Cyclopropane, bent bonding

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... 

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. 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

In keeping with the "bent-bond" description of Figure 3.10, the carbon-carbon bond distance in cyclopropane (151 pm) is slightly shorter than that of ethane (153 pm) and cyclohexane (154 pm). The calculated values from molecular models (see Learning By Modeling) reproduce these experimental values. 

Strong sp -sp a bonds are not possible for cyclopropane, because the 60° bond angles of the ring do not permit the orbitals to be properly aligned for effective overlap (Figure 3.10). The less effective overlap that does occur leads to what chemists refer to as bent bonds. The electron density in the carbon-carbon bonds of cyclopropane does not lie along the internuclear- axis but is distr-ibuted along an arc between the two carbon atoms. The r-ing bonds of cyclopropane are weaker than other carbon-carbon a bonds. 

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. 

Cyclopentenones. from 1.4-diketones. 886-887 Cyclopropane, angle strain in, 115 bent bonds in. 115 from alkenes. 227-229 molecular model of, 111. 115 strain energy of, 114 torsional strain in, 115 Cystathionine, cysteine from. 1177 Cysteine, biosynthesis of, 1177 disulfide bridges from, 1029 structure and properties of, 1018 Cytosine, electrostatic potential map of, 1104... 

FIGURE 1.4 The bent bonds ( tear drops ) of cyclopropane. 

Figure 3.81 Geminal interactions in cyclopropane and propane, showing bent cyclopropane (a) bond

Calculations have shown that there are two ways in which cyclopropane may be protonated at one of the carbons (comer protonated) and at one of the C—C bonds (edge protonated). They have about the same energy, and are involved in the movement of a proton around the ring. The calculations indicate that cyclopropane is unusually basic for a hydrocarbon. The reason can be seen in the stmctures of the protonated cyclopropanes (Fig. 15.4). The bent bonds permit a proton to bond to a C C bond without coming close to the positively charged carbon nuclei. 

The increased reactivity of cyclopropanes results from the presence of bent bonds which can interact with electrophiles, and can be more easily cleaved thermally than ordinary C C bonds. One indication of the consequences of the distortion is found in the strain energies (SE)37 that are calculated as the difference between the observed heat of formation and that estimated for a strain-free model. One might, for example, consider cyclohexane as strain-free, and then a model for cyclopropane would be half the heat of formation for cyclohexane. The available data for heats of formation of cyclopropane and cyclobutane derivatives are given in Table 2,38 The heat of formation of cyclohexane is — 29.4 kcal/mol, and the strain energy of cyclopropane is 12.7-0.5( — 29.4) or 27.5 kcal/mol. 

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. 

Double and triple bonds are represented with bent bonds formed with flexible couplings. Substances that require models with bent bonds normally are found to be much less stable and, therefore, chemically more reactive than molecules which can be constructed with straight sticks. Figure 1-4 shows the double bond of ethylene, the triple bond of acetylene, and the distorted bonds of cyclopropane. 

FIGURE 7. Forster-Coulson-Moffltt bent bond orbitals of cyclopropane. Reprinted from E. Honegger, E. Heilbronner and A. Schmelzer, Nouv. J. Chim., 6,519 (1982) by permission of Gauthier-Villars Publishers... 

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