Inorganic — phase-transfer catalysis
Phase-transfer catalysis (anion shuttle)
How the Phase-transfer catalysis (anion shuttle) 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: Q⁺X⁻ (e.g. Bu₄N⁺Br⁻, 1–10 mol%), NaCN (aq), alkyl bromide (organic). Conditions: Biphasic H₂O / toluene, 25–80 °C, vigorous stirring. Bonds formed: C–CN; Q⁺Br⁻; bonds broken: C–Br. Stereochemistry: SN2 inversion where applicable. Rate and selectivity: Interfacial anion exchange and SN2 in organic phase.
At a glance
Step 1: Anion exchange at the interface
A lipophilic quaternary ammonium salt (like a tetrabutylammonium halide) exchanges its counterion at the interface of an aqueous phase, picking up a reactive anion (like hydroxide or cyanide) that's otherwise trapped in water.
Step 2: Lipophilic ion pair crosses into the organic phase
Because the large, bulky ammonium cation shields the anion and makes the whole ion pair lipophilic, it can now cross into the organic phase — carrying a reactive, normally water-bound nucleophile into direct contact with an organic-soluble electrophile for the first time.
Step 3: SN2, then the catalyst returns
Once in the organic phase, the anion performs whatever ordinary reaction it would (SN2, elimination, etc.) exactly as covered throughout this library — the catalyst then shuttles the spent counterion (like the halide displaced in an SN2) back to the aqueous phase to pick up another equivalent of active anion, closing the cycle.
Abbreviations and terms
- X
- Halogen / leaving group. Cl, Br or I (sometimes any leaving group).
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
How it compares with related reactions
| Phase-transfer catalysis (anion shuttle) | Asymmetric phase-transfer catalysis | Catalytic hydrogenation of an alkene (Pd/C, H₂) | |
|---|---|---|---|
| Reagents | Q⁺X⁻ (e.g. Bu₄N⁺Br⁻, 1–10 mol%), NaCN (aq), alkyl bromide (organic) | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr | H₂ (1 atm), Pd/C |
| Conditions | Biphasic H₂O / toluene, 25–80 °C, vigorous stirring | Toluene/CH₂Cl₂, 0 °C, vigorous stirring | EtOH or EtOAc, 25 °C |
| Stereochemistry | SN2 inversion where applicable | High ee: chiral ion pair shields one enolate face | Syn addition from the face bound to Pd |
| Rate / selectivity | Interfacial anion exchange and SN2 in organic phase | Alkylation of the ion pair is rate-limiting | Fast; limited by H₂ and substrate access to the surface |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Phase-transfer catalysis (anion shuttle)?
Q⁺X⁻ (e.g. Bu₄N⁺Br⁻, 1–10 mol%), NaCN (aq), alkyl bromide (organic). Conditions: Biphasic H₂O / toluene, 25–80 °C, vigorous stirring.
Which bonds are formed and which are broken?
Formed: C–CN; Q⁺Br⁻. Broken: C–Br.
What is the stereochemical outcome?
SN2 inversion where applicable
What controls the rate or the selectivity?
Interfacial anion exchange and SN2 in organic phase
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam