Can be azo dyes obtained by grinding under solvent-free conditions?
The solid-solid reactions of some electron-donors with sulfanilic acid in the presence of solid sodium nitrite afford azo dyes by self-catalyzed diazotization of sulfanilic acid (2) under solvent-free conditions with moderate yields. Also the reactions of some electron-donors with diazotization of o-nitroaniline (5), m-nitroaniline (6) and p-nitroaniline (7) in the presence of solid sodium nitrite catalyzed by p-toluenesulfonic acid (PTSA) afford azo dyes under solvent-free conditions in good yields. This new method totally avoids the use of acids, alkalies, and toxic and/or expensive solvents in diazotization and diazo coupling reactions.
