Azo‐Coupling Reactions Used in Analytical Chemistry: The Role of Reactants,Intermediates, and Aqueous Medium

A quantum‐chemical study of the analytically important azo‐coupling reactions of naphthalen‐1‐amine ( 1 ) with naphthalene‐1‐diazonium ( 2 ) and 4‐sulphobenzenediazonium ( 8 ) cations has been carried out. The reactions have been found to be frontier‐controlled, and their regioselectivity is unequivocally determined by the thermodynamics of the intermediate σ‐complexes, as well as by the aqueous medium. The latter defines high positional selectivity, providing the decision between two possible reaction routes predicted on the basis of quantum‐chemical computations for separate molecular systems. As a result, azo coupling occurs at the 4‐ rather than at the 2‐position of 1 . Furthermore, the aqueous medium makes the selection of analytical forms – protonated azo‐coupling products with a quinonehydrazone‐type structure.