Pima County Community College

OCHEM 1

Chapter 4 Organic Compounds: Cycloalkanes and Their Stereochemistry

SHORT ANSWER

1)Draw 7 constitutional isomers of a cycloalkane with the formula C₆H₁₂. Name each of the isomers you drew.

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IUPAC Naming Instructions: Provide IUPAC names for each structure.

3. Name:

ANS:

trans-1-tert-butyl-4-ethylcyclohexane or trans-1-(1,1-dimethylethyl)-4-ethylcyclohexane

Drawing Instructions: Draw structures corresponding to each of the following names.

4. Draw: cis-1-sec-butyl-2-ethylcyclopentane

5. Draw: 3-cyclobutylpentane

6. Draw: 1-chloro-2-isopropylcyclopentane ANS:

7. Draw: 3,5-dicyclohexylnonane

8. Circle all bridgehead carbons in the following structure.

9. Draw the two stereoisomers of 1,3-dibromocyclobutane.

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Exhibit 4-1

Refer to the structure below to answer the following question(s):

11. Refer to Exhibit 4-1. Which of the labeled bonds in the structure are equatorial bonds?

12. Refer to Exhibit 4-1. Which of the labeled bonds are trans to bond b?

13. Refer to Exhibit 4-1. Which labeled bonds have a 1,3-diaxial interaction with each other?

ANS:

b and c

Exhibit 4-2

For each substituted cyclohexane below, draw its ring-flip isomer. Circle the most stable conformation.

14. 
    Refer to Exhibit 4-2.

15. Refer to Exhibit 4-2.

16. Refer to Exhibit 4-2.

Exhibit 4-3

The following question(s) refer to the structure of camphor shown below.

17. Refer to Exhibit 4-3. Circle all bridgehead carbons in camphor.

18. Refer to Exhibit 4-3. Camphor is an example of a: fused bicyclic molecule.

bridged bicyclic molecule. fused tricyclic molecule. bridged tricyclic molecule.

19. Below are the two chair conformations of a 1,2,4-trimethylcyclohexane. Estimate the amount of 1,3-diaxial strain in each conformer and predict which conformer is most stable.

20. The energy difference of 3.8 kJ/mol between gauche and anti butane corresponds to an equilibrium constant, K_eq, of approximately 1.9. Calculate the percentage of each conformer at equilibrium.

21. In general, 5-alkyl substituents in 1,3-dioxane exhibit a smaller equatorial preference than they do in cyclohexane. To what might you attribute this observation?

Exhibit 4-4

Label each pair of compounds below as: conformational isomers stereoisomers

constitutional isomers identical

22.                 

23.                         

24.                 

25.                 

26.                 

Exhibit 4-5

For each pair of molecules below, circle the most stable.

27.     

28.

29.     

30. (-)-Menthol can be isolated from the peppermint plant and is responsible for the characteristic flavor and taste of peppermint. The structure of (-)-menthol is:

On the chair template provided below, draw the two chair conformations that are in equilibrium for (-)-menthol.

31. D-Pinitol is an interesting hexahydroxy cyclohexane, whose structure is shown below.

On the templates provided, draw the two chair conformations that are in equilibrium for D-pinitol.

Circle the most stable conformation.

MULTIPLE CHOICE

1. The two structures show below represent:

constitutional isomers stereoisomers

cis-trans isomers both b and c

a, b and c

2. What relationship exists between the following two structures?

identical molecules                                                 stereoisomers

constitutional isomers                                           different molecules

3. Consider the two methyl groups indicated with letters in the following molecular model.

These two groups are:

A: axial B: axial                                                     A: equatorial B: axial

A: axial B: equatorial                                          A: equatorial B: equatorial

4. Which of the following would produce the greatest amount of 1,3-diaxial strain when substituted for Cl in the following structure?

1. CN                                                                       
2. C(CH₃)₃
3. OH
4. CO₂H

5. Which of the following can exist as cis-trans isomers?

1. cyclohexane
2. methylcyclohexane
3. 1,1-dimethylcyclohexane 1,2-dimethylcyclohexane both c and d
4. all of the above

6. The following substituents are attached to a cyclohexane ring. Which substituted cyclohexane is correctly named as a "cyclohexyl alkane"?

1. tert-butyl
2. 2-methylpentyl cyclopentyl octyl
3. both b and d
4. none of the above

7. In methylcyclohexane:

1. all carbon atoms are sp³ hybridized.
2. ring-carbon atoms are sp² hybridized and the methyl group is sp³ hybridized.
3.  all bond angles are approximately 120°.
4. ring-bond angles are approximately 120° and the ring-methyl bond angle is approximately 109°.

8. Which of the following would have the smallest strain energy (kJ/mol)?

1. cyclobutane                                             
2. cyclohexane
3. cyclopentane                                           
4. cyclooctane

9. Consider the following table.

Based on the data in the table, which of the following compounds would have the largest strain energy?

1. cyclopropane                                           
2. cyclopentane
3. cyclobutane                                             
4. cyclohexane

10. Consider the following table.

What is the approximate strain energy per CH₂ for cyclopropane?

1. 12 kJ                                                            
2. 110 kJ
3. 37 kJ                                                                    
4. 230