Lecture Outline Compounds with 2 or More Stereocenters

Name ______________________________________ If you're not totally fried after Wednesday evening's exam, you can read section 7.5. 1 If a molecule has 5 tetrahedral stereocenters, how many stereoisomers could exist? Is this a guaranteed number, a maximum or a minimum? 2 Q: Dude! We just had an exam and we're totally fried. Gimme a break, Dude. A: Whatever. 3 OK, what was the Stupid Pencil Trick demonstrated in Wednesday's class? (a) One pencil in each ear and then spin around cw or ccw while clucking like a chicken. (b) Hold a bunch of pencils in a tetrahedral arrangement and then quickly do the chiropermu-switcharoo to generate the R or S label. Cool. How does that work??? (c) Something involving tracing circles in the air — what the heck were you thinking anyway? That's about the dumbest thing I ever saw. (d) Uh, I missed it. Will it be on You-Tube™? (A: I hope not.) 4 What's in Prof. Snyder's basement right now? (a) four feet of water (b) household trash from the past three years and an open window (c) hundreds of broken plastic models of cyclohexane (d) all of the above (e) squirrels 5 If the answer to #4 were "squirrels" (hypothetically, of course), what would be some good names for them? Please feel free to submit at least 2 or 3 (not that it really is squirrels... I'm just saying if). Lecture outline Compounds with 2 or more stereocenters — CH3CH2 CH CH Cl Br CH3 2 scs, each can be R or S, so 4 combos possible Draw them below by attaching Hs, Me, and Et Br C Br C C Cl C Cl Br Br C C C C Cl Cl Diastereomers are stereoisomers that are not enantiomers That's it. Don't make it more difficult. Note that enantiomers have opposite configuration at every sc. (Why must this be true?) If two compounds have the same config at some scs, but opp config at others, they must be diastereomers. Note how this fits the definition above; however this is not the definition of diastereomers! For example, these pairs are also diastereomers: CH3 CH3 and and Br Br Here's another example — O tartaric acid HO O C CH CH C OH OH OH 2 scs, so let's draw the 4 stereoisomers... CO2 H CO2 H C C C CO2 H CO2 H C CO2 H CO2 H C CO2 H CO2 H C C C What happened? A cmpd with 2 or more scs that is achiral — is called "meso". (meso just means achiral with scs) Stereochemistry of cyclic cmpds is easiest to see if the ring is treated as being planar trans Br Br cis Br Br Br Br — even cyclohexanes can be treated as planar, e.g., CH3 CH3 CH3 CH3 CH3 CH3 ring-flip is fast at room temp ! time-averaged symmetry (as seen in flat drawing) This is more the rule than the exception — i.e. most achiral cmpds have chiral conformations. Just about any achiral molecule with any flexibility at all will have chiral conformations, but they all come in pairs — if an achiral structure can distort in a "right-handed" way, then it can distort just as easily in the corresponding "left-handed" way, so all chiral conformations must come in enantiomeric pairs. As a result, no properties associated with chirality are observed. Butane is another example. The gauche conformers are enantiomeric, but the anti has a plane of symmetry, so the molecule is achiral overall. Let's review the definition of achiral — plane of symmetry or inversion center "in any reasonable (i.e. energetically accessible) conformation" — for our purposes here, a flat ring can be considered "reasonable". Other "reasonable" conformations would be eclipsed conformers, boat cyclohexanes, and other higher-E structures — whether they're energy minima or maxima doesn't matter — they just have to be accessible at ordinary temperatures. (Examples of energetically unreasonable things would be structures with square planar Cs (nothing would be chiral!), structures with perpendicular pi bonds, etc.) A planar N is also energetically accessible. This is why amines, like the one below, are effectively achiral. the rapid inversion passes through an achiral structure with a planar N. You can think of this example as well as the cyclohexane above as having "time-averaged symmetry" as a result of the rapid interconversion of enantiomeric forms. N Et H Et Me H Me N It's interesting to think about the implications of working at very low temp — 77K for example. At that temp, the interconversion would be completely shut down. The two cyclohexanes, as well as the two amines could be separated — at least in principle — and they would behave like any other stably chiral molecules.