The properties of the cycloalkanes differ from those of their straight chain analogs. Cycloalkanes have higher boiling points, melting points, and densities than their straight-chain analogs. Increased London forces are due to more rigid and symmetric cyclic systems. The smaller cycloalkanes with an odd number of carbon atoms have lower melting points than expected compared to cycloalkanes with an even number of carbon atoms. This is attributed to crystal-packing forces between the two types of cycloalkanes.

Ring Strain and the Structure of Cycloalkanes 4-2 The heats of combustion of the cycloalkanes reveal the presence of ring strain. Cyclopropane (60 o ) and cyclobutane (90 o ) possess C-C-C bond angles significantly different from o and are strained. This is termed ring strain. Cyclohexane has essentially no ring strain. The ring strain in cycloalkanes can be estimated by comparing the theoretical heats of combustion to the measured heats of combustion: Four groupings: Small (3,4) Common (5,6,7) Medium (8-12) Large (>12)

Strain affects the structures and conformational function of the smaller cycloalkanes. Cyclopropane: All of the methylene hydrogens are eclipsed (eclipsing strain) There is no available bond rotation to achieve a staggered conformation. The C-C-C bond angles are 60 o, far from the unstrained value of o. The C-C bond energy is about 65 kcal mol -1 The ring of cyclopropane is easily opened, for instance by hydrogenation:

Cyclobutane: Cyclobutane is puckered with a bending angle of about 26 o. The bent cyclobutane molecules flips rapidly from one puckered conformation to another. The puckered conformation partially relieves the strain caused by the otherwise eight eclipsing hydrogens. The C-C bond strength is about 63 kcal mol -1. Cyclobutane also undergoes ring opening but is less reactive than cyclopropane.

Cyclopentane: A regular pentagon has interior angles of 108 o (close to tetrahedral). However, cyclopentane is puckered, not planar. The puckering in cyclopentane relieves some of the hydrogen eclipsing, however, it somewhat increases the bond strain. The observed structures balances these two opposing factors to achieve a structure of lowest energy. There are two puckered conformations, the envelope and the half chair. There is little energy difference between them and they rapidly interconvert.

Cyclohexane: A Strain-Free Cycloalkane 4-3 The chair conformation of cyclohexane is strain free. Cyclohexane has several conformations, one of which is called the chair conformation. In the chair conformation, eclipsing of the hydrogens is completely prevented, the C-C-C bond angles are very nearly tetrahedral, and the molecule is nearly strain free.

Cyclohexane also has several less stable conformations. A second, less stable conformation of cyclohexane is the boat form, which is less stable than the chair form by 6.9 kcal mol -1. The higher energy is due to the eclipsing of the 8 hydrogens at the base of the boat, and the transannular (steric crowding across a ring) strain between the two hydrogens in the boat framework.

The boat form of cyclohexane is flexible and actually represents a transition state between two slightly more stable twist-boat (or skew- boat) conformations. The stabilization of the twist forms to the boat form is about 1.4 kcal mol -1, The activation energy for interconversion of the chair form and boat forms is 10.8 kcal mol -1. Normally cyclohexane exists primarily as the chair confomer with very small amounts of the twist boat form and no actual boat form.

Cyclohexane has axial and equatorial hydrogen atoms. In the chair form, cyclohexane has two types of hydrogens: Axial 6 H parallel to the principle molecular axis Equitorial 6 H perpendicular to the principle molecular axis