Reactions of Diazonium Salts Involving Displacement of Nitrogen

The diazonium group, $-\text{N}_2^+$, is an outstanding leaving group -- nitrogen

gas is an exceptionally stable, low-energy molecule to depart as -- so a very wide range

of nucleophiles and metal catalysts can displace it from the ring, installing a new

substituent at exactly the ring position the original amino group occupied.

Sandmeyer reaction. Treating the diazonium salt with a cuprous halide in the

corresponding hydrohalic acid replaces $-\text{N}_2^+$ with that halogen:

$\text{Ar}-\text{N}_2^+ + \text{CuCl}/\text{HCl} \to \text{Ar}-\text{Cl} + \text{N}_2$

(similarly $\text{CuBr}/\text{HBr}$ gives $\text{Ar}-\text{Br}$). The SAME transformation

carried out with plain copper powder (rather than a cuprous salt) in the corresponding

acid is called the Gattermann reaction -- a cheaper variant of essentially the same

copper-catalysed radical mechanism.

Iodide. Aryl iodides are obtained even more simply, by treating the diazonium salt

with aqueous potassium iodide directly -- no copper catalyst is required at all,

because iodide is nucleophilic enough on its own: $\text{Ar}-\text{N}_2^+ + \text{KI} \to \text{Ar}-\text{I} + \text{N}_2 + \text{KX}$.

Fluoride: the Balz-Schiemann reaction. The diazonium salt is first converted to its

tetrafluoroborate salt, $\text{Ar}-\text{N}_2^+\ \text{BF}_4^-$, which is then isolated,

dried, and carefully HEATED (thermal decomposition, not a solution reaction) to give the

aryl fluoride directly: $\text{Ar}-\text{N}_2^+\ \text{BF}_4^- \xrightarrow{\Delta} \text{Ar}-\text{F} + \text{N}_2 + \text{BF}_3$. This is the standard laboratory route to

an aryl fluoride, since fluorine cannot be introduced by direct electrophilic aromatic

substitution at all.

Cyanide. Treating the diazonium salt with $\text{CuCN}$ (a Sandmeyer-type reaction)

replaces $-\text{N}_2^+$ with $-\text{CN}$, giving an aromatic NITRILE

($\text{Ar}-\text{CN}$) -- a valuable route to an aromatic carbonitrile that direct

substitution chemistry cannot achieve.

Hydroxyl. Simply boiling the diazonium salt with water (no catalyst) hydrolyses it

to phenol: $\text{Ar}-\text{N}_2^+ + \text{H}_2\text{O} \xrightarrow{\Delta} \text{Ar}- \text{OH} + \text{N}_2 + \text{H}^+$ -- this is also, as noted in Section 25.14, exactly

what happens if a diazonium solution is simply allowed to warm above its stable range.

Deamination. Treating the diazonium salt with hypophosphorous acid,

$\text{H}_3\text{PO}_2$, and water replaces $-\text{N}_2^+$ with plain hydrogen: …