Electronegativity - Localized Chemical Bonding - Introduction - March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

Part I. Introduction

Chapter 1. Localized Chemical Bonding

1.G. Electronegativity

The electron cloud that bonds two atoms is not symmetrical (with respect to the plane that is the perpendicular bisector of the bond) except when the two atoms are the same and have the same substituents. A symmetrical electron cloud typically occurs when there is a bond between two identical atoms, and an unsymmetrical electron cloud occurs when there are two different atoms. When there are two different atoms, and one is more electronegative than the other, the electron cloud is necessarily distorted toward one side of the bond or the other, depending on which atom (nucleus plus electrons) maintains the greater attraction for the cloud. This attraction is called electronegativity;25and it is greatest for atoms in the upper-right corner of the periodic table and lowest for atoms in the lower-left corner. Thus a bond between fluorine and carbon (C–F) shows distortion of the electron cloud associated with the bond, so that there is a higher probability of finding the electrons near the fluorine than near the carbon. Such a bond is said to be polarized, and the C–F bond is an example of a polarized covalent bond. The polarization gives the fluorine a partial negative charge (δ) and the carbon a partial positive charge (δ+).

A number of attempts have been made to set up quantitative tables of electronegativity that will indicate the direction and extent of electron-cloud distortion for a bond between any pair of atoms. The most popular of these scales, devised by Pauling, is based on bond energies (see Sec. 1.L) of diatomic molecules. It is rationalized that if the electron distribution were symmetrical in a molecule A–B, the bond energy would be the mean of the energies of A–A and B–B, since in these cases the cloud must be undistorted. If the actual bond energy of A–B is higher than this (and it usually is), it is the result of the partial charges (the charges attract each other and make a stronger bond, which requires more energy to break). It is necessary to assign a value to one element arbitrarily (F = 4.0). Then the electronegativity of another is obtained from the difference between the actual energy of A–B and the mean of A–A and B–B (this difference is called Δ) by the formula

equation

where xA and xB are the electronegativities of the known and unknown atoms and 23.06 is an arbitrary constant. Part of the scale derived from this treatment is shown in Table 1.1.26,27

Table 1.1 Electronegativities of Some Atoms on the Pauling26 and Sanderson27 Scales.

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Other treatments28 have led to scales that are based on different principles, for example, the average of the ionization potential and the electron affinity,29 the average one-electron energy of valence-shell electrons in ground-state free atoms,30 or the “compactness” of an atom's electron cloud.24 In some of these treatments, electronegativities can be calculated for different valence states, for different hybridizations (e.g., sp carbon atoms are more electronegative than sp2, which are still more electronegative than sp3),31 and even differently for primary, secondary, and tertiary carbon atoms. Also, electronegativity values can be calculated for groups rather than atoms (Table 1.2).32

Table 1.2 Some Group Electronegativities Relative to H = 2.17632

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Electronegativity information can be obtained from NMR spectra. In the absence of a magnetically anisotropic group33 the chemical shift of a img or a img nucleus is approximately proportional to the electron density around it, and hence to the electronegativity of the atom or group to which it is attached. The greater the electronegativity of the atom or group, the lower the electron density around the proton, and the further downfield the chemical shift [relative to tetramethylsilane (TMS) as zero ppm]. An example of the use of this correlation is found in the variation of chemical shift of the ring protons in the series toluene, ethylbenzene, isopropylbenzene, and tert-butylbenzene (there is a magnetically anisotropic group here, but its effect should be constant throughout the series). The electron density surrounding the ring protons decreases34 in the order given.35 However, this type of correlation is by no means perfect, since all the measurements are made in a powerful field, which itself may affect the electron density distribution. Coupling constants between the two protons of a system –CH–CH–X have also been found to depend on the electronegativity of X.36

When the difference in electronegativities is great, the electron density in an orbital may be effectively localized on only one nucleus. This is an ionic bond, which is seen to arise naturally out of the previous discussion. It is possible to view polarized covalent bonds as intermediates between ionic and covalent bonds. With this view, the extent of electron-cloud distortion is expressed as the percent ionic character of a bond. In this model, there is a continuous gradation from ionic to covalent bonds.