Enantiomer Question

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Hey everyone,

I know that R and S are enantiomers of each other.

1.) But are D and L enantiomers of each other too?
2.) Are D and L vs. R and S diastereomers of each other, or what?


Thank you!

 

[Jengreef]

I got tripped over on a question like this last night. For an R configuration molecule to be an enantiomer of an S configuration molecule, the entire molecule must be the non-superimposable image of the other. The change of a single configuration alone will not make it an enantiomer (eg. 2R,3S-dichloropentane with 2R,3R-dichloropentane are diastereomers despite the change of the configuration of carbon 3).

For D and L sugars, their classification depends on the last chiral carbon. On the Fischer projections, this is the bottom-most chiral carbon. If the last chiral carbon is R, then the sugar itself is classified as a D sugar. If the last chiral carbon is S, then is considered an L sugar.

This is what Berkley Review tells me anyway. Anyone feel free to correct me.

 

[sciencebooks]

Hey everyone,

I know that R and S are enantiomers of each other.

1.) But are D and L enantiomers of each other too?
2.) Are D and L vs. R and S diastereomers of each other, or what?


Thank you!

1. Not necessarily. D and L refers only to the last chiral alcohol group (usually drawn at the bottom, furthest from the carbonyl function).

So what we have is:

• f there are MULTIPLE chiral centers, every chiral center must have opposite configurations for D and L sugars to be enantiomers.
• If only some chiral centers have opposite configurations, they are diastereomers.
• If only ONE chiral center has an opposite configuration (so only at the D and L final chiral center, in this case), they are epimers.

2. D and L only refer to which side alcohol is on. R and S refer to absolute configuration using the Cahn-Ingold-Prelog rules and applying them to Fischer projections.

***Unless you're refering to d and l (lower case)... As in, the direction of plane polarized light?

 

[indianjatt]

1. Not necessarily. D and L refers only to the last chiral alcohol group (usually drawn at the bottom, furthest from the carbonyl function).

Actually, if you have the same molecule (say D-glucose and L-glucose), they ARE enantiomers of each other. The only thing that changes between a D and L sugar is the spatial orientation of the molecule. Yes, D-glucose and L-mannose are diastereomers, but if you are concerned with D/L nomenclature for the same molecule, they will be enantiomers.


2. D and L only refer to which side alcohol is on. R and S refer to absolute configuration using the Cahn-Ingold-Prelog rules and applying them to Fischer projections.
D/L are actually assigned based on the Cahn Ingold Prelog rules, not just sides (although because of the general convention for carbohydrate depiction - fischer projections, looking at if the OH is on the right/; left works). However, D/L is only assigned based on the chirality of the highest priority carbon and not based on all stereocenters, which can be confusing.

 

[sciencebooks]

1. Not necessarily. D and L refers only to the last chiral alcohol group (usually drawn at the bottom, furthest from the carbonyl function).

Actually, if you have the same molecule (say D-glucose and L-glucose), they ARE enantiomers of each other. The only thing that changes between a D and L sugar is the spatial orientation of the molecule. Yes, D-glucose and L-mannose are diastereomers, but if you are concerned with D/L nomenclature for the same molecule, they will be enantiomers.

Hm, this is totally true. Don't know why I totally overlooked that entire point. I think I'm SO used to only seeing D glucose, for example, that I never really considered L glucose but obviously they'd have to have SOMETHING BIG in something to retain the name. Whoops!