Chemistry 350 Organic Chemistry I

Study Guide :: Unit 4

Organic Compounds: Cycloalkanes and their Stereochemistry

Unit Preview

This unit deals with the concept of stereochemistry and conformational analysis in cyclic compounds. It discusses the causes of various ring strains and their effects on the overall energy level of a cycloalkane. We shall stress the stereochemistry of alicyclic compounds. You will find it advantageous to keep your molecular models available as you work through this unit.

Unit Objectives

When you have completed Unit 4, you should be able to

  1. fulfill all of the detailed objectives listed under each individual section.
  2. draw the cis-trans isomers of some simple disubstituted cycloalkanes, and write the IUPAC names of such compounds.

4.1  Naming Cycloalkanes

Objectives

After completing this section, you should be able to

  1. name a substituted or unsubstituted cycloalkane, given its Kekulé structure, shorthand structure or condensed structure.
  2. draw the Kekulé, shorthand or condensed structure for a substituted or unsubstituted cycloalkane, given its IUPAC name.
  3. draw all possible cycloalkane structures (substituted or unsubstituted) that correspond to a given molecular formula.

Learning Activities

Read 4.1 Naming Cycloalkanes and do any associated exercises.

4.2  Cis-Trans Isomerism in Cycloalkanes

Objectives

After completing this section, you should be able to

  1. recognize that a formula of the type Cn H2n may represent a cycloalkane.
  2. draw structural formulas that distinguish between cis and trans disubstituted cycloalkanes.
  3. construct models of cis- and trans-disubstituted cycloalkanes using ball-and-stick molecular models.

Learning Activities

  1. Read 4.2 Cis-Trans Isomerism in Cycloalkanes and do any associated exercises.
  2. Build cycloalkane models: It is difficult to make models of cyclopropane and cyclobutane using the model kit provided with this course: this difficulty is a reflection of the concept of ring strain, which will be introduced in Section 4.3. However, you can make an approximate model of cyclopropane if you use the curved rods to represent the carbon-carbon bonds. The result is not perfect, but it should help you to visualize cis and trans isomers. Similarly, you can make an imperfect model of cyclobutane by using two straight rods and two curved rods to represent the carbon-carbon bonds. Remember, however, that in practice, all of the $\ce{\sf{C-C}}$ bonds in cyclobutane are identical. You can construct models of cyclopentane and cyclohexane using the normal rods to represent carbon-carbon bonds.

4.3  Stability of Cycloalkanes: Ring Strain

Objectives

After completing this section, you should be able to

  1. describe the Baeyer strain theory.
  2. describe how the measurement of heats of combustion can be used to provide information about the amount of strain present in an alicyclic ring.
  3. explain the inadequacies of the Baeyer strain theory.
  4. determine which of two similar compounds is likely to be the most stable, by assessing such factors as angle strain, torsional strain and steric strain.

Learning Activities

  1. Read 4.3 Stability of Cycloalkanes: Ring Strain and do any associated exercises.
  2. Build cycloalkane models: To obtain a better appreciation of the concept of angle strain and if you have not already done so, you should try building molecular models of cyclopropane, cyclobutane, cyclopentane and cyclohexane using the “carbon” balls to represent the ring carbons.

4.4  Conformations of Cycloalkanes

Objectives

After completing this section, you should be able to

  1. describe, and sketch the conformation of, cyclopropane, cyclobutane and cyclopentane.
  2. describe the bonding in cyclopropane, and hence account for the high reactivity of this compound.
  3. analyse the stability of cyclobutane, cyclopentane and their substituted derivatives in terms of angular strain, torsional strain and steric interactions.

Learning Activities

Read 4.4 Conformations of Cycloalkanes and do any associated exercises.

4.5  Conformations of Cyclohexane

Objectives

After completing this section, you should be able to

  1. explain why cyclohexane rings are free of angular strain.
  2. draw the conventional shorthand structure of a cyclohexane ring.

Learning Activities

  1. Read 4.5 Conformations of Cyclohexane and do any associated exercises.

  2. Construct a model of the chair conformation of cyclohexane using six black “carbon” balls and six rods to form $\ce{\sf{C-C}}$ bonds. Insert twelve rods to form $\ce{\sf{C-H}}$ bonds into the vacant holes, and attach twelve of the one-hole hydrogen atoms to these bonds. Keep this model with you as you proceed to the next section. The twist-boat conformation of cyclohexane is obtained by twisting the boat conformer slightly; it is somewhat less stable than the chair conformation.

4.6  Axial and Equatorial Bonds in Cyclohexane

Objectives

After completing this section, you should be able to

  1. sketch the shorthand structure of cyclohexane, with axial and equatorial hydrogen atoms clearly shown and identified.
  2. identify the axial and equatorial hydrogens in a given sketch of the cyclohexane molecule.
  3. explain how chair conformations of cyclohexane and its derivatives can interconvert through the process of ring flip.

Learning Activities

  1. Read 4.6 Axial and Equatorial Bonds in Cyclohexane and do any associated exercises. Note: You will probably find it advantageous to refer to your model of cyclohexane as you do these problems.

  2. When studying this section, keep the model of cyclohexane in front of you. To understand the rest of this unit, you must be able to distinguish between axial and equatorial substituents in a cyclohexane ring, and you must be able to represent such structures on paper. Do not destroy your model of cyclohexane. Use your model of cyclohexane to ensure that you understand the concept of ring flip. Observe how flipping the ring converts every axial hydrogen to equatorial and vice versa. After you have carried out a ring flip on your model of cyclohexane, repeat the procedure for a monosubstituted cyclohexane. Refer to the example of methylcyclohexane equilibrium for assistance if necessary. To make a suitable model, replace any one of the “hydrogen” balls with a methyl group or just a ball of a different colour.

4.7  Conformations of Monosubstituted Cyclohexanes

Objectives

After completing this section, you should be able to

  1. account for the greater stability of the equatorial conformers of monosubstituted cyclohexanes compared to their axial counterparts, using the concept of 1,3‑diaxial interaction.
  2. compare the gauche interactions in butane with the 1,3‑diaxial interactions in the axial conformer of methylcyclohexane.
  3. arrange a given list of substituents in increasing or decreasing order of 1,3‑diaxial interactions.

Learning Activities

  1. Read 4.7 Conformations of Monosubstituted Cyclohexanes and do any associated exercises.

  2. As usual, you will find that molecular models are invaluable aids to achieving this objective. We suggest that you start by making a model of methylcyclohexane. You can now observe how close the methyl hydrogens are to the axial hydrogens attached to carbons 3 and 5. Next, sight along the $\ce{\sf{C{1}-C{2}}}$ bond, holding the model in such a way that the ring-methyl $\ce{\sf{C-C}}$ bond is pointing upwards. The “Newman projection” of C1 can now be drawn.

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    Figure 4.1: Newman projection of C1 in methylcyclohexane

    To complete the projection, you must show the atoms that are attached to carbon atom 2, just as you did in Unit 3 when you were drawing Newman projections for the simple alkanes. After looking along the $\ce{\sf{C{1}-C{2}}}$ bond of your model you should be able to draw the following diagram.

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    Figure 4.2: Newman projection of the $\ce{\sf{C{1}-C{2}}}$ bond of methylcyclohexane

    The gauche interaction between the methyl substituent and the ring methylene (CH3) group is quite clear from this diagram. You may now add additional detail, as shown in the Newman projection of methylcyclohexane in the LibreText.

4.8  Conformations of Disubstituted Cyclohexanes

Objective

After completing this section, you should be able to use conformational analysis to determine the most stable conformation of a given disubstituted cyclohexane.

Learning Activities

Read 4.8 Conformations of Disubstituted Cyclohexanes and do any associated exercises.

4.9  Conformations of Polycyclic Molecules

Objective

After completing this section, you should be able to draw the structures and construct molecular models of cis- and trans-decalin and of norbornane.

Learning Activities

  1. Read 4.9 Conformations of Polycyclic Molecules and do any associated exercises.
  2. Make models of cis- and trans-decalin. You should have enough “carbon” balls in your model set to enable you to construct a model of the testosterone molecule shown at the bottom of the LibreText reading. Molecules of this type are biologically very important.

Summary

In this unit, we carried out a fairly detailed study of alicyclic compounds, particularly cyclohexane and its derivatives. We paid particular attention to the stereochemistry of these compounds.