(2r,3s)-2,3-dihydroxybutanedioic acid and (2s,3r)-2,3-dihydroxybutanedioic acid

[aurevoir0711]

Aren't these two molecules enantiomers of each other? The answer states that these two molecules are the same molecules (not enantiomers), but I can't figure out why that is... I would sincerely appreciate your help. Thank you.

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[sbook2]

Aren't these two molecules enantiomers of each other? The answer states that these two molecules are the same molecules (not enantiomers), but I can't figure out why that is... I would sincerely appreciate your help. Thank you.

Those molecules are superimpossable on one another and are therefore the same molecule. If you have a molecule kit I would build them so that you can see that they are in fact the same. Or draw out the fisher projections and try doing some mental gymnastics.

For them to be enantiomers they would be (2R,3R) - 2,3-dihydroxybutandioic acid and (2S,3S) - 2,3-dihydroxybutandioic acid

You can also check out this website I found down where they discuss tartaric acid (which is 2,3-dihydroxybutanedioic acid):
https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/sterism3.htm

 

[aldol16]

Have you drawn these molecules out? There's an internal plane of symmetry such that if you flip the stereocenters like you have done, you get the same molecule. In other words, it's a meso compound.

Without drawing it, it's hard to explain, but I'll try anyway. So you have a four-carbon chain with the terminal carbons being the C of a carboxylic acid. Each of the middle two carbons has an OH attached to it. They both either come out of the page or go back into the page. So imagine they come out of the page, which corresponds to one of the "enantiomers" you list (bonus: which one is it?). Now imagine viewing the molecule from the other side of the page. You would see both hydroxyls coming out of the page at you. You didn't flip any stereocenters, yet you arrived at the other "enantiomer" you listed. Therefore, both "enantiomers" are the same molecule!

 

[aurevoir0711]

That makes sense now. I drew the molecules with COOH on both ends and everything clicks. Thanks for the responses people!