Preparation and Physical Properties of Diazonium Salts

Diazotization is the reaction by which an AROMATIC primary amine is converted

into an arenediazonium salt, using sodium nitrite and a mineral acid (usually

$\text{HCl}$) generated as nitrous acid in situ, held at the tightly controlled

temperature of $0$–$5^\circ\text{C}$:

The low temperature is not incidental -- it is essential, because the diazonium salt,

while far more stable than its aliphatic counterpart (Section 25.9), is still a fairly

reactive species that decomposes readily if warmed even modestly, releasing nitrogen gas

and giving phenol as the major product (via hydrolysis of the diazonium ion by water,

Section 25.15). Diazotization is therefore always carried out with the reaction flask

cooled in ice, and the diazonium salt produced is used immediately, in situ, for the next

step rather than stored.

Why aromatic diazonium salts are relatively stable while aliphatic ones are not. The

positive charge on the terminal nitrogen of $\text{Ar}-\text{N}_2^+$ can be delocalised

by resonance into the attached aromatic ring -- an option an aliphatic diazonium ion,

with no adjacent $\pi$-system to delocalise into, simply does not have. This resonance

stabilisation is large enough to let the aromatic salt persist as a genuine, if

temperature-sensitive, species in cold solution.

Physical properties. Arenediazonium salts are colourless (or pale yellow) crystalline

solids, freely soluble in cold water, and reasonably stable as a COLD AQUEOUS solution --

but they are markedly UNSTABLE in the dry solid state (some, particularly the perchlorate …