2.1: Conformations of Ethane Conformational isomerism involves rotation about sigma bonds, and does not involve any differences in the connectivity or geometry of bonding. Two or more structures that are categorized as conformational isomers, or conformers, are really just two of the exact same molecule that differ only in terms of the angle about one or more sigma bonds. 2.1.1: Conformations of chain alkanes 2.2: Conformations of Other Alkanes Ethane has only two conformers of note - staggered and eclipsed. Alkanes that are more complex than ethane, like propane and butane have a greater variety in possible conformers and their relative energies. 2.3: 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. 2.4: 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. 2.4.1: Conformations of cyclic alkanes 2.5: 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. 2.5.1: Conformational analysis 2.6: 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. 2.7: 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. 2.8: pka and pH