Small Rings Cyclopropane and Cyclobutane

Conformational analysis is far simpler in cyclopropane than in any other cycloalkane. Cyclopropane s three carbon atoms are, of geometric necessity, coplanar, and rotation about its carbon-carbon bonds is impossible. You saw in Section 3.4 how angle strain in cyclopropane leads to an abnormally large heat of combustion. Let s now look at cyclopropane in more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry. 

FIGURE 3.16 Bent bonds in cyclopropane. The orbitals involved in carbon-carbon bond formation overlap in a region that is displaced from the internuclear axis. Orbital overlap is less effective than in a normal carbon-carbon ct bond, and the carbon-carbon bond is weaker. 

FIGURE 3.17 Distribution of two products at equilibrium plotted as a function of the difference in free energy (AG°) at 25°C between them. 

In addition to angle strain, cyclopropane is destabilized by torsional strain. Each C—H bond of cyclopropane is echpsed with two others. 

PROBLEM 3.7 The heats of combustion of ethylcyclopropane and methylcy-clobutane have been measured as 3352 and 3384 kJ/mol (801.2 and 808.8 kcal/mol), respectively. Assign the correct heat of combustion to each isomer. 

FIGURE 3.18 Non-planar ( puckered ) conformation of cyclobutane. The nonplanar conformation is more stable because it avoids the eclipsing of bonds on adjacent carbons that characterizes the planar conformation. 

The Baeyer strain theory is useful to us in identifying angle strain as a destabilizing effect. Its fundamental flaw is its assumption that the rings of cycloalkanes are planar. With the exception of cyclopropane, cycloalkanes are nonplanar. Sections 3.5-3.13 describe the shapes of cycloalkanes. WeTl begin with cyclopropane. 

Chapter 3 Alkanes and Cycloalkanes Conformations and cis-trans Stereoisomers 

Cycloalkane Number of CH2 groups Heat of combustion Heat of combustion per CH2 group  

Neighboring C—H bonds are eciipsed in any pianar oyoioaikane. Thus aii pianar oonformations are destabiiized by torsionai strain. 

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Three- and four-membered rings (cyclopropane and cyclobutane derivatives) were treated in heat of formation calculations by MM3. Each of them required its own structural parameter set and its own ring constants (R3 and R4, respectively). Four-membered rings have also been treated by MM4. These small ring compounds present no particular difficulties and are well treated in the same way as are the alkanes. 

Note that special atom types are defined for carbon atoms involved in small rings, like cyclopropane and cyclobutane. The reason for this will be discussed in Section 2.2.2. 

Small ring hydrocarbons have a wide range of thermal reactivity, with cyclopropane and cyclobutane being quite stable thermally. With these compounds, the thermolysis is known to proceed via initial cleavage of one C—C bond giving a diyl, which has a relatively high energy. 

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