Bonding in cyclopropane

Copyright 2001 John Wiley Sons, Inc. ISBNs 0-471-35833-9 (Hardback) 0-471-22041-8 (Electronic) 

Orbital Interaction Theory of Organic Chemistry, Second Edition. Arvi Rauk 

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FIGURE 3 10 Bent bonds in cyclopropane (a) The orbitals involved in carbon-carbon bond formation overlap in a region that is displaced from the internuclear axis (b) The three areas of greatest negative electrostatic potential (red) correspond to those predicted by the bent bond description... 

For a review of various descriptions of the bonding in cyclopropane, see A. de Meijer, Angew. Chem. Int. Ed. Engl. 18 809 (1979) K. B. Wiberg, in The Chemistry of the Cyclopropyl Group, Z. Rappoport, ed., John Wiley Sons, New York, Chapter 1, 1987 B. Rozsondai, in The Chemistry of the Cyclopropyl Group, Vol. 2, Z. Rappoport (ed.), John Wiley Sons, New York, Chapter 3, 1995. 

Figure 4.1 (a) Rotation occurs around the carbon-carbon bond in ethane, but (b) no rotation is possible around the carbon-carbon bonds in cyclopropane without breaking open the ring. 

Cyclopropane, C.H, is a hydrocarbon composed of a three-membered ring of carbon atoms, (a) Determine the hybridization of the carbon atoms, (b) Predict the CCC and HCH bond angles at each carbon atom on the basis of your answer to part (a), (c) What must the real CCC bond angles in cyclopropane be (d) What is the defining characteristic of a cr-bond compared with a ir-bond, for example (e) How do the C—C cr-bonds in cyclopropane extend the definition of conventional o-bonds (f) Draw a picture depicting the molecular orbitals to illustrate your answer. 

The strained C,C bond in cyclopropanes is prone to heterolysis, particularly, in the presence of substituents stabilizing the resulting charges. These facts can be used to interpret easy methanolysis of cyclopropanes (385), resulting in regeneration of the starting functionalized AN (382) in high yields (Eq. 1) (493). 

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. 

The strain energies of cyclopropane (26.5 kcaFmol) and cyclobutane (26 kcal/ mol) are nearly the same despite the apparent much greater bond angle deformation with the former. One reason is that the weak C—C bonds in cyclopropane are compensated in part by the stronger C—H bonds. The increased strength results from the greater s character, and it is known that C—H bond strengths increase with... 

The vibrational spectra of several small ring compounds have been studied in sufficient detail so as to provide force constants for the vibrational modes7 Cyclopropane has shorter CH bonds with larger stretching force constants than for cyclobutane. The CH bonds in cyclopropane resemble those in ethylene, corresponding to their increased i-orbital character. 

Figure 4.11. Bonding in cyclopropane a) 2p and sp2 hybrid orbitals (b) Walsh MOs. One of the degenerate HOMOs, W2, and the LUMO, W4, will interact in fashion with a substituent on the ring.

There is much evidence, chiefly derived from nmr coupling constants, that the bonding in cyclopropanes is not the same as in compounds that lack small-angle strain.280 For a... 

There are several ways to describe the a bonds in cyclopropane. The most simple is to identify the C H bonds as coming from the straightforward sp3 hybrids on the carbon atoms and the Is orbitals on the hydrogen atoms 1.32 in... 

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