Fused benzene rings unsubstituted

These substitution reactions probably all occur on the conjugate acid species as supported by the fact that the corresponding cations, such as A-methylquinolinium, undergo nitration at approximately the same rate. 

The relative reactivity of different positions toward electrophilic substitution is conveniently studied by acid-catalyzed deuterium exchange reaction rates can be followed by NMR and introduction of deuterium hardly affects the reactivity of the remaining positions. In D2S04 both quinoline and quinoline 1-oxide react as the conjugate acid at positions 8 5, 6 7 3. 

If the hetero ring is in the form of a pyridone, pyrone or A-oxide, or contains a strongly electron-donating substituent (OR or NR2), electrophilic substitution into the hetero ring can compete with substitution into a fused benzene ring. In some such compounds, substitution occurs entirely in the 

In other compounds, reaction can occur in both rings. Under such circumstances the orientation can depend on the conditions frequently reaction in the benzene ring involves the cationic species, whereas that in the pyridine ring involves the free base. Thus, the temperature dependent nitration of quinoline 1-oxide (578) reflects the decrease in intrinsic acidity as the temperature rises, which in turn increases the available amount of free base species. 

Coumarins undergo nitration readily in the 6-position while bromination results in substitution at the 3-position as a consequence of addition-elimination. 

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Unsubstituted pyrazoles and indazoles are cleaved by strong bases. Indazoles undergo ring opening more readily than pyrazoles since the fused benzene ring enhances the ease of removal of the C-3 proton in the initiating step. 

The 7r-electron excessive character of pyrrole and indole renders both systems extremely susceptible to electrophilic attack and the fused benzene rings of carbazole also undergo electrophilic substitution more readily than does an unsubstituted benzene ring. In contrast, the 2/7-isoindole system only survives intact during electrophilic substitution reactions under the mildest of conditions and the system is more susceptible to [ 4 + 2] cycloaddition reactions than is pyrrole. 1,1-Disubstituted IH-isoindoles generally undergo nucleophilic addition-elimination reactions across the 2,3-bond or yield products derived from an initial electrophilic attack at the 2-position. 

In fused ring systems, the positions are not equivalent and there is usually a preferred orientation even in the unsubstituted hydrocarbon. The preferred positions may often by predicted as for benzene rings. Thus it is possible to draw more canonical forms for the arenium ion when naphthalene is attacked at the a position than when it is attacked at the p position, and the a position is the preferred site of attack,though, as previously mentioned (p. 682), the isomer formed by substitution at the p position is thermodynamically more stable and is the product if the reaction is reversible and equilibrium is reached. Because of the more extensive delocalization of charges in the corresponding arenium ions, naphthalene is more reactive than benzene and substitution is faster at both positions. Similarly, anthracene, phenanthrene, and other fused polycyclic aromatic hydrocarbons are also substituted faster than benzene. 

Quinazolin-4(3//)-one and derivatives 17 with substituents at the benzene ring and/or the 2-position are conveniently prepared in high yield by fusing anthranilic acid derivatives 16 with amides or thioamides. The synthesis was first reported by von Niementowski in 1895 and is best suited for the preparation of 2-unsubstituted quinazolin-4(3i/)-ones by the fusion of anthranilic acid derivatives with formamide. Homologous acylamides require higher tempera-... 

Benzimidazole is a fused aromatic imidazole ring system where a benzene ring is fused to the 4 and 5 positions of an imidazole ring. Benzimidazoles are also known as benziminazoles and 1,3-benzodiazoles [1,2]. They possess both acidic and basic characteristics. The NH group present in benzimidazoles is relatively strongly acidic and also weakly basic. Another characteristic of benzimidazoles is that they have the capacity to form salts. Benzimidazoles with unsubstituted NH groups exhibit fast prototropic tau-tomerism, which leads to equihbrium mixtures of asymmetrically substituted compounds [1]. 

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