Carbohydrate chain-shortening — oxidative
Ruff degradation
How the Ruff 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) Br₂, H₂O; 2) Fe³⁺ (Fe₂(SO₄)₃ or Fe(OAc)₃), H₂O₂. Conditions: Aqueous, pH ~ 3–5, 25–40 °C. Bonds formed: CO₂; new C=O at old C2; bonds broken: C1–C2 bond; C1 carboxylate. Stereochemistry: Stereocentres C3 onward retained. Rate and selectivity: Radical decarboxylation fast; yields typically moderate (30–50 %).
At a glance
Step 1: Oxidize the aldose to the aldonic acid
An aldose is first oxidized (typically with bromine water) to an aldonic acid, converting the terminal aldehyde into a carboxylic acid.
Step 2: Fenton radical chemistry decarboxylates
Fe³⁺ and hydrogen peroxide generate a hydroxyl radical (Fenton-type chemistry, covered earlier), which oxidatively decarboxylates the aldonic acid — the same radical decarboxylation logic as the Hunsdiecker reaction, just driven by peroxide instead of Br₂.
Step 3: Radical oxidized to the shorter aldose
The result is an aldose with one fewer carbon, terminating in a new aldehyde — an oxidative alternative to the Wohl degradation that reaches the same kind of product through entirely different (radical, rather than nitrile-elimination) chemistry.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- OAc
- Acetoxy. –OC(=O)CH₃, an acetate group.
- H2O2
- Hydrogen peroxide. Oxidant; its anion HOO⁻ is an excellent nucleophile.
- HBr
- Hydrobromic acid. Strong acid; bromide is a good nucleophile.
How it compares with related reactions
| Ruff degradation | Wohl degradation | Acid-catalyzed glycoside hydrolysis | |
|---|---|---|---|
| Reagents | 1) Br₂, H₂O; 2) Fe³⁺ (Fe₂(SO₄)₃ or Fe(OAc)₃), H₂O₂ | 1) NH₂OH; 2) Ac₂O, NaOAc; 3) NaOMe, MeOH (then deacetylation) | Dilute HCl or H₂SO₄ (H₃O⁺), H₂O |
| Conditions | Aqueous, pH ~ 3–5, 25–40 °C | Oxime at 25 °C; Ac₂O ~100 °C; NaOMe 0–25 °C | Heat 60–100 °C; excess water |
| Stereochemistry | Stereocentres C3 onward retained | Stereocentres C3 onward retained; C2 becomes the new aldehyde | Both anomers form (planar oxocarbenium) |
| Rate / selectivity | Radical decarboxylation fast; yields typically moderate (30–50 %) | Cyanohydrin collapse is fast; dehydration to nitrile needs Ac₂O | 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 Ruff degradation?
1) Br₂, H₂O; 2) Fe³⁺ (Fe₂(SO₄)₃ or Fe(OAc)₃), H₂O₂. Conditions: Aqueous, pH ~ 3–5, 25–40 °C.
Which bonds are formed and which are broken?
Formed: CO₂; new C=O at old C2. Broken: C1–C2 bond; C1 carboxylate.
What is the stereochemical outcome?
Stereocentres C3 onward retained
What controls the rate or the selectivity?
Radical decarboxylation fast; yields typically moderate (30–50 %)
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- Curved-arrow notation: rules and the mistakes students make