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Significance of Enantiomer?


Anchovyforestbane
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Even if the given compound is chiral (nonsuperimposable), wouldn't it simply depend on perspective whether the compound is levorotary or dextorotary? And for antichiral compounds, enantiomers don't seem to mean much. 
It was once described to me that the answer has to do with the given compound's ability to refract light, but I never received elaboration on how this works or how it effects stereochemistry. 
Can someone explain these things?

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I am unsure of what you are wanting to know here, so forgive me if I am off base. The l and d notation refers to the direction a chiral molecule rotates polarised light. To my knowledge, this property doesn't really affect stereochemistry, it's more that it is a consequence of it (i.e. only chiral molecules are able to do it). The reason molecules are able to exist as either l or d is because two enantiomers will rotate polarised light to the same magnitude, but in opposite directions. For example, if one enantiomer of a molecule rotates light by +10o (d), then the other enantiomer will have an optical rotation of -10o (l). 

 

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1 hour ago, Anchovyforestbane said:

I see. I have one more question; why else could it be that different enantiomers have different pharmaceutical properties?

Left-sided molecules can't react with right-side-compatible molecules... they don't 'fit'. If the body uses molecules with only one chirality for some process, the other won't work with it.

Edited by StringJunky
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2 hours ago, StringJunky said:

Left-sided molecules can't react with right-side-compatible molecules... they don't 'fit'. If the body uses molecules with only one chirality for some process, the other won't work with it.

How specifically does the direction in which a molecule directs light effect how they fit together?

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5 hours ago, Anchovyforestbane said:

How specifically does the direction in which a molecule directs light effect how they fit together?

It doesn’t. The stereochemistry and how they occupy space is what matters. A good illustration of why this matters in drug design is your feet. They are mirror images of one another but nonsuperimposable in the same way enantiomers are. You will probably know from experience that if you try to put your left shoe on your right foot or vice versa, it won’t fit very well. The same is often true for chiral drug molecules and their molecular targets (eg. enzymes). In the drug world, the quintessential example of why chirality matters a lot is thalidomide, which I’ll let you look up. 

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10 minutes ago, hypervalent_iodine said:

It doesn’t. The stereochemistry and how they occupy space is what matters. A good illustration of why this matters in drug design is your feet. They are mirror images of one another but nonsuperimposable in the same way enantiomers are. You will probably know from experience that if you try to put your left shoe on your right foot or vice versa, it won’t fit very well. The same is often true for chiral drug molecules and their molecular targets (eg. enzymes). In the drug world, the quintessential example of why chirality matters a lot is thalidomide, which I’ll let you look up. 

Say you had two circles, each with a tangent shaded circle to their left. These images are identical, but if you rotate one of these figures upside down it becomes nonsuperimposable with the other, despite nothing having changed. What confuses me, is how enantiomers are any different. It seems like whether a chiral compound is D or L would simply depend on perspective; so what exactly is causing the difference on a molecular scale?

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I agree with Sensei. Get a model kit or even just some tooth picks and blu tack. Make something with four different substituents and make its mirror image, then try and rotate the mirror image so that it is identical to the first molecule. You’ll find that it doesn’t work because they are not superimposable. If you repeat the process but with two substituents that are the same, you should find that you can rotate them to look identical (hence it is not chiral). The example you gave doesn’t work since by your own logic, the two mirror images are superimposable.

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On 11/17/2020 at 5:48 PM, Anchovyforestbane said:

Even if the given compound is chiral (nonsuperimposable), wouldn't it simply depend on perspective whether the compound is levorotary or dextorotary? And for antichiral compounds, enantiomers don't seem to mean much. 
It was once described to me that the answer has to do with the given compound's ability to refract light, but I never received elaboration on how this works or how it effects stereochemistry. 
Can someone explain these things?

 

You have posted in the chemistry section and also asked about the implications for drug molecules.

So physical properties such as optical rotation of polarisation are not important here.

We are looking at chemical properties.

This is important because we are no longer talking about one molecule here but two or more molecules and the effect on their (chemical) interaction.

This is the significance you were seeking.

Enantiomers arise when the configuration of a molecule is such that it is chiral or posseses 'handedness'.

The left and right handed versions are called enantiomers. One is the enantiomer of the other.

Since we are talking about at least two different molecules for a chemical reaction, we must recognise that either or both can be chiral and possess two enantiomers.

I will call the first molecule the reagent molecule and the second the environment molecule.

So with a chiral reagent we have two situations:

A chiral reagent and a non chiral environment  Fig1

A chiral reagent and a chiral environment  :  Fig2

I hope you understand the 3D notation for the configuration of the molecules  ask if you do not.

enant1.thumb.jpg.e00f43bacd256a15ef40a2751ae0eb19.jpg

I suggest you use sensei's models if you have access to them or make your own with potatoes and cocktail sticks.

You will see that the functional groups on the enantiomers of the different molecules line up differently for a chiral environment, so will react differently.

but for a non chiral environment both reagent enantiomers line up the same so will react the same.

 

Edited by studiot
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2 hours ago, studiot said:

I hope you understand the 3D notation for the configuration of the molecules  ask if you do not.

I do, do not worry.

 

2 hours ago, studiot said:

 

enant1.thumb.jpg.e00f43bacd256a15ef40a2751ae0eb19.jpg

I suggest you use sensei's models if you have access to them or make your own with potatoes and cocktail sticks.

You will see that the functional groups on the enantiomers of the different molecules line up differently for a chiral environment, so will react differently.

but for a non chiral environment both reagent enantiomers line up the same so will react the same.

 

Ahhh, I see, this makes a lot of sense. I thank you for explaining.

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