It is amazing that I have been able to hold out this long without thrusting some of my subject matter onto the Reader. But when Cooley’s ‘looking glass’ theory came up in our readings… well, how can I resist the temptation to discuss the chemistry of enantiomers? Impossible! It is too perfect an opportunity to ignore!Bear with me, Reader, if you think you do not like chemistry. This is a fascinating topic, and we will have some fun with it, and then I promise to show you why it is even relevant to our discussion at all.
First, a flight of fancy. Imagine yourself peeling a plump, fresh orange. You dig your nail under the rind and pull back, tearing it open. Some juice trickles over your finger, a fine mist forms in the air. Do you smell it? That happy, fresh burst of citrus? Yes, that is the smell of chemistry!
Actually, it is the smell of myriad different compounds wafting through the air, from the orange peel to the receptors in your nose. Each receptor is designed to recognize a different kind of compound, and in the case of the orange, the specific molecule your nose is detecting is a hydrocarbon called D-limonene. (By hydrocarbon, I mean to say, a compound made of the elements hydrogen and carbon.)
But it gets much more interesting, because now we are going to do some house work. Yes, that’s right, we’re going to polish a beautiful wooden desk, and because it is made of wood we are going to use a furniture polish that smells like pine (it is only polite to make the dead tree feel at home). Naturally, the piny fragrance of the polish is completely different from the scent of the orange, and with your new knowledge of chemistry you can now readily attribute this phenomenon to the different chemical compounds which are present in the furniture polish stimulating a different set of receptors in your nose. Quite simple. There is nothing remarkable about that!
But we are not giving our noses enough credit, because the compound in the furniture polish which our noses recognize as completely different from the one in the orange is, in fact, L-limonene. It is the mirror image, or enantiomer, of the D-limonene in the orange. The two molecules are exactly the same in every way, except for their geometry in 3-D space. To help you see the difference, I have produced an image of the D-limonene and L-limonene, facing one another as if in a reflection. If you were to construct physical models of each of these images, you would find you would not be able to superimpose them. They are fundamentally different in shape.
(A note for interpreting this kind of diagram: the ‘sticks’ represent chemical bonds. Notice in some places there are double bonds. At each ‘corner,’ where the bonds meet, there is an atom. The black corners represent carbon atoms; the white ones are hydrogen. Also notice how carbon always forms four bonds, and hydrogen can only make one.)
Do not be discouraged if you have trouble realizing the difference between these two molecules. In fact, it is a difference that many organic chemistry students find difficult to appreciate. During a test they will sit at their desks and construct models of the molecules using pencils as bonds and erasers as atoms. Or they will turn their papers upside-down and cock their heads to the side. I’ve seen some try to use their hands and fingers to illustrate the geometry, like some kind of strange chemistry shadow-puppeteering. Obviously, people’s noses are much smarter than they are!
The lack of equivalence between an object and its reflection is not a novel concept for most people. For example, it is immediately apparent when we look into a mirror while wearing a T-shirt that has something written on it - the lettering is all wrong. But in fact, every time we look in the mirror, no matter what we are wearing, an inaccurate image is reflected back to us. The subtle asymmetries of our facial features – the kinds of subtle asymmetries that our brains have evolved to notice - are all inverted. So in fact, most of the time, we don’t really know what we look like. Lest you think the inaccuracy trivial, I’ll remind you of the two limonenes. If somehow it were possible for D-limonene to look into the mirror and see his reflection, he would surely mistake himself for L-limonene, and probably end up applying for a job at the Pine-Sol factory. Imagine his embarrassment when the boss declares, “You smell like an orange, you ridiculous thing! What are we supposed to do with you?”
(It makes you wonder if the company hasn’t developed its new ‘Orange Energy’ formulation to accommodate all the misguided applicants.)
1 comment:
Thanks for writing this.
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