Naphthalene canonical structures

With naphthalene the calculation is very much more involved. The number of canonical structures is here forty-two (Fig. 2), so that the... 

A possibly more reliable prediction can be made on the basis of Sherman s wave function for naphthalene,16 by considering all 42 canonical structures. The fractional double bond character of a bond can be considered to be given approximately (neglecting non-orthogonality of the canonical wave functions) by the expression... 

Comparison of the bond lengths with those of benzene, naphthalene and pyridine suggests that whilst there is certainly aromatic character some bond fixation occurs in the naphthyridines. It is probable that canonical structure (1) (of, for example, 1,5-naphthyridine) and to a lesser extent structures (7) and (8) are the major contributors to the ground state, but the charged structures (9) and (10) must also be included in order to account for the chemical properties of the naphthyridines. 

Figure 10. Schematic illustration of a tendency of each benzene fragment in naphthalene to retain its aromaticity by producing cis 1,3-butadiene partial localization in its twin-ring as described by the resonance structures (7a) and (7b) yielding the resulting predominant canonical structure (7c). This intuitive argument is supported by the (HF/6-31G ) bond distances and the corresponding 7r-bond orders given within parentheses.

Aromatic hydrocarbons such as naphthalene (19) also undergo electrophilic substitution, although now not all ring positions of the parent hydrocarbon are equivalent. Nitration occurs almost exclusively in the 1- or a-position of naphthalene. Consideration of the contributing structures to the hybrid carbocation indicates why this is so. For a-attack, the canonical structures include 20, 21 and 22. Whereas in 20 and 21 the stable aromatic sextet is preserved, in 22 the aromaticity is disrupted. However, for attack at the 2- or j3-position, only one structure, 23, can... 

THE STRUCTURE OF MOLECULES Table XXI. Canonical Structure of Naphthalene... 

Treating naphthalene comparably reveals three canonical structures, 3, 4, and 5. Note the standard use of a double-headed arrow to interelate resonance contributors. This must never be confused with the use of opposing straight fish-hook arrows which are used to designate an equilibrium between two species resonance contributors have no separate existence they are not in equilibrium one with the other. 

When an aromatic compound adds MA, generally, the resonance stabilization of such a system is destroyed. When loss of resonance energies are significant, high activation energies are anticipated for these reactions. Thus, simple aromatic compounds tend to be less reactive as dienes. In the series benzene, naphthalene, and anthracene, examination of canonical structure suggests that the middle ring of anthracene has the most dienelike character. The normal order of reactivity observed, therefore, is benzene < naphthalene < anthracene. 

In contrast to benzene, the bond lengths in naphthalene are not all equal, as illustrated in 4. The resonance energy of naphthalene is 255 kJ mol", which is higher than, though not twice that of, benzene (151 kJ mol" ). In the canonical forms 5 and 7 that contribute to the valence bond structure for naphthalene, only one of the two rings is fully benzenoid. Naphthalene is less aromatic than benzene, which accounts for its higher reactivity towards electrophilic attack compared with benzene. 

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