Cyclobutane eclipsing 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. 

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. 

Knowing the importance of angle and eclipsing strain in the small-ring cycloalkanes, we should expect that these strains would become still more important in going from cyclobutane to bicyclo[1.1.0]butane or from cyclooctane to pentacyclo[4.2.0.02,5.03 8.04,7]octane (cubane). This expectation is borne out by the data in Table 12-6, which gives the properties of several illustrative smallring polycyclic molecules that have been synthesized only in recent years. 

The total barrier to planarity of thietane is 274 cm" (784 cal/mole) so that at room temperature only about 25% of the molecules occupy vibrational levels above the barrier. The barrier is greater than that in oxetane but less than that in cyclobutane. Ring-strain favors planarity whereas eclipsing of methylene protons favors torsion and nonplanarity, a-methylthietan possesses equatorial and axial con-formers observable at room temperature with an inversion barrier of 341 cm (975 cal/mole). 3,3-dimethylthietane has a barrier of 300cm (857 cal/mole). °... 

Tables 3-1 through 3-3 contain data on which empirical calculations of chemical shifts can be made. The tables represent a fraction of the data available on the fundamental alkane, alkene, and aromatic structures. Moreover, corrections must be applied in order to avoid nonadditivity caused primarily by steric effects. Thus, three groups on a single carbon atom, two large groups cis to each other on a double bond, or any two ortho groups can cause deviations from the parameters listed in the tables. If sufficient model compounds are available, the corrections shown can be applied. Further empirical calculations are possible for any structural entity, so that the eclipsing strain in cyclobutanes, the variety of steric interactions in cyclopentanones, or the variations in angle strain in norbornanes may be taken into account.

If we assumed cyclobutane to be planar, then the angles between the bonds would be 90 ° rather than the optimal 109.5 and all the C-H bonds would be eclipsed (7.8). In practice, it is not quite as bad as this—the ring flexes slightly (the interplanar angle in 7.9 is about 25 °) to relieve some of both angle and eclipsing strain (7.9). 

Cyclopropane and cyclobutane exhibit considerable angle and eclipsing strain. Cyclopropane relieves this by forming banana bonds and cyclobutane by flexing from the planar. 

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. 

Cyclobutane, in fact, is not a planar molecule. To reduce torsional strain, this compound attains the above nonplanar folded conformation. Hydrogen atoms are not eclipsed in this conformation and torsional strain is much less than in the planar structure. However, in this form angles are less than 90°, which means a slight increase in angle strain. 

To achieve a bond angle of 90°, cyclobutane must be planar. This would force the adjacent C-H bonds to be eclipsed and would also raise the energy of the system. As a compromise, cyclobutane folds diagonally by 35°. While this raises the angle strain somewhat, it decreases the eclipsing interactions so that the lowest possible energy is attained. While each methylene group is less strained than one in cyclopropane, the total molecular strain is similar to that of cyclopropane. 

Comparison of the heats of combustion of cycloalkanes (Table 9.1) shows that cyclopropane, cyclobutane, and cyclononane yield more energy per methylene group than the other cycloalkanes. This can be attributed to strain resulting from bond-angle distortion (Baeyer strain), eclipsed conformations (Pitzer strain), and trans-annular, repulsive van der Waals interactions. Common (five- and six-membered) rings and large (more than twelve-membered) rings have little or no strain. This... 

Cyclopropane (Fig.J) is a flat molecule in respect of C-atoms, with the hydrogen atoms situated above and below the plane of the ring, so it has no conformational isomers. Cyclobutane can form three distinct shapes-a planar shape and two butterfly shapes (fig.K). Cyclopentane can also form a number of shapes or conformations. The planar structures for cyclobutane and cyclopentane are too strained to exist in practice because of eclipsed C-H bonds. 

FIGURE 3-14 The ring strain of a planar cyclobutane results from two factors 109.5° tetrahedral / / eclipsed rih eclipsed jyb... 

In fact, in any planar conformation all the C-H bonds will be eclipsed with their neighbours. In cyclobutane, the ring distorts from a planar conformation in order to reduce the eclipsing interactions, even though this reduces the bond angles further and so increases the bond angle strain. 

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