Diazo coupling electron density

The difference in position of attack on primary and secondary aromatic amines, compared with phenols, probably reflects the relative electron-density of the various positions in the former compounds exerting the controlling influence for, in contrast to a number of other aromatic electrophilic substitution reactions, diazo coupling is sensitive to relatively small differences in electron density (reflecting the rather low ability as an electrophile of PhN2 ). Similar differences in electron-density do of course occur in phenols but here control over the position of attack is exerted more by the relative strengths of the bonds formed in the two products in the two alternative coupled products derivable from amines, this latter difference is much less marked. 

A great deal of evidence has accumulated to show that a metal ion in a chelate ring can alter the electron density in a molecule. One method of obtaining quantitative data on such changes in electron density is to compare the rates of electrophilic attack at a point in the unchelated and chelated molecules as, for example, in the iodination of 8-hydroxyquinoline chelates (Section II,A). Another such kinetic study has been carried out on the diazo coupling of 8-hydroxyquinoline-5-sulfonic acid and its zinc(II) chelate with diazotized sulfanilic acid (141)-... 

Amino-l,2,4-thiadiazoles are diazotized at — 10°C in phosphoric acid, and couple with naphthol to give moderate yields of diazo dyes.182,388 In their limited stability, the 3-diazonium salts resemble those of 3-aminoiso-thiazole, but differ from the more stable 1,2,4-thiadiazole- and isothiazole-5-diazonium salts a consideration of the 7t-electron densities leads to the opposite conclusion.182... 

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