CHAPTER 5 HW: STEREOISOMERS

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1 CAPTER 5 W: STEREISMERS CIRALITY AND ENANTIMERS 1. Define the following terms: a. Chiral A chiral substance has the possibility of having right and left handed versions (or it can have an enantiomer). b. Enantiomers (and give an example) Stereoisomers that are non-superimposable mirror images: they mirror but are different spatially, in 3D.. Put an asterisk (*) on all chiral centers in these compounds. Some may have no chiral centers and some may have more than one. F * * no stereocenters * * * * no stereocenters 3. Find the chiral center in each compounds, then draw both enantiomers. * a. b. * *. Draw the enantiomer of each compound. Compound Enantiomer Compound Enantiomer F F C 3 C 3 Page 1

2 DIASTEREMERS AND MES CMPUNDS 5. Define the following terms and give an example of each: a. Diastereomers Stereoisomers that are not enantiomers. The atoms are connected in the same order, but they do not mirror, and they are still different in 3D. b. Meso compound A meso compound has two or more chiral centers, but also has an internal plane of symmetry and is therefore achiral (is superimposable with its mirror image). 6. Draw a diastereomer of each compound. (several answers: keep at least one center, switch at least one) Compound A diastereomer Compound A diastereomer C 3 C 3 C 3 C 3 7. Draw two stereoisomers of the structure below that are diastereomers of each other. 8. Draw a compound with a molecular formula of C 5 10 that is (there are several answers) A chiral compound. A meso compound. An achiral compound that is not a meso compound. A constitutional isomer of your previous structure. Page

3 9. Identify the relationship between each pair of compounds. Are they identical, constitutional isomers, enantiomers, diastereomers, or not isomers? Relationship Enantiomers Constitutional isomers C C 3 C C 3 C C 3 C C 3 Relationship Diastereomers Identical Relationship Identical Not isomers (C 5 vs. C 7 ) 10. A meso compound has no enantiomer, but can have a diastereomer. Explain and give examples. 1 3 A meso compound is achiral, meaning it can t have an enantiomer. It is superimposable with its mirror image (e.g. structure 1 is the mirror image of structure - and they are identical, not different as they would be if they were enantiomers). But compound (meso) is a diastereomer of compound 3- they are stereoisomers that don t mirror. 11. Explain why compound M has no enantiomer and why compound N has no diastereomer. C M C C N C 3 Compound M has an internal mirror plane, so is achiral, meaning it cannot have an enantiomer. Compound N has only 1 chiral center, so can only have an enantiomer. You need at least chiral centers to have diasteromers. 1. For n chiral centers in a compound, there may be n possible stereoisomers.,-dichloropentane has two chiral centers, but only has three stereoisomers. Explain why this is, using structures. ne of the three stereoisomers (3) in,-dichloromethane is meso, since it has an internal plane of symmetry. When a meso compound exists, there will be less than n stereoisomers. 1 3 Page 3

4 R,S DESIGNATINS 13. Circle the four different groups around each chiral center, then rank each group from 1- using priorities for R,S nomenclature Determine the R,S configuration of each chiral center. 3 1 a. 3 C b. c. 1 R C C C S ( is ) 1 looks R, so is S d. e. N R R 1 3 S 1 looks R, so is S 15. Ketamine is an anesthetic used in human and veterinary medicine. It is also used by some recreationally as a club drug. It is often administered as a racemic mixture, although the more active form is the S enantiomer. Which of the structures below is (S)-ketamine? 3 NC NC Why is cis-1-chloro--methylcyclopentane an incomplete name for this structure C 3 C 3 Cis does differentiate this structure from trans, but there are two different structures that have the cis orientation ( enantiomers). R,S naming is necessary to distinguish between the two different enantiomers. Page

5 17. Give the IUPAC name for each compound, including R,S designations. Structure Name (R)-,,5-trichlorohexane (3R,5S)-3,5-dicyclopropyloctane Structure C C 3 C 3 F Name (1R,3R)-1-ethyl-3-fluorocyclohexane (3S,S,5S)-1,3-dibromo-5-chloro- -methylhexane 18. Draw the structure from the following names, including dashed and wedged bonds where appropriate. Structure C 3 C 3 Name (R)-3-methylhexane (3S,6R)-6-isopropyl-3-methyldecane 19. Multiple choice: choose the best answer(s). a. The enantiomer of (S,3S)-3-bromo--chloropentane is: (S,3R)-3-bromo--chloropentane (R,3R)-3-bromo--chloropentane (R,3S)-3-bromo--chloropentane b. The following are diastereomers of (1R,3S)-1-ethyl-3-methylcyclohexane: (1R,3R)-1-ethyl-3-methylcyclohexane (1S,3R)-1-ethyl-3-methylcyclohexane (1S,3S)-1-ethyl-3-methylcyclohexane Page 5

6 FISCER PRJECTINS 0. Draw the following: (several answers for the diastereomer and constitutional isomer) Draw the enantiomer Draw a diastereomer Draw a constitutional isomer C 3 C 3 C 3 C 3 C C C 3 C C 3 C 3 C C 3 1. Draw each structure in a Fischer projection, then decide whether the compound is chiral or achiral. Structure C C 3 C 3 Fischer or or or C C 3 C 3 Chiral or achiral? Chiral Achiral Chiral (not quite symmetric: different ends). Draw Fischer projections of the following compounds to help determine their relationship. Are they identical, enantiomers, diastereomers, constitutional isomers, or not related? 3 C C C C C 3 C 3 C C 3 3 C C C C C 3 (C ) 3 C 3 C C 3 C 3 (C ) 3 C 3 They are enantiomers (non-superimposable mirror images) Page 6

7 3. Identify the relationship between each pair of compounds. Are they identical, constitutional isomers, enantiomers, diastereomers, or not isomers? C 3 C 3 C C 3 C C 3 1 C 3 C 3 1 C 3 C C C 3 3 looks S, so R (C 3 # is in front) C C 3 3 S (C 3 # is in back) C C 3 Relationship Enantiomers (do R,S) Identical (rotate 180 ) C 3 C 3 C 3 C C 3 C 3 C 3 C 3 C C 3 C 3 C 3 C 3 C 3 Relationship Diastereomers Enantiomers PTICAL ACTIVITY AND PYSICAL PRPERTIES F STEREISMERS. Define the following terms. a. Plane-polarized light Light which has been passed through a polarizing filter so the waves are oscillating in one direction. b. ptical rotation A measurement of how a sample (typically of a chiral compound) rotates plane-polarized light. c. Racemic mixture A mixture that has equal amounts of enantiomers (50-50 mix). d. Enantiomeric excess (ee) In a sample which includes both enantiomers, the ee is the % difference between the amount of the major enantiomer and the amount of the minor enantiomer. For example, an 80:0 mixture of enantiomers has an ee of 60 (80 0 = 60%). Page 7

8 5. The product mixture from a chemical reaction is reported as 98 ee. What does this mean? Give the percentages of the components present in the mixture. 98 ee means there is 98% excess of one enantiomer. The mixture is 99% one enantiomer and 1% the other (99% 1% = 98%) 6. Would the following pairs of compounds have identical or different physical properties (boiling points, densities, etc.)? int: determine if they re identical, enantiomers, or diastereomers. iefly explain. ief explanation Identical properties: they are enantiomers. The only phys. prop. that enantiomers don t share is optical rotation. Different properties: they are diastereomers. ief explanation Identical properties: they are identical molecules. Identical properties: they are enantiomers. Page 8

9 7. Multiple choice. iefly explain each choice. a. Compound A has an optical rotation of 13., or [a ]= Compound B should have an optical rotation of (circle one): [a ]= +13. [a ]= -13. [a ]= 0 [a ]= some other number A B These are enantiomers, and their optical rotation has the same magnitude but opposite sign. b. Compound C has a boiling point of 130 C. Compound D should have a boiling point of (circle one): b.p. = 130 b.p. = 0 C b.p. = some other number C 3 C 3 C C 3 D C 3 These are identical (both are cis-1,-dimethylcyclohexane). c. Compound E has an optical rotation of -8.3, or [a ]= Compound F should have an optical rotation of (circle one): [a ]= -8.3 [a ]= +8.3 [a ]= 0 [a ]= some other number Molecule F is achiral it is a meso compound, so it has an optical rotation of 0. E F E and F are diastereomers, so they have different physical properties, and -8.3 and 0 are different. d. Compound G has a density of 0.91 g/ml. Compound should have a density of (circle one): d = 0.91 g/ml d = some other number G These are diastereomers, so they don t have the same physical properties. You can t predict the density of one diastereomer based on the properties of another. Page 9