Nucleophilic aromatic substitution — heterocycle
Chichibabin reaction: amination of pyridine
How the Chichibabin reaction 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: NaNH₂ (sodium amide). Conditions: Toluene or xylene, 100–130 °C (or liquid NH₃, KMnO₄). Bonds formed: C–N; H₂; bonds broken: C–H (as H⁻); N–H. Stereochemistry: None (aromatic product); 2-selective. Rate and selectivity: Slow addition; H⁻ expulsion drives off H₂ gas.
At a glance
Step 1: Amide attacks C2
Sodium amide's nucleophilic nitrogen attacks the carbon adjacent to pyridine's ring nitrogen — that position is activated toward nucleophilic attack the same way nitro groups activate SNAr on benzene rings.
Step 2: σ-adduct (Meisenheimer-type)
This forms an anionic, resonance-stabilized intermediate where the negative charge is delocalized onto pyridine's electronegative ring nitrogen — the heterocyclic equivalent of a Meisenheimer complex.
Step 3: Hydride loss gives H₂
Loss of hydride (as H₂ gas, with the help of another equivalent of NaNH₂) restores aromaticity, giving 2-aminopyridine — a route to functionalize electron-poor heterocycles that resist ordinary EAS.
Abbreviations and terms
- NaNH2
- Sodium amide. Very strong base (pKa of NH₃ ≈ 38); makes acetylides and arynes.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- KMnO4
- Potassium permanganate. Strong oxidant: diols (cold, basic) or cleavage (hot).
- SNAr
- Nucleophilic aromatic substitution. Addition–elimination at an electron-poor arene via a Meisenheimer complex.
- EAS
- Electrophilic aromatic substitution. An electrophile replaces H on an arene via a Wheland intermediate.
- Meisenheimer
- Meisenheimer complex. The anionic cyclohexadienyl intermediate of SNAr stabilised by electron-withdrawing groups.
How it compares with related reactions
| Chichibabin reaction | SNAr on 1-fluoro-2,4-dinitrobenzene | Bromination of benzene (FeBr₃-catalyzed) | |
|---|---|---|---|
| Reagents | NaNH₂ (sodium amide) | NaOH (aq); 1-fluoro-2,4-dinitrobenzene (Sanger-type substrate) | Br₂ with FeBr₃ (Lewis-acid catalyst) |
| Conditions | Toluene or xylene, 100–130 °C (or liquid NH₃, KMnO₄) | H₂O/EtOH or DMSO, 25–50 °C | Anhydrous, 25 °C, dark |
| Stereochemistry | None (aromatic product); 2-selective | None (planar aromatic) | None (planar, achiral) |
| Rate / selectivity | Slow addition; H⁻ expulsion drives off H₂ gas | Addition is rate-limiting; F ≫ Cl, Br, I; ortho/para EWGs accelerate | Slow step = arenium formation; deactivated rings react poorly |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Chichibabin reaction?
NaNH₂ (sodium amide). Conditions: Toluene or xylene, 100–130 °C (or liquid NH₃, KMnO₄).
Which bonds are formed and which are broken?
Formed: C–N; H₂. Broken: C–H (as H⁻); N–H.
What is the stereochemical outcome?
None (aromatic product); 2-selective
What controls the rate or the selectivity?
Slow addition; H⁻ expulsion drives off H₂ gas
Study guides that help with this reaction
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
- How to answer mechanism questions in an exam