Electronegative oxygen rings

Due to the presence of two electronegative oxygen rings, the pyrazine and quinoxaline rings are poor substrates for electrophilic aromatic substrates. These nitrogen atoms do increase the heterocycles reactivity toward nucleophilic aromatic reaction. This reaction is usually done starting... 

Another seven-membered ring that shows some aromatic character is tropone (44). This molecule would have an aromatic sextet if the two C=0 electrons stayed away from the ring and resided near the electronegative oxygen atom. In fact, tropones are stable compounds, and tropolones (45) are found in nature. However, analyses of dipole moments, NMR spectra, and X-ray diffraction measurements show that tropones and tropolones display appreciable bond alternations. ... 

Hie phosphorus atom is in an approximately tetrahedral environment using four er bonds of approximately sp3 character. We should expect the more electronegative oxygen atoms to bond to s-poor orbitals on the phosphorus and the two oxygen atoms in the ring do attract hybridizations of about 20% s.40 The most electropositive substituent on the phosphorus is the bromine atom and Bent s rule would predict an s-rich... 

A In benzenesulfonic acid, strongly electronegative oxygen atoms are bonded to the atom attached to the aromatic ring. Hierefore electron density is withdrawn from the ring by both mesomeric and inductive effects. The canonical forms for the carbocaiion resulting from para attack by an electrophile are shown in Scheme 2.13, Application of the principles described in the chapter should enable... 

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