Alternating polarity of atoms in valence bond structures

ALTERNATING POLARITY OF ATOMS IN VALENCE BOND STRUCTURES 

The bond energy of ionic states of individual bonds is strengthened if alternate positive and negative charges arise on adjacent atoms. As an example, we shall consider first, sulphamide S02(NH2)2 for which the valence bond structure 

In the case of benzene, resonance is generally considered as occurring between the two Kekul and the three Dewar structures, but other structures involving a separation of charge can be conceived e,g. 

Such structures do not produce any resultant polarity of the molecule since for each bond the two forms and G G+ contribute equally to the 

Other examples of stabilization in this way are given by benzene-sulphonic acid and nitrobenzene  

Alternating polarity may also occur in aliphatic molecules, but is generally present to a lesser extent than in the aromatic series. The resonance in the non-substituted hydrocarbons between the forms C+ C and C C+ will be negligible on the introduction of an atom e.g. chlorine, which forms a partially ionic bond with carbon, however, alternating polarity can be induced and resonance between the forms X and XI will occur. 

2 Debye, P. Polar Molecules New York, 1929 8 Kirkwood, J. G. Phys. Z 33 (193a) 259 

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