Angle strain in cyclopropanes

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. 

If the C s of the cyclobutane ring were coplanar, they would form a rigid square with internal bond angles of 90°. The deviation from 109.5° would not be as great as that for cyclopropane, and there would be less angle strain in cyclopropane. However, this is somewhat offset by the fact that the eclipsing strain involves four pairs of H s, one pair more than in cyclopropane. 

Angle strain in cyclopropane. The bond angles are compressed to 60° from the usual 109.5° bond angle of sp3 hybridized carbon atoms. This severe angle strain leads to nonlinear overlap of the sp3 orbitals and bent 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. 

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planai geometry by adopting the nonplanai puckered conformation shown in Figure 3.11. 

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... 

It might be expected that the strain in cyclopropane, in which the C-C-C bond angles are distorted from 109°28 to 60°, would weaken the bonds and thus lead to an increase in the bond distance. This effect is not observed, however, the carbon-carbon distance in cyclopropane being the same as in the other hydrocarbons to within the accuracy of the investigation. There is even some small indication that the C-C distance in cyclic aliphatic hydrocarbons is slightly smaller (by about 0.01 A.) than the normal distance, the three values reported being 1.53, 1.52, and 1.53 A. 

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

Cyclopropane possesses both significant angle and torsion strain. Strain in saturated cyclic hydrocarbons is determined from the heat of combustion, and comparisons are made per CH2 group in a molecule. For cyclopropane the molar enthalpy change of combustion is 2090 kJ mol-1, i.e. 696.5 kJ mol-1 per CH2 group. The corresponding figure for cyclohexane is 3948 kJ mol-1, i.e. 658 kJ mol-1 per CH2. If cyclohexane is taken to be strain free (which is a reasonable approximation), one can conclude that the strain in cyclopropane is (696.5 - 658) = 38.5 kJ mol-1 per CH2. It should be noted that these experimentally deter-... 

The bond angles in planar cyclobutane would have to be compressed from 109.5° to 90°, the bond angle associated with a planar four-membered ring. Planar cyclobutane would then be expected to have less angle strain than cyclopropane because the bond angles in cyclobutane are only 19.5° away from the ideal bond angle. 

Free radical halogenatlon of cyclopropane itself is very difficult because the C—H bonds are so strong (recall Problem 3 of Chapter 4). Electrophilic reactions (e.g., additions to a double bond) are not u.seful because the ring is easily broken open. Nucleophilic substitutions are very difficult because of angle strain in the transition states of the reactions, te.. 

The strain in cyclopropane and cyclobutane is largely a result of angular strain due to compression of the tetrahedral angle of 109 28 in the carbon-carbon bonding. In cyclopentane and cyclohexane this angular strain is reduced by the molecule adopting a puckered conformation. Thus, the conformation of the cyclopentane molecule is that of four carbon atoms in a plane, with one carbon atom some 0 5 A below this plane Fig. 1 (a). This is a mean value about which the puckering oscillates, and in addition there is a... 

Torsional strain is the second factor in cyclopropane s large ring strain. The three-membered ring is planar, and all the bonds are eclipsed. A Newman projection of one of the carbon-carbon bonds (Fig. 3-16) shows that the conformation resembles the totally eclipsed conformation of butane. The torsional strain in cyclopropane is not as great as its angle strain, but it helps to account for the large total ring strain. 

---