Ch5 problems - )f the mull. s 2”. This ;ed stereo ecule...

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Unformatted text preview: )f the mull. s 2”. This ;ed stereo ecule C011.— 1tical willl impound. tically se- it to maize - rs are the ith plane- polariza- _ her coulr is called :asured in tion, [a]. Ition. The iomers is e config- m be as- of Cahn. ck draw- _1 (_'|||I stir;- can be introduced into an achiral compound by i“ Mai-ll halogenation. When the transition states are enan~ mum-rig (related as object and mirror image), the result Li- :I racemate becauSe the faces of the planar radical re— :ch nr equal rates. “mar-:11 halogenation of a chiral molecule containing one “flu-“1.77;? r will give a racemate if the reaction takes Page: at the stereocenter. When reaction elsewhere leads [[1 Hit-SI diastereomers, they will be formed in unequal :mu 'Illlll-‘t- tallest. Problems 207 14. The preference for the formation of one stereoisomer, when Several are possible, is called stereoselectivity. 15. The separation of enantiomers is called resolution. It is achieved by the reaction of the racematc with the pure enantiomer of a chiral compound to yield separable di- astereomers. Chemical removal of the chiral reagent frees both enantiomers of the original racemate. Another way of separating enantiomers is by chiral chromatography on an optically active support. 3“. (traits- each of the following common objects as being either chiral or achiral. Assume in. t'flt'll case that the object is in its simplest form, without decoration or printed labels. is: A ladder; (b) a door; (c) an electric fan; (d) a refrigerator; (e) Earth; (f) a baseball; lg] a baseball bat; (h) a baseball glove; (i) a flat sheet of paper; (j) a fork; (k) a spoon; 41] a knife 3?. Each part of this problem lists two objects or sets of objects. As precisely as you can, describe the relation between the two sets, using the terminology of this chapter; that is, gpucii‘y whether they are identical, enantiomeric, or diastereomeric. (a) An American toy Lu,“- mmpared with a British toy car (same color and design but steering wheels on op- Isrsitc sides); (b) two left shoes compared with two right shoes (same color, size, and style}; (c) a pair of skates compared with two left skates (same color, size, and style); rd} a right glove on top of a left glove (palm to palm) compared with a left glove on top of a right glove (palm to palm; same color, size, and style). For each pair of the following molecules, indicate whether its members are identical, mluetural isomers, conformers, or stereoisomers. How would you describe the relation between conformations when they are maintained at a temperature too low to permit rhea“ to interconvert? /CH3 (EH3 (:1) CH3CH2CHzcl-I and CH3CH2CHCH2CH3 CH3 Cl Cch2CH2 OH CH3CH 0H (0) and H H H H (‘11 E? ('11 (e) CH3(‘ICH2CH2CH3 and CH3CHCH2CHCH3 Br H (b) H3C>g and 5<CH3 H CH3 CH3 Br H. H Br (d) and H H H H H H H 3 m \(::;0CH3 and \C;:OCH3 HBC/ Cl H/ C1 H C H (g) CH3 (1!) W and H CH3 C] H H3 H 208 Chapter; STEREflISflMERS 29. Which of the following compounds are chiral? (Hint: Look for stereocenters.) (a) 2-Methylheptane (b) 3-Methylheptane (c) 4-Methy1heptane (d) 1,1-Dibrornopropane (e) 1,2-Dibromopropane (f) 1,3-Dibromopropane (c) F (g) Ethene, H2C=CH2 (h) Ethyne, HCECH (i) Benzene, Q (Note: Like ethene, benzene contains all 5192- (e) F hybridized carbons and is therefore planar.) (RH H (j) Epinephrine, H0 CHCHzNHCHg (k) Vanillin, H0 CH (3) l: HO _ CH30 H . i) (EH (H) 0(1le (I) I: . . . HOC 0 (l) Citric acid, Hoccnzpcnzcon (m) Ascorbic acid, 0 H /C\ _ 0 OH HO OH (k) H0 CH3 (|ng I Cl' . . N Ii (11) p-Menthane-1,8~dlol (terpin hydrate), (0) Mcperidine (dememl), (“n / OCHZCHg ‘ ' H3C—C—OH C/ | H H CH3 0 \ I to} / 30. éach of the following molecules has the molecular formula C5H120 (check for I ourself). Which ones are chiral? . .53. l-m' earl (a) (b) (C) /Y\OH 11: least /Kp/ OH I [m {EH 3-1. Assign 1 HT the f: 0H 0H HIIH'F. at Wullitl f. 31. Which of the following cyclohexane derivatives are chiral? For the purpose of determin' H_ ing the chirality of a cyclic compound, the ring may generally be treated as if it were on _ ‘ planar. \, CH3 CH3 CH3 CH3 II CH3 /,CH3 I (a) (b) (c) N (d) ‘CH3 ' I H (EH 1e] ‘1 . 3 / . @or each pair of structures shown, indicate whether the two species are constitutional iso— mers, enantiomers, diastereomers of one another, or identical molecules. “1 CH3 CH3 CH3 CH3 /_ \ (a) Cl’7l\ and C1 (b) and )Y‘Cl Cl H H C1 C] H _ H F hag—L. Problems 209 .) CH3 CHZF CH3 CH3 heptane }\ d m r and (d) a“ “C1 ’mOPTOPflnc ' FCl H H c1 C1 H F H CH3 F CH3 F l” and HJY‘CHs (D and Am F (:1 H Cl c1 CH H U (:1 /,C1 (,C1 C1 xii“ W [i and U (11) CL and d ——f C1 “(:1 C1 \C1 Cl Cl c1 c1 * OCHzCH_,= r’ I’ " “ H Br H C} H Br Br H I | x \ \ x | w] ’ 311d A( (P) I and >)\ )r H c1 H Br Br H H \Br 'nr each of the following formulas, identify every structural isomer containing one or . mum stereoeenters, give the number of stereoisomers for each, and draw and fully name Ill least one of the stereoisomers in each case. I tall C7H16 (b) ch18 (c) CsHm, with one ring @ssigu the appropriate designation of configuration (R or S) to the stereocenter in each car the following molecules. (Hint: Regarding cyclic structures containing stereocenters, treat the ring as if it were two separate substituents that happen to be attached to each rilhcr at the far end of the molecule—look for the first point of difference, just as you u'nllltl for acyclic structures.) Edetemjn' H3C H H OH Cl H Fit were {a} I’ (b) (c) " (d) H Br ” CH3 OH F F OH H 0 0 . H (21>O< OH M (EH3 (e) Mm (f) (g) HBC \H (h) HSC \H OH utional isoi O 0 “F H (3) CH3 \NHZ \H OCH3 CH3O 210 Chapter 5 STEREGISHMERS 35. ark the stereocenters in each of the chiral molecules in Problem 30. Draw any single stereoisomer of each of these molecules, and assign the appropriate designation (R or S) to each stereocenter. 36 ’l‘he two isomers of carvone [systematic name: 2-methyl-5-(1~methylethenyl)—2— cyclohexenone] are drawn here. Which is R and which is S? O O H2C=C\ “‘H HZC =c\ “H CH3 CH3 (+)-Carvone (—)~Carvone (In cutaway seeds) (In spearmint) 3’7.'Il)raw structural representations of each of the following molecules. Be sure that your structure clearly shows the configuration at the stereocenter. (Hint: You may find it use- ful to first draw the enantiorner whose configuration is easiest for you to determine and then, if necessary, modify your structure to fit the one requested in the problem.) (a) (R)—2-chloropentane; (h) (S)-2—methyl—3-bromohexane; (e) (S)~1,3-dichlorobutane; (d) (R)-2—chloro-1,1,I-trifluoro—B-methylbutane. 38.\1)raw structural representations of each of the following molecules. Be sure that your structure clearly shows the configuration at each stereocenter. (a) (R)-3-brorno-3— methylhexane; (b) (3R,5.S')-3,5-dimethylheptane; (c) (2R,3S)-2-bromo—3—methylpentane; (d) (.5)- l , 1,2—trimethy1cyclopropane; (e) ( 1S,2.S')- l-chIoro— l —trifluoromethyl-2— methylcyclobutane; (f) (1R,2R,3S)-1,2—dichloro-3-ethylcyclohexane. 39 Draw and name all possible stereoisomers of (CH3J2CHCHBrCHC1Ci-13. 40. For each of the following questions, assume that all measurements are made in lO-cm polarimeter sample containers. (3) A 10—mL solution of 0.4 g of optically active 2—butanol in water displays an optical rotation of —0.56°. What is its specific rota— tion? (1)) The specific rotation of sucrose (common sugar) is +664 What would be the observed optical rotation of such a solution containing 3 g of sucrose? (c) A solution of pure (S)—2—bromobutane in ethanol is found to have an observed a = 57.3“. If [a] for (S)-2-bromobutane is 23.1, what is the concentration of the solution? Natural epinephrine, [MET = —50, is used medicinally. Its enantiomer is medically worthless and is, in fact, toxic. You, a pharmacist, are given a solution said to contain 1 g of epinephrine in 20 mL of liquid, but the optical purity is not specified. You place it in a polarinrpeter (IO—cm tube) and get a reading of —2.5°. What is the optical purity of the sample? Is it safe to use medicinally? 42. Sodium hydrogen (S)-glutarnate [(S)-monosodium glutamate], [edgyC = +24, is the ac- tive flavor enhancer known as MSG. The condensed formula of MSG is shown in the margin. (3) Draw the structure of the S enantiomer of MSG. (b) If a commercial sample NH; fl) l HOOZH‘CHCHZCH2CO’N3’“ 0 of MSG Were found to have a [{1}]in 2 +8, what would be its optical purity? What would be the percentages of the S and R enantiomers in the mixture? (c) Answer the . . 25°C 5 same questions for a sample With [ab = +16. 4 s 43. he molecule in the margin is menthol, with the stereochemistry omitted. (3) Identify 811 3 stereocenters in menthol. (b) How many stereoisomers exist for the menthol structure? 0H (c) Draw all the stereoisomers of menthol, and identify all pairs of enantiomers. @- Natural (—)-menthol, the essential oil primarily responsible for the flavor Menthol and aroma of peppermint, is the 1R,ZS,5R-stereoisomer. (a) Identify (—)-menthol from the structures you drew for Problem 43, part (b). (b) Another of the naturally occurring diastereomer (+)—isomentl compound. I standing of 1 stability orde menthol, ant 4.5, {'1} the stereo and (+)—neot natural sourc exhibits [0L]D dti. For each of t identical or e CH: l ,C Br’ I \ C] CH« tell Cl+ 0C} 4?. Determine th 43. The compour is 105. (a) I oilier enan’tiot have any 0th: ||I.'t| you drth than for (c). clmw um: .19. Write the cor ca! Lit-signalit Hurt-trim. of t] ifillmurs of tin Innt'ruu: How “it'd in equal R or S. “U {illluiiufil 5|}. Monochlrwim Information a cinpcntuue at 5|. Draw all post “wiper-i1} whet] :Inmlmts5 and 52. IIIIIH'I . u how |I‘|t||'-II." n‘Fl‘ei Problems 2 l 1 .ny Single Liifl_.m~:-.ariiiit=irs of menthol is (+)—isomenthol, the lS,2R,5R-stereoisomer. Identify m (R 0r 5! I , I iqnmPnthol among your structures. (c) A third is (+)—neomenthol, the lS,2S,5R- nufipfiflnz‘l. Find (+)-neomenthol among your structures. ((1) Based on your under- 2_ (mush-r: of the conformations of substituted cyclohexanes (Section 4-4), what is the whim} order (from most stable to least) for the three diastereomers, menthol, iso- |-,,._.nt.'-.n! and neomenthol? 4.1 [Ll litt: stereoisonrlers described in the two problems above, (—)-menthol ([ot][J : —51) I am! (+)—neomcnthol ([040 = +21) are the major constituents in mentha oil, their main “mm-.1: source. The menthol-neomenthol mixture in a natural sample of mentha oil mimic; [or]D = #33. What are the percentages of menthol and neomenthol in this oil? Mr :0; finch of the following pairs of structures, indicate whether the two compounds are firmit‘nl or enantiomers of each other. ; (EHchg (szCHg, CH3 1 EH3 “1] ’,C\ and /C\ (b) H+Cl and ,,C\ ' 1 CH CH CH CH \ ‘Br Cl‘ 1 Br 3375:” Br (31 2 3 3 2 Cl Br H 1 use- mine and CH3 OCH3 1:1 NH2 in] CI+CF3 311d F3C +CH3 (d) H2N""¢“'C02H and H+CH(CH3)2 ane; l OCH: C1 CH(Cli3)2 COIZH at your @fltcrminc the R or S designation for each stereocenter in the structures in Problem 46. '3‘ I tilt. The compound pictured in the margin is a sugar called (—)—arabinose. Its specific rotation H\ /,0 pentane" it; #105. (a) Draw the enantiomer of (—)—arabinose., (b) Does (—)—arabinose have any C olltei' enantiomers? (c) Draw a diastereomer of (-)-arabinose. (d) Does (—)—arabinose HO H Iiitt-t- any other diastereomers? (e) If possible, predict the specific rotation of the structure lhal you drew for (a). (f) If possible, predict the specific rotation of the structure that you H OH tltuw for (c). (g) Does (—)-arabinose have any optically inactive diastereomers? If it does, ts axe than one. H OH -f optically CH OH =cific rois— @‘Erilc the complete IUPAC name of the following compound (do not forget stereocherni- _2 Id be the cal designations). (—)-Arabmose ulution of Ta} for (IIHZCH3 ,-'C\ lically H I CH2CH2C1 C5H10C12 :ontain Cl no place I purity Reaction of this compound with 1 mol of C12 in the presence of light produces several isomers of the formula C5H9Cl3. For each part of this problem, give the following infor— is the ac_ mation: How many stereoisomers are formed? If more than one is formed, are they gener- in the ated in equal or unequal amounts? Designate every stereocenter in each stereoisomer as R or S. magpie (a) Chlorination at C3 (b) Chlorination at C4 (c) Chlorination at C5 :r the . launchini'irniriou of methylcyclopentane can result in several products. Give the same infnml-ltinll as that requested in Problem 49 for the monochlorination of methylcy— ienfify a“ Elnyruulnc at C1, C2, and C3. ucture? 3 - Imu- all possible products of the chlorination of (S)-2-bromo—l,l-dimethylcyclobutane. Stitch";- whether they are chiral or achiral, whether they are formed in equal or unequal the flaw! amounts, and which are optically active when formed. 1TH2 from the 53- mil-strata how to resolve racemic 1-phenylethanamine (shown in the margin), using the C5H5CHCH3 :urring Mtthod of reversible conversion into diastereorners. 1-Phenylethanamine 212 Chapter 5 STEREOISDMERS 53. Draw a flowchart that diagrams a method for the resolution of racemic 2-hydroxy- propanoic acid (lactic acid, Table 5—1), using (S)-1-phenylethanamine. @iow many different stereoisomeric products are formed in the monobromination of (a) racemic trans-1,2-dimethylcyclohexane and (b) pure (R,R)—l,2-dirnethy1cyclohexane? (c) For your answers to (a) and (b), indicate whether you expect equal or unequal amounts of the various products to be formed. Indicate to what extent products can be separated on the basis of having different physical properties (dig, solubility, boiling point). 55. Make a model of cis-1,2-dimethylcyclohexane in its most stable con- formation. If the molecule were rigidly locked into this conformation, would it be chiral? (Test your anSWer by making a model of the mirror image and checking for superimposability.) Flip the ring of the mode]. What is the stereoisomeric relation between the original conformation and the conformation after flipping the ring? How do the results that you have obtained in this problem relate to your answer to Problem 31(a). 56. Morphinane is the parent substance of the broad class of chiral molecules known as the morphine alkaloids. Interestingly, the (+) and (-) enantiomers of the compounds in this family have rather different physiological properties. The (—) compounds, such as mor- phine, are “narcotic analgesics” (painkillers), whereas the (+) compounds are “antitus— sives” (ingredients in cough syrup). Dextromethorphan is one of the simplest and most common of the latter. CH30 Morphinane Dextromethorphan (a) Locate and identify all the stereocenters in dextromethorphan. ([1) Draw the emo— tiomer of dextrornethorphan. (c) As best you can (it is not easy), assign R and S configu— rations to all the stereocenters in dextromethorphan. 57. We will learn in Chapter 18 that hydrogens on the carbon atom adjacent to the carbonyl functional group (C20) are acidic. The compound (S)-3-methyl—2-pentanone, loses its optical activity when it is dissolved in a solution containing a catalytic amount of base. Explain. ' O 6% H CH3 (S)-3-Methyl-2-pentanone 58. The enzymatic introduction of a functional group into a biologically important molecule is not only specific with regard to the location at which the reaction occurs in the mol- ecule (see Chapter 4, Problem 43), but also usually specific in the stereochemistry obtained. The biosynthesis of epinephrine first requires that a hydroxy group be intro- duced specifically to produce (—)-norepinephrine from the achiral substrate dopamine. (The completion of the synthesis of epinephrine will be presented in Problem 65 of Chapter 9.) Only the (-.) enantiomer is functional in the appropriate physiological manner, so the synthesis must be highly stereoselective. \ H0 Do; in: Is the c would the he of equa status? (1:) uunsititm s chiral or c; Team Problt Ell. Suuiiea ha' tlguitlsl Gel u dccalifi-t just like a [a] line yo 1!»: turns r. stercocht'n isumcn: an l_l'l]' .I'tllliml 'Illilll-lttl: wit‘ activity M hh'mu'heu ||l.'|| at {33. Illill none l “1-: stereo: [if-l The st: ring Tunic-J, alert—somep- iihfiolutc ct l‘mprn FL‘SEll. all. WhirEi rot I'Jflli l.".' [in I1. ...
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Ch5 problems - )f the mull. s 2”. This ;ed stereo ecule...

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