Separating Enantiomers

[SaintJude]

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The treatment of a racemic amine with an enantiomeric pure carboxylic acid results in

A. Two enantiomeric transition states equal in energy
B. Two diastereomeric transition states equal in energy.
C. Two enantiomeric transition states that are different in energy
D. Two diastereomeric transition states that are different in energy.

😕





Answer choice is....D.

 

[Ssina]

I think since by reacting an amine with carboxylic acid will yield the amid bond... and carboxylic acid was pure enantiomer, it will not change it's chirality... so if S in the beginning it'll be S while at the transition state. But since the amine is 50/50 of S and R, then it will have (theoretically, if all else the same) half the compounds with S (carboxylic side)- R (on amine)... and half S(carboxylic side)- S(amine)... that'll be diastereomeric since one chirality changed... as far as energy goes, my guess would be steric where maybe the carbonyl is more reactive with one more than the other amine groups!

was this a free standing Q, any bg info given?!

 

[SaintJude]

Free-standing. I mean separating enantiomers is on that beowulf-length official mcat topics list, so I guess it's expected you should be able to figure it out. Apparently...

I do understand that in order to separate a racemic mixture of a chiral compound you can frequently apply an enantiomeric pure amine in order to obtain its diastereomeric salts. Separate those salts and then reconvert them back to enantiomers. These diastereomers can then be separated via simple crystallization or chromatography. Then via acidification you can isolate each enantiomer. So maybe you can do the same, just switched.

 

[SaintJude]

Found a site that explains this process. Apparently one salt with crystallize faster in the other due to the different transition states. Thanks Ssina.

http://www.chem.ucla.edu/~bacher/Specialtopics/Resolution.html

 

[Ssina]

Found a site that explains this process. Apparently one salt with crystallize faster in the other due to the different transition states. Thanks Ssina.

http://www.chem.ucla.edu/~bacher/Specialtopics/Resolution.html

👍
thanks for sharing!
here also... totally forgot this is called resolution!

 

[dranet]

👍
thanks for sharing!
here also... totally forgot this is called resolution!

Thankyou for this link. I did this the 2nd lot of Ochem lab classes. Very juicy subject. I will be reading this one because it is a different approach, we were selecting the enantiomer, this one is looking at both.