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Chapter 3

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CHM 2210

Chapter 3 Wednesday, July 10, 2013 12:07 AM 1. Chirality is the term assigned to a moleculewith no plane of symmetry a. Asymmetric,so the mirror image is non-superimposable 2. A chiral molecule has at least one stereogeniccenter present 3. When a compound is chiral, it will have one nonsuperimposable mirror image, called its enantiomer 4. There are a few ways to draw an enantiomer a. The easiest way is to place the "mirror" behind the molecule,because the skeleton of the moleculeis drawn the exact same way except all dashes become wedges and all wedges becomedashes i. b. When dealing with bicyclic compounds, the easiest way is to place the mirror on the side of the compound i. 5. Switching any two groups on a chirality center will invert the configuration a. b. It doesn’t matter which two groups are switch c. If you switch two groups a second time, the configuration will switch back 6. An easy way to rotate the molecule is as follows a. First, assign priority to groups i. b. Find the fourth position and switch it with the group that is on the dash i. c. Then switch the other two groups i. d. Two switches have been performed,so the final drawing must have the same configuration as the original drawing i. But now, the molecule has been redrawn in a perspective that exhibits the fourth priority dash on a dash 7. Determining absolute configuration a. The atom with the highest atomic number (biggest) is assigned priority of 1 b. Lowestatomic number is assigned 4 i. c. Rotate the moleculeso the fourth priority is directing behind the page (dashes) i. d. Look to see if the 1-2-3 sequence is clockwiseor counterclockwise i. Counterclockwiseis S 1) ii. Clockwiseis R 1) e. 8. Optical Rotation a. Enantiomers exhibit identical properties, such as boiling and meltingpoint (diastereomers do not!) i. b. Enantiomers do however rotate plane-polarized light differently c. They also react with other chiral compounds differently from each other d. When many waves of light are traveling in the same direction, they each have a different polarization, randomly oriented with respect to one another e. When light passes through a polarizing filer, only photons of a particular polarization are allowed to pass through, give rise to plane polarized light i. f. The rotation of plane-polarized light causes by optically active compoundscan be measured using a polarimeter i. g. When a solution of a chiral compound is placed in a polarimeter,the observed rotation (α) will be depended on the number of molecules that the light encounters as it travels through the solution i. If the concentration is doubled, the observed rotation willdouble h. The same is true for the distance the light travels through the solution (path length) i. By using a standard concentration (1g/ml)and pathlength (1dm), we can obtain the specific rotation i. ii. The specific rotation is a physical constant, like its melting or boiling point j. A compound that rotates light clockwiseis called dextrorotatory,d, and is symbolizedby + (dexter is +) i. The opposite is levorotatory,l, which is symbolized by - 9. Types of Configurational Isomers a. Can be either enantiomersor diastereomers b. Enantiomersare configurational isomers that are nonsuperimposablemirror images of each other i. All chiral centers are different (reversed) c. Diastereomersare nonsuperimposableand that are not mirror images of each other i. If one of two chiral centers differ (or a few but not all), the compounds are diastereomers d. e. When the orientation of a moleculeremains constant and three substituents change their location, this implies that the compound has been rotated about that chiral center i. ii. 1) **picture a pencil skewering the bond with nothing on it in the picture and twisting! f. Meso compounds are individual structures which contain a mirror plane slicing through the middle of the compound and an even number of chiral centers i. Optically inactive ii. May be identified by an inversion center in the middle or a mirror plane in the middle iii. A meso compound has the same number of R stereocenters as S iv. 10. Stereochemistryin Reactions a. When reactions involve chiral centers, they are often selective for one Steroisomersover another b. The type of stereoisomerformed depends on the chirality of the starting reagents c. When two enantiomers are formed, they are formed in equal quantity, and product is said to be racemic to be racemic i. Enantiomerscan be formed in a unequal ratio if a chiral catalyst is present ii. This leads to the concept of enantiomericexcess d. When two diastereomers are formed, they are formed unequally, so the product mixture has a major and minor compound and can rotate light 11. Enantiomericexcess a. When a mixture is not in a 50-50 ratio, then the net rotation of light is not zero b. The farther the value deviates from zero, the greater the difference in concentrationof the two enantiomers c. 12. Substitution reactions a. Involve the exchange of one functional group for another i. b. In every sub reaction, there is an electrophile and a nucleophile c. Leaving groups withdraw electrondensity via induction, rendering
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