Stable benzenoid system

In general, electron-releasing groups (e.g. —NH2, —OH) diminish or prevent covalent hydration by decreasing the electron deficiency in the nucleus. This diminution becomes ineffective if a new kind of stabilizing resonance is facilitated by the substituent, e.g. the urea-type resonance and the 4-aminopyridine-type resonance in 2- and 6-hydroxypteridine, respectively. The reluctance of the anions of these substances to form hydrates is attributed to the stable benzenoid system, e.g. 42, in the anhydrous anion compared with the predominantly lactam form of the neutral species, e.g. 43. 

Sol 4. (i) The first step involves a comotatory 67u-electron cyclization of I to give an intermediate II, which undergoes [1,9] hydrogen shift to give a stable benzenoid system III. This is followed by a [1,3] hydrogen shift and opening of an oxazoline ring to form a stable naphthalene system V. 

It is unclear how such a profound anpirical prediction, which characterizes the most stable benzenoid systems, could have been overlooked or ignored for such a long time by so many theoretical chanists. Clar s approach to novel representation of benzenoid systems may have appeared to some as a phantasm, but Clar, as we have already mentioned, has reported on a number of experimental results that support his... 

A stable ferrocene-TCNE adduct has been prepared and its structure, closely analogous to charge-transfer complexes of benzenoid systems has been determined to be that shown as 4.20 Whether a charge-transfer species involving the iron atom rather than the ring is possible has not been deter-... 

The generalization also holds for benzazepines. In fact, the only stable 2iT-2-benzazepines synthesized so far are those in which the 5,6-carbon bond forms part of a fused ring system (ie.g. 63). 1,5-H shift to produce the more stable 3//-2-benzazepine is suppressed as it involves disruption of the benzenoid system (see Section 5.16.3.6). 

Finding upper and lower bounds for the K-value of benzenoid systems is intimately related to the identification of benzenoids with extremal (minimum and maximum) number of Kekule structures. Clearly, such benzenoids are expected to possess unusual chemical properties (e.g. to be highly reactive or exceptionally stable). 

Use of synonyms to cover plagiarism has been known, particularly in social sciences [22]. In mathematics and natural sciences, use of synonyms is not so infrequent as it may offer alternative labels that may have distinct descriptive features. Thus, for example, Clar referred to benzenoid systems having only k electron aromatic sextets and empty rings as the fully benzenoid systems [23]. Polansky and Gutman have referred to the same as the all benzenoid systems [24], and Dias refers to these most stable benzenoids as the total resonant sextet benzenoids [25]. There are no problems with synonyms in chemistry/or informed persons, but uninformed chemists, as was the anonymous referee of our paper on ring currents, can be confused, which is, of course, their problem ... 

In case of trans-isomer conrotatory ring opening is symmetry allowed under thermal conditions giving rise to stable benzene. For this reason derivatives of trans-isomer of Dewar-benzene is not known even if they spontaneously undergo conrotatory ring-opening to give benzenoid system of all cis-double bond. 

Clar s Structure Rules Benzenoid having a single Clar structure with only aromatic Jt-sextets and empty rings, the fully benzenoid systems described by Clar, are the most stable benzenoids having several Clar structures are the next most stable and benzenoids having a single Clar structure with rings with isolated CC double bonds are the least stable. 

The reactivity of the amino groups at the pteridine nucleus depends very much upon their position. All amino groups form part of amidine or guanidine systems and therefore do not behave like benzenoid amino functions which can usually be diazotized. The 4-, 6-and 7-amino groups are in general subject to hydrolysis by acid and alkali, whereas the 2-amino group is more stable under these conditions but is often more susceptible to removal by nitrous acid. 

The mechanism for these electrophilic substitution reactions involves formation of a dication intermediate (13) which, as in the case of benzenoid substitution reactions, loses a proton and reverts to the original stable system. 

As expected, annulation to one or two benzene rings increases the overall stability of the azepine system (Section 5.16.1.2). Even so, some systems, e.g. (4) and (9), are too unstable as the NH tautomers to permit isolation, and as yet are known only as the oxo or hydro derivatives or, as in the case of the dibenzazepine (9), as the more stable fully benzenoid 5//-tautomer. 

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