mark / anti addition with 2 products; enantiomers. One of which has R,R and the other S,S.

Seems like you are right unless we are both missing something.

I think I got it. The example is wrong. But the answer is right. Answer C has 3 chiral centers, I forgot to include the carbon attached to ethyl. The ethyl carbon can be S or R, so we have a diastereomer between the wedge ethyl and the dash ethyl with the same R/S designation for the Bromines.
At least I think....

nvm the example is correct too... lol.

 

[Dusk]

If you have n chiral centers, then you can have n^2 possible stereoisomers. So if you have 2 chiral centers eg. then you can have 4 potential isomers. I say potential bc you can have meso. The thing to remember is that with enantiomers EVERY stereocenter must be inverted. The consequence of that is that there can only be ONE enantiomer for any given molecule. The other stereoisomers are diastereomers. In fact, ANY stereoisomer that is not and enantiomer is a diastereomer.

awesome explanation, thanks!! 👍

 

[rmm30]

I think I got it. The example is wrong. But the answer is right. Answer C has 3 chiral centers, I forgot to include the carbon attached to ethyl. The ethyl carbon can be S or R, so we have a diastereomer between the wedge ethyl and the dash ethyl with the same R/S designation for the Bromines.
At least I think....

i think that's right.