Nitroalkane to carbonyl
Nef reaction
How the Nef 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: 1) NaOH or NaOEt; 2) cold aq. H₂SO₄ (pH < 1). Conditions: Nitronate at 0–25 °C; then strong acid, 0 °C. Bonds formed: C=O; N₂O (from HNO); bonds broken: C–N; α C–H; N–O. Stereochemistry: α-Stereocentre lost at nitronate stage. Rate and selectivity: Acid quench must be strongly acidic (pH < 1) to avoid oximes.
At a glance
Step 1: Base forms the nitronate
Base deprotonates a primary or secondary nitroalkane's alpha carbon, forming a resonance-stabilized nitronate anion — the same acidic-alpha-carbon chemistry covered in the Henry reaction.
Step 2: Acid protonates, water adds
Acidic hydrolysis protonates the nitronate at oxygen, generating a nitroso intermediate that tautomerizes and hydrolyzes further, passing through an unstable nitrilic acid-type species.
Step 3: Collapse to the ketone
Loss of a nitrogen-containing byproduct (as N₂O or related species) delivers a carbonyl compound directly — this reaction is essentially the reverse-engineered payoff of using a nitro group as a masked carbonyl throughout a synthesis, exactly the way the Henry reaction installs one.
Abbreviations and terms
- NaOEt
- Sodium ethoxide. Alkoxide base matched to ethyl esters (avoids transesterification).
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
- pKa
- Acid-dissociation constant (–log). Lower pKa = stronger acid; used to judge which proton a base removes.
How it compares with related reactions
| Nef reaction | Clemmensen reduction | Corey-Seebach reaction (dithiane umpolung) | |
|---|---|---|---|
| Reagents | 1) NaOH or NaOEt; 2) cold aq. H₂SO₄ (pH < 1) | Zn(Hg), conc. HCl | 1) HS(CH₂)₃SH, BF₃·OEt₂ 2) n-BuLi, THF 3) R′–X 4) HgCl₂/H₂O (or NCS, AgNO₃) |
| Conditions | Nitronate at 0–25 °C; then strong acid, 0 °C | Reflux, heterogeneous (metal surface) | Deprotonation –30 °C to 0 °C; alkylation –78 °C → 25 °C |
| Stereochemistry | α-Stereocentre lost at nitronate stage | Not stereocontrolled; mechanism debated | None at carbanion; electrophile attack at planar/axial C2 |
| Rate / selectivity | Acid quench must be strongly acidic (pH < 1) to avoid oximes | Works best for aryl ketones; avoid acid-sensitive groups | Deprotonation (pKa ≈ 31) fast with n-BuLi; SN2 alkylation rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Nef reaction?
1) NaOH or NaOEt; 2) cold aq. H₂SO₄ (pH < 1). Conditions: Nitronate at 0–25 °C; then strong acid, 0 °C.
Which bonds are formed and which are broken?
Formed: C=O; N₂O (from HNO). Broken: C–N; α C–H; N–O.
What is the stereochemical outcome?
α-Stereocentre lost at nitronate stage
What controls the rate or the selectivity?
Acid quench must be strongly acidic (pH < 1) to avoid oximes
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go