Permanent Polarization of Single Bonds

Since it is possible for some of these factors to operate in opposition to each other, it is necessary to be able to evaluate, qualitatively at least, both the direction in which they operate and their relative magnitudes. 

The permanent polarization of single bonds, or induction-static (/ ), as it is often called, originates whenever there is a covalent bondTBetween atoms which are electrically dissimilar. In a covalent bond between two like atoms the shared electron pair is midway between the two nuclei, and the valence electrons of each atom completely neutralize the positive charge of each atomic kernel (Fig. 1). In a covalent bond between two atoms such as carbon and chlorine, however, chlorine has a greater affinity for electrons than carbon. 

Consequently tHe shared pair, together with the valence electrons of the carbon atom, is attracted towards chlorine, and the electrical centers of the valence electrons and the atomic kernels no longer coincide 

Electrical centers of the valence electrons and the atomic kernels coincide 

An atom or group is sometimes defined as elec-tronegative (—18) if the electron pair which it shares with a carbon atom is farther from the carbon atom to which it is connected than the shared pair of a carbon-hydrogen bond is from the carbon atom. Conversely, an atom or group is said to be electropositive (+/ ) if the shared electron pair is closer to the carbon atom with which it is connected than the shared electron pair of a carbon-hydrogen bond is to that carbon atom. The convention may be diagrammed as follows  

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In discussing the permanent polarization of single bonds we cannot help wondering how far along a carbon chain this factor exerts its influence. From an examination of the ionization constants of the aliphatic chloroacids we see (Table 2) that two methylene groups interposed between the carboxyl group and the carbon to which the chlorine atom is attached almost destroy the effect of the permanent polarization of the carbon-chlorine bond upon the carboxyl group. 

Whenever an atom with an unshared electron pair is connected to a carbon-carbon double bond, there will be a tendency for that pair to form a double bond with the carbon atom to which it is attached and for the original double bond to become abnormally polarized in the di ction away from that atom. This is undoubtedly one of the most difficult principles of resonance with which to become familiar because such an atom can at the same time attract electrons by a permanent polarization of the single bond. Consider, for example, the structure of vinyl chloride. There are two resonance structures of the double bond which must be considered (XI and XII). Experience has shown that the contribution of... 

The polarizability of a double bond is related to resonance in the same way that the polarizability of a single bond is related to permanent polarization. The carbonyl group, as we have seen, can be best represented as a resonance hybrid of the structures LXXIX and LXXX in... 

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