Double atoms 421 bond conjugated layer

Inspection of empirical formulas and the corresponding structures of starting material and product reveals that III is solely a dimer of I. Two important facts, however, conspire against this outer-layer simplicity. First, if the deep-blue compound I is allowed to dimerize by itself (and this in fact occurs in solution), the colorless dimeric structure IV (but not III) results. Second, there is an enamine in the medium, which obviously makes the difference. The role of this enamine should not be taken lightly, particularly if the conjugation of the sulfur atoms in I imbues this compound with a behavior reminiscent of oxygen homolog systems, that is, quinonoids. The exocyclic double bond is amenable to a Michael-type 1,4 addition by suitable nucleophiles while the thioketone sulfur is potentially a nucleophilic center (see Scheme 28.1). 

In what are called conjugate stems the concentrations of electrons, normally shown as double bonds, are delocalized (i.e. more diffused). By conjugate system is meant a molecule (or portion thereof) in which every second bond would conventionally be written as a double bond (examples benzene, butadiene, pyridine). The new electronic equilibrium is illustrated by benzene of which six of the valency electrons (one from each carbon atom) unite in forming the 7r-layer which completely covers both faces of the molecule. This condition is maintained by resonance. However, resonance can exist without conjugation, as in amides, which have a structure intermediate between 8.1) and 8.2), but not otherwise representable. The double-headed arrow is the accepted sign for resonance. Note that in the canonical forms [such as 8.1) and 8.2)] which make up the resonance hybrid, no atom ever shifts its position only electrons and charges are mobile. 

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