Cyclopropane, angle strain bent bonds

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FIGURE 4.9 (a) Orbital overlap in the carbon-carbon bonds of cyclopropane cannot occur perfectly end-on. This leads to weaker bent bonds and to angle strain, p) Bond distances and angles in cyclopropane. (c) A Newman projection formula as viewed along one carbon-carbon bond shows the eclipsed hydrogens. (Viewing along either of the other two bonds would show the same picture.) (d) Ball-and-stick model of cyclopropane. 

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. 

Bent bonds (Section 4.4) The bonds in small rings such as cyclopropane that bend away from the internuclear line and overlap at a slight angle, rather than head-on. Bent bonds are highly strained and highly reactive. 

The noteworthy point is a shortening of C-C bond distance as compared to the aliphatic C-C bond distance which is 1.54 A. This shortening manifests itself in the development of special character of the bonds. Since the sp orbitals of carbon now cannot overlap as effectively as they do in alkanes where perfect end-on overlap occurs the bonds in cyclopropane becomes bent and weak and also lead to an angle strain and so the molecule has greater potential energy. So this bent or banana bond accounts for the most of the ring strain. 

Many of the unique properties of cyclopropanes, and to a lesser extent, cyclobutanes, are derived from the formation of bent bonds. They may act in a fashion similar to 7r-bonds in interacting with electron-deficient centers, and are more easily cleaved thermally via electrophilic attack than are ordinary C-C bonds. The strain energy associated with bond angle deformation is also an important quantity, especially when considering thermal reactions. 

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. ... 

Figiire9.3. Angle strain, a) Maximum overlap permitted for open-chain or large-ring compounds. (6) Poor overlap for cyclopropane ring. Bent bonds have much p character. 

Another field where care must be exercised when using the hybrid orbital concept deals with bent bonds and molecular strain. Cyclopropane is the best example. The molecular strain is usually justified by the occurrence of bent CC bonds, and these are interpreted in terms of hybrid orbitals that cannot have axes making angles of 60° to coincide with the C-C nuclear axes (Fig. 8.11). 

Compression of the ideal 109.5° Csp bond angle to an internal bond angle of 60° in cyclopropane results in considerable angle strain, which is manifested by bent bonds. Six pairs of eclipsed hydrogens adds an additional 6 kcal/mol of torsional strain. Total strain is 27 kcal/mol. 

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