Electronegativity for atoms

Here is a summary of electronegativities for atoms commonly involved in organic reactions. 

Table 2.22 Recommended values of electronegativities for atoms in molecules...

An extended set of physicochemical descriptors was used in this study, including, for example, partial atomic charge and effective polari2 ability of the protons, average of electronegativities of atoms two bonds away, or maximum, T-atomic charge of atoms two bonds away. 

The strength of an acid depends on the atom to which the proton is bonded The two mam factors are the strength of the H—X bond and the electronegativity of X Bond strength is more important for atoms m the same group of the periodic table electronegativity is more important for atoms m the same row Electronegative atoms elsewhere m the molecule can increase the acidity by inductive effects... 

Typical coupling constants for isothiazoles are given in Table 5. The electronegative nitrogen atom reduces 3,4 and V3.5 from the values of 3.50 Hz and 0.27 Hz, respectively, in thiophene. The V values correlate quite well with rr-bond orders calculated by MO methods (74CJC833). 

Some force fields make special provision for the mutual electrostatic potential energy of pairs of atoms that have different electronegativities. If atom A has a formal charge of i2a and atom B (distant J ab from Qa) has a formal charge of (2b, then their mutual potential energy is... 

Whereas this study focused on reaction protocol, the effect of the methylene source on selectivity was another important factor which demanded attention. Earlier studies have demonstrated that substitution of chloroiodomethane for diiodomethane leads to an increased reaction rate (Scheme 3.10) [22]. It is, thus, surprising that the use of chloroiodomethane in sub-protocol la leads a slower, less selective reaction. In contrast to the use of diiodomethane ( 100% conversion at 300 min), the reaction of chloroiodomethane only reaches 58% conversion after 300 min. Selectivity is severely reduced, dropping to 75 25 er. The failure of this reagent in the chiral process may be attributed to the obvious differences between the highly polarizable iodine and the more electronegative chlorine atom, although an exact analysis of the difference is not clear. 

The conjugate addition of a nucleophile to an a,fi-unsaturated aldehyde or ketone is caused by the same electronic factors that are responsible for direct addition. The electronegative oxygen atom of the a,/3-unsaturated carbonyl compound withdraws electrons from the /3 carbon, thereby making it electron-poor and more electrophilic than a typical alkene carbon. 

Bond Energies and the Relative Electronegativity of Atoms.—In Table II there are collected the energies of single bonds obtained in the preceding sections. One additional value, obtained by a method to be described later, is also included 1.44 v. e. for N N. Under each bond energy is given the value for a normal covalent bond, calculated from additivity, and below that the difference A. It is seen that A is positive in twenty of the twenty-one cases. The exception, C I, may be due to experimental error, and be not real. 

The electronegativity map may be used with considerable confidence in predicting bond energies, especially for atoms which lie near each other on the map. It will be observed that the difference in bond energy of H A and... 

Selenoaldehydes 104, like thioaldehydes, have also been generated in situ from acetals and then directly trapped with dienes, thus offering a useful one-pot procedure for preparing cyclic seleno-compounds [103,104], The construction of a carbon-selenium double bond was achieved by reacting acetal derivatives with dimethylaluminum selenide (Equation 2.30). Cycloadditions of seleno aldehydes occur even at 0 °C. In these reactions, however, the carbon-selenium bond formed by the nucleophilic attack of the electronegative selenium atom in 105 to the aluminum-coordinated acetal carbon, may require a high reaction temperature [103], The cycloaddition with cyclopentadiene preferentially gave the kinetically favorable endo isomer. 

To summarize, there are two important trends electronegativity (for comparing atoms in the same row) and size (for comparing atoms in the same column). The first factor (comparing atoms in the same row) is a much stronger effect. In other words, the difference in stability between and F is much greater than the difference in stability between 1 and F . 

For a solution of ammonia in acetone, we must examine both components. Acetone has an electronegative oxygen atom with nonbonding pains, whereas NH3 has a polar N—bond. Consequently, a mixture of these two compounds displays hydrogen bonding between the hydrogen atoms of ammonia and oxygen atoms of acetone ... 

Most substances contain covalent bonds rather than ions. Nevertheless, the electrons in a bond between atoms of two different elements, such as FeO or CO, are polarized toward the more electronegative atom (see Figure 19-3 for electronegativities). For oxidation number purposes, we imagine that these electrons are transferred completely to the more electronegative atom. 

The designation of hard acids is not restricted to metal cations. For example, in BF3 the small boron atom in its +3 oxidation state is bonded to three highly electronegative fluorine atoms. All the B—F bonds are polarized away from a boron center that is already electron-deficient. Boron trifiuoride is a hard Lewis acid. 

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