Carbon atom, electronegativity

In the HMOa) method, the carbon-atom electronegativity is assumed to depend linearly on the net atom charge qr. A more attractive Coulombic parameter ar is attributed to a positively charged carbon atom, because of fewer electrons to screen the core. The proposed change in electronegativity is o>Pqr the parameter o> has an optimized value [Pg.296]

Previous studies with a variety of datasets had shown the importance of charge distribution, of inductive effect), of r-electronegativity, resonance effect), and of effective polarizability, aeffi polarizability effect) for details on these methods see Section 7.1). All four of these descriptors on all three carbon atoms were calculated. However, in the final study, a reduced set of descriptors, shown in Table 3-4, was chosen that was obtained both by statistical methods and by chemical intuition. 

The nitrogen ending atoms being more electronegative possess a tt electronic density greater than that of the carbon atoms directly linked to them. 

Nucleophilic Reactions. The strong electronegativity of fluorine results in the facile reaction of perfluoroepoxides with nucleophiles. These reactions comprise the majority of the reported reactions of this class of compounds. Nucleophilic attack on the epoxide ring takes place at the more highly substituted carbon atom to give ring-opened products. Fluorinated alkoxides are intermediates in these reactions and are in equiUbrium with fluoride ion and a perfluorocarbonyl compound. The process is illustrated by the reaction of methanol and HFPO to form methyl 2,3,3,3-tetrafluoro-2-methoxypropanoate (eq. 4). 

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. 

Information on partially and fully saturated heterocycles is much more limited and is summarized in Figure 3. As would be expected, the downfield shift of the a-carbon atom decreases with decreasing electronegativity of the heteroatom in the sequence O < NH < S < CH2. 

If an H atom in an alkane R-// is replaced by a substituent X, the C chemical shift 8c in the a-position increases proportionally to the electronegativity of X (-/ effect). In the (1-position, Sc generally also increases, whereas it decreases at the C atom y to the substituent (y-effect, see Section 2.3.4). More remote carbon atoms remain almost uninfluenced (dSc 0). 

It is also useful for structure elucidation that Jqh increases with the electronegativity of the heteroatom or substituent bound to the coupled carbon atom (Table 2.6, from top to bottom). 

How does the Hiickel ff-electron model deal with pyridine Nitrogen is more electronegative than carbon, so the N atom ought to have a higher (more negative) T-electron charge than a carbon atom in benzene. It is conventional to write the heteroatom (X and Y are used to denote heteroatoms such as N and O) parameters in terms of the standard ac and yScc... 

When two resonance forms are nonequivalent, the actual structure of the resonance hybrid is closer to the more stable form than to the less stable form. I bus, we might expect the true structure of the acetone anion to be closer to the resonance form that places the negative charge on an electronegative oxygen atom than to the form that places the charge on a carbon atom. 

Organic acids are characterized by the presence of a positively polarized hydrogen atom (blue in electrostatic potential maps) and are of two main kinds those acids such as methanol and acetic acid that contain a hydrogen atom bonded to an electronegative oxygen atom (O-H) and those such as acetone (Section 2.5) that contain a hydrogen atom bonded to a carbon atom next to a C=0 bond (Q=C-C-H). 

Elements such as oxygen, nitrogen, fluorine, and chlorine are more electronegative than carbon, so a carbon atom bonded to one of these atoms has a partial positive charge ( -1-). Conversely, metals are less electronegative than... 

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