Strain, angle

Solution Eclipsing strain has a valu e of approximately 1 Kcal/mole for two adjacent, eclipsed bonds. In planar cyclopentane 10 pairs of eclipsed hydrogens are found, thus 10 Kcal of eclipsing strain is present. 

As planar cyclopentane folds, the number of eclipsed hydrogens decreases, and thus eclipsing strain decreases. 

In puckered cyclopentane, only two pairs of eclipsed hydrogens are found 1 and 3, 2 and 4. Hence only 

Therefore, as planar cyclopentane becomes puckered, a loss of 8 Kcal of eclipsing strain enables the puckered form to become more stable. 

In planar cyclobutane, each C-C-C bond angle measures 90 . Thus the angle strain is 

---

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... 

A second but much less stable nonplanar conformation called the boat is shown in Eigure 3 14 Like the chair the boat conformation has bond angles that are approximately tetrahedral and is relatively free of angle strain It is however destabi hzed by the torsional strain associated with eclipsed bonds on four of its carbons The... 

Angle strain destabilization that results from distortion of bond angles from then-normal values... 

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

FIGURE 5 5 Ball and spoke and space filling models of as and trans 2 butene The space filling model shows the serious van der Waals strain between two of the hydrogens in as 2 butene The molecule ad justs by expanding those bond angles that increase the separation between the crowded atoms The combi nation of angle strain and van der Waals strain makes as 2 butene less stable than trans 2 butene... 

Learning By Modeling contains a model of dynemicin A which shows that the C=C—C=C—C=C unit can be incorporated into the molecule without much angle strain... 

Speculation about the stability of Ceo centered on the extent to which the aromaticity associated with its 20 benzene rings is degraded by their non planarity and the accompanying angle strain It is now clear that Ceo is a relatively reactive substance reacting with many substances toward which ben zene itself is inert Many of these reactions are char acterized by addition to buckminsterfullerene converting sp hybridized carbons to sp hybridized ones and reducing the overall strain... 

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... 

Angle strain is the main source of strain in epoxides but torsional strain that re suits from the eclipsing of bonds on adjacent carbons is also present Both kinds of strain are relieved when a ring opening reaction occurs... 

The most striking chemical property of epoxides is their far greater reactivity toward nude ophilic reagents compared with that of simple ethers Epoxides react rapidly with nude ophiles under conditions in which other ethers are inert This enhanced reactivity results from the angle strain of epoxides Reactions that open the nng relieve this strain... 

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 medium-size rings (7 to 12 ring atoms) are relatively free of angle strain and can easily take a variety of spatial arrangements. They are not large enough to avoid all nonbonded interactions between atoms. 

The twist form is also called the flexible form, because without angle strain a left-handed twist (c) can be converted into a right-handed one (d), by way of a third form ca. 7 kJ mol higher in energy (and actually not strictly an intermediate, since it occupies a maximum on the energy profile) which is the boat form (e). 

Exocyclic unsaturation can stabilize small ring heterocycles. In three-membered rings it is difficult to separate the contributions from increased angle strain and from electronic interactions between the unsaturation and the heteroatom. In four-membered rings such separation has been done 74PMH(6)199, p. 235). The CRSEs change from oxetane... 

DE, since the 7c-electron energy is 4a + 4/3, the same as that for two independent double bonds. Thus, at this level of approximation, HMO theory prediets no stabilization for eyelobutadiene from delocalization and furthermore predicts that the molecule will have unpaired electrons, which would lead to very high reactivity. In addition, cyclobutadiene would suffer angle strain, whieh is not present in benzene. The extreme instability of eyelobutadiene is then understandable. Higher-level MO ealculations modify this picture somewhat and predict that eyelobutadiene will be a rectangular molecule, as will be diseussed in Chapter 9. These ealculations, nevertheless, agree with simple HMO theory in... 

Bicyclo[1.1.0]butane is an example of a molecule in which severe angle strain results in decreased stability and greatly enhanced reactivity. The bicyclo[1.1.0]butane ring has a strain energy of 63.9kcal/mol, and the central bond is associated with a relatively high... 

---