Cyclopropane hybridization

Conformational analysis is far simpler m cyclopropane than m any other cycloalkane Cyclopropane s three carbon atoms are of geometric necessity coplanar and rotation about Its carbon-carbon bonds is impossible You saw m Section 3 4 how angle strain m cyclopropane leads to an abnormally large heat of combustion Let s now look at cyclopropane m more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry... 

It may be proper at this stage to lead the reader back to the stage where we constructed the localized orbitals of a CH2 group. At that time two valence orbitals were set aside—the 2pv orbital, and the outer (2s, 2pr) hybrid. Both of these orbitals lie in the. r, y plane. Now in our description of cyclopropane, we used bond orbitals to describe the CC bonding these bond orbitals are derived from in-plane (xy y) hybrids on each carbon. The two hybrids which are required on each carbon atom—in ordet to participate in two bond orbitals—are built precisely from the 2py orbital and the (2s, 2pj.) out combination on each CH2 group. 

In the interaction of the local 2pv orbitals, two more bonding molecular orbitals are formed against one less bonding. In all previous cases the opposite occurred. This is due to the negative overlap between adjacent 2py orbitals—whether, by convention, all positive lobes point in the clockwise direction, or whether all positive lobes point in the anticlockwise direction. The two bonding 2pv combinations in fact fall below the two antibonding (hybrid 2s, 2px) combinations. The former each have two electrons while the latter are empty. The six electrons of the three C—C bonds are nicely accounted for. The method creates simultaneously the acc and or c molecular orbitals of cyclopropane (note that the latter three lie relatively close in energy). 

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. 

Dichlorocarbene adds to 150 to give the monoadduct 158 as well as the two bis-adducts 159 and 160 (equation 18)101. An ortho -bis-adduct could not be detected among the cyclopropanation products—possibly because of steric hindrance in the immediate vicinity of 158. The anti-arrangement of the two cylopropane rings in 159 and 160 was established by X-ray structural analysis101. Formally, the latter adducts represent hybrids of [6]radialenes and [6]rotanes ( rotaradialenes ). 

Figure 3.21 The bent C—C bond of cyclopropane, showing (a) the ate NBO and (b) the he bonding hybrid (sp3,46), which is oriented 24.3° outside the line of C—C centers.

The equilibrium hybrids and NBOs of the final titanacyclopropane ring bonds are shown in Fig. 4.73. Although some bond strain is evident, the degree of bondbending appears appreciably less than that in cyclopropane (cf. Fig. 3.21). The relatively gentle degree of bending of metal hybrids is particularly noteworthy in... 

The central carbon atom in methylenecyclopropane is almost sp-hybridized, like that in aUene the carbon atoms in cyclopropane are almost sp -hybridized, cf de Meijere A (1979) Angew Chem 91 867 Angew Chem Int Ed Engl 18 809... 

Many of the reactions discussed are not suitable to establish such a relationship, either because of the general stereochemical course of the reaction type, or because of the inherent symmetry of the substrate. Any reaction in which the pyramidal sp hybridized cyclopropane centers are converted to planar sp hybridized ones will lose any memory of the radical cation chirality or stereochemistry, unless it is transferred to a new chiral center generated in the course of the conversion. On the other hand, there are several reaction types which, given appropriate substrates, may be used to probe the stereochemical course of a cyclopropane radical cation reaction. For example, several hydrogen migrations have shown elements of stereoselectivity. Similarly, oxygenation reactions may have the potential to reveal some degree of stereoselectivity. 

Ethynylated dihydrofullerenes serve as precursors for buckydumbbells (Scheme 3.3). Coupling of the desilylated compound 10 with CuCl leads to the dimer 11 [24]. Reaction of Cjq with the acetylide Li-C=C-Li leads to the dimer with a bridge consisting of one acetylene unit only. Electronic interaction between the fullerene-units in these two buckydumbbells is negligible. Further examples of CgQ-acetylene-hybrids synthesized by using cyclopropanation reactions are shown in Chapter 4. 

In cyclopropane, each C atom is still -hybridized, so we should have a bond angle of 109.5°, but each C atom is at the corner of an equilateral triangle, which has angles of 60° As a result, there is considerable angle strain. The sp hybrids still overlap but only just This gives a very unstable and weak structure. The angle strain can be defined as the strain induced in a molecule when bond angle deviates from the ideal tetrahedral value. For example, this deviation in cyclopropane is from 109.5 to 60°. 

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