Charge separated resonance structure, bond

In Chapter 10 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for syntheses of heterocyclic compounds and for carbon-carbon bond formation. Table 6.2 lists some of the types of molecules that are capable of dipolar cycloaddition. These molecules, which are called 1,3-dipoles, have it electron systems that are isoelectronic with allyl or propargyl anions, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a 1,3-relationship, and it is this structural feature that leads to the name 1,3-dipolar cycloadditions for this class of reactions.136... 

Thieno[3,4-d]thiepin (58) superficially resembles the benzothiepins although X-ray structure analysis reveals that the thiepin ring is in fact planar (69JA3995). The bond length for the central C—C bond (1.46 A) is also consistent with the valence bond structure (59a). The stability of this system has been attributed disparately to contributions from the oppositely charged separated resonance structures (59a and 59b) (70JA1453,73JA2860). 

Draw charge-separated resonance structures to give a representation of the bonding in PF5 such that the octet rule is strictly obeyed. 

The reason for the stability of arenediazonium salts, relative to their alkane counterparts, is resonance and the high energy of the aryl cations formed by loss of nitrogen. One of the electron pairs making up the aromatic rr system can be delocalized into the functional group, which results in charge-separated resonance structures containing a double bond between... 

The H of RCHO does not stabilize the charge-separated resonance structure, so it contributes less to the hybrid. The C=0 has more double bond character, higher wavenumber... 

Structures B and C have charge separation and fewer bonds than A, making them less stable resonance structures and only minor contributors to the resonance hybrid. B and C both contain a double bond between the centrsil carbon atoms, however, so the hybrid must have a partial double bond there. This makes the c entral C—C bond shorter than a C-C single bond in an alkane. 

In valence bond terms the pyrazine ring may be represented as a resonance hybrid of a number of canonical structures (e.g. 1-4), with charge separated structures such as (3) contributing significantly, as evidenced by the polar character of the C=N bond in a number of reactions. The fusion of one or two benzene rings in quinoxaline (5) and phenazine (6) clearly increases the number of resonance structures which are available to these systems. 

The effect of substituents on colour in substituted anthraquinones may be rationalised using the valence-bond (resonance) approach, in the same way as has been presented previously for a series of azo dyes (see Chapter 2 for details). For the purpose of explaining the colour of the dyes, it is assumed that the ground electronic state of the dye most closely resembles the most stable resonance forms, the normal Kekule-type structures, and that the first excited state of the dye more closely resembles the less stable, charge-separated forms. Some relevant resonance forms for anthraquinones 52, 52c, 52d and 52f are illustrated in Figure 4.3. The ground state of the parent compound 52 is assumed to resemble closely structures such as I, while charge-separated forms, such as structure II, are assumed to make a major contribution to the first excited state. Structure II is clearly unstable due to the carbocationic centre. In the case of aminoanthraquinones 52c and 52d, donation of the lone pair from the... 

A good illustration of this concept is seen in a series of nitrophenols. The nitro group itself has to be drawn with charge separation to accommodate the electrons and our rules of bonding. However, resonance structures suggest that there is electron delocalization within the nitro group. 

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