Reactivity of Six-membered Rings

Nucleophilic attack at ring carbon occurs in benzenes only when electron-withdrawing substituents are present. Even with pyridine, only the strongest nucleophiles react. This is because the formation of the initial adduct (2) involves de-aromatization of the pyridine ring and, once formed, many such adducts tend to re-aromatize by dissociation (1 2). Benzo fusion decreases the loss in aromaticity for the formation of the adduct and thus quinoline (3) and especially acridine (4) react more readily with nucleophiles. 

Reaction with nucleophiles by deprotonation at a CH bond occurs in pyridine much more readily than in benzene. The reactivity order is y 3 a rather than y, a (3 because of lone pair lone pair repulsion in the a-deprotonated species (c/. discussion in Section 3.2.1.8). 

Extrapolation from benzene through pyridine to the diazines and then to the triazines and tetrazines delineates the main trends of azine chemistry. 

4 Pyridones, N-oxides and related compounds betainoid rings 

Three types of compound can be considered (see Section 2.2.1.2) to be derived from a cationic ring carrying a negatively charged substituent (7)  

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