# 5.E: Stereoisomerism of Organic Molecules (Exercises)

Exercise 5-1 Draw structures showing the configuration of the following compounds. You may wish to review the nomenclature of alkenes, given in Section 3-3.

a. trans-3-methyl-3-hexene
c. (cis-1-propenyl)cyclobutane
d. trans-1,2-di-(cis-1-propenyl)cyclobutane

Exercise 5-2 Geraniol is a naturally occurring compound found in certain grasses and is used in perfumes to simulate the odor of roses. The IUPAC name for this compound is 3,7-dimethyl-trans-2,6-octadien-1-ol (where "-1-ol" signifies that there is an $$\ce{OH}$$ group on the lowest-numbered carbon of the chain). Select the correct structure from among the following:

Exercise 5-3 Identify the chiral carbon atoms by an asterisk (*) in each of the following structures. If no chiral carbons are present, write achiral.

a.

b. $$\ce{CH_3CH_2CBr=CH_2}$$

c.

d.

e.

f.

Exercise 5-4 How many chiral centers are evident in the structure of cholesterol? Identify them by the number of the carbon atom.

Exercise 5-5 The work of the German chemist Wislicenus on hydroxypropanoic acids was influential in the development of van't Hoff's ideas on stereoisomerism. By 1869, Wislicenus had established that there are three isomeric hydroxypropanoic acids, let us call them A, B, and C, of partial structure $$\ce{C_2H_4(OH)(CO_2H)}$$. Isomer A was isolated from sour milk and Isomer B from a meat extract. Both A and B had the same physical properties, except for optical rotation, wherein A was levorotatory and B was dextrorotatory. Isomer C was not optically active and had considerably different physical and chemical properties from A or B. Work out structures A, B, and C in as much detail as you can from the information given.

Exercise 5-6 Examine the structures of $$\beta$$-carotene and vitamin A shown in Section 2-1 and Section 3-0 and determine the configuration at each of the double bonds in the chain attached to the ring(s). Are these substances chiral or achiral?

Exercise 5-7 Draw the staggered conformations of each of the following compounds using the indicated convention:

a. 2,3-dimethylbutane (sawhorse)
b. 1,2-dibromo-1,1,2,2-tetrafluoroethane (Newman)
c. the d,l isomers of 1-chloro-1-fluoroethane (Newman)

Exercise 5-8 Draw the conformation of 2,2,5,5-tetramethylhexane that you expect to be of lowest energy.

Exercise 5-9 Draw a staggered conformation in both the sawhorse and Newman representations that corresponds to the configurations shown in the projection formulas a-c.

a.

b.

c.

Exercise 5-10 Draw projection formulas that correspond to the specific configurations shown in the following structures:

a.

b.

c.*

Exercise 5-11 This exercise can clarify for you the constraints on manipulating projection formulas. You will be helped by checking the configuration with models, as in Figure 5-12. The idea is to determine what effect there is, if any, on the configuration represented by the formula by making various changes in the projection. Your answer for each part should be that the operation changes, or does not change, the configuration.

a. interchange of substituents across the horizontal bond:

b. interchange of other substituents:

c. $$180^\text{o}$$ rotation in plane of paper:

d. $$90^\text{o}$$ rotation in plane of paper:

e. end-over-end flip outside plane of paper:

Exercise 5-12 Analysis of the crystals of a particular tartaric acid show them to be made up of equal amounts of the following conformations:

Use ball-and-stick models to determine the relationship between these two conformations and also whether this tartaric acid is meso-tartaric acid, an optically active tartaric acid, or racemic acid. Give your reasoning.

Exercise 5-13 Write structures for all the configurations possible for 2,4-dibromopentane. Which stereoisomers are enantiomers? Which are diastereomers? What combination of isomers would give a racemic mixture? Which isomer is achiral?

Exercise 5-14 From the compounds listed select all those that may have achiral meso configurations and draw the configurations for each of them.

a. 1,2-dichlorocyclopropane
b. 1,4-dichlorocyclohexane
c. 1,3-dichlorocyclohexane
d. 2,3-dichloropentane
e. 2,3,4-trichloropentane
f. 2,3,4,5-tetrachlorohexane

Exercise 5-15 Look carefully at each pair of structures shown below and decide whether they are identical. If you are uncertain, use molecular models.

a.

b.

c.

d.

e.

f. $$\ce{Cl-C \equiv C-Br}$$ $$\ce{Br-C \equiv C-Cl}$$

g.

Exercise 5-16 The two structures shown in each of the following pairs are isomers. Determine whether they are position, configurational, or conformational isomers. Use of models will be very helpful.

a.

b.

c.

d.

e.

Exercise 5-17 Which of the following compounds could exist as cis-trans configurational isomers?

a. 1,2-dibromoethane
b. 2,3-dibromopropene
c. dibromoethyne
d. 1,3-dibromopropene

Exercise 5-18 Which of the following compounds can exist as (1) a pair of enantiomers, (2) a pair of cis-trans isomers, and (3) as a cis pair of enantiomers and a trans pair of enantiomers?

a. 3-chloro-1-butyne
b. 4-chloro-1-butyne
e. 4-chloro-2-pentene
f. 5-chloro-2-pentene

Exercise 5-19 Write structures showing the specified configurations for each of the following compounds. Make your drawings as clear as possible so there is no ambiguity as to structure or configuration:

a. cis-1,2-diphenylethene
b. trans-2-chloro-2-butene
c. trans-1-propenylbenzene
h. cis-1-tert-butyl-4-methylcyclohexane

Exercise 5-20 Write structural formulas showing configuration for all of the possible cis-trans isomers of the following compounds:

a. 1,2,3-trimethylcyclopropane
b. 1,3-dichlorocyclopentane
d. 1-(3-methylcyclobutyl)-3-methylcyclobutane

Exercise 5-21 Would you expect cis- or trans-1,2-dimethylcyclopropane to be the more stable? Explain.

Exercise 5-22 Draw suitable formulas for all of the position and configurational isomers possible (include optical isomers but not conformational isomers) for the following compounds of molecular formula:

a. $$\ce{C_3H_5Cl}$$ (five)
b. $$\ce{C_5H_{10}}$$ (thirteen)
c. $$\ce{C_4H_7Cl}$$ (nineteen)

Exercise 5-23 Show how the sawhorse and Newman conventions can be used to represent the different possible staggered conformations of the following substances:

a. chloroethane
b. 1,2-dichloro-1-fluoroethane
c. 1,2-dichloroethane
d. 2,3-dimethylbutane

Exercise 5-24 Determine which of the following compounds are chiral and which are achiral. Indicate each chiral atom with an asterisk (*), noting that more than one may be present in some examples.

a. 2,3-dimethylpentane
b. 2,3-dimethyl-2-pentene
c. 2-bromo-3-chlorobutane

d.

e.

f.

g.

Exercise 5-25 Write structures that fit the following descriptions:

a. An achiral isomer of dimethylcyclohexane that has the methyl groups on different carbons.

b. All the chiral isomers of formula $$\ce{C_5H_{12}O}$$.

c. A compound of formula $$\ce{C_4H_5Cl}$$ that has just one double bond and is chiral.

d.* The conformation of 2,5-dimethylhexane you would anticipate to be the most stable.

Exercise 5-26 If you have a set of molecular models with which you can make or use bent bonds for double bonds, construct each of the following molecules and determine if stereoisomerism is possible and, if so, identify the type of stereoisomers.

a. $$\ce{ClCH=C=CHCl}$$
b. $$\ce{ClCH=C=C=CHCl}$$

c.

d.*

Exercise 5-27 Designate the configuration of the compounds whose structures are drawn below using the cis-trans terminology.

a.

b.

c.

d.

e.

Exercise 5-28 Draw sawhorse formulas as in Figure 5-10 for the following cyclohexane derivatives:

a. 1,1,3,3-tetramethylcyclohexane
b. cis-1,2-dimethylcyclohexane

c.

Exercise 5-29 Determine the relationship between the pairs of compounds written as perspective formulas as being enantiomers, diastereomers, conformational isomers, cis-trans isomers, or some combination of these. Models will be very helpful.

a.

b.

c.

d.

e.

f.

Exercise 5-30 This is a problem similar to 5-29, except that the structures are written mostly as projection formulas of the Fischer or Newman type. Determine the relationship between the pairs of compounds as one of the following: identical, position isomers, enantiomers, diastereomers, conformational isomers, or cis-trans isomers. ($$\ce{D}$$ stands for deuterium, the hydrogen isotope of mass 2.)

a.

b.

c.

d.

e.

f.

Exercise 5-31 Draw structures for all the possible configurational isomers of the following compounds. In Part a, $$\ce{D}$$ stands for deuterium, the hydrogen isotope of mass 2.

a. ethene-1,2-$$\ce{D_2}$$ (1,2-dideuterioethene)
b. 3-phenoxy-1-butene
c. 4-iodo-2-pentene
d. 2-chloro-3-phenylbutane
e. 2,3-diphenylbutane
f. 3-chlorocyclohexene
g. 3-chlorocyclooctene (use models)
h. 4-chloromethylcyclohexane
i. 3-chloromethylcyclohexane
j. 1-methyl-4-(1-propenyl)cyclohexane
k. 1-methyl-3-(1-propenyl)cyclohexane

Exercise 5-32 Determine which of the following conformations is identical with its mirror image (models will be very helpful). For the purpose of this part of the problem, assume that the compounds are locked in the conformations shown. For Parts a-d, determine which of these substances becomes achiral on free rotation.

a.

b.

c.

d.

e.

f.

Exercise 5-33 Redraw the perspective drawings a, b, and c as Fischer projection formulas, leaving the configuration at the chiral centers unchanged. Similarly, redraw d and e in perspective, using a staggered sawhorse representation for e.

a.

b.

c.

d.

e.

Exercise 5-34 Use the $$D$$,$$L$$ system to designate the configuration at each chiral center in Structures a-e in Exercise 5-33.

Exercise 5-35 This problem is designed to illustrate chirality, asymmetry, and dissymmetry with simple models or common objects.

a. Bend three pieces of wire into a hair-pin shape with equal legs. Now take one piece and make a $$90^\text{o}$$ bend in one of the legs in the middle to give (1). Bend up both legs equally of another piece to give (2), and one up and the other down to give (3). Determine whether (1), (2), and (3) are chiral or achiral, and asymmetric, dissymmetric, or symmetric. (See footnote $$^1$$, Section 5-1B)

b. Classify each of the following as chiral, achiral, asymmetric, dissymmetric, or symmetric: a cup, a shirt, a bicycle, a tennis racket, an automobile, a penny, a pair of scissors, a flat spiral (4), and a conical spring (5). Indicate any ambiguities that may be involved.

Exercise 5-36 The structures of some biochemically interesting compounds are shown below. Mark the chiral carbons in each and calculate by the $$2^n$$ rule how many stereoisomers might be expected. Explain why only one pair of enantiomers is known for camphor (ball-and-stick models will be very helpful here). How many different stereoisomers would you expect actually could be prepared of the quinine and codeine structures?

## Contributors

• John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."