Carbohydrate chain-shortening
Wohl degradation
How the Wohl degradation 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) NH₂OH; 2) Ac₂O, NaOAc; 3) NaOMe, MeOH (then deacetylation). Conditions: Oxime at 25 °C; Ac₂O ~100 °C; NaOMe 0–25 °C. Bonds formed: C=N, C≡N; new C=O at old C2; HCN; bonds broken: C1–C2 bond; C=O (C1). Stereochemistry: Stereocentres C3 onward retained; C2 becomes the new aldehyde. Rate and selectivity: Cyanohydrin collapse is fast; dehydration to nitrile needs Ac₂O.
At a glance
Step 1: Aldose → oxime → nitrile
An aldose's aldehyde first condenses with hydroxylamine to form an oxime, then dehydration (with acetic anhydride) converts it to a nitrile — pulling the terminal carbon's chemistry away from the sugar's usual hemiacetal behavior.
Step 2: Base expels cyanide
Base-mediated elimination (a retro-nucleophilic-addition, essentially cyanide leaving in reverse of how the Kiliani-Fischer synthesis adds it) cleaves off that terminal carbon as HCN.
Step 3: Net result: one carbon shorter
The result is an aldose with one fewer carbon — mechanistically, this whole sequence is the Kiliani-Fischer chain-extension run in reverse, useful for working backward through a sugar's stereochemical relationships.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- MeOH
- Methanol. Polar protic solvent (also a nucleophile in solvolysis).
- AcOH
- Acetic acid. Weak acid and polar protic solvent.
- NaOMe
- Sodium methoxide. Alkoxide base matched to methyl esters.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- Ac2O
- Acetic anhydride. Acetylating agent.
How it compares with related reactions
| Wohl degradation | Ruff degradation | Acid-catalyzed glycoside hydrolysis | |
|---|---|---|---|
| Reagents | 1) NH₂OH; 2) Ac₂O, NaOAc; 3) NaOMe, MeOH (then deacetylation) | 1) Br₂, H₂O; 2) Fe³⁺ (Fe₂(SO₄)₃ or Fe(OAc)₃), H₂O₂ | Dilute HCl or H₂SO₄ (H₃O⁺), H₂O |
| Conditions | Oxime at 25 °C; Ac₂O ~100 °C; NaOMe 0–25 °C | Aqueous, pH ~ 3–5, 25–40 °C | Heat 60–100 °C; excess water |
| Stereochemistry | Stereocentres C3 onward retained; C2 becomes the new aldehyde | Stereocentres C3 onward retained | Both anomers form (planar oxocarbenium) |
| Rate / selectivity | Cyanohydrin collapse is fast; dehydration to nitrile needs Ac₂O | Radical decarboxylation fast; yields typically moderate (30–50 %) | Aglycone departure from the protonated glycoside is rate-limiting (A-1 mechanism) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Wohl degradation?
1) NH₂OH; 2) Ac₂O, NaOAc; 3) NaOMe, MeOH (then deacetylation). Conditions: Oxime at 25 °C; Ac₂O ~100 °C; NaOMe 0–25 °C.
Which bonds are formed and which are broken?
Formed: C=N, C≡N; new C=O at old C2; HCN. Broken: C1–C2 bond; C=O (C1).
What is the stereochemical outcome?
Stereocentres C3 onward retained; C2 becomes the new aldehyde
What controls the rate or the selectivity?
Cyanohydrin collapse is fast; dehydration to nitrile needs Ac₂O
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- Curved-arrow notation: rules and the mistakes students make