The mystery of the multiple menthols
I am facing a problem I cannot find an answer for, anywhere. My company manufactures a topical analgesic. We use camphor and menthol crystals. We have never had problems until a new question surfaced recently. Are the crystals the same as dl-Camphor and dl-Menthol? There are separate CAS numbers for all four (crystals and dl-).
I have found, through research, that the dl- has something to do with the rotation of the molecules That doesn't tell me (as a layman) what the difference is, if indeed they are different. Do DL- chemicals have different properties? Are they interchangeable? There must be some reason for having two different forms of these chemicals but I can't find those reasons. It's probably not as big of an issue as I have made it in my mind, but I can't explain it to anyone else and that is becoming somewhat of a stumbling block.
If you can help I would really appreciate it.
the quick answer:
The crystals you have been using are not the same as the DL versions you are thinking of purchasing. If you have been using crystals, stick with them. DL versions are (typically) less expensive, and they are mixtures that contain half of what you want and half of something else.
I'm betting you want to purchase L-menthol, also known as "1R, 2S, 5R-menthol", also known as (-)-menthol. These are three different names for the same molecule! This is because chemists have different systems of naming things, which sounds dumb, but it isn't. You use the system that is relevant to the discussion.
There are seven other forms of the menthol molecule, though you are less likely to bump into them. Each of those may have several names as well. No wonder you are confused. The one mentioned above is the most common one, the principal constituent of peppermint oil.
As far as camphor, you want D-camphor, also identical to R-camphor, also identical to (+)-camphor. Again, that is the common form and the form you get from the camphor laurel. Both of these "natural" versions of camphor and menthol can be synthesized, too.
more detail for budding chemists out there
Now, what you are dealing with when you see D- or L- before a chemical name are two mirror-image forms of the same molecule. They are not the same. A "DL-something" is a 50-50 mixture of both D and L forms.
Will D and L behave the same?
It depends; in biological systems, which also have this "handedness", no. In purely physical, non-biological tests, like measuring melting points and density, yes.
Technically these two forms, D- and L- are called enantiomers (eh-NAN-tee-oh-mers). Enantiomers always come in pairs. You can't have three enantiomers for example. Just like you can't have three mirror images.
Molecules that have this ability to exist as two forms that are NOT the same are called chiral (pronounced KY-ral), which means "handed".
Two mirror images of an amino acid, thanks to Wikipedia entry on "chiral". Note that since the molecule is not flat (the "COOH" portion is furthest from you and the blue "R" ball is closest to you), they are not identical.
How can you tell if a molecule is chiral?
If you can look at an object and its mirror image, it is pretty easy to see whether or not they are the same. You do the superimposibility test, a fancy name for imagining one object coexisting in the same exact space as the other, where all parts overlap. Can they do this with every part overlapping exactly?
This is proof that one of the most useful skills in science is having an imagination. I make my students practice this superimposibility test on various objects with their imaginary mirror image. A perfect sphere is identical to its reflection, so it does not have this property of handedness. It is thus not chiral, or "achiral" (ay-KY-ral). Some objects are chiral, some are achiral. Hands are chiral:
The best analogy to D and L versions of a molecule that I know, is your right and left hands. They have the same structure, the same overall geometry, except they are mirror images of each other. Also, if you try to put a right glove on a left hand you know they are not the same. Or, if you try to put your right hand on top of your left so both palms are down, the fingers don't line up. Your thumb will be where the pinkie is.
Your right and left hand can not be superimposed. They are identical looking mirror images (unless you have a scar or something on one) that are not identical. They are chiral objects.
If you are dealing with molecules which are merely objects that are too small to see, the same principal applies. But since you can't see them you don't know which enantiomer you have! All you can do is look at a three dimensional picture of a molecule's representation on paper, and do the superimposibility test with its mirror image to see whether or not it can exist as a pair of enantiomers. Then you have to worry, as you do, about which one to use in your formula.
You might determine which of the two enantiomers you possess if you put your sample in this rather simple device called a polarimeter. This table top device shines polarized light through a sample of of your molecule dissolved in solution in a clear tube.
Chiral molecules either rotate plane-polarized light to the left or to the right. Ones that rotate light to the right get a + designation and ones that rotate light to the left get a - designation. (This designation has nothing to do with other unrelated systems you see such as D vs. L, or R vs. S. Those systems have to do with the shape of the molecule, roughly speaking.)
The guts of a polarimeter
The polarized light coming out of the end of the tube after passing by these chiral molecules has an altered orientation which is detected and displayed for you to read. That is why you might see the mysterious designation of "+44" on a label of natural camphor. It means pure natural camphor rotates plane polarized light by 44 degrees to the right in a polarimeter. Its mirror image would rotate polarized light 44 degrees to the left and get a "-" designation. (Officially, it is best to put the "-" or "+" inside of parentheses before the name of a molecule so that it is easier to see; a tiny minus sign might get lost if you have a lot of specks and stuff on your document!)
Your basic polarimeter
If you have equal amounts of both enantiomers in the tube their effects cancel each other out and cause a net of zero rotation of the light and you might mistakenly think your molecule is achiral.
Now you don't a polarimeter! (Although they are fun and I would never want to stop anyone from playing with one.) I would just read the bottle label, and maybe if I wasn't sure I would do a melting point test to determine purity (described below.) A respectable chemical company should put one of the three designation systems on the label for which enantiomer you have.
But I wanted to explain why you see the term "rotation"or "optical activity" or
"+" or "-" that you mention. All chiral molecules have this property of rotating plane-polarized light. They are "optically active", whereas achiral molecules don't do anything exciting with plane polarized light. Polarized light just goes right past them, unaffected.
Not all molecules have this ability to exist as enantiomers, it depends on their geometry. I make my class practice by looking at everyday objects (chair, pencil, mugs) to imagine whether these objects' mirror images would be identical to the original. There are several simple tricks to learn in doing this which I don't have time to go into here; I can elaborate more in a comment if someone wants me to. They are great imagination exercises. Here are a few:
The plane of symmetry trick
One of the easiest tricks is to look for a plane of symmetry. If an object has bilateral symmetry, it will be achiral. (In other words, if you can imagine a mirror plane dividing the object into two equal identical halves which are reflections of each other, it is achiral.) Notice how your hand cannot be dived into two equal halves that are reflections of each other; this asymmetry leads to chirality.
The stereocenter trick
There are other tricks; you learn to look on a molecule for a carbon bonded to four different things, and you can find one, that is a potential "chiral" site, a region of asymmetry which may give rise to a mirror image that is not identical:
A carbon bonded to four different things is a potential site where chirality can arise; these two models are not identical due to the tetrahedral arrangement of bonds around the central atom.
This region of asymmetry is called a stereocenter.
Why are there eight menthols?
To further complicate the question of which menthol you want, there are actually 3 stereocenters on menthol. Each can exist in a left/right configuration. This does not give rise to multiple enantiomers; remember you can only have two enantiomers, mirror images always come in pairs! But it does give rise to four pairs of enantiomers. Collectively these related structures are called stereoisomers. If you pick two stereoisomers that are not mirror images of each other, they are called diastereomers.
Imagine that you are some strange creature with three hands, just as menthol has three stereocenters. They could be
left left left, or
right right right, or
left right right, or
right left left, or
right right left, or
left left right, or
right left right, or
left right left!
So you see there are eight possible arrangements, four pairs of mirror images, thus there are eight possible menthol molecules.
If the number of stereocenters on a molecule are N, then the number of possible molecules is 2 to the Nth power. (There are some odd cases where a mirror plane of symmetry causes a molecule with multiple stereocenters to be achiral so we have to say "possible" stereoisomers rather than being definite about it.) So there are 2 x 2 x 2 = 8 menthols possible, but peppermint plants make only one of them. It is the "1R, 2S, 5R" version. Here are all the possibilities:
The wedged lines and hatched lines are the standard way chemists try to show bonds coming toward you, out of the plane of the paper (wedge) or away from you, into the plane of the paper (hatch). So, these are not flat molecules. The ones on top are mirror images of the ones directly beneath them.
The numbers refer to each point (carbon 1, carbon 2, or carbon 5) on the ring structure that has this handedness, and the S or R designation refers to whether it is "right" or "left", known as configuration.
(Carbons in a ring are normally just depicted as points on the ring rather than "C's". Also, since a carbon almost always has four bonds, and hydrogen always has one bond, any missing bonds to these points have to be to a hydrogen, which is not shown. The hydrogens are left off of the ring to keep the structure from looking too complicated. So if you have trouble finding the carbons that are bonded to four different things here, it might because it is an abbreviated structure that assumes you know this method of portraying molecules.)
The natural menthol molecule you want is always 1R, 2S, 5R, or (-)-menthol. It should naturally crystallize, as the presence of other stereoisomers will prevent the crystals from packing easily.
A fast way to estimate whether a solid is pure
Also, any pure solid should have a well-defined, narrow melting point range, which can easily be measured in a device called a melting point apparatus, if you are interested in investing in such a thing. They have fancy expensive ones these days but I'm happy using an old fashioned cheap one. The classic brand is Meltemp. You plug it in, put your sample in a glass capillary tube, heat it up, watch your crystals melt through a nifty little magnifying lens, and immediately record the temperature that it melts at. If it melts over a range or if it melts lower than it ought to, it is likely impure.
It is a handy thing to know that pure solids have a sharp, well-defined melting points, while impure ones melt below their standard melting points, and more over a range of temperatures. Thus you can quickly get a feel for how pure your solid is or is not.
If you know it is L or D can you automatically say if it is R or S or + or - ?
No. The designations of R vs. S, or D vs. L, are different systems chemists use to portray the shape of the molecule around the "handed" spot on the molecule, so are geometrical designation. The other system you will see is + vs. -. That has to do with whether the molecule will rotate left or right when hit by polarized light in a polarimeter. These three systems are all separate from one another and you should never try to link one to another; for example, an "R" molecule might be either D or L or + or -. So don't assume that D means R or L or - or + for example, that is a common point of confusion. These are three completely separate systems, each useful in their own way.
How many camphors are there?
Camphor is actually simpler than menthol. Although it has two sites of chirality, which in theory gives it four possible stereoisomers, due to some constraints on how a bicyclic ring can, or actually can't, turn itself inside out without breaking itself apart, it actually only comes in only two possible forms. This is sort of a fun advanced organic chemistry question but I won't get into details here. (It's fun if you have the time.) So, there is natural camphor, which is the D (+) form, and that is probably what you want.
I will leave it to advanced chemistry students to try rotating these mirror images around in your mind to verify that they are not superimposable! Some people need physical models, others are particularly gifted at this visualization. If you can do this in your head, you are a natural born organic chemist!
What's the difference between synthetic and natural molecules?
Actually, none, if they are identical enantiomers. It is misleading to judge molecules from their source, because their behavior depends only on their structure. Molecules are not haunted by their origins. But! It is not so easy for us in our labs to make pure enantiomers as it is for organisms to do it.
Nature tends to make only one enantiomer, because its machinery (enzymes) used to make biomolecules also has this property of "handedness".
When you synthesize molecules in a lab that have this handed character, they tend to be an equal mixture of 50-50 "left" and "right" handed molecules. This is because we normally start with smaller, simpler molecules that are not handed in the first place, or sometimes because we start with a mixture of 50-50 D and L. This DL mixture that is typical of what you synthesize in a lab is called a racemic mixture (ray-SEEM-ic).
So when organisms make biomolecules they tend to be one hand only. Nature tends to make L-amino acids but not D-amino acids, and D-sugars but not L-sugars, for example. There are some exceptions.
When we make chiral molecules from scratch in the lab, they tend to be a racemic 50-50 mixture of both enantiomers. More effort must be exerted to separate the enantiomers.
That is why when I first learned about amino acids found in meteorites from outer space, I was dying to know whether they were racemic or handed. Astronomer David Levy was kind enough to write me back, revealing these extraterrestrial amino acids are, alas, racemic, which implies they are generated by random physical, not organic, processes. Still, it is exciting they can arise spontaneously in space by random reactions. The ideas behind how life arose with some slight enantiomeric excess that tipped the balance in favor of creating mainly L-amino acids and D-sugars that now dominate our biological landscape is beyond the scope of this discussion, but an exciting one to contemplate.
The DL-versions of the stuff you are looking at purchasing is the racemic mixture. I am betting it is less expensive. It takes a little more effort to separate them out into their pure forms. So, chemical companies will often sell both the racemic mixture, labeled "DL", and they also sell the more expensive purified enantiomers, labeled D or L. Once it is purified by a lab into the pure enantiomer that you want, you can assume it will behave exactly the same way as the one made by a plant or animal.
There is a claim I ran across saying that making pure, synthetic menthol is more sustainable and produces less CO2 than isolating it from plants.
I haven't verified this statement, and would like to know who is making it, but it is an interesting one; I suppose you would have to look at how much fuel and energy it costs to raise a crop of peppermint and extract the oil, compared to making it from small molecules in the lab. If I had a business that used great gobs of (-)-menthol I'd try to choose the form that generated the least CO2 in concentrating it.
The crystals that you have been using are likely to to be pure just because, I am guessing, it is easier for pure molecules to pack together into a crystal than a racemic mixture. The difference between crystals and amorphous goo or glassy solids that fracture unevenly is that in crystals the units pack neatly and orderly together. This is most likely if the solid is pure. Does the source of your crystals use any designation on the label like D, L, R, S, +, or -?
Now, if you are just using these topically, I would not worry excessively much about using a racemic mixture. Especially if it is cheaper. They may, however, be half as concentrated, since they are diluted by half with the mirror image molecule. It would be changing your formula.
It is likely that one of the mirror image forms of both these molecules won't have the effect you are used to. Topically, you may not notice this so much, but taken internally, it is a very important issue!
Biological systems perceive mirror image forms of a molecule in dramatically different ways
The principal molecule responsible for spearmint flavor has a mirror image enantiomer which give caraway its flavor. They look the same, but have dramatically different effects in a biological system, our mouth!
R-carvone smells like spearmint, S-carvone smells like caraway
Our taste buds are three dimensional, and as such they respond differently to different enantiomers of the same molecule. Just like a right glove won't fit in on a left hand, a pepperminty menthol molecule won't fit on a caraway receptor on the tongue, and vice versa.
If you are taking molecules internally, you DO have to worry about enantiomerism. D-thyroxine is a heart drug, whereas L-thyroxine is a thyroid medication. They don't have opposite effects, they have unrelated effects.
Why don't enantiomers have opposite effects?
This is a common misperception. They have unrelated, different effects, but they are not like good and bad twins with opposite personalities! Here's proof: Imagine you have a house key that fits your front door lock (like a chiral drug interacts with an enzyme or receptor). If you have a mirror image of your house key, it won't fit into your front door lock, and it won't fit into your back door lock, but it might go into some random dude's lock who lives in Paraguay or something, just by chance. Does that make sense?
The mirror image enantiomer of a drug is not likely to have an effect on the same system that the drug works on. It may, however, have an effect on a completely unrelated system, just by accident.
So, if you use these topically, and switch from the pure forms to the racemic mixtures, you might expect the result to be different. Probably not harmful, and probably cheaper, but different. More diluted.
To keep consistency in your product you might want to opt for the pure stuff.
Thank you so much for your explanation of the Crystal v. dl- thing. You must be a great professor. You were able to make something complex understandable and I appreciate your ability to do that, Again, thank you so much.
You can follow this conversation by subscribing to the comment feed for this post.