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2.01 R,S Nomenclature Answers.Cdx Chem S-20ab Purple Book Solutions Week 2 - Page 1 of 27 R and S Nomenclature 1. Circle all the stereocenters in the following molecules. For each stereocenter, assign an R or S designation according to the standard rules. Cl H R H R H R OH S H S H3C S S O R S HO N 2. Draw an unambiguous structural representation of the following molecules: (S)-4,5-dimethyl-(Z)-2-hexene (R)-3,5,6-trimethyl-(E)-3-heptene H H 3. Provide the best possible systematic name for the following compound: (S, E)-5-isopropyl-2,3,7-trimethyl-4-octene Chem S-20ab Purple Book Solutions Week 2 - Page 2 of 27 Chirality 1. For each of the following species, indicate if it is chiral or achiral. For those molecules that are chiral, circle all stereocenters, and draw the structure of the enantiomer. H H F F H C CH 3 Cl Cl 3 chiral H CH3 H3C H chiral H CH3 achiral H Br Br H H Br Br H chiral H Br Br H achiral ("meso") H3C CH3 H3C CH3 H3C CH3 H3C CH3 chiral Chem S-20ab Purple Book Solutions Week 2 - Page 3 of 27 Enantiomers and Diastereomers 1. For each of the following pairs of stereoisomers, indicate if they are enantiomers, diastereomers, or identical compounds. diastereomers identical enantiomers identical CH3 CH3 Br Br CH Br 3 diastereomers H3C H3C Br CH3 Chem S-20ab Purple Book Solutions Week 2 - Page 4 of 27 Identifying Isomers 1. For each of the following pairs of isomers, indicate whether they are Identical, Enantiomers, Diastereomers, or None of the above by circling the appropriate term for each. Identical Enantiomers Diastereomers None HN HN Me H Identical Enantiomers Diastereomers None H Me Identical Enantiomers Diastereomers None HN HN H H Me Me Identical Enantiomers Diastereomers None H Identical Enantiomers Diastereomers None H Identical Enantiomers Diastereomers None Chem S-20ab Purple Book Solutions Week 2 - Page 5 of 27 Stereochemical Concepts 1. Which of the following are true? Give counterexamples for those that are false. (a) In some cases, constitutional isomers are chiral. T (b) In every case, a pair of enantiomers have a mirror-image relationship. T (c) Mirror-image molecules are in all cases enantiomers. F identical CH3 CH3 (d) If a compound has an enantiomer, it must be chiral. T H (e) Every chiral compound has a diastereomer. F has no diastereomer F H C 3 Cl (f) If a compound has a diastereomer it must be chiral. F and are diastereomers H Br (g) Any molecule containing an asymmetric carbon must be chiral. F achiral ("meso") Br H (h) Any molecule with a stereocenter must have a stereoisomer. T (i) Some diastereomers have a mirror image relationship. F; no diastereomers have a mirror image relationship. (j) Some chiral compounds are optically inactive. F; all PURE chiral compounds are optically active (k) All chiral molecules have no plane of symmetry. T H Br (l) If a compound has a stereocenter, it has an enantiomer. F achiral ("meso") Br H (m) A meso compound will necessarily have at least two diastereomers. T Chem S-20ab Purple Book Solutions Week 2 - Page 6 of 27 Working with Stereoisomers 1. Consider the species 2,3-butanediol, whose structural formula is: HO OH 2,3-butanediol H3C CH3 a) There are three distinct, pure, configurational stereoisomers of 2,3-butanediol, all of which have the same structural formula above. Using any unambiguous notation, draw the three different stereoisomers in the boxes below. (The boxes have been arbitrarily labeledA,B,andC.) ABC HO OH HO OH HO OH H3C CH3 H3C CH3 H3C CH3 b) By circling the appropriate terms below, indicate whether each of the three species you drew above is chiral or achiral. A: chiral achiral B: chiral achiral C: chiral achiral c) Two of the above species have the same melting point (19°C). The other species has a different melting point (34°C). Circle the letter of the species which has a melting point of 34°C: ABC What characteristic of the other two stereoisomers causes them to share the same melting point (19°C)? B and C are enantiomers, and enantiomers always have identical physical properties; for instance, they will have the same melting point. Chem S-20ab Purple Book Solutions Week 2 - Page 7 of 27 Principles of Reaction Stereochemistry 1. Each of the following statements is false. Discuss why the statement is false. Then make the statement true by changing only the underlined word or phrase. Two molecules which are diastereomers should be expected to have the same melting point. enantiomers Two molecules which are enantiomers would exhibit different rates of reactivity with an achiral reagent. diastereomers An achiral starting material can react with an achiral reagent to give a single chiral product. single achiral product or racemic mixture of products Two molecules which are enantiomers would exhibit identical reaction rates when reacting with a chiral reagent (such as a biological enzyme). different When an achiral starting material is treated with an achiral reagent to yield a pair of diastereomers,both products must always be produced in equal amounts. enantiomers Chem S-20ab Purple Book Solutions Week 2 - Page 8 of 27 Mechanism and Stereochemistry 1. a) In the box below, draw the product of the following reaction. Your product must match the chemical formula provided. Indicate stereochemistry clearly, if appropriate. O OH Br2 Br (+/-) Chemical Formula: C6H11BrO b) Provide a complete curved arrow mechanism for the transformation indicated above. Explain the stereochemical outcome, if any. Br Br Br OH OH Bromonium ion can form OH attacks from the opposite above or below the plane, leading to the plane, leading to a diastereoselectivity (although, racemic mixture of since only one sterocenter is bromonium ion formed in this reaction, only one intermediates. This in pair of enantiomers is possible. turn leads to a racemic solv mixture of products. H O O Br Br c) Explain the regioselectivity of this reaction; in other words, why is the particular isomer you drew in (a) the one that's formed. The intermediate bromonium ion is unsymmetrical: there is much more partial positive charge on the tertiary carbon. This is similar to the relative stabilization conferred by tertiary vs. secondary carbocations. The nucleophile (ROH) then reacts by breaking the weaker of the two C–Br bonds. Chem S-20ab Purple Book Solutions Week 2 - Page 9 of 27 Stereochemistry of Alkene Additions 1. Fill in each box with the organic product(s) of the indicated transformation. Be sure to indicate the correct stereochemistry. (If the product is formed as a racemic mixture, please indicate this!) Br Br + Br Br2 Br (racemic mixture) Br Br ** Note: The chair forms are shown just + to give you extra practice converting planar drawings to chairs. Br Br Br Br + OH Br2,H2O OH (racemic mixture) OH Br + Br OH OH OH + 1. BH3,THF – (racemic mixture) 2. H2O2,OH H OH + OH H (achiral) H2 ,Pt/C Chem S-20ab Purple Book Solutions Week 2 - Page 10 of 27 Stereochemistry of Alkene Additions 1. Fill in each box with the starting materials thatwouldleadtotheindicatedproduct.Besuretoindicatethe correct stereochemistry. H Br Br2 Br H HO H 1) BH3,THF - 2)H2O2,OH Me H (+/-) H D2, Pt/C D D (+/-) Br O HO Me 2 Me Et Et Br H (+/-) Chem S-20ab Purple Book Solutions Week 2 - Page 11 of 27 Cyclohexanes 1. Look at a chair. Notice the axial and equatorial substituents. Pay close attention to the parallel lines! A chair has 3 sets of parallel lines, plus vertical lines for axial groups. Once that is drawn in, every subsequent line must be parallel to a line that has already ben drawn! Try drawing t-butyl groups in the equatorial positions. 2. Draw each of the following dimethyl cyclohexanes (on the planar ring), then identify whether the substituents would be either: "Axial and Equatorial" or "Both Equatorial" 1,2-cis 1,3-cis 1,4-cis Ax. and Eq. Both Eq. Ax. and Eq. 1,2-trans 1,3-trans 1,4-trans Both Eq. Ax. and Eq. Both Eq. 3. Draw each of the following in the most stable chair conformation: Et Chem S-20ab Purple Book Solutions Week 2 - Page 12 of 27 Chair Conformations 1. Consider the following molecule: CH3 Cl H3C CH3 a) This molecule is expected to have two relatively stable "chair" conformations. Draw clear representations of the two different conformations in the boxes below. (The boxes have been arbitrarily labeled A and B.) In addition, please be sure to draw the isopropyl substituent in its best conformation in each case. AB H3C H3C H H Cl CH3 H3C H Cl H3C H CH3 b) Which of the two chair conformations is more stable? (circle one of the statements below) 1) A is more stable than B. 2) Both conformations are equally stable. 3) B is more stable than A. Chem S-20ab Purple Book Solutions Week 2 - Page 13 of 27 Bicyclic Compounds and Bredt's Rule 1. Draw each of the following bicyclic compounds in a good "perspective" drawing. CH3 CH3 trans-decalin: two chairs CH3 CH3 CH3 norbornane skeleton: CH H3C 3 CH3 2. The molecule trans-cyclooctene is known to exist. (It is chiral, by the way). Why is the analagous molecule trans-cyclohexene unstable? Far too strained to have a trans-alkene in a six-membered ring. H H = H H H H trans-cyclooctene trans-cyclooctene trans-cyclohexene???? (planar representation) (perspective representation) 3.
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