Benzenoid compounds, examples

For benzenoid compounds, structures can be established by the historically important substitution method (Section 1-1F) or with the aid of correlations between spectroscopic properties and positions of substitution, as we indicated in Section 22-3. Also, it is often possible to identify the isomers by converting them to compounds of known structure. For example, trifluoromethylbenzene on nitration gives only one product, which has been shown to be the 3-nitro derivative by conversion to the known 3-nitrobenzoic acid by concentrated sulfuric acid ... 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

Another example of bromotropone contraction is derived from Nozoe s benzotropazine chemistry [Scheme 146 89H(29)1459]. Condensation of tropone 552 with further o-aminothiophenol (117b) gave benzenoid compound 554 instead of the expected tris(benzazino)tropylidene. Presumably, this condensation takes place via initial cyclization (to benzothiazine), then cyclocondensation (to spirobenzothiazoline), followed by tautomerization (to norcaradiene 553), ring contraction, and dehydrogenation. 

Photodimerization is common for anthracenes, less so for naphthalenes, and until this year only one example of this reaction with a benzenoid compound had appeared. Thus irradiation of... 

The simple unsubstituted heterocyclic systems show a wide range of electronic absorption, from the simple 200 nm band of furan, for example, to the 340 nm maximum shown by pyridazine. As is true for benzenoid compounds, the presence of substituents that can conjugate causes profound changes in electronic absorption, but the many variations possible are outside the scope of this section. 

Parallel with these thermodynamic data, Norman and Smith have shown that OH-, and not H02-, is largely responsible for hydroxylation of benzenoid compounds, and olefinic bonds exhibit a reactive preference for hydroxyl entities. Provided that there is no further participation between the radicals and anions Qfor example, that shown in equation (46)] or... 

The examples given in the table are based on chemical oxidation. By contrast, anodic oxidation of iodine in trimethyl orthoformate provides the iodonium ion which is effective for the iodination of appropriate substituted benzenoid compounds (ref.37) and has been considered superior to the iodonium ion developed in acetonitrile solution. Thus the additbn of the anolyte (4 moles) from trimethyl orthoformate (TMOF), elemental iodine and lithium perchlorate trihydrate, by anodic oxidation at ambient temperature in a divided electrolytic cell equipped with a ceramic diaphragm and platinum electrodes, to anisole (1 mole) in TMOF afforded iodoanisoles in 69% yield, in the proportions, 4-iodomethoxybenzene (87%), and the 2-iodo isomer (13%) with none of the 3-isomer. 

Structures determined by modern X-ray techniques show that significant nonplanarities exist in benzenoid compounds of this type. This finding infers that nonplanar polycyclic aromatic compounds are quite common. Structures 7-9, which have twist angles ranging from 30° to 42°, are examples... 

Photoaddition of thiochromone 1,1-dioxide in the presence of benzene, for example, gave a 2 1 adduct (180) in 32% yield. A correlation was observed between the behaviour of the benzenoid compounds and their ionization potentials.5-Aryl esters of thiophen-2-thiolocarboxylic acids undergo a phytochemical transformation into a mixture of products, including the thienobenzothiopyranones (Scheme 4)/ ... 

Classes of chemical compounds having different function groups and odor descriptors, some of which are useful to the flavor or perfume industries were selected for this initial study. For example, alcohols, aldehydes, pyrazines and various benzenoid compounds which have been isolated in the volatiles of cooked meat as reviewed by Hornstein (22) were studied. For each class of chemical compounds literature threshold values obtained only from one laboratory were used in order to prevent errors associated with technique or methodology between laboratories that occur for threshold determinations as discussed by Guadagni al. ) and Powers and Ware (23). 

Benzenoid compounds can also react by addition with chlorine atoms, for example, the irradiation of benzene and chlorine gives a mixture of stereoisomeric hexachlorocyclohexanes (Scheme 4.60). 

The term aromatic was first applied in the early part of the nineteenth century to compounds having an aromatic odour, which had been isolated from many volatile vegetable oils, for example benz-aldehyde (from oil of bitter almonds), methyl salicylate (from oil of wintergreen), anethole (from aniseed) and many others. Around 1800, substances designated as aromatic also included non-benzenoid compounds, largely consisting of terpenoid natural oils. (For an... 

Benzotropolones take part in electrophilic substitution reactions, for example halogenation [293,328,331] and coupling with aryldiazonium salts [332]. They may undergo rearrangement to benzenoid compounds in some instances, for example treatment of 3,4-benzotropolone with excess bromine gives 2,3-dibromonaphtho-quinone and with excess concentrated nitric acid leads to 2,4-di-nitro-l-naphthol [333]. Like other tropolones, 3,4-benzotropolone is photochemically converted into a bicycloheptadienone [334]... 

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