Nucleophilic aromatic substitution
SNAr on 1-fluoro-2,4-dinitrobenzene
How the SNAr on 1-fluoro-2,4-dinitrobenzene 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: NaOH (aq); 1-fluoro-2,4-dinitrobenzene (Sanger-type substrate). Conditions: H₂O/EtOH or DMSO, 25–50 °C. Bonds formed: C–O σ; Meisenheimer complex (transient); bonds broken: C–F σ. Stereochemistry: None (planar aromatic). Rate and selectivity: Addition is rate-limiting; F ≫ Cl, Br, I; ortho/para EWGs accelerate.
At a glance
Step 1: Attack at the C–F carbon
Unlike ordinary aryl halides, this ring is flanked by strong electron-withdrawing nitro groups. Hydroxide attacks the carbon bearing fluorine directly — the nitro groups will stabilize the negative charge that's about to form.
Step 2: Meisenheimer complex
This gives an anionic, sp³ Meisenheimer complex: the negative charge is delocalized off the ring and onto the oxygens of the nitro groups, which is what makes this pathway viable at all.
Step 3: Fluoride leaves; aromaticity returns
Fluoride leaves, aromaticity is restored, and the phenol product forms. Unlike EAS, this proceeds through an anionic intermediate — the electronics are essentially reversed.
Abbreviations and terms
- DMSO
- Dimethyl sulfoxide. Polar aprotic solvent; also the oxidant in Swern/Kornblum-type reactions.
- EtOH
- Ethanol. Polar protic solvent.
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- 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.
- Wheland
- Wheland intermediate (arenium ion). The cyclohexadienyl cation formed when an electrophile adds to a benzene ring.
- Meisenheimer
- Meisenheimer complex. The anionic cyclohexadienyl intermediate of SNAr stabilised by electron-withdrawing groups.
How it compares with related reactions
| SNAr on 1-fluoro-2,4-dinitrobenzene | Chichibabin reaction | Bromination of benzene (FeBr₃-catalyzed) | |
|---|---|---|---|
| Reagents | NaOH (aq); 1-fluoro-2,4-dinitrobenzene (Sanger-type substrate) | NaNH₂ (sodium amide) | Br₂ with FeBr₃ (Lewis-acid catalyst) |
| Conditions | H₂O/EtOH or DMSO, 25–50 °C | Toluene or xylene, 100–130 °C (or liquid NH₃, KMnO₄) | Anhydrous, 25 °C, dark |
| Stereochemistry | None (planar aromatic) | None (aromatic product); 2-selective | None (planar, achiral) |
| Rate / selectivity | Addition is rate-limiting; F ≫ Cl, Br, I; ortho/para EWGs accelerate | Slow addition; H⁻ expulsion drives off H₂ gas | 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 SNAr on 1-fluoro-2,4-dinitrobenzene?
NaOH (aq); 1-fluoro-2,4-dinitrobenzene (Sanger-type substrate). Conditions: H₂O/EtOH or DMSO, 25–50 °C.
Which bonds are formed and which are broken?
Formed: C–O σ; Meisenheimer complex (transient). Broken: C–F σ.
What is the stereochemical outcome?
None (planar aromatic)
What controls the rate or the selectivity?
Addition is rate-limiting; F ≫ Cl, Br, I; ortho/para EWGs accelerate
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