Kinetics and mechanisms of aryldiazonium Ions in aqueous solutions

Abstract

In aqueous acid solution and in mixed alcohol-water solvents ([H3O+ ] > 10–2 M), in the dark and in the absence of reductants, the spontaneous decomposition of aryldiazonium, ArN2 + , salts proceeds through borderline SN1 (DN + AN) -SN2 mechanisms. The rate constant values depend strongly on the nature of the substituents attached to the aromatic ring of ArN2 + and, for those with electron-withdrawing substituents, on solution composition. The product distribution is proportional to the composition of the solvation shell of the ipso carbon, which reflects the composition of the water/cosolvent mixture. However, upon decreasing moderately the acidity, reactions involving the formation of diazohydroxides, ArN2OH, diazoethers, ArN2OR, and diazoates, ArN2O− , become competitive and may even be the main decomposition pathway. The stability of ArN2OH, ArN2OR, and ArN2O− species (which may coexist with ArN2 + in solution) is intimately related to the Z-E (syn-anti, cis-trans) isomerization of the O-adducts, so that they may undergo further reactions when they are components of a Lewis acid-base equilibrium, or undergo homolytic scission to produce homolytic reduction products. In this book chapter, we aim to provide the reader with a practical and (hopefully) useful view of the complex chemistry of ArN2 + in aqueous and mixed alcohol-water solutions, mainly covering the kinetics and mechanisms of the reactions. In a last section, we introduce some analytical methods for the determination of diazonium salts and their degradation products.