After you have completed Chapter 4, you should be able to
- fulfill all of the detailed objectives listed under each individual section.
- draw the cis-trans isomers of some simple disubstituted cycloalkanes, and write the IUPAC names of such compounds.
- define, and use in context, the key terms introduced in this chapter.
This chapter deals with the concept of stereochemistry and conformational analysis in cyclic compounds. The causes of various ring strains and their effects on the overall energy level of a cycloalkane are discussed. We shall stress the stereochemistry of alicyclic compounds.
- 4.1: Naming Cycloalkanes
- Cycloalkanes have one or more rings of carbon atoms, and contain only carbon-hydrogen and carbon-carbon single bonds. The naming of cycloalkanes follows a set of rules similar to that used for naming alkanes.
- 4.2: Cis-Trans Isomerism in Cycloalkanes
- Stereoisomers are molecules that have the same molecular formula, the same atom connectivity, but they differ in the relative spatial orientation of the atoms. Di-substituted cycloalkanes are one class of molecules that exhibit stereoisomerism. Di-substituted cycloalkane stereoisomers are designated by the nomenclature prefixes cis (Latin, meaning on this side) and trans (Latin, meaning across).
- 4.3: Stability of Cycloalkanes - Ring Strain
- Small cycloalkanes, like cyclopropane, are dramatically less stable than larger cycloalkanes due to ring strain. Ring strain is caused by increased torsional strain, steric strain, and angle strain, in the small, nearly planar ring of cyclopropane. Larger rings like cyclohexane, have much lower ring straing because they adopt non-planar conformations.
- 4.4: Conformations of Cycloalkanes
- Overall ring strain decreases in cycloalkane rings that are large enough to allow the carbon-carbon bonds to rotate away from planar structures. For this reason, cyclopentane is significantly more stable, than cyclopropane and cyclobutane.
- 4.5: Conformations of Cyclohexane
- Rings larger than cyclopentane would have angle strain if they were planar. However, this strain, together with the eclipsing strain inherent in a planar structure, can be relieved by puckering the ring. Cyclohexane is a good example of a carbocyclic system that virtually eliminates eclipsing and angle strain by adopting non-planar conformations.
- 4.6: Axial and Equatorial Bonds in Cyclohexane
- The hydrogens of cyclohexane exist in two distinct locations - axial and equatorial. The two chair conformations of cyclohexane rapidly interconverts through a process called ring flip.
- 4.7: Conformations of Monosubstituted Cyclohexanes
- Mono-substituted cyclohexane prefers the ring flip conformer in which the substituent is equatorial. 1,3-diaxial interactions occur when the substituent is axial, instead of equatorial. The larger the substituent, the more pronounced the preference.
- 4.8: Conformations of Disubstituted Cyclohexanes
- The most stable configurational isomer of a disubstituted cyclohexane will be the isomer that has the most stable conformational isomer.
- 4.9: Conformations of Polycyclic Molecules
- Polycyclic molecules are common and important in nature. Biologically important polycyclic molecules are found in cholesterol, sex hormones, birth control pills, cortisone, and anabolic steroids
Thumbnail: Ball-and-stick model of cyclobutane. (Public Domain; Ben Mills via Wikipedia)