As a result, IV and V are chiral molecules and are enantiomers.

Transcription

1 5.1 lassify each of the following objects as to whether it is chiral or achiral: (a) A screwdriver achiral (d) A tennis shoe chiral (g) A car chiral (b) A baseball bat achiral (e) An ear chiral (h) A hammer achiral (c ) A golf club chiral (f) A woodscrew chiral 5.2 If models are available, construct the 2-butanols represented in ig5.3 and demonstrate for yourself that are not mutually superposable. (a) Make similar models of 2-bromopropane. Are they superposable? 3 3 Solution: 3 and 3 (b) Is a molecule of 2-bromopropane chiral? Solution: No (c) Would you expect to find enantiomeric forms of 2-bromopropane? Solution: No 5.3 Demonstrate the validity of what we have represented in ig. 5.5 by constructing models. Demonstrate for yourself that III and IV are related as an object and its mirror image and that they are not superposable (i.e., that III and IV are chiral molecules and are enantiomers). (a) Now take IV and exchange the positions of any two groups. What is the new relationship between the molecules? (b) Now take either models and exchange the positions of any two groups. What is the relationship between the molecules now? X X W Z Y Mirror Y Z W III IV Y Z X W Y W Z X V VI (a) Occasionally, if you exchange the positions of X and Y, we get V. By a series of rotations, you will be able to get III which is identical with V and is the mirror image of IV.

2 As a result, IV and V are chiral molecules and are enantiomers. (b) If you take IV for example, exchange the two groups such as X and Z,you will get VI. By a series of rotations, you will get a compound which is identical with V. As a result, V are identical with VI. 5.4 Some of the molecules listed here have stereo centers; some do not. Write three-dimensional formulas for both enantiomers of those molecules that do have stereo centers. (a) 2-luoropropane (b) 2-Methylbutane (c) 2-halorobutane (d) 2-Methyl1-1-butanol (e) 2-omopentane (f) 3-Menthylpentane (g) 3-Methylhexane (h) 1-hloro-2-methylbutane * Stands for stereo center (a) No stereo center (b) No stereo center l l (c) 3 * * O * * O (d) (e) 3 * * 3 (f) No stereo center 3 2 * * 2 3 (g) l * * l (h) 5.5 Which atom is the stereocenter (a) of limonene and (b) of thalidomide? Draw bond-line formulas for the limonene and thalidomide enantiomers, showing the stereocenter in each using wedge-dashed

3 wedge notation. 3 O 3 2 O O O 5.6 Show how a square planar structure for carbon compounds can be elimniated from consideration by considering 2 l 2 and 2 l as examples of disubstituted methanes. (a) ow many isomers would be possible in each instance if the carbon had a square planar structure? (b) ow many isomers are possible in each instance if the carbon is tetrahedral? onsider l as an example of a trisubstituted methane. (c) ow many isomers would be possible if the carbon atom were square planar? (d) ow many isomers are possible for l if carbon is tetrahedral? (a) l l 2 l 2 l l 2 l l l (b) 1,1 (c) l l l l (d) l l l 5.7 Which of the listed in problem 5.1 possess a plane of symmetry and are, therefore, achiral?

4 (a) A screwdriver (b) A baseball bat (c)a golf club (d) A tennis shoe (e) An ear (f) A woodscrew (g)a car (h) A hammer The follow objects have a plane of symmetry (a), (b), and (h). 5.8 Write three-dimensional fomulas and designate a plane of symmetry for all of the achiral molecules in Problem 5.4. (a) 2-luoropropane (e) 2-omopentane (b) 2-Methylbutane (f) 3-Methylpentane (c) 2-hlorobutane (g) 3-Methylhexane (d) 2-Methyl-1-butanol (h) 1-hloro-2-methylbutane In each instance below, the plane defined by the page is a plane of symmetry. (a) (b) (f) Write the enantiomeric forms of bromochlorofluoromethane and assign each enantiomer its correct (R) or (S) designation. Answers: (c) (a) (b) l (d) (a) (R) (b) l (d) (c) (S) 5.10 Give ( R ) and ( S ) designations for each pair of enantiomers given as answers to Problem 5.4. (a) 2-luoropropane (b) 2-Methylbutane (c) 2-hlorobutane (d) 2-Methyl-1-butanol (e) 2-omopentane (f) 3-Methylpentane (g) 3-Methylhexane (h) 1-hloro-2-methylbutane Answers: (a) no enantiomers (b) no enantiomers (c)

5 l 3 l R-2-hlorobutane S-2-hlorobutane (d) 2 O 2 O S-2-Methyl-1-butanol R-2-Methyl-1-butanol (e) R-2-omopentane S-2-omopentane (f) no enantiomers (g) S-3-Methylhexane R-3-Methylhexane (h) 2 l 2 l S-1-hloro-2-methylbutane R-1-hloro-2-methylbutane 5.11 List the substituents in each of the following sets in order of priority, from highest to lowest: (a) l, O, S, (b) 3, 2, 2l, 2O

6 (c), O, O, 3 (d) (3) 2, ( 3 ) 3,., = 2 (e), N ( 3 ) 2, O 3, 3 (a) l > S > O> (b) 2 > 2 l > 2 O > 3 (c) O > O > 3 > (d) ( 3 ) 3 > = 2 > ( 3 ) 2 >. (e) O 3 > N ( 3 ) 2 > 3 > 5.12 Assign (R) or (S) designations to each of the following compounds. (a) 3 2 l (b) 2 5 (S) 2 O (c) ( 3 ) 3 (R) 3 ( 3 ) 3 (S) 5.13 Tell whether the two structures in each pair represent enantiomers or two molecules of the same compound in different orientations. l (a) l and enantiomers 3 l (b) l and 3 the same

7 3 2 O O 2 3 (c) 3 and 3 enantiomers 5.14 Shown below is the configuration of (+)-carvone.(+)-arvone is the principal component of caraway seed oil and is responsible for its characteristic odor. (-)-arvone, its enantiomer, is the main component of spearmint oil and give its characteristic odor. The fact that the carvone enantiomers do not smell the same suggests that the receptor sites in the nose for these compounds are chiral, and only the correct enantiomer will fit its particular site.give the correct (R) and (S) designations for (+) and (-)-carvone. O O * the enantiomer is * S-(+)-arvone R-(-)arvone 5.15 A sample of 2-methyl-1-butanol (see section 5.7) has a specific rotation, [α], equal to (a) What is the % enantiomeric excess of the sample? (b) Which enantiomeric is in excess, the ( R) or the (S)? Answer : (a) Enantiomeric excess= ( ) / ( ) = 20% (b) R Write three-dimensional formulas for the (S) isomers of (a) ibuprofen (b) methyldopa, and (c) penicillamine.

8 1 Ibuprofen 2 Methyldopa 3 Penicillamine 3 OO OO O 3 2 N 2 N 2 O 3 3 S 2 O O (a) If 3 and 4 are enantiomers. What are 1and 4? (b) What are 2 and 3 and 2 and 4? (c) Would you expect 1 and 3 to have the same melting point? (d) The same boiling point? (e) The same vapor pressure? (a) 1 and 4 are diastereomers. (b) 2 and 3; 2 and 4 are both diastereomers. (c) 1 and 3 don't have the same melting point. (d) 1 and 3 don't have the same boiling point. (f) 1 and 3 don't have the same vapor pressure Which of the following would be optically active? (a) A pure sample of A (c) A pure sample of (b) A pure sample of B (d) An equimolar mixture of A and B

9 A B (a) (b) Because A and B are chiral compounds, they are optically active. is meco compound, is achiral. The mixture of (d) is racemic form. It is optically inactive The following are formulas for three compounds, written in noneclipsed conformations. In each instance tell which compound (A, B or ) each formula represents This is compound A B 5.20 write three-dimensional formulas for all of the stereoisomers of each of the following compounds. Label pairs of enantiomers and label meso compounds. (a) 3 ll 3 (d) 3 O 2 l 3 (b) 3 O 2 O 3 (e) 3 3 (c) 2 l 2 l Answer (a) l 3 l 3 3 l l l l meso compounds racemic form (b)

10 3 3 3 O O O O O O meso compounds enantiomers (c) 2 l 2 l 2 l 2 l 2 l 2 l meso compounds enantiomers (d) O O O O l l l l enantiomers enantiomers (e) enantiomers enantiomers 5.21 Give names that include (R) and (S) designations for compounds B and in Section 5.11A.

11 B. 3 3 (2S, 3S)-2, 3-dibromobutane. 3 3 (2R, 3S)-2, 3-dibromobutane 5.22 Give names that include (R) and (S) designations for your answers to problem (a) 3 ll 3 (b) 3 O 2 O 3 (c) 2 l 2 l (d) 3 O 2 l 3 (e) 3 3 (a) l l l l l l (1) (2) (3) (1) :(2S,3R)-2,3-Dichlorobutane (2) :(2S,3S)-2,3-Dichlorobutane (3) :(2R,3R)-2,3-Dichlorobutane (b) O O O O O O (1) (2) (3)

12 (1) :(2S, 4R)-2,4-pentanediol (2) :(2S, 4S)-2,4-pentanediol (3) :(2R, 4R)-2,4-pentanediol (c) 2 l 2 l 2 l 2 l 2 l 2 l (1) (2) (3) (1) :(2R, 3S)-1,4-Dichloro-2,3-difluorobutane (2) :(2R, 3R)-1,4-Dichloro-2,3-difluorobutane (3) :(2S,3S)-1,4-Dichloro-2,3-difluorobutane (d) O O O O l l l l (1) (2) (3) (4) (1) :(2S,4R)-4-hloro-2-pentanol (2) :(2S,4S)-4-hloro-2-pentanol (3) :(2R,4R)-4-hloro-2-pentanol (4) :(2R,4S)-4-hloro-2-pentanol (e) (1) (2) (3) 3 (4) (1) :(2S,3R)-2-omo-3-fluoro-butane

13 (2) :(2S,3S)-2-omo-3-fluoro-butane (3) :(2R,3R)-2-omo-3-fluoro-butane (4) :(2R,3S)-2-omo-3-fluoro-butane 5.23 hloramphenicol (below) is a potent antibiotic, isolated from streptomyces venezuelae, that is particularly effective against typhoid fever. It was the first naturally occurring substance shown to contain a nitro ( NO 2 ) group attached to an aromatic ring. Both stereocenters in chloramphenicol are known to have the (R) configuration. Identify the two stereocenters and write a three-dimensional formula for chloramphenicol. NO 2 O NOl 2 2 O hloramphenicol NO 2 O O 2 NOl (a) Is trans-1,2-dimethylcyclopentane (5) superposable on its mirror image (i.e., on compound (6)? (b) Is cis-1,2-dimethylcyclopentane (7) superposable on its mirror image? (c) Is cis-1,2-dimethylcyclopentane a chiral molecule? (d) Would cis-1,2-dimethylcyclopentane show optical activity? (e) What is the stereo isomeric relationship between 5 and 7? (f) Between 6 and 7? Answer (a) No. (b) Yes. (c) No.

14 (d) No. (e) They are diastereomers. (f) They are diastereomers too Write structural formulas for all of the stereoisomers of 1,3-dimethylcyclopentane. Label pairs of enantiomers and meso compounds of they exist. a pair of entantiomers, meso compounds 5.26 Write formulas for all of the isomers of each of the following. Designate pairs of enantiomers and achiral compounds where they exist. (a) 1-omo-2-chlorocyclohexane is-1-omo-2-chlorocyclohexane has a pair of enantiomers l l (1S,2R)-1-omo-2-chlorocyclohexane (1R,2S)- 1-omo-2-chlorocyclohexane Trans-1-omo-2-chlorocyclohexane has a pair of enantiomers l (1R, 2R)- 1-omo-2-chlorocyclohexane l (1S, 2S)-1-omo-2-chlorocyclohexane (b) 1-omo-3-chlorocyclohexane is-1-omo-3-chlorocyclohexane has a pair of enantiomers l l (1R, 3S)- 1-omo-3-chlorocyclohexane (1S, 2R) - 1-omo-3-chlorocyclohexane

15 Trans-1-omo-3-chlorocyclohexane has a pair of enantiomers l l (1R, 3R)- 1-omo-3-chlorocyclohexane (1S, 3S) - 1-omo-3-chlorocyclohexane (c) 1-omo-4-chlorocyclohexane Both trans-1-omo-4-chlorocyclohexane and cis-1-omo-4-chlorocyclohexane are achiral l l is-1-omo-4-chlorocyclohexane are achiral Trans-1-omo-4-chlorocyclohexane 5.27 Give the (R-S) designation for each compound given as an answer to Problem (a) 1-omo-2-chlorocyclohexane (c) 1-omo-4chlorocyclohexane (b) 1-omo-3chlorocyclohexane (a) l l l l Enantiomers Enantiomers (b) l l l l Enantiomers Enantiomers (c) l achiral l achiral 5.28 Write three-dimensional formulas for the starting compound, the product, and all of the

16 intermediates in a synthesis similar to the one just given that relates the configuration of (-)-glyceraldehyde with (+)-lactic acid. Label each compound with its proper (R)-(S) and (+)-(-) designation. O OO go NO 2 O O (S)-(-)-Isoserine 2 O (S)-(-)-Glyceraldehyde 2 O (S)-(+)-Glyceric acid O OO OO NO 2 Zn, 3 O + O O OO 2 N (S)-(-)-Isoserine (R)-(+)-3-omo-2-hydroxypropanoic acid (S)-(+)-Lactic acid 5.29 ow would you synthesize (R)-1-deuterio-2-methylbutane? int: onsider one of the enantiomers of 1-chloro-2-methylbutane in Section 5.7 as a starting compound. Target molecule: (R)-1-deuterio-2-methylbutane 2 D Me The reaction should be as follow: 2 D 2 l + LiAlD4 T Me 5.30 Which of the following are chiral and, therefore, capable of existing as enantiomers? (a) 1,3-Dichlorobutane (e) 2-omobicyclo [1.1.0] butane (b) 1,2-Dibromopropane (f) 2-luorobicyclo [2.2.2] octane (c) 1,5-Dichloropentane (g) 2-hlorobicyclo [2.1.1] hexane (d) 3-hylpentane (h) 5-hlorobicyclo [2.1.1] hexane a, b, f and g are chiral, therefore, capable of existing as enantiomers

17 5.31 (a) ow many carbon atoms does an alkane (not a cycloalkane) need before it is capable of existing in enantiomeric forms? (B) Give correct names for two sets of enantiomers with this minimum number of carbon atoms. (a) An alkane needs at least 7 carbon atoms to exist in enantiomeric forms. (b) (s)-3-methyl-hexane (R)-3-Methyl-hexane (s)-2,3-dimethyl-pentane ( R)-2,3-Dimethyl-pentane 5.32 (a) Write the structure of 2,2-dichlorobicyclo[2.2.1]heptane. (b) ow many stereocenters does it contain? (c) ow many stereoisomers are predicted by the 2 n rule? (d) Only one pair of enantiomers is possible for 2,2-dichlorobicyclo[2.2.1]heptane. Explain. l (a) l (b) Two stereogenic carbons. (c) 2 2 (d) l * * l l * * l Mirror

18 Look at the picture,the * is chiral centers, the carbon carbon bond can not be changed, so the R or S can not be changed. So, there is only one pair Shown below are Newman projection formulas for (R, R)-, (S, S)-2,3-dichlorobutane. and (R, S)-2,3-dichlorobutane. (a) Which is which? (b) Which formula is a meso compound? 3 l 3 l l 3 l l l A B (a) A is (R, S)-2,3-dichlorobutane. B is (S, S)-2,3-dichlorobutane. is (R, R)-2,3-dichlorobutane. (b) A is a meso compound write appropriate structural formulas for (a) a cycle molecule that is a constitutional isomer of cyclohexane; (b) molecules with the formula 6 12 that contain one ring and that are enantiomer of each other; (c) molecules with the formula 6 12 that contain one ring and are diastereoisomers of each other; (d) molecules with the formula 6 12 that contain no ring and that are enantiomers of each other, and (e) molecules with the formula c6h12 that contain no ring and that are diastereomers of each other. (a) etc. (b) etc. (c) etc.

19 (d) (e) etc onsider the following pairs of structures. Identify the relationship between them by describing them as representing enantiomers, diastereomers, constitutional isomers, or two molecules of the same compound. (a) 3 3 same compound (b) 3 3 enantiomers (c) Diastereomers (d)

20 same compound (e) same compound (f) constitutional isomers (g) 3 3 diastereomers (h) l l l l Enantiomers (i) l l l l same compound (j)

21 l l l l enantiomers (k) 3 3 l l l l 3 3 diastereomers (l) 3 3 l l (m) 3 3 same compound diastereomers (n) constitutional isomers (o) l l l l diastereomers (p)

22 l l l l l l l l same compound (q) same compound 5.36 Discuss the anticipated stereochemistry of each of the following compounds. (a) l===l (b) l===l 2 (c) 2 ===l We know that carbon-carbon double bond was in a plane. So the three double bonds are all in their own plane. But we know that the P orbitals of a carbon are vertical. I first set a model as follows, which shows A B D are in a plane. D A B ollowing the model, we see for (a) (b) (c) they have a symmetrical plane. So they are achrial. But A is different from B, is different from D, so it has cis-trans isomers. But, only structure (a) has cis- trans isomer There are four dimethylcyclopropane isomers. (a) Write three-dimensional formulas for these isomers. (b) Which of these isomers are chiral? (c) If a mixture consisting of 1 mol of each of these isomers were subjected to simple gas chromatography, how many fractions would be obtained and which compounds would each fraction contain? (d) ow many of these fractions would be optically active? Answer (a)

23 (b) are chiral. (c)there are three fractions, one is a pair of enantiomers, the other two are cis-1,2dimethylcyclopropane and 1,1-dimethylcyclopropane. (d) None of them (use models to solve this problem) (a) Write a conformational structure for the most stable conformation of trans-1,2-diethylcyclohexane and writer its mirror image. (b) Are these two molecules superposable?(c) Are they interconvertible through a ring flip?(d) Repeat the process in part (a) with cis-1,2-diethylcychexane. (e) Are these structures superposable?(f) Are they interconvertrible? (a) trans-1,2-diethylcyclohexane (b) These two molecules are not superposable (c) No, they are not. (d) cis-1,2-diethylcychexane (e) No (f) Yes 5.39 (Use models to solve this problem) (a) Write a conformation structure for the most stable conformation of tran-1,4-diethylcyclohexane and for its mirror image. (b)are these structure superposable? (c) Do they represent enantiomers? (d) Does trans-1,4-diethylcyclohexan have a stereoisomer, and if so what is it? (e) Is this stereoisomer chiral?

24 (a) (b) yes (c) no (d) yes Its structure is as follow: (e) It is achiral (Use models to solve this problem.) Write conformational structures for all of the stereoisomer of 1,3-diethylcyclohextane. Label pairs of enantiomers and meso compounds if they exist. A or B Structure B and structure are enantiomers. Structure A is meso compound Tartaric acid [O 2 (O)(O)O 2 ] was an important compound in the history of stereochemistry. Two naturally occurring forms of tartaric acid are optically inactive. One form has a melting point of 206, the other a melting point of 140. The inactive tartaric acid with a melting point of 206 can be separated into two optically active forms of tartaric acid with the same melting point (170 ). One optically active tartaric acid has [α] D 25 =+12, and the other, [α] 25 D =-12. All attempts to separate the other inactive tartaric acid (melting point 140 ) into optically active compounds fail. (a) Write the three-dimensional structure of the tartaric acid with melting point 140. (b) What are possible structures for the optically active tartaric acids with melting points of 170? (c) an you be sure which tartaric acid in (b) has a positive rotation and which has a negative rotation? (d) What is the nature of the form of tartaric acid with a melting point of 206?

25 (a) OO O OO O (b) OO O O OO O OO OO O (c) No (d) It contains 50% of each isomer, called racemic form (a) An aqueous solution of pure stereoisomer X of concentration 0.10g/mL had observed rotation -30 degree in a 1.0 dm tube at nm (the sodium D line) and 25 centigrade. What do you calculate its [α] D to be at this temperature? (b) Under identical conditions but with concentration 0.050g/mL, a solution of X had observed rotation +165 degree. Rationalize how this could be and recalculate [α] D for stereoisomer X. (c) If optical rotation of a substance studied at only one concentration is 0 degree, can it definitely be concluded to be achiral? Racemic? (a) [α] D =α/(c*l)=(-30)/(0.1*1.0)=-300 centigrade (b) [α] D =α/(c*l)=165/(0.050*1.0)=3300 centigrade. The results are just the same, because 3300=360* (c) No. The apparent 0 degree could actually be + 0, or 360 degree If a sample of a pure substance that has two or more stereocenters has an observed rotation of 0 o, it could be a racemate. ould it possible be a pure stereoisomer? ould it possibly be a pure enantiomer? Yes, it could be a meso form or an enantiomer whose stereocenters, by rare coincidence, happen to cancel each others activity Unknown Y has a molecular formula of 3 6 O 2. It contains functional group that absorbs infrared radiation in the 3200 to 3550 cm -1 region (when studied as a pure liquid; i.e., neat ), and it has no absorption in the 1620 to 1780 cm -1 region. No carbon atom in the structure of Y has more than one oxygen atom bonded to it, and can exist in two (and only two) stereoisomeric forms. What are the structures of these forms of Y?

26 O 2 2 O Oxiranyl-methanol O 2 O 2 Oxiranyl-methanol