Rearrangement — azoxy to hydroxyazo
Wallach rearrangement
How the Wallach rearrangement 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: Azoxybenzene, conc. H₂SO₄. Conditions: Strong acid, 25–100 °C. Bonds formed: C–O (para, on ring); N=N retained; bonds broken: N–O. Stereochemistry: None; para-selective. Rate and selectivity: Loss of water from the dication is rate-limiting.
At a glance
Step 1: Double protonation at oxygen activates N–O
Strong acid protonates an azoxybenzene's oxygen, activating the N–O bond and generating a delocalized cationic intermediate spread across the N=N–O framework — conceptually related to the nitrenium-type delocalization seen in the Bamberger rearrangement.
Step 2: Ring π pushes toward N; water leaves
One aromatic ring's π electrons shift density toward the electron-poor nitrogen framework internally, placing positive character specifically at that ring's para carbon — the same kind of intramolecular charge delocalization used throughout this family of aromatic rearrangements.
Step 3: Water attacks C-4; rearomatisation
Water attacks that activated para position and loss of a proton restores aromaticity, giving a para-hydroxyazobenzene — net effect, an oxygen has migrated from the N=N linkage directly onto one aromatic ring, entirely under strongly acidic, non-nucleophile-first conditions.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
How it compares with related reactions
| Wallach rearrangement | Bamberger rearrangement | Beckmann rearrangement | |
|---|---|---|---|
| Reagents | Azoxybenzene, conc. H₂SO₄ | N-Phenylhydroxylamine, dilute H₂SO₄ (aq) | H₂SO₄, PCl₅, TsCl or PPA |
| Conditions | Strong acid, 25–100 °C | H₂O, 25–60 °C | Acid, 100–130 °C (industrial: oleum) |
| Stereochemistry | None; para-selective | None; para-selective | Group anti to OH migrates, with retention |
| Rate / selectivity | Loss of water from the dication is rate-limiting | Loss of water to give the nitrenium ion is rate-limiting | Migration is rate-limiting; leaving-group ability of N–OX matters |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Wallach rearrangement?
Azoxybenzene, conc. H₂SO₄. Conditions: Strong acid, 25–100 °C.
Which bonds are formed and which are broken?
Formed: C–O (para, on ring); N=N retained. Broken: N–O.
What is the stereochemical outcome?
None; para-selective
What controls the rate or the selectivity?
Loss of water from the dication is rate-limiting
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti