L .Isomer Confusion. D

[floodgate]

First off, I'm not sure this is the right board for this, but it fit my thread's concept best in my opinion. Feel free to move it if it is out of place.

Also, I don't claim to know much more than the average person about chemistry, so please excuse me if my questions are obvious or don't make much sense.

Anyways, correct me if I'm wrong, please.

I often hear about different isomers of the same chemical. For example,Levo-methamphetamine and Dextro-methamphetamine, where there levo is the left side of the compound, and dextro is the right side. I will be using methamphetamine for most of my questions simply because I've read the most about it in regards to isomers. Racemic mixtures contain both levo- and dextro- forms of the compound. In psychotropic applications, -levo isomers of a certain substance are known to usually last longer, and -dextro isomer forms of the same substance are known to require a lower dose and have stronger effects.

Here come the questions:

-If levo and dextro refer to the "direction" of a single molecule, then must EVERY molecule that is able( chiral?) to have a "right and a left form" be either levo or dextro? So would any single random methamphetamine molecule HAVE to be one or the other (levo-methamphetamine or dextro-methamphetamine)?

-Are most substances of -levo and -dextro form? I was reading about chirality and from what I've read, achiral molecules are achiral when they have an "improper rotation". Is the rotation of a molecule dependent on properties of that specific molecule, or properties of the general chemical it is. For example, could a molecule of methamphetamine have it's rotation become improper? If so, does this happen in normal situations? or is it a rare occurrence that is not commonly observed in every day chemistry. What is the correct terminology for specifically declaring an achiral compound? (like, "neutral-methamphetamine" )

-Is it known what determines the levo/dextro ratio of a resulting chemical after a chemical reaction that produces a chiral compound? Is it dependent on the specific reaction that results in the final compound? Or is it determined primarily by other things such as the properties of the compound being formed or by variables such as temperature, light, and humidity

-Do the dextro/levo forms of a compound react the same (or at least similarly enough) to all chemical reactions that would normally be plausible with any given chemical? For example, if I had some (I'm not sure if these compounds make sense, but try to understand it hypothetically ) pure "levo-acetic acid"(or pure dextro-acetic acid) and normal, from the store (racemic?) sodium bicarbonate, and mixed them together in the famous baking soda and vinegar acid-base reaction, would it still yield sodium acetate and carbonic acid ( which then decomposes to water and carbon dioxide)? If so, would the reaction happen at the same speed? Are there specific reactions that are significantly different when compounds are levo/dextro/racemic?

-Would a 90/10 percent mixture of levo and dextro isomers of a compound that underwent a chemical change with 100% yield result in a 90/10 percent ratio of levo- and dextro- in the resulting substance? If so, would a pure levo-(or dextro) substance that underwent the same chemical change yield a pure levo-(or dextro) substance?

-Are there general methods to convert a dextro- substance to levo-, or the other way around? Are there general methods to convert a isomer mixture with a much higher levo- or dextro- ratio to it's counterpart to a mixture that was closer to 50/50? If there are no general methods, are there specific methods that have been established to work well for certain types of compounds? Where can I find examples of some of these methods?


Too much writing for now, I'm starting to confuse myself. I know some of my questions are probably horribly stated and amateur-ish, but I'm trying to learn and -any- input or help or sources to study would be greatly appreciated.

Thank you very much,

-f l o o d g a t e -

 

[Temeraroius]

I would like answers to this too...

-Do the dextro/levo forms of a compound react the same (or at least similarly enough) to all chemical reactions that would normally be plausible with any given chemical? For example, if I had some (I'm not sure if these compounds make sense, but try to understand it hypothetically ) pure "levo-acetic acid"(or pure dextro-acetic acid) and normal, from the store (racemic?) sodium bicarbonate, and mixed them together in the famous baking soda and vinegar acid-base reaction, would it still yield sodium acetate and carbonic acid ( which then decomposes to water and carbon dioxide)? If so, would the reaction happen at the same speed? Are there specific reactions that are significantly different when compounds are levo/dextro/racemic?

I would think they dont react the same way, simply because if they behave differently in your body (which is basically just more complex chemical reactions involving enzymes etc), they would behave differently outside of it too. Im pretty sure the synthesis/extraction method for the two isomers is different. Then again I'm no chemist either.

 

[shibireru]

Neither am I a chemist. Let me get that out of the way.

I would like answers to this too...

I would think they dont react the same way, simply because if they behave differently in your body (which is basically just more complex chemical reactions involving enzymes etc), they would behave differently outside of it too. Im pretty sure the synthesis/extraction method for the two isomers is different. Then again I'm no chemist either.

It needs to be kept in mind that the main reason one enantiomer of a given prototypical substance may have effects in the body and the other none or a very different constellation of effects or comparatively weaker effects of the same class - or vice versa - is that most pharmacologic agents achieve their effects by inducing conformational changes in receptor proteins, which are generally highly elaborate concave molecules which envelop or partially envelop their ligands. Just as a glove made for the right hand doesn't fit on the left and one made for the left doesn't fit the right, a receptor that can be activated by the dextroratory or levoratory enantiomer of a particular substance may not respond in the same way to the levoratory or dextroratory form, respectively.

I would think that when you're reacting simpler molecules with one another, where one is not fitting inside of the other or bonding to multiple points on the other, you'd see essentially the same products, whether your reagents are levoratory or dextroratory. Though, whether those products would have the same biological activity regardless of handedness is a different matter entirely.

But again, I am not a chemist.

-Is it known what determines the levo/dextro ratio of a resulting chemical after a chemical reaction that produces a chiral compound? Is it dependent on the specific reaction that results in the final compound? Or is it determined primarily by other things such as the properties of the compound being formed or by variables such as temperature, light, and humidity

Let's continue with the traditional hand heuristic/analogy. Suppose you could produce a human hand in a way similar to the way in which you synthesize complex molecules. Also, for the sake of argument, let's imagine that the body of the hand is symmetrical and that there are a total of six bonding points on each. Let's say now that the thumb can be attached to either the first or the last bonding point of the six bonding points but on no other and only one thumb can be attached to a given "hand body" at a time. If you combine a bunch of, ummmm, "hand bodies" 8) with thumbs and a catalyst, you'll find that you have a roughly equal number of hands with the thumb on the right and hands with the thumb on the left. Now we want to attach the index finger. Well, the index finger must be in juxtaposition with the thumb for various reasons, so it bonds either directly to the left or directly to the right of the thumb depending on to which bonding point the thumb has bonded. Etc... and so on, until you'll finished synthesizing your hands. Even though all the products of your reactions are hands, they are not all identical to one another: some are left hands and others are right hands. In fact, the mixture is roughly 50-50. This is what's called a racemic mixture.

(This analogy was pretty flawed but I think it gets the job done for the most part.)

Many pharmaceuticals are racemic because isolation can be extremely tedious and costly - or at any rate the costs of isolation certainly exceed whatever costs inhaere in disaffecting a small number of consumers by asking them to ingest twice as much of a substance than they would otherwise have to ingest. It's not hard to decrease the amount of a filler or binding agent and increase the share of realestate of the active principle or to merely double the size of the pill, or so I am disposed to think. Typically, the only reason pharmaceutical companies bother with isolating one or the other is that enantiopure pharmaceuticals can be patented when the patent on the racemic form has run out. (Amazingly, they can also patent that time-released bullshit and charge you more per ounce than if you were buying gold even if the immediate release form is only 5 bucks. Bupropion anyone?)

 

[Ylide]

Ok, a lot has been said already but let me see if I can give some quick and dirty responses now and after I've gotten some sleep come in and expand on them.

Alright first off, levo and dextro are prefixes that refer to rotation of plane polarized light. In my opinion they're actually a very shitty way to talk about chirality as they actually have no bearing on the absolute configuration of the molecule. Use of the R and S notation is much preferred as they actually are based on molecular structure. Also levo and dextro can hide the fact that a molecule could have multiple stereo centers, whereas in R and S they are explicitly shown.

A molecule that is chiral can not all of a sudden become achiral, and vice versa, without some sort of reaction taking place. The quick and easy rule is that chiral molecules have stereogenic centers i.e. carbon atoms that are bonded to four different things. So unless bond breaking and formation happens a chiral center isn't going to go away or appear. Also molecules will not convert from one enantiomer to another spontaneously.

Enantiomers, by their definition, have the exact same physical and chemical properties assuming they are in an achiral environment. If they are in a chiral environment then all bets are off. So (R)-2-Butanol will react in exactly the same manner as (S)-2-Butanol with methyl iodide.

I'll come back and try to clean this up a bit later, but for now that should help answer a couple of your questions.

 

[nuke]

Here's some basic notes of stereochemistry:
http://pirate.shu.edu/~sowajohn/shortcourse/part3/part3.html
http://chem.pdx.edu/~wamserc/C334F99/4notes.htm

-If levo and dextro refer to the "direction" of a single molecule, then must EVERY molecule that is able( chiral?) to have a "right and a left form" be either levo or dextro? So would any single random methamphetamine molecule HAVE to be one or the other (levo-methamphetamine or dextro-methamphetamine)?

Yes, although there are different forms of stereoisomers including diastereomers, what you're referring to is an enantiomer.

-Are most substances of -levo and -dextro form? I was reading about chirality and from what I've read, achiral molecules are achiral when they have an "improper rotation". Is the rotation of a molecule dependent on properties of that specific molecule, or properties of the general chemical it is. For example, could a molecule of methamphetamine have it's rotation become improper? If so, does this happen in normal situations? or is it a rare occurrence that is not commonly observed in every day chemistry. What is the correct terminology for specifically declaring an achiral compound? (like, "neutral-methamphetamine" )

Achiral compounds do not have four different substituents around at least one of the carbons in the compound.

-Is it known what determines the levo/dextro ratio of a resulting chemical after a chemical reaction that produces a chiral compound? Is it dependent on the specific reaction that results in the final compound? Or is it determined primarily by other things such as the properties of the compound being formed or by variables such as temperature, light, and humidity

The stereochemistry is usually determined exclusively by the reaction mechanics and are variable for different reactions.

-Do the dextro/levo forms of a compound react the same (or at least similarly enough) to all chemical reactions that would normally be plausible with any given chemical? For example, if I had some (I'm not sure if these compounds make sense, but try to understand it hypothetically ) pure "levo-acetic acid"(or pure dextro-acetic acid) and normal, from the store (racemic?) sodium bicarbonate, and mixed them together in the famous baking soda and vinegar acid-base reaction, would it still yield sodium acetate and carbonic acid ( which then decomposes to water and carbon dioxide)? If so, would the reaction happen at the same speed? Are there specific reactions that are significantly different when compounds are levo/dextro/racemic?

Acetic acid has no stereoisomers because the molecule is sp2 hybridized -- for enantiomers the carbon (chiral carbon) must be an sp3 hybridized carbon (though cis/trans stereoisomerism can result from sp2 hybridization!).
Different enantiomers do tend to have different chemical properties, but whether or not they'll have a bearing on a reaction depends on the reaction itself -- some reactions are used specifically for the purpose of only chemically changing one enantiomer so that the other can be isolated.

-Would a 90/10 percent mixture of levo and dextro isomers of a compound that underwent a chemical change with 100% yield result in a 90/10 percent ratio of levo- and dextro- in the resulting substance? If so, would a pure levo-(or dextro) substance that underwent the same chemical change yield a pure levo-(or dextro) substance?

This is entirely dependent on the stereochemistry of the reaction.

-Are there general methods to convert a dextro- substance to levo-, or the other way around? Are there general methods to convert a isomer mixture with a much higher levo- or dextro- ratio to it's counterpart to a mixture that was closer to 50/50? If there are no general methods, are there specific methods that have been established to work well for certain types of compounds? Where can I find examples of some of these methods?

Not usually, generally it's impossible to do this with enantiomers because you'd be rearranging sigma bonds and carbons don't generally like that (the energetics are very unfavourable). Sometimes certain conditions (eg exposure to light, acidic or basic conditions) can transform one stereoisomer to another, though. See epimerization.

-Are there general methods to convert a dextro- substance to levo-, or the other way around? Are there general methods to convert a isomer mixture with a much higher levo- or dextro- ratio to it's counterpart to a mixture that was closer to 50/50? If there are no general methods, are there specific methods that have been established to work well for certain types of compounds? Where can I find examples of some of these methods?

The answer is not really -- there are some biochemistry methods to do this but they're few and far between. Stereochemistry is usually established during the synthetic method by which the molecule is produced.

 

[ranunky]

dud you got it wrong from the start. the dextro and levo has no predictable association with the configuration of the molecule, it refers to the effect of the pure isomer on plane polarised light. levo compounds will rotate the axis of polarisation to the left, dextro will rotate it to the right.

You can verify this effect at home easily with two sheets of polaroid plastic (from 3d glasses or the photo shop) and some glucose (it's always pure d-glucose that occurs in nature) from the pharmacy or wherever.

Place the two sheets over each other and you will see that as you rotate the top one, when it's at 90 degrees to the bottom one, all the light will be blocked. Now put one on top of a light source and place a beaker filled with glucose solution on top of that. put the other sheet on top of the beaker and rotate it until it blocks the light, observe that the angle required is different as the light has been magically rotated by the glucose! My heart skipped a beat or two the when I did this one.

Acetic acid has no stereoisomers because the molecule is sp2 hybridized -- for enantiomers the carbon (chiral carbon) must be an sp3 hybridized carbon (though cis/trans stereoisomerism can result from sp2 hybridization!).

don't confuse the chap, he hasn't learned about electron shells or bonding or any of that crap lol! The easiest way to tell if an atom is chiral is to look at what's attached to. If each of the 4 bonds is attached to a different type of group, then its chiral. Eg. Acetic acid on carbon is attachd to an oxygen, a hydroxyl, and another carbon, three different groups => not 4 different groups, not chiral the other carbon is attached to 3 hydrogens and another carbon => 2 different groups, not chiral.

Now, once you know which carbon is chiral you can determine whether is is an R or an S isomer by looking at it's attached groups and following a set of rules you can find on wikipedia. R and S designation is in no way linked to D and L. Sometimes the R isomer is D sometimes it's L and same for S. There is no way you canfigure out whether it's D or L without either looking it up or isolating the isomer and doing the polarising experiment i described above.

 

[Black]

dud you got it wrong from the start. the dextro and levo has no predictable association with the configuration of the molecule, it refers to the effect of the pure isomer on plane polarised light. levo compounds will rotate the axis of polarisation to the left, dextro will rotate it to the right.

no. (+) and (-) indicate the rotation of polarised light. D and L indicate the configuration compred to glyceraldehyde, which it's outdated. using R and S is applicable to every chiral molecule and gives a realiable, absolute conformation and therefore preferable.

 

[ranunky]

Thanks black. That D/L designation is gay as hell.

 

[MurphyClox]

Just as a sidenote: The CIP-System has its flaws, too, although they are comparably negligible. Example: Draw a glucose-molecule, once in the open-chain form and then in the closed-ring form as well. Determine the stereochemistry at C4 for both cases.... Weird, isn't it?! - Murphy

 

[Black]

Just as a sidenote: The CIP-System has its flaws, too, although they are comparably negligible. Example: Draw a glucose-molecule, once in the open-chain form and then in the closed-ring form as well. Determine the stereochemistry at C4 for both cases.... Weird, isn't it?! - Murphy

argh! :D
yes it's weird, but it's the best system we have. iirc there are also some amphetamine analogues of which S is the active form as opposed to R, because the same groups point in the same direction as with the parent compound 8)