Cyclobutane and Cyclopropane

An analogous estimate can be applied to cyclobutane. In this case we consider the distorted methane molecule with one of the valence angles fixed at 90°. In our model, the response of the HOs to the deformation is proportional to the deviation of the valence angle from the tetrahedral one. The deviation of the C-C-C angle from the tetrahedral one in cyclobutane (19.5°) amounts to 40% of that in cyclopropane. Therefore, we can expect that about the same ratio will be observed for the deviations of the H-C-H valence angle from the tetrahedral one in the cyclobutane and cyclopropane molecules. In fact, the ratio got in the SLG-MINDO/3 numerical experiment is about 39%. 

The Bond-Forming Initiation Theory gives a good interpretation of the observed spontaneous polymerizations of captodative monomers. The tetramethylene diradicals already implicated as initiators in the thermal (spontaneous) polymerizations of vinyl monomers can be particularly stabilized by captodative substituents. For comparison, and to initiate the polymerization of third monomers, captodative cyclobutanes and cyclopropanes are particularly appropriate precursors for generating tetra- and trimethylene diradicals. In particular the extensive work of Viehe [3,45,46] showed that thermolysis of captodative substituted cyclopropanes leads to trimethylene captodative diradicals at reasonable temperatures. Their initiating abilities for polymerization have not yet been determined. 

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... 

It is well-recognized that the hydrocarbons cyclopropane and cyclobutane have nearly identical strain energies, and so these microcycles have been quite naturally paired in numerous treatments of molecular strain. How similar are cyclobutylamine (12, X = NH2, 13, = 4, X = NH2) and cyclopropylamine (2, X = NH2,13, = 3, X = NH2) and other correspondingly monosubstituted cyclobutanes and cyclopropanes What about... 

In this respect the nearly identical strain energies of cyclobutane and cyclopropane are somewhat surprising. One of the possible origins of this phenomenon is probably the two 1-3 interactions in cyclobutane. Such an interaction was estimated by Bauld and CO workers to induce a destabilizing effect of 10-16 kcal mol" The gap between the expected and observed strain energies of bicyclobutane is suggested to be due to a single repulsive interaction of this type as shown in (5) between the two methylene carbons . 

Cyclopentane is appreciably less strained than cyclobutane and cyclopropane, and the strain energy relative to cyclohexane is ca. 6.45 kJ mol-1 per CH2 group. In order to lessen the torsion strain that would occur in a planar conformation, in which every C-H bond is involved in two eclipsing interactions, cyclopentane adopts a puckered conformation (see Dunitz, Further Reading). This has four carbons approximately planar, with the fifth carbon bent out of this plane in such a way that the molecule resembles a small near-square envelope 9. A Newman projection of 9 is shown in 10. 

Cycloheptane, Cyclopentane, Cyclobutane and Cyclopropane Derivatives.- The theds of bicyclo[3.1.0]hexan-2-ones from a D-ribose derivative is depicted in Scheme... 

The group increments method suggests that we can predict AH ° for any organic molecule—a powerful tool, indeed. Let s consider some more examples. Table 2.5 shows calculated and experimental AH° values for simple cycloalkanes. The calculated AHf° for cyclo[u] is just w(-4.93 kcal/mol). For cyclohexane the result is quite good, but then things start to go downhill. The error for cyclopentane is considerable, while cyclobutane and cyclopropane are completely off. 

Higher overtones of cyclopentane, cyclobutane, and cyclopropane have also been studied with regard to their information content on equatorial and axial conformations. It is generally acknowledged that these higher overtones are explained by local mode theory. 

The parameter sets were modified slightly, as described in detail in the paper. This was done partly to conform to our new forms of potential energy functions, see sections 9 1, 9 2 and 11.6.2, partly to take into account the special problems encountered with torsional angles in spiro compounds containing small rings. The parameter sets were checked on cyclohexane, cyclopentane, cyclobutane and cyclopropane with good results except for the vibrational spectrxim of cyclopropane and the structure of cyclobutane which came out planar as in some of its derivatives. 

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