|MadSci Network: Chemistry|
Hi Tania, Wow! this is quite a complicated question. First, "optical activity" is the phenomenon that is observed when plane polarized light is passed through a solution of an optically active compound and is found to have rotated upon exiting the solution. This means the polarized light that exits the solution is polarized at a different angle than the light entering the solution. The angle between the two planes of polarization is the "rotation" of that substance for a specific set of conditions (solvent, concentration, wavelength, and path length). This phenomenon is one of the many ways light interacts with matter to yield interesting information about the matter. The property of a particular molecule which predicts whether or not it will be optically active is its symmetry. Specifically, a molecule which possesses a plane of symmetry will _not_ be optically active and a molecule which does not possess a plane of symmetry _will_ be optically active. This is the defining criterion for chirality and, thus, a "chiral" molecule will be optically active and an "achiral" molecule will not. In order to understand this rule some information about the 3-dimensional structure of molecules is required. Virtually all molecules with more then three atoms possess three dimensional structures. These structures are composed of atoms bonded to each other in various patterns and shapes and they become exponentially more complex as the numbers of atoms involved go up. A molecular structure, then, possesses a plane of symmetry if, and only if, it can be divided in half such that the two halves are mirror images of each other. Or, if the mirror image of the entire molecule is identical (superimposable upon) to the original molecule then it possesses a plane of symmetry. If a molecule is not identical to its mirror image than it is chiral. The classic example of a chiral structure is the hand. Your right hand is the mirror image of your left hand; however, they are not identical and they are not superimposable. Two molecules which are not identical but are mirror images of each other are termed "enantiomers"....your right hand is the enantiomer of your left hand (and vice versa). When a photon interacts with a chiral molecule it is rotated by a certain amount that is specific to that molecule. The enantiomer of that molecule will rotate a photon exactly the same amount in the opposite direction. Thus, a mixture of equal amounts of enantiomers (called "racemic") will not be optically active because half the molecules are rotating light in one direction and the other half are rotating it exactly the same amount in the other direction. The net rotation is zero in that case. Unless specific measures are taken to prevent it, _all_ chemical reactions that generate chiral molecules from achiral molecules do so in racemic fashion (that is, both enantiomers are produced in equal amounts so that no optical activity is produced). Where, then, does optical activity come from?....The answer is, exclusively, life. Optically active compounds are found free of their enantiomers in living things, from humans all the way down to viruses (the borderline between living and nonliving). Here is the answer to your very first question: the use of optically active compounds is life, itself! Virtually every important molecule in your body is optically active and must be to function correctly. Synthetic chemists have learned in the last 30yrs or so how to steal (or borrow in the case of catalysts) optical activity from natural sources to produce optically active (enantiomerically pure) synthetic compounds which is why there are optically active products available today that do not occur in nature. However, this was not always the case and has had disastrous results in some cases. Thalidomide was once sold as theracemic mixture which resulted in terrible birth defects in babies born to mothers who used it. It was later found that one enantiomer was responsible for the desired, safe, sedative effect of the drug and the other enantiomer was responsible for the terrible side effects. Had the drug been sold in its optically active form (free of the deleterious enantiomer) the birth defects would likely have been avoided. One enantiomer of amphetamine is sold in some over-the-counter inhalers for asthma; however, it is the other enantiomer that is illegal because of its addictive euphoric effect. There are many, many examples of pharmaceuticals that are sold in their racemic form (Advil, Prozac, to name a couple) where it is likely only one of the enantiomers is responsible for the desired activity. The other enantiomer could be causing side effects (like in Thalidomide) or doing nothing at all. In the light of recent advancements in synthetic methodology, the FDA is currently debating whether or not (or to what extent) to require pharmaceuticals to be optically active. Sugar is optically active because it is harvested from the natural source. Other optically active compounds that are generally available are tartaric acid (or any of its salts), ascorbic acid (vitamin C), vit B12, vit E (if it is natural....the synthetic is sold in its racemic form), virtually all amino acids are harvested from natural sources and are sold in health food stores, patchouly oil, sodium naproxin (this is one of the few enantiomerically pure synthetic pharmaceuticals), camphor, Sudafed. This is by no means a complete list but it should help in finding some good examples of optical activity. Thanks for the interesting question! Jeremy.
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