Diazonium salt formation
Diazotization of aniline
How the Diazotization of aniline works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: NaNO₂, HCl (HONO in situ). Conditions: H₂O, 0–5 °C (ice bath). Bonds formed: N–N bond; N≡N⁺; bonds broken: N–H (x2), O–H; C–N retained. Stereochemistry: None. Rate and selectivity: Keep cold: diazonium salts decompose above ~5 °C.
At a glance
Step 1: Nitrosonium ion from HONO
Nitrous acid (generated in situ from NaNO₂ and HCl) is protonated and loses water, generating a nitrosonium ion (NO⁺) — a small, reactive electrophile.
Step 2: N-nitrosation of aniline
Aniline's nucleophilic nitrogen lone pair attacks this nitrosonium ion, forming an N-nitrosamine, which tautomerizes and loses water in a sequence of proton transfers to build up the N=N triple-bond-adjacent framework.
Step 3: Aryl diazonium salt
The result is an aryl diazonium salt, stable only at low temperature — this is the very first step that has to happen before any of the diazonium chemistry covered elsewhere in this library (Sandmeyer, Balz-Schiemann, azo coupling) can proceed.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
How it compares with related reactions
| Diazotization of aniline | Balz-Schiemann reaction | Carbodiimide-mediated amide (peptide) coupling | |
|---|---|---|---|
| Reagents | NaNO₂, HCl (HONO in situ) | ArNH₂ → ArN₂⁺ (NaNO₂, HCl); then HBF₄ or NaBF₄; heat | R–CO₂H, R′–NH₂, DCC (dicyclohexylcarbodiimide) |
| Conditions | H₂O, 0–5 °C (ice bath) | 0–5 °C for salt; dry salt pyrolysed at 100–200 °C (or in PhCl/hexane) | CH₂Cl₂ or DMF, 0 °C → 25 °C (often + HOBt/DMAP) |
| Stereochemistry | None | Not relevant | Racemisation of α-stereocentres possible via oxazolone (HOBt suppresses) |
| Rate / selectivity | Keep cold: diazonium salts decompose above ~5 °C | N₂ loss (aryl cation formation) is rate-limiting; salt must be dry | O-Acylisourea is short-lived; DCU precipitates and drives the reaction |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Diazotization of aniline?
NaNO₂, HCl (HONO in situ). Conditions: H₂O, 0–5 °C (ice bath).
Which bonds are formed and which are broken?
Formed: N–N bond; N≡N⁺. Broken: N–H (x2), O–H; C–N retained.
What is the stereochemical outcome?
None
What controls the rate or the selectivity?
Keep cold: diazonium salts decompose above ~5 °C
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go