Strain of cyclopentane

Problem 4,10 How many H eclipsing interactions would be present if cyclopentane were planar Assuming an energy cost of 4.0 kj/mol for each eclipsing interaction, how much torsional strain would planar cyclopentane have Since the measured total Strain of cyclopentane is 26 kj/mol, how much of the torsional strain is relieved by puckering ... 

The furanose rings of the deoxyribose units of DNA are conformationally labile. All flexible forms of cyclopentane and related rings are of nearly constant strain and pseudorotations take place by a fast wave-like motion around the ring The flexibility of the furanose rings (M, Levitt, 1978) is presumably responsible for the partial unraveling of the DNA double helix in biological processes. 

The hydroxylation of cyclohexane by a strain of Xanthobacterium sp. (Trickett et al. 1990). In cell extracts, a range of other substrates was oxidized including cyclopentane, pinane, and toluene (Warburton et al. 1990). 

In a similar way the potential constant method as described here allows the simultaneous vibrational analysis of systems which differ in other strain factors. Furthermore, conformations and enthalpies (and other properties see Section 6.5. for examples) may be calculated with the same force field. For instance, vibrational, conformational, and energetic properties of cyclopentane, cyclohexane and cyclodecane can be analysed simultaneously with a single common force field, despite the fact that these cycloalkanes involve different distributions of angle and torsional strain, and of nonbonded interactions 8, 17). This is not possible by means of conventional vibrational spectroscopic calculations. 

Equivalently, the enthalpy of reaction 47 is equated to that of the analogous (unsubstituted, saturated, carbocyclic) reaction 48. The difference is 29 kJmol , taken as the strain energy of cyclopentane. 

As an example, the heat of formation of cyclopentane in the gas phase is 18.4 kcal/mol. The Franklin group equivalent for CH2 is —4.93 kcaFmol, and thus an unstrained model for cyclopentane is five times this quantity or —24.6 kcaF mol. The difference between the two values gives the strain energy as 6.2 kcal/ mol. The strain energies given in Figure 15.1 were obtained in this way. 

Pinacol rearrangement driven by the release of the ring strain of a cyclobutane ring has been employed in an extremely efficient manner to form cyclopentanone derivatives. Experimentally. the Lewis acid mediated aldol condensation of benzaldehyde with l,2-bis(trimethyl-siloxy)cyclobutcne at —78 C gave the pinacol 1 in its silylated form.35,36 Subsequent treatment of this pinacol with trifluoroacetic acid at room temperature afforded 2-phenyl-cyclopentane-l,3-dione (2) in 97% yield.35,36... 

Build a model of cyclopentane. Examine the various types of strain present in the planar and nonplanar geometries. 

Conformational analysis of rings larger than cyclohexane is more complicated. These rings are also less common than cyclohexane, so we discuss their conformations only briefly. As can be seen from Table 6.1, the seven-membered ring compound cycloheptane has only a small amount of strain. Obviously, it is nonplanar to avoid angle strain. It does have some torsional strain, but the overall strain is comparable to that of cyclopentane. It is a fairly common ring system. 

In cyclopentane, as opposed to cyclobutane and cyclopropane, the bond angles have values close to the optimum. Therefore, the strain in the molecule arises essentially from bond opposition and is partly relieved by puckered conformations. Two flexible forms of cyclopentane exist, namely the so-called envelope (LXXVlIIa) and half-chair (LXXVIIIb) forms. The former has four carbons in the same plane, and... 

The pseudorotation circuit of cyclopentane is essentially of constant strain, and therefore without maxima and minima in contrast, the fully planar conformer is less stable by ca. 5 kcal/mol [101]. An extensive treatment of the conformational aspects of cyclopentane and of many five-membered-ring compounds has been published by Legon [92]. 

If cyclopentane were planar, it would have ten hydrogen-hydrogen interactions with a total energy cost of 40 kJ/mol. The measured total strain energy of 26 kJ/mol indicates that 14 kJ/mol of eclipsing strain in cyclopentane (35%) has been relieved by puckering. 

These are not frequently used monomers. There exists a clear connection between the strain of various members in the series of cyclic hydrocarbon molecules and their heats of combustion (see Table 1). The high heats of combustion of the first members are the consequence of the C—C bond angle deviation from 109°28. In cyclohexane, the most stable cycloalkane which can exist in the chair conformation, the C—C bond angle value deviates very little from that observed in unstrained compounds. Cyclopentane exhibits the smallest deviation of the C—C bond angle from the theoretical value. Its higher heat of combustion is due to steric interactions of pairs of neighbouring hydrogen atoms. A similar situation is observed with cycloheptane [12a]. 

Strain energies of some cyclic and bicyclic compounds are shown in Table 4. As reported by J. D. Cox and G. Pilcher801, the strain energies of cyclopentane, tetrahyd-rofuran, and 1,3-dioxolane are very close, i.e. the replacement of -CH2- by O-atom in a cyclic compound has no significant effect on the conventional strain energy in these monocyclic compounds. 

---