Angle strain cyclopropane

Cyclopentenones. from 1.4-diketones. 886-887 Cyclopropane, angle strain in, 115 bent bonds in. 115 from alkenes. 227-229 molecular model of, 111. 115 strain energy of, 114 torsional strain in, 115 Cystathionine, cysteine from. 1177 Cysteine, biosynthesis of, 1177 disulfide bridges from, 1029 structure and properties of, 1018 Cytosine, electrostatic potential map of, 1104... 

Conformational analysis is far simpler m cyclopropane than m any other cycloalkane Cyclopropane s three carbon atoms are of geometric necessity coplanar and rotation about Its carbon-carbon bonds is impossible You saw m Section 3 4 how angle strain m cyclopropane leads to an abnormally large heat of combustion Let s now look at cyclopropane m more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry... 

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planar geometry by adopting the nonplanar puckered confer matron shown m Figure 3 11... 

Cyclopropane is planar and destabilized by angle strain and torsional strain Cyclobutane is nonplanar and less strained than cyclopropane... 

Thus epoxides like cyclopropanes have significant angle strain They tend to undergo reactions that open the three membered nng by cleaving one of the carbon-oxygen bonds... 

Cyclobutane has less angle strain than cyclopropane (only 19.5°). It is also believed to have some bent-bond character associated with the carbon-carbon bonds. The molecule exists in a nonplanar conformation in order to minimize hydrogen-hydrogen eclipsing strain. 

The Baeyer strain theory is useful to us in identifying angle strain as a destabilizing effect. Its fundfflnental 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. We ll begin with cyclopropane. 

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

Cyclopropane is the most strained of all rings, primarily because of the angle strain caused by its 60° (,-C-C bond angles. In addition, cyclopropane also has considerable torsional strain because the C-H bonds on neighboring carbon atoms are eclipsed (Figure 4.4). 

Cyclobutane has less angle strain than cyclopropane but has more torsional strain because of its larger number of ring hydrogens. As a result, the total strain for the two compounds is nearly the same—110 kj/mol (26.4 kcal/mol) for cyclobutane versus 115 kj/mol (27.5 kcal/mol) for cyclopropane. Experiments show that cvclobutane is not quite flat but is slightly bent so that one carbon atom lies about 25° above the plane of the other three (Figure 4.5). The effect of... 

Cyclopropane (115 kj/mol strain) and cyclobutane (110.4 kj/mol strain) have both angle strain and torsional strain. Cyclopentane is free of angle strain but has a substantial torsional strain due to its large number of eclipsing interactions. Both cyclobutane and cyclopentane pucker slightly away from planarity to relieve torsional strain. 

Four-membered rings also exhibit angle strain, but much less, and are less easily opened. Cyclobutane is riiore resistant than cyclopropane to bromination, and though it can be hydrogenated to butane, more strenuous conditions are required. Nevertheless, pyrolysis at 420°C gives two molecules of ethylene. As mentioned earlier (page 177), cyclobutane is not planar. 

The Origin of Ring Strain in Cyclopropane and Cyclobutane Angle Strain and torsional Strain... 

The noteworthy point is a shortening of C-C bond distance as compared to the aliphatic C-C bond distance which is 1.54 A. This shortening manifests itself in the development of special character of the bonds. Since the sp orbitals of carbon now cannot overlap as effectively as they do in alkanes where perfect end-on overlap occurs the bonds in cyclopropane becomes bent and weak and also lead to an angle strain and so the molecule has greater potential energy. So this bent or banana bond accounts for the most of the ring strain. 

---