Electronegativity of substituents

Allylic radical are relatively stable, and the pentadienyl radical is particularly stable. In such molecules, (E), E), (E),(Z), and (Z),(Z) stereoisomers can form. It has been calculated that (Z),(Z)-pentadienyl radical is 5.6 kcal mol less stable than the ( ),( )-pentadienyl radical. ° It is noted that vinyl radical have (E) and (Z) forms and the inversion barrier from one to the other increases as the electronegativity of substituents increase. Enolate radicals are also known. ... 

The relative insensitivity of Sp to changes in electronegativity of substituents which is observed for P fluoro-compounds is also evident for penta-arylphosphoranes. Thus the placement of />ara-substituents on the phenyl rings of pentaphenylphosphorane has very little effect and Sp is -f 88 1 p.p.m. for (16 Y = H, Me, or Cl). The effect on Sp of the introduction of an amino-group in the bis-biphenylenephosphorane system (17) is similar to that of an alkyl group (see Table 3). ... 

Examination of the data for derivatives 6 allows one to observe the influence of the electronegativity of substituents both on the ethereal oxygens and on the peroxidic atoms. 

The less steep slope of the correlation line, as compared with the slope found for the analogous relation in aminosilanes [ (SiN) = OAl SiC) — 7.3]103, results from the lower electronegativities of substituents on the N atom in the silazanes. 

Theory suggests and experiment confirms that coupling constants can be related to a number of physical parameters. Among the most important are (1) hybridization, (2) dihedral bond angles, and (3) electronegativity of substituents. 

According to Ernst et al. empirical parabolic dependence of Si chemical shifts on the sum of electronegativities of substituents Rj at the tetra-coordinate central... 

Figure 3. Changes in chemical shift with electronegativity of substituent Y...

The second group of activators are substances which react with the lactam or its anion to yield an A -substituted lactam [107, 113—116] in a sufficiently fast reaction e.g., isocyanates, anhydrides, esters, ketenimines. The activating effect of Af-substituted lactams was found to incresise with increasing electronegativity of substituents [84, 111, 117]. Another group of activating substances are precursors of compounds of the second group. For example, AT-2,2-trisubstituted j3-oxoamides [118—120] decompose at elevated temperatures very easily into ketone and isocyanate, which is a very effective activator. 

We consider (Sec. 1.23) electronic effects to be of two kinds inductive effects related to the electronegativity of substituents and resonance effects. In the case of carbonium ions, we shall see (Sec. 8.21), a resonance effect involves overlap of the empty p orbital of the electron-deficient carbon with orbitals on other, nearby atoms the result is, of course, that the p orbital is no longer empty, and the electron-deficient carbon no longer so positive. Maximum overlap depends on coplanarity in this part of the molecule, and it is here that we find the second advantage of flatness in a carbonium ion. 

For example, in any substituted ethylene—or in any pair of geometric isomers— J is always larger between tram protons than between cis protons furthermore, the size of J varies in a regular way with the electronegativity of substituent so that one can often assign configuration without having both isomers in han< ... 

Reductive removal of electronegative Of-substituents from ketones, acids and derivatives... 

The factor (1-tr) converts the general resonance condition for the nucleus P to the specific resonance condition for the individual phosphorus atom in the actual compound that we want to measure. The parameter phosphorus atom is subject to. These are normally limited to diamagnetic factors, such as electronegativity of substituents, and diamagnetic anisotropy due to participation in double bonds or shielding cones of adjacent functional groups. 

Having seen that phosphorus chemical shifts are indeed predictable in terms of electronegativity of substituents and zr-bond involvement of phosphorus, we will now turn to the question of bonding to other moieties. We clearly expect a downfleld shift, since the electron density on phosphorus will be reduced upon utilization of the lone pair for a donor bond. 

---