enantiomer sesolution...

[predent7]

Advertisement - Members don't see this ad

sorry enantiomer resolution...

If you want to separate l-dopa and d-dopawhat process will work ?
a. distillation
b. recrystallization
c. formation of diasteremers with an enantiomerically pure compound.

Why is not a or b, but c ?
Thanks for your help !

 

[herkulease]

enantiomers have identical physical and chemical properties.

The only difference is that rotate light in opposite directions.

Left or right, +/-

Remember L and D in this case tell you nothing other than they rotate it differently. Do not assume that L means left or (-) or D means Right/ (+).

given that they have identical physical and chemical properties.

Distillation is out. They'll both boil at the same temperate.

crystallization is also out. Both would become crystals at the same freezing point.

that only leaves you with choice C.

 

[predent7]

wow, thanks now I got it but what does following mean ?
thanks.

c. formation of diasteremers with an enantiomerically pure compound.

 

[UndergradGuy7]

Enantiomers have the same physical properties like boiling point etc.

Diastereomers do not have the same properties as enantiomers.

So say you have (S, S)-xxxx and (R, R)-xxxx they are enantiomers. They have the same properties. So what you do is make the (S, S)-xxxx into a (S, R)-xxxx. Now (S, R) and (R, R) are mixed and they are diastereomers. They have different properties and can be separated.


Oh and enantiomerically pure compound means you use a reagent that only reacts with one of the enantiomers to form a diastereomer, so that both enantiomers don't react.

xxxx is just a random name for the molecule.

 

[orgohacks]

Enantiomers have the same physical properties like boiling point etc.

Diastereomers do not have the same properties as enantiomers.

So say you have (S, S)-xxxx and (R, R)-xxxx they are enantiomers. They have the same properties. So what you do is make the (S, S)-xxxx into a (S, R)-xxxx. Now (S, R) and (R, R) are mixed and they are diastereomers. They have different properties and can be separated.


Oh and enantiomerically pure compound means you use a reagent that only reacts with one of the enantiomers to form a diastereomer, so that both enantiomers don't react.

xxxx is just a random name for the molecule.

For resolution, this isn't quite right. While changing (S,S) into (S,R) would indeed make it a diastereomer, it's difficult to do this selectively on the (S,S) without also affecting the (R,R).

The classic example of resolution is when we add an enantiopure compound to the mixture that makes a salt. Let's take racemic tartaric acid for instance. You can make a salt from an acid by adding an amine. So to resolve a racemic mixture of carboxylic acids, what we can do is add an ENANTIOPURE amine, cheaply & readily available from nature, like (&#821😉-strychnine or (&#821😉-brucine. This will make the following two salts from racemic tartaric acid.

(&#821😉-strychnine(+)-tartaric acid
(&#821😉-strychnine(&#821😉-tartaric acid.

these are diastereomers (we'd need to have (+)-strychnine present to make enantiomers) therefore they have different physical & chem properties, including crystallization properties. This allows you to separate.



hope this helps - James