Effect of substituents on reactivity

The effect of substituents on reactivity may be understood in terms of Fig. 31. The ground state profile (unbroken bold line) is the result of the mixing of DA, D+A- and 3D 3A configurations (107). Substituent effects on reactivity appear to be dominated by their effect on D+A. Substituents that stabilize D+A lower its energy along the entire reaction co-ordinate (broken line) and lead to a lower energy reaction profile (broken bold line). 

EFFECT OF SUBSTITUENTS ON REACTIVITY AND ORIENTATION IN ELECTROPHILIC AROMATIC SUBSTITUTION... 

Six-membered rings are considered before five-membered ones because they have been studied in greater detail and consequently their reactions are better understood. Because rate constants for quater-nization reactions have been correlated with values pertaining to the conjugate acids of heteroaromatic nucleophiles, substituent effects on acidities will be discussed prior to kinetic results. Acidity investigations suffer from fewer complications than N-alkylation and therefore provide results that offer considerable insight into the electronic effects of substituents on reactivity. Our review mentions only incidentally such related reactions, as oxidation - and acylation at an annular nitrogen atom. 

These rearrangements may involve formation of diradicals as discrete intermediates, or they may be concerted processes in which bond breaking and bond making occur simultaneously. Their insensitivity to nitric oxide and oxygen and the absence of dimeric products suggest that long-lived radicals are not intermediates. The effects of substituents on reactivity can be accounted for in terms of either a concerted or a diradical mechanism. However, the... 

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