Cyclopropane ring deformation

Temperature = 25°C pressure = 1 bar a-CsN bond not included -NsO bond not included c No ring deformation correction d No cyclopropane ring in method e-C=N- bond not included f Some nitrogen groups not in table 8 No peroxide group in table h Average deviation from experimental values... 

The three carbon atoms of the cyclopropane ring lie in a plane. Therefore the angle strain is expected to be considerable because each C-C-C valence angle must be deformed 49.5° from the tetrahedral value. It is likely that some relief from the strain associated with the eclipsing of the hydrogens of cyclopropane is achieved by distortion of the H-C-H and H-C-C bond angles ... 

Fig. 2.3 Experimental deformation charge density in the cyclopropane ring of 7-dispiro[2.0.2.1 Jheptane carboxylic add. Reproduced with permission from [95], copyright 1996 International Union of Crystallography...

Figure 9 Deformation electron density map in a cyclopropane ring confirms the concept of bent bonds (dashed lines).

FIGURE 2. Dynamic filtered deformation electron-density distribution of cyclopropane (1). Sections (a) in the ring plane and (b) in the crystallographic mirror plane, perpendicular to the ring plane. Contours are at 0.05 e A 3 intervals, dashed lines represent negative areas. Reproduced by permission of the International Union of Crystallography from Reference 23b... 

The relation between strain and reactivity may be illustrated with electrophilic reactions of cyclopropane and cyclobutane (218). The decrease in strain is essentially equal in both compounds, but cyclopropane is highly reactive, whereas cyclobutane is essentially inert. Acid-catalyzed ring opening, for example, is controlled by the basicity of the two cycloalkanes. The latter are only slightly deformed by protonation and little strain is therefore released. 

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