Electronegativity, carbon compared with

The property that most separates acetylene from ethane and ethylene is its acidity. It, too, can be explained on the basis of the greater electronegativity of 5/r-hybridized carbon compared with sp and sp. ... 

As may be seen in figure 4 and as noted above for C60 the fullerene n molecular orbitals are composed of carbon atomic orbitals which contain a substantial amount of 2s orbital character and as the carbon 2s orbital lies so much lower in energy than the 2p orbital the resulting orbitals are expected to exhibit an enhanced electronegativity when compared with their planar counterparts. Furthermore the fullerenes are not only of intermediate hybridization, but probably exhibit variable hybridization as they pass through different states of reduction, for the pyramid-alization of anions is well known. Based on figure 3 it seems that the reduction potential of C60 is lowered by about 0.8V as a result of rehybridization effects. 

The NMR spectral properties of the parent heterocycles are summarized in Table 12. The signal for the pyrrole a-carbon is broadened as a result of coupling with the adjacent nitrogen-14 atom (c/. Section 3.01.4.3). While the frequencies observed for the /3-carbon atoms show a fairly systematic upheld shift with increasing electronegativity of the heteroatom, the shifts for the a-carbon atoms vary irregularly. The shifts are comparable with that for benzene, S 128.7. 

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

The above differences in behaviour between the germanium- and tin-centred radicals can therefore be ascribed to (a) the greater difference in electronegativity between tin and carbon as opposed to germanium and carbon, which manifests itself in the greater influence of the pyramidal nature of the tin radicals, and (b) the poorer overlap of the Sn 5p-C 2p as compared with Ge 4p-C 2p orbitals. 

---