And bond polarity

When factors work in the same direction (effective nuclear charge and bond polarity in the case of C — N < C — C), we can make confident predictions. When factors work in the opposite direction ( value and bond multiplicity in the case of H — C < C 1 O), we can explain the observed values but could not predict them. 

A proton transfer reaction involves breaking a covalent bond. For an acid, an H — X bond breaks as the acid transfers a proton to the base, and the bonding electrons are converted to a lone pair on X. Breaking the H — X bond becomes easier to accomplish as the bond energy becomes weaker and as the bonding electrons become more polarized toward X. Bond strengths and bond polarities help explain trends in acidity among neutral molecules. 

More than just a few parameters have to be considered when modelling chemical reactivity in a broader perspective than for the well-defined but restricted reaction sets of the preceding section. Here, however, not enough statistically well-balanced, quantitative, experimental data are available to allow multilinear regression analysis (MLRA). An additional complicating factor derives from comparison of various reactions, where data of quite different types are encountered. For example, how can product distributions for electrophilic aromatic substitutions be compared with acidity constants of aliphatic carboxylic acids And on the side of the parameters how can the influence on chemical reactivity of both bond dissociation energies and bond polarities be simultaneously handled when only limited data are available ... 

Table 4.14. Relative energies (/srei) and NBO characteristics of optimized pi and tw isomers o/Ir(CH2)H, Os(CH2)2, and W(CH2)3 (see Fig. 4.18), showing Lewis densities (pf), hybridizations (Iim), and bond polarizations (100cm2)...

Table 4.17 summarizes the optimized bond angles xmh and hmh and M—X and M—H hybrid and bond-polarity descriptors for these species. 

The question of protonation sites is one of the basic questions in the behaviour of complex organic molecules in solution, since protonated molecules are intermediates in synthetic organic chemistry, and the knowledge of protonation sites is important for the theory of reaction mechanisms of acid-catalysed reactions. It is also of fundamental importance for structural theory in general, since it is intimately connected with the concepts of mesomerism, electron density and bond polarization. 

Despite our earlier enunciated electronegativity and bond polarity logic, we must forego nearly all comparison with the free (uncomplexed) carbanions. Unlike the rather stable cyclopropyl anion, the cyclobutyl and cyclopentyl ions are unbound with regard to loss of their extra electron. That is, the gas phase ionization process to form the radical from the carbanion, Ru R" -E e, is energetically favorable. 

For structural elucidation by means of carbon-13 coupling constants, an empirical approach is often sufficient Carbon-13 one-bond coupling constants, particularly JCH values roughly correlate with carbon hybridization and bond polarity. The latter is greatly affected by electron-withdrawing heteroatoms or substituents. These relations will be outlined in the following sections. 

Jcn also reflects hybridization and bond polarity. Amides, for example, display larger 27cn magnitudes than amines [135] due to the amide resonance, which increases the n character of CN bonding and, in addition, makes the amide nitrogen more positive. 

Relating Eg to electronegativity provides a theoretical basis of this concept. Here is a definition without assumptions and derived directly from first principles. It is a function of the electronic configuration of atoms only and emerges naturally as the response of an atom to its environment. It is indeed the tendency of an atom to interact with electrons [117] and the fundamental parameter that quantifies chemical affinity and bond polarity. 

As you might expect from the discussion of electronegativity and bond polarity in Section 6.4, the carbon-magnesium bond is polarized, making the carbon atom both nucleophilic and basic. An electrostatic potential map clearly shows the electron-rich (red) character of the carbon bonded to magnesium ... 

Ionic Versus Covalent Character and Bond Polarities... 

Predict molecular polarity from three-dimensional geometry and bond polarity. 

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