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Chem 341 • Organic Chemistry I Lecture Summary 25 • October 24, 2007 Chapter 9 - Stereochemistry Chirality Many molecules in nature are ‘handed’. That is, they can exist as a special kind of stereoisomer that we call Enantiomers. Enantiomers are compounds that are mirror images and are not superimposable on each other (not identical). Just like you have a left-hand and a right-hand, some molecules are Chiral. (Note that molecules that are identical with their mirror images are called achiral.) Due to the tetrahedral nature of sp3-hybridized carbons, if all four groups on a carbon are different, their mirror images are not identical. See below, for example, a simple methyl group with four different colored groups attached. It’s mirror image is a different molecule. If you try to line them up and superimpose them, you weill find that two of the colors do not match up. Thus, this is a chiral molecule. Chirality and Biology The molecules of living systems are made up mostly of compounds that contain these type of stereogenic carbons - carbons with four different groups attached. Mainly these include sugars which make up the backbone of DNA and amino acids which are the basic structures of proteins. Thus, biological processes rely on the interactions of chiral molecules. Many drugs only work by fitting one enantiomer of the drug into an enzyme receptor like a key in a lock. A classic example of this is the tradegy of the drug, thalidomide. This drug was made as a 50:50 mixture of enantiomers and given to pregnant women to ease morning sickness. One enantiomer of the drug had the beneficial effects while the mirror image caused severe birth defects. ©2007 Gregory R. Cook North Dakota State University page 1 Chem 341 O stereogenic carbon O N * O stereogenic carbon N * O N H O (R)-thalidomide beneficial sedative O N H O O (S)-thalidomide causes birth defects Optical Activity Enantiomers have identical physical properties in all respects but one. Their ability to rotate the plane of plane polarized light. All other properties such as melting points, boiling points, spectroscopic absorptions, etc. are all identical. As shown below, when light is passed through a polarizing filter the waves are all traveling in one plane. When those light waves strike a chiral molecule, the plane of the waves as they travel through are rotated. What you see if you are looking directly at the light as it comes toward your eye would be a rotation to the left or right from the original plane. Enantiomers will rotate the light an equal amount in opposite directions. Samples that rotate plane polarized light are said to be “optically active”. Achiral molecules do not rotate the plane of polarized light. light travels in waves but oscillates in all three dimensions along the path of travel Light is polarized and oscillates in a single dimension when the polarized light passes through a chiral sample, it is bent out of the original plane chiral sample Note that the enantiomer will bend the light in the opposite direction chiral sample enantiomer polarizing filter A racemic mixture of enantiomers is a 50:50 mixture of the mirror images. A racemic mixture will not be optically active as they rotation of plane polarized light will be exactly cancelled out by the presence of both enantiomers in equal amounts. Mixtures of enantiomers that are not equal amounts will show rotation of plane polarized light in the amount of the excess of one enantiomer over the other. That is, the effect will not be completely cancelled out. R and S Configurations of Stereocenters As enantiomers are stereoisomers that are only different at the asymmetric carbon (stereogenic carbon), we need a way to distinguish them in the name. We use the Cahn-Ingold-Prelog priority rules (as we did with E and Z) to assign a ranking of priority to the four different groups. If we visualize the molecule pointing the lowest priority group away (usually H) the three remaining groups define a circle. If you travel from highest priority (1) to lowest (3) you will go clockwise in the R-enantiomer whereas you will travel counterclockwise in the S-enantiomer. ©2007 Gregory R. Cook North Dakota State University page 2 Chem 341 You can also use your hands to help you by pointing your thumb toward the lowest priority group and seeing which hand you need to curl your fingers from highest priority to lowest priority. The right hand would be the R-enantiomer (latin - rectus = right) and the left hand would be the S-enantiomer (latin sinister = left). 4 H 3 Cl 1 1 2 (R)-2-chlorobutane 2 Cl H 4 3 (S)-2-chlorobutane Quiz of the day ©2007 Gregory R. Cook North Dakota State University page 3 Chem 341