Diazonium Group Substitution with Halogens and Cyanide: Sandmeyer and Schiemann Reactions

The Sandmeyer reaction treats arenediazonium salts with copper(I) salts of halides or cyanide to form aryl halides and aryl nitriles. The reaction proceeds via an aryl copper intermediate. Aryl nitriles produced can be further hydrolyzed to yield carboxylic acids, making this reaction valuable for synthesizing substituted benzenes from primary arylamines.

Fluorine reacts too violently with arenediazonium salts in Sandmeyer conditions, while iodine reacts too slowly. Instead, aryl fluorides are prepared via Schiemann reactions using fluoroboric acid, and aryl iodides are formed by treating arenediazonium salts with potassium iodide, which proceeds readily.

The Schiemann reaction converts arenediazonium salts to aryl fluorides using fluoroboric acid. The reaction generates diazonium fluoroborate as a precipitated salt, which is isolated, dried, and heated until decomposition occurs, yielding the corresponding aryl fluoride product. This method enables direct fluorination of aromatic rings.

Aryl nitriles produced from Sandmeyer reactions undergo facile hydrolysis to yield carboxylic acids. This conversion is very straightforward and efficient, making the Sandmeyer reaction a crucial step in converting aryl amines to substituted benzonitrile intermediates that can be further functionalized into carboxylic acids.

The sequence begins with nitration of benzene, followed by reduction to yield amino benzenes. These primary arylamines undergo diazotization to form arenediazonium salts, which then undergo substitution reactions like Sandmeyer or Schiemann reactions to install various functional groups on the aromatic ring.

Arenediazonium salts can be substituted with halides (chloride, bromide, fluoride, iodide), hydroxyl groups, nitriles, and other functional groups. The choice of reagent and reaction conditions determines which group is installed, enabling diverse aromatic substitution patterns from a common diazonium intermediate.

Although the Sandmeyer reaction mechanism is complex, it is believed to progress through an aryl copper intermediate. Copper(I) salts facilitate the substitution of the diazonium group, enabling formation of aryl halides and nitriles with good efficiency and selectivity from arenediazonium precursors.