Chirality-Steve - This was provided from my tutor to walk me through Chirality. PDF

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This was provided from my tutor to walk me through Chirality....

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“Chirality"   

Assigning R/S designations Enantiomers vs. Diastereomers Meso compounds

If a carbon is bonded to 4 different "legs" something very interesting and important for organic happens: The same molecule can exist as two *mirror-image* isomers. Such carbons are called "chiral carbons," or "stereocarbons" or "stereocenters" or "asymmetric carbons." The pair of mirror-image compounds are called "enantiomers." These compounds are also called "optical isomers," because it turns out they *rotate polarized light.* (Optical isomers are one of the two classes of "stereoisomers," the other being "geometric isomers," cis & trans. Some texts call these "conformational isomers.") Enantiomers have identical *physical* properties, like bp, which means you can't separate them by distillation (which is a very common way to separate liquids with different bp’s). But, they can have much different *chemical* properties in living critters. As noted above, chiral compounds rotate polarized light. The direction is given by the letters "l" (lower-case L) and "d," for "levo" and "dexter." Levo is left across the top, so CCW, while dexter is CW, or right across the top. But with that said, we don't pay much attention to the way the *light* rotates, because no one has found a way to predict that from looking at the molecule. (Note that lower-case "L" and "d" are NOT the same as upper-case L & D, which is a notation used with sugars. While sugars rotate light, the designations refer to whether the OH group just above the bottom alcohol in a Fischer projection is on the left or right. We'll talk about that later.) So, some chemists came up with a system that classifies chiral molecules by the weight of the first atoms connected to the chiral carbon. This designation is called "absolute configuration" but is commonly called simply "R/S," where R stands for "right" and S stands for "sinister," one of the many Latin terms for "left." But *note* that R and S do NOT predict the way *light* will rotate -- because as noted above, no one has found a way to do that by inspection. So, an "R" can rotate light either direction. All we know is that if an R rotates one way, the S enantiomer will rotate it the opposite way. Most professors typically have several test questions asking whether a chiral carbon is R or S, so being able to do this *fast* and accurately is crucial. The system is called the "CIP rules"-- the initials of the chemists who devise the system. The steps are very simple, and with an hour of practice you can assign R or S in ten or 15 seconds. Here's how:

1. Rank the heaviest 3 atoms attached to the C, by weight, with heaviest as #1. If two first atoms tie, go to the next atom in the leg, and repeat until you get a difference. Call these legs 1, 2 and 3. 2. Now, in a tetrahedral shape one leg is coming toward you, one leg goes back into the paper and two legs are in the plane of the paper. So, only three cases are possible: a) If the lightest leg is going away from you, connect the legs 1-2-3 and the direction of your arc is the answer: If it's curving to the right over the top, it's R; b) If the lightest leg is coming toward you, connect 1-2-3 and reverse the direction to get the answer; c) If the lightest leg is "in the plane," exchange it with the leg that's going into the paper, redraw your arc, then reverse the answer (cuz when you only have 3 numbers, swapping any two reverses the rotation). That's 12 lines (not including spaces). Very short and simple. I'll try to do a Powerpoint with several practice problems before your next test. === DIASTEREOMERS n

If a molecule has "n" chiral carbons, you can have a max of 2 stereoisomers. So, with two chiral carbons, for example, you can have 4 stereoisomers: RR, SS, RS and SR. If you have identical formulas, and the designations on ALL the chiral carbons change, those two compounds are "enantiomers." By contrast, if any but NOT all the letters change, you have "diastereomers." (Obviously, if none of the letters change, it's the same compound.) Diastereomers are useful because they have very slightly different bp's, so can be separated by very careful distillation. But that's really a pain, so if you only want to produce one of two enantiomers, far better to use reactions that produce just the one you want, instead of both! (And there's a way to do that, as you'll see.) Mixtures containing roughly equal amounts of both enantiomers are called "racemic." === MESO COMPOUNDS

These are unique compounds cuz even though they have chiral carbons, the molecule *doesn't* rotate polarized light, so it's called "achiral." How can chiral carbons not rotate light? Because the molecule has a plane of symmetry, with each carbon having identical legs, with one an R and the other S. Thus the two C's rotate light in opposite directions but by the same amount, which doesn't produce any *net* rotation. Note that any time you have two chiral carbons with a visible plane of symmetry (a "mirror plane"), it's meso. But you often see problems where there's no visible plane, but the C's have the same legs attached to 'em. In that case assign R or S to each. If one is R and the other S, it's meso even if you don't have a visible plane of symmetry. By contrast, if you have a ring with two C's, each of which has 3 visible legs (so *could* be chiral), and both lie ON the plane of symmetry, molecule isn't meso cuz neither of the C's is chiral. It's a good trick question....