[GoodGirl]

I have a test in organic chemistry soon, and i'm trying to solve some earlier tests, but i just cant understand this problem: In the following reaction we get a mix of the following stereoisomers, which one is correct? The solution to this problame states that A is the correct answer. I understand that B and C are exactly the same molecule, just flipped over, so thats wrong. I have learned that you have to invert the R/S-configuration when the molecule is drawn whit a fischer structure and have found out the following: A have (R,R) configuration and D have (S,S) configuration, and are therfore enantiomers, so thats also wrong. But i just cant see why A and B is the correct answer, and not A and C or C and D? Can someone explain this to me?

[ringhio]

I have a test in organic chemistry soon, and i'm trying to solve some earlier tests, but i just cant understand this problem: In the following reaction we get a mix of the following stereoisomers, which one is correct? The solution to this problame states that A is the correct answer. I understand that B and C are exactly the same molecule, just flipped over, so thats wrong. I have learned that you have to invert the R/S-configuration when the molecule is drawn whit a fischer structure and have found out the following: A have (R,R) configuration and D have (S,S) configuration, and are therfore enantiomers, so thats also wrong. But i just cant see why A and B is the correct answer, and not A and C or C and D? Can someone explain this to me?

you get only A and B because they have R,R and R,S configurations: you don't have to invert configuration of the C-Cl bond, it remains R because itsn't involved in the reaction

[GoodGirl]

why cant it be C and D when C have (S,R) configuration and D have (S,S) configuration? As far as i see the C-Cl bond isn't involved in the reaction either here. The only differens is that the Cl is on the right side instead of the left.

[sjb]

B and C (and A and D) are not the same molecules. Look into Fischer projections and how they represent stereochemistry.

[GoodGirl]

I have been looking into fischer projections, but i still dont understand.

[GoodGirl]

Im also wonderig why its right to place the Cl-group on the left side when its cearly facing toward us. I would therefore think it would be correct to place the Cl-group on the left side of the fischer projection. Can anyone explain this?

[ringhio]

this is the correct Fischer projection of your ketone. When you do the reaction you modify only C2 configuration and create two new stereoisomers, but configuration around C3 doesn't change and that way is the only way to write right configuration around C-Cl bond.

[kriggy]

I would like to point out, that since the molecule contains already one stereocenter, then one of the products will be in excess of the other. Probably the S-R isomer will be in excess copared to the R-R because the chlorie will block the accces to the carbonyl from that side.