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Keys to the Chapter 79 that have the same atoms linked in the same order (i.e., identical connectivity), but that do not have identical three-dimensional shapes. The first example in Section 5-1, 2-bromobutane, is one of a vast number of mole- cules that are chiral. Chiral molecules can exist as either of two stereoisomeric shapes, which are related to each other as an object is to its mirror image. Before going any further, let's make one point clear: Every mol- ecule has a mirror image, obviously. What makes a chiral molecule special is that it is not identical to its mir- ror image. Methane is identical to its mirror image; it is not chiral. The two possible shapes of a chiral mol- ecule differ in the same way that a right-handed object differs from a left-handed object, as gloves, shoes, and hands do. Chirality, therefore, is "handedness" on a molecular level. The two mirror-image shapes of a chiral molecule are called enantiomers. What makes a molecule chiral? The most common of several types of structural features that can make a molecule chiral is the presence of a carbon atom attached to four different atoms or groups (an asymmetric carbon atom, an example of what is called a stereocenter). At this point it is worthwhile to dust off your set of models and start manufacturing chiral molecules. Prove to yourself that the model of the mirror image of one cannot be superimposed on the original. This is the first step toward developing the ability to visualize this relationship clearly. 5-2. Optical Activity The physical difference between enantiomers is subtle that they end up for the most part displaying identi- cal physical and chemical properties. They can be distinguished from each other only after interacting with something else that is, itself, already "handed." By analogy, a right and a left glove will have the same weight, color, and texture. However, interaction with, for instance, a right hand will immediately distinguish them. The fact that we can tell them apart just by looking at them reflects the fact that our brain's interpretation of the signals from our binocular visual system is "handed" in its ability to perceive depth as well as left-right rela- tionships. For chiral molecules, the counterpart to this is their interaction with plane-polarized light: The plane of polarization of plane-polarized light is rotated when it passes through a solution of one enantiomer of a chi- ral molecule. This phenomenon, labeled optical rotation, is the most common way of detecting chiral mole- cules and is described in considerable detail in this section. Molecules possessing the ability to rotate the plane of polarized light are said to be optically active, or to display optical activity; and another term for enantiomers is optical isomers. One further term of importance is the one given to a mixture of equal amounts of the two enantiomers of a chiral molecule: racemic mixture. The two enantiomers of a chiral molecule each rotate light by equal amounts, but in opposite directions, so the racemic displays no optical activity because the two components exactly cancel out each other's rotations. Again, because of the large number of new terms and ideas associated with this material, it merits careful study, with a good set of models close at hand. 5-3. The R-S Sequence Rules Like all the rules of nomenclature, the R-S system for molecules containing stereocenters has one purpose: the concise, unambiguous description of a single chemical structure. In most cases the system is not particularly difficult to apply, because the assignment of priorities to the groups on an asymmetric carbon is usually straight- forward and use of models to view the stereocenter properly takes care of the rest. Models should be made of the examples in the chapter so that you can confirm their R or S designations. Priority rule 2 is an occasional source of trouble until the concept of "first point of difference" is well un- derstood. If this gives you trouble, take it stepwise: 1. Write the substituent groups to be compared side-by-side, with the bond of attachment to the asymmetric atom on the left. 2. Starting at the left, look at the first atom in each substituent chain and identify the atoms attached to it in descending order of priority. If the highest priority atoms in each are the same, work your way down in priority, looking for the first nonidentical atoms (first point of difference). If no differences are found at this stage, move out from the first to the second atom in the chain and repeat the process. If there is branching here. the direction of this move should be chosen to examine the highest priority atom for