Diazonium Salts — Preparation & Reactions

What Diazonium Salts Are

Diazonium salts carry the general formula $\ce{Ar-N2^+ X^-}$, where $\ce{Ar}$ is an aryl group and $\ce{X^-}$ may be $\ce{Cl^-}$, $\ce{Br^-}$, $\ce{HSO4^-}$, $\ce{BF4^-}$ and so on. The $\ce{-N2^+}$ unit, written $\ce{-N#N^+}$, is called the diazonium group. Salts are named by adding "diazonium" to the parent hydrocarbon name, followed by the anion: $\ce{C6H5N2^+Cl^-}$ is benzenediazonium chloride and $\ce{C6H5N2^+HSO4^-}$ is benzenediazonium hydrogensulphate.

The distinction NEET tests repeatedly is between two families. Primary aliphatic amines form alkyldiazonium salts that are so unstable they cannot be isolated at all. Primary aromatic amines form arenediazonium salts that are stable for a short time in solution at low temperature (273–278 K). That single difference — resonance stabilisation by the ring — is the entire reason this subtopic exists.

Diazotisation: Making Benzenediazonium Chloride

Diazotisation is the conversion of a primary aromatic amine into a diazonium salt. Benzenediazonium chloride is prepared by treating aniline with nitrous acid at 273–278 K. Nitrous acid is unstable and cannot be stored, so it is generated in situ in the reaction mixture from sodium nitrite and hydrochloric acid. The overall equation is:

$$\ce{C6H5NH2 + NaNO2 + 2HCl ->[273-278\ K] C6H5N2^+Cl^- + NaCl + 2H2O}$$

Because the salt decomposes easily, it is not generally stored. It is used immediately after preparation, while still ice-cold. Both NCERT and NIOS stress this point — the salt is consumed in the next step rather than isolated as a dry solid.

Physical character of the salt

Benzenediazonium chloride is a colourless crystalline solid, readily soluble in water and stable in the cold, but it reacts with water when warmed. It decomposes easily in the dry state — a property that makes the dry solid hazardous. By contrast, benzenediazonium fluoroborate ($\ce{C6H5N2^+BF4^-}$) is water-insoluble and stable at room temperature, which is precisely why it can be isolated, dried and then decomposed in the Balz–Schiemann route to aryl fluorides.

Stability and Why Aliphatic Salts Fail

The stability of the arenediazonium ion is explained by resonance. The positive charge on the $\ce{-N2^+}$ group is not localised; it is delocalised into the benzene ring, distributing the charge over several atoms. A more dispersed charge means a more stable cation, which is why benzenediazonium chloride can survive in cold solution long enough to react usefully.

An alkyldiazonium ion has no aromatic ring to share its charge. The instant a primary aliphatic amine meets nitrous acid, the unstable salt forms and immediately loses nitrogen quantitatively, generating a carbocation that is trapped by water to give an alcohol. NEET frames this as a true/false comparison; the wording is worth memorising verbatim.

$$\ce{R-NH2 + HNO2 -> [R-N2^+] ->[H2O] R-OH + N2 ^ + HCl}$$

The quantitative evolution of nitrogen from aliphatic amines is not merely a curiosity. NCERT notes it is used in the estimation of amino acids and proteins, since each free $\ce{-NH2}$ liberates a measurable volume of $\ce{N2}$.

Reactions Displacing Nitrogen

The diazonium group is an excellent leaving group. Reactions of arenediazonium salts split into two categories: (A) displacement of nitrogen, where $\ce{-N2^+}$ is replaced by another group and $\ce{N2}$ escapes as gas, and (B) retention of the diazo group, the coupling reactions. We take displacement first.

Sandmeyer reaction (Cl, Br, CN)

The chloride, bromide and cyanide nucleophiles can be introduced into the ring in the presence of the corresponding cuprous (Cu(I)) salt. This is the Sandmeyer reaction.

$$\ce{C6H5N2^+Cl^- + CuCl ->[\Delta] C6H5Cl + N2}$$ $$\ce{C6H5N2^+Cl^- + CuBr ->[\Delta] C6H5Br + N2}$$ $$\ce{C6H5N2^+Cl^- + CuCN ->[\Delta] C6H5CN + N2}$$

Gattermann reaction (Cl, Br)

Alternatively, chlorine or bromine can be introduced by treating the diazonium solution with the corresponding halogen acid in the presence of copper powder. This is the Gattermann reaction. The yield in the Sandmeyer reaction is found to be better than in the Gattermann reaction.

$$\ce{C6H5N2^+Cl^- ->[Cu/HCl] C6H5Cl + N2}$$

Fluorine, Iodine, Hydroxyl and Hydrogen

Replacement by fluoride — Balz–Schiemann reaction

Fluorine resists direct introduction into the ring. When arenediazonium chloride is treated with fluoroboric acid ($\ce{HBF4}$), the arenediazonium fluoroborate precipitates. This solid, being stable and isolable, is filtered, dried and then heated, whereupon it decomposes to yield the aryl fluoride. This two-step sequence is the Balz–Schiemann reaction.

$$\ce{C6H5N2^+Cl^- + HBF4 -> C6H5N2^+BF4^- v + HCl}$$ $$\ce{C6H5N2^+BF4^- ->[\Delta] C6H5F + N2 + BF3}$$

Replacement by iodide

Iodine is not easily introduced into the benzene ring directly. But when the diazonium salt solution is simply treated with potassium iodide, iodobenzene is formed — no copper catalyst required. This is the cleanest way to put iodine on an aromatic ring.

$$\ce{C6H5N2^+Cl^- + KI -> C6H5I + KCl + N2}$$

Replacement by hydroxyl

If the temperature of the diazonium salt solution is allowed to rise up to 283 K, the salt is hydrolysed to phenol. This is the same decomposition that makes warming the salt dangerous in synthesis — here it is harnessed deliberately.

$$\ce{C6H5N2^+Cl^- + H2O ->[283\ K] C6H5OH + N2 + HCl}$$

Replacement by hydrogen

Mild reducing agents such as hypophosphorous acid (phosphinic acid, $\ce{H3PO2}$) or ethanol reduce diazonium salts to arenes, removing the diazonium group entirely. The reducing agents are themselves oxidised — $\ce{H3PO2}$ to phosphorous acid, and ethanol to ethanal.

$$\ce{C6H5N2^+Cl^- + H3PO2 + H2O -> C6H6 + N2 + H3PO3 + HCl}$$

Replacement by the nitro group

When diazonium fluoroborate is heated with aqueous sodium nitrite in the presence of copper, the diazonium group is replaced by a $\ce{-NO2}$ group, giving the nitroarene.

Retention of N₂: Azo Coupling and Dyes

In the second category the diazo group is retained. The benzenediazonium ion is a weak electrophile that attacks an electron-rich aromatic ring — a phenol or an aromatic amine — at the para position. The product is an azo compound in which the two rings are joined through the $\ce{-N=N-}$ bridge. This is an electrophilic substitution reaction, and the coupling almost always occurs para to the activating $\ce{-OH}$ or $\ce{-NH2}$ group.

With phenol, benzenediazonium chloride gives p-hydroxyazobenzene; with aniline it gives p-aminoazobenzene:

$$\ce{C6H5N2^+Cl^- + C6H5OH -> p\text{-}HO\text{-}C6H4\text{-}N=N\text{-}C6H5 + HCl}$$ $$\ce{C6H5N2^+Cl^- + C6H5NH2 -> p\text{-}H2N\text{-}C6H4\text{-}N=N\text{-}C6H5 + HCl}$$

The azo products possess an extended conjugated system spanning both rings through the $\ce{-N=N-}$ link. This conjugation absorbs visible light, so the compounds are typically coloured and are widely used as azo dyes.

Synthetic Importance

Diazonium salts are powerful intermediates precisely because they accomplish what direct substitution on benzene cannot. NCERT lists the decisive cases:

Aryl fluorides and iodides cannot be prepared by direct halogenation. The cyano group cannot be introduced by nucleophilic substitution of chlorine in chlorobenzene — yet cyanobenzene is easily obtained from a diazonium salt via the Sandmeyer reaction. The diazonium route therefore allows the introduction of $\ce{-F}$, $\ce{-Cl}$, $\ce{-Br}$, $\ce{-I}$, $\ce{-CN}$, $\ce{-OH}$ and $\ce{-NO2}$ into the ring, and the reductive removal of the diazo group ($\ce{-H}$) lets a chemist use the amino group as a temporary directing handle and then erase it.

This last trick underlies classic conversions such as aniline to 1,3,5-tribromobenzene: the $\ce{-NH2}$ group directs bromine to all three positions ortho/para to it, after which diazotisation and reduction with $\ce{H3PO2}$ remove the nitrogen, leaving the symmetric tribromobenzene that could never be made by brominating benzene directly.

The replacement of the diazo group by other groups is helpful in preparing those substituted aromatic compounds which cannot be prepared by direct substitution in benzene or substituted benzene. — NCERT Class XII, Section 9.10

Worked Conversions