Chapter 5 (Stereochemistry, Questions) PDF

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Chapter 5: Stereochemistry (Questions) 1. What is the relationship between the following two compounds? (Stereoisomers, Identical, Constitutional isomers or different compounds).

a.

b. c.

d.

2. Which of the following is the definition of chirality? A) The existence of a molecule with a mirror image. B) The existence of a molecule with a carbon atom that has four different substituents. C) The existence of a molecule that is superimposable on its mirror image. D) The existence of a molecule that is not superimposable on its mirror image. 3. Which of the following statements is not true? A) A molecule that is superimposable on its mirror image is said to achiral. B) A molecule that is not superimposable on its mirror image is said to be chiral. C) A molecule that is superimposable on its mirror image is said to be chiral. D) A carbon atom bonded to four different groups is a stereogenic center.

4. Which of the following statements is true? A) Enantiomers are mirror images that are not superimposable. B) A molecule that is superimposable on its mirror image is chiral. C) A chiral molecule usually contains a plane of symmetry. D) An achiral molecule does not contain a plane of symmetry.

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5. Which of the following statements is true? A) Achiral molecules usually contain a plane of symmetry. B) With one tetrahedral stereogenic center, a molecule may or may not be chiral. C) With two or more stereogenic centers, a molecule is always chiral. D) Chiral molecules usually contain a plane of symmetry. 6. Which of the following is the correct definition for a pair of enantiomers? A) A pair of stereoisomers that have a plane of symmetry. B) A pair of stereoisomers that are not mirror images of each other. C) A pair of stereoisomers that are not superimposable mirror images of each other. D) A pair of stereoisomers that are superimposable mirror images of each other.

7. Which of the following statements is true? A) The presence of a plane of symmetry makes a molecule chiral. B) An achiral molecule must have one or more stereogenic centers. C) All molecules that have stereogenic centers are chiral. D) Stereoisomers that are not superimposable on their mirror image are enantiomers.

8. Which of the following molecules has a plane of symmetry?

9. Which of the following molecules are chiral? I. cis-1,3-Dibromocyclohexane II. 1-Bromo-1-methylcyclohexane III. trans-1-Bromo-3-methylcyclohexane IV. cis-1-Bromo-3-methylcyclohexane 10. Which of the following molecules are achiral?

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11. How many chirality centers are present in each of the following compounds?

12. How many chirality centers are possible for a molecule with formula? CH3CH(OH)CH(OH)CH3

Dimethycyclopropane

CH3CHBrCH(OH)CH3

Dimethycyclopropane

13. Rank the following groups in order of decreasing priority according to the CahnIngold-Prelog rule.

14. Which of the following structures has a different configuration from the other three?

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15. Assign the R or S configuration to each of the following compounds:

16. Which of the following is the definition for a pair of diastereomers? A) A pair of stereoisomers with stereogenic centers which is not chiral. B) A pair of stereoisomers that are superimposable mirror images of each other. D) A pair of stereoisomers that are not superimposable mirror images of each other. D) A pair of stereoisomers that are not mirror images of each other.

17. Which of the following statements about meso compounds is not true? A) A meso compound generally has a plane of symmetry. B) A meso compound and its mirror image are identical. C) A meso compound is achiral. D) A meso compound is chiral.

18. Which of the following compounds is a meso compound? (2R, 3S)-dichlorobutane

C) (2R, 3S)-3-chloro-2-butanol

(2R, 3R)-dichlorobutane

D) (2R, 3R)-3-chloro-2-butanol

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19. Which of the following compounds are meso?

20. A natural product was isolated in the laboratory, and its observed rotation was +10° when measured in a 1 dm sample tube containing 1.0 g of compound in 10 mL of H2O. What is the specific rotation of this compound?

21. A solution containing 0.08 g/mL of a pure R enantiomer in a 1 dm polarimeter rotates plane polarized light by +4°.

What is the specific rotation of the S

enantiomer?

22. How much of the R enantiomer is present in 10 g of a mixture which has an enantiomeric excess of 30% of the S isomer?

23. A solution containing 0.4 g/mL of a pure S enantiomer is a 1 dm polarimeter rotates plane polarized light by +5.6°. What is the rotation of a solution containing 0.8 g/mL of the S enantiomer in the same polarimeter.

24. A reaction mixture is three parts of one enantiomer and one part of the other. What is the percent ee of this mixture?

25. Tell whether the compounds of each pair are enantiomers, diastereomers, constitutional isomers, or not isomeric.

CH2OH

CH2OH

C(O)CH3

C(O)CH3

H

OH HO

H

H

OH

H

OH

H

OH HO

H

H

OH

H

OH

CH2Br

CH2Br

CH2OH

(A)

CH2OH (B)

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CHO

CHO

CHO

HO

H

H

OH

H

H

OH

H

OH

HO

CH2CH3

CHO OH HO

H

H

OH

H

C(O)CH3

CH2CH3

CH2OH (D)

(C)

26. Compound D with the molecular formula C6H12 is optically inactive but can be resolved into enantiomers. On catalytic hydrogenation, D is converted to E (C6H14) and E, is optically inactive. Propose structures for D and E.

27. Draw the enantiomers of 1,3-dibromobutane (CH2BrCH2CHBrCH3) and label them as (R) and (S).

28. When one of the enantiomers of butan-2-ol (CH3CHOHCH2CH3) is placed in a polarimeter, the observed rotation is 4.05° counterclockwise. The solution was made by diluting 6.00 g of butan-2-ol to a total of 40.0 mL, and the solution was placed into a 200-mm polarimeter tube for the measurement. Determine the specific rotation for this enantiomer of butan-2-ol (CH3CHOHCH2CH3). 29. Looking at the portion of cellulose and that of starch as presented in slides (3), the two molecules appear to be identic when referring to their structures. But there is a difference between them. Explain that difference. 30. Consider a molecule with two stereogenic centres. How many stereoisomers that molecules will have if you ruled out the possibility of the presence of a meso structure? How many stereoisomers will have in presence of meso structure?

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