Ch 3 : Cycloalkanes

Chapter 3: Conformations of Alkanes and Cycloalkanes

Cycloalkanes

Cycloalkanes just means "cyclic alkanes" - ring systems formed of only C-C and C-H bonds.
Such structures are commonly encountered in natural compounds such as steroids, with cyclopentanes and cyclohexanes being the most common.
Other than cyclopropane (which must be planar), cycloalkanes are "puckered" to relieve some of the ring strain by lowering angle and torsional strains.
Ring strain : cyclopropane > cyclobutane > cyclopentane > cyclohexane

The structures of some of the smaller cycloalkanes are shown below with the planar structures for contrast. In each case, manipulate the JMOL images to look for the deviation from planarity and the effect this has on the eclipsing interactions of adjacent H atoms and C-C bonds. In order to be able to compare the strain in each member of the cycloalkane series, the heat of combustion per methylene (i.e. -CH₂-) is also given. The smaller this number is the less ring strain there is.

C₃H₆ CYCLOPROPANE DHc / CH₂ = -697 kJ/mol (-166.6 kcal/mol)

C₄H₈ CYCLOBUTANE DHc / CH₂ = -681 kJ/mol (-162.7 kcal/mol)
	planar	most stable structure

C₅H₁₀ CYCLOPENTANE DHc / CH₂ = -658 kJ/mol (-157.3 kcal/mol)
	planar	most stable structure