Nomenclature of Cycloalkanes
Cycloalkanes are named with the prefix cyclo- followed by the name of the alkane with the same number of carbons. Cycloalkanes exhibit cis/trans isomerism. When the substituents are on the same side of the molecule, they are said to be cis; when they are on opposite sides, they are said to be trans.
Physical Properties
They have higher melting and boiling points than the corresponding alkanes with the same number of carbons. The rigidity of the ring allows for a greater number of intermolecular interactions, which need to be broken by the input of energy to transition the molecules to the gas phase.
Ring Strain
Small cycloalkanes (cyclopropane, cyclobutane) exhibit significant strain due to bond angles and eclipsing interactions. Larger cycloalkanes like cyclopentane and cyclohexane are almost free of strain.
Conformational Isomers in Cyclohexane
Cyclohexane adopts a chair conformation to avoid eclipsing interactions between hydrogens. The chair form of cyclohexane contains two types of hydrogens: axial hydrogens that are perpendicular to the plane of the molecule, and equatorial hydrogens that are in the same plane.
Equatorial-Axial Equilibrium in Substituted Cyclohexanes
Cyclohexane exhibits a conformational equilibrium that interconverts equatorial hydrogens to axial and vice versa. When a cyclohexane is substituted, the conformation that places more groups in the equatorial position is the most stable, with the conformational equilibrium shifted towards that conformation.
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