Ruff degradation template

D-Glucose is oxidised by bromine water to D-gluconic acid; its calcium salt with Fe³⁺ and H₂O₂ loses one electron, the carboxyl radical loses CO₂, and oxidation of the α-hydroxy radical gives the aldehyde of D-arabinose (simplified radical mechanism).

Mechanism step by step

An aldose loses C1 as CO₂: the calcium aldonate with H₂O₂ and Fe³⁺ gives the aldose with one carbon fewer

1. Oxidation – Bromine water oxidises the aldehyde to D-gluconic acid (summarised); CaCO₃ then gives the Ca²⁺ salt
2. Electron transfer – Fe³⁺ takes one electron from the carboxylate: a carboxyl radical (summarised)
3. Loss of CO₂ – The C1–C2 bond breaks homolytically: CO₂ and an α-hydroxy radical at the old C2
4. Oxidation to the aldehyde – A second Fe³⁺ takes one electron and H⁺ is lost: D-arabinose (summarised)

Key points

• C1 leaves as CO₂ and C2 becomes the new aldehyde carbon, so the C2 stereocentre is destroyed: D-glucose and D-mannose (C2 epimers) both give D-arabinose.
• H₂O₂ re-oxidises Fe²⁺ to Fe³⁺, so only a catalytic amount of iron is needed.
• The reverse of the Kiliani–Fischer synthesis; it shortens a D-aldose to the next lower D-aldose (the C4/C5 configuration is kept).
• Simplified radical mechanism: the details of the Fe³⁺/H₂O₂ step are not fully settled.

Drawn with 132 atoms, 8 arrows, 9 shapes – every one editable in MolDraw.

Sources

• Wade, Organic Chemistry, 8th ed., Pearson 2013, ch. 23 Textbook