Electronegativity of Carbon Atoms

Note that the various definitions do not reach a numerical consensus on the relative electronegativity of sp, sp, and sp carbon, although the order is consistent. We are 

Description of Molecular Structure Using Valence Bond Concepts 

If we compare the values of propanoic acid (4.87), propenoic acid (4.25), and propynoic acid (1.84), we get some indication that the hybridization of carbon does exert a substantial polar effect. The acidity increases with the electronegativity of the carbon group. 

Orbitals of different hybridization on the same carbon are also thought of as having different electronegativities. For example, in strained hydrocarbons such cyclopropane the C-H bonds are more acidic than normal. This is attributed to the additional s character of the C-H bonds, which compensates for the added p character of the strained C-C bonds. ° 

An important qualitative result emerges from these numbers. Bond strength is increased by electronegativity differences. This is illustrated, for example, by the strength of the bonds of fluorine with the other second-row elements. 

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Because the electronegativity of carbon atoms follows the order sp > sp > sp, ethyne is a stronger acid than ethene, and ethene is a stronger acid than ethane. 

Even more important is the fact that the formation of the triol carbocations (PAHTC) has not been correctly calculated. Any treatment based on a simple Hiickel-MO or PMO calculations for odd AH ions neglect the effect of the differently charged carbon atoms and hence, must be in error. The ionic charge distributed over the aromatic system affects the electronegativity of carbon atoms in specific ways and this has a profound effect on the 7i-energy. Breakdowns of both the PMO and HMO approximations with ionic reaction intermediates are documented in the work of Dewar and Thompson [36,70], Streitwieser et al [35,71] and Szentpaly [39]. The reactivity patterns with radical and ionic reaction intermediates of PAH are different [34-39]. It has been pointed out by Dewar [36] that the PMO method works better for radical than ions, and adequate modifications of the PMO method have been developed for ionic intermediates [16,38,39]. 

The chemical shift is related to the part of the electron density contributed by the valence electrons, ft is a natural extension, therefore, to try to relate changes of chemical shift due to neighbouring atoms to the electronegativities of those atoms. A good illustration of this is provided by the X-ray photoelectron carbon Is spectmm of ethyltrifluoroacetate, CF3COOCH2CH3, in Figure 8.14, obtained with AlXa ionizing radiation which was narrowed with a monochromator. 

Fig. 35. (a) Correlation between the photolysis rate of halomethyl-cobalamins and the electronegativity of the alkyl ligand. (The electronegativity values are shown as the sums of the Pauling electronegatives of the atoms attached to the methyl carbon.) (b) Correlation between pKa for displacement of benzimidazole and alkyl group electronegativity for a series of halomethyl-cobalamins... 

During initiation, degradation begins at sites on the polymer that are susceptible to nucleophilic attack. These reactive sites are typically associated with nearby electronegative groups that enhance the reactivity of carbon atoms, as shown in Fig. 9.1. In general, we represent the initiation step as shown in Fig. 9.2. 

We have shown19 that there are separate but nearly parallel linear relationships between the methyl deviations, 6(CH3—Z), of methyl substituted species CH3—Z and the electronegativity of the atom in Z bonded to carbon for functionality in the second row of the periodic table (Z = —OH, —NH2, —CH3, —B<) and in the third row of the periodic table (Z = —Cl, —SH, —Al<). If the methyl deviations for CII3B1 and (CH3)4Ge, both containing fourth-row atoms, are now calculated and the results plotted versus the Pauling electronegativity, the two new points fall on the line established by the third-row series (Figure 1). 

When some boron atoms in non-classical boranes are exchanged by isolobal C+ units, the multicenter bonding MOs look qualitatively the same, but the contribution of carbon hybrid orbitals is larger than those from boron atoms [compare Figures 3.2-3(b) and (c)]. This polarization is due to the higher electronegativity of carbon versus boron atoms. 

The p-carbon signals in heterocyclohexanes 52 (143) respond to the electronegativity of endocyclic substituents similarly to those of carbon atoms in p positions with respect to exocyclic substituents. The slopes of chemical shift vs. electronegativity plots for the p-methylene groups in 52 and 53 are negative (Table 10). For the quaternary P-carbon atom in 53, however, a slight positive slope is observed (164). 

The Pauling electronegativities of carbon and tellurium are, respectively, 2.5 and 2.1. This, in addition to the large volume of the tellurium atom (atomic radius 1.37, ionic radius 2.21), promotes easy polarization of Te-C bonds. The ionic character of the bonds increases in the order C(sp ) Te>C(sp ) Te>C(sp)-Te, in accordance with the electronegativity of carbon accompanying the s character (Table 1.1). 

The C—C and C—B interatomic distances in carboranes can also be related to the coordination numbers of the skeletal atoms. Two factors tend to make these distances shorter than the B —B distances in comparable boranes the preference of the carbon atoms for sites of low coordination number and the greater electronegativity of carbon than boron, which increases the electron density in the region of the carbon atoms and so strengthens the bonds that they form. Table IX lists some C—C distances for closo- and wido-carboranes 13, 20, 21, 26, 98,121,168) and metal-acetylene 50, 58,112) complexes, relating them... 

In the case of insertion toward fluorine (41), there is an even greater amount of positive-positive charge repulsion between the ortho and ipso carbons than in the transition state and this effect is responsible, in part, for a higher activation barrier for insertion toward F to form 41 than away from fluorine to form 40. Therefore, the origin of the pronounced influence of ortho,ortho-difluoro substitution on the lifetime of singlet arylnitrene and the increased activation energy of its cyclization is the result of combination of the steric effect and the extraordinary electronegativity of fluorine atom. 

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