Al-catalyzed disproportionation
Tishchenko reaction
How the Tishchenko 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: 2 RCHO, cat. Al(OR)₃ (e.g. Al(OEt)₃ or Al(OiPr)₃). Conditions: Neat or toluene, 0–25 °C. Bonds formed: Ester C–O and C=O; C–H (new alkoxide CH₂); bonds broken: Aldehyde C=O; hemiacetal C–H. Stereochemistry: None (acetaldehyde example). Rate and selectivity: 1,5-hydride shift in a six-membered Al-bound cycle is turnover-limiting.
At a glance
Step 1: Alkoxide adds to a bound aldehyde
An aluminum alkoxide catalyst attacks one aldehyde's carbonyl carbon, forming a tetrahedral aluminum hemiacetal-like intermediate — the same nucleophilic addition logic as MPV reduction's first step.
Step 2: Intramolecular hydride transfer
This intermediate's alkoxide oxygen (or hydride, delivered intramolecularly) reduces a second aldehyde molecule bound to the same aluminum center — a self-contained, catalytic hydride-transfer disproportionation reminiscent of the Cannizzaro reaction, but requiring only catalytic aluminum rather than stoichiometric hydroxide.
Step 3: Ester released, catalyst regenerated
The result is one ester formed directly from two equivalents of the same aldehyde — one molecule is oxidized to the acyl portion, the other reduced to the alkoxy portion, both joined in a single catalytic cycle.
Abbreviations and terms
- OEt
- Ethoxy. –OCH₂CH₃.
How it compares with related reactions
| Tishchenko reaction | Acid-catalyzed hydration of an alkene | Cannizzaro reaction of benzaldehyde | |
|---|---|---|---|
| Reagents | 2 RCHO, cat. Al(OR)₃ (e.g. Al(OEt)₃ or Al(OiPr)₃) | Dilute H₂SO₄ or H₃PO₄ (H₃O⁺), H₂O | Conc. NaOH or KOH (50 %) |
| Conditions | Neat or toluene, 0–25 °C | 25–100 °C; heat and excess water favour the alcohol | Aqueous/alcoholic, 25–80 °C; aldehyde lacks α-H |
| Stereochemistry | None (acetaldehyde example) | Planar carbocation: racemic; Markovnikov regiochemistry | Not applicable (no new stereocentre) |
| Rate / selectivity | 1,5-hydride shift in a six-membered Al-bound cycle is turnover-limiting | Protonation of the alkene is rate-determining | Second order in aldehyde, first order in HO⁻ (rate-limiting hydride transfer) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Tishchenko reaction?
2 RCHO, cat. Al(OR)₃ (e.g. Al(OEt)₃ or Al(OiPr)₃). Conditions: Neat or toluene, 0–25 °C.
Which bonds are formed and which are broken?
Formed: Ester C–O and C=O; C–H (new alkoxide CH₂). Broken: Aldehyde C=O; hemiacetal C–H.
What is the stereochemical outcome?
None (acetaldehyde example)
What controls the rate or the selectivity?
1,5-hydride shift in a six-membered Al-bound cycle is turnover-limiting
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go