4: Organic Compounds - Cycloalkanes and their Stereochemistry
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.
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- 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).
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- 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.
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- 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.
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- 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.
Thumbnail: Ball-and-stick model of cyclobutane. (Public Domain; Ben Mills via Wikipedia )