Aromatic Substitution by Diazonium Ions

Among the reagents that are classified as weak electrophiles, the best studied are the aryl diazonium ions. These reagents react only with aromatic substrates having strong ERG substituents, and the products are azo compounds. The aryl diazonium 

Aryl diazonium ions are stable in solution only near or below room temperature, and this also limits the range of compounds that can be successfully substimted by diazonium ions. 

Kinetic investigations revealed second-order kinetic behavior for substimtion by diazonium ions in a number of instances. In the case of phenols, it is the conjugate base that undergoes substitution.This finding is entirely reasonable, since the depro-tonated oxy group is a better electron donor than the neutral hydroxy substituent. The reactivity of the diazonium ion depends on the substituent groups that are present. Reactivity is increased by EWG and decreased by ERG.  

Because of the limited range of aromatic compounds that react with diazonium ions, selectivity data comparable to those discussed for other electrophilic substitutions are not available. Diazotization, since it involves a weak electrophile, would be expected to reveal high substrate and position selectivity. 

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