Q4(c). Periodic acid oxidation of glucose and fructose

Introduction. HIO₄ cleaves every C–C bond between two carbons that carry OH or C=O groups (through a cyclic periodate ester), one HIO₄ per bond broken. The products depend on the end groups: a CH₂OH end gives HCHO, an internal CHOH gives HCOOH, an aldehyde CHO gives HCOOH, and an internal C=O (ketone) gives CO₂ when cleaved on both sides.

Figure: Open-chain forms; each short rule between two carbons is one C–C bond cleaved by one HIO₄.

1. Glucose (an aldohexose): C-1 is CHO, C-2 to C-5 are CHOH, C-6 is CH₂OH. Five C–C bonds are cut. C-1 (aldehyde) and the four internal CHOH carbons C-2 to C-5 each become HCOOH: 5 HCOOH; C-6 gives one HCHO. C₆H₁₂O₆ + 5 HIO₄ → 5 HCOOH + HCHO + 5 HIO₃
2. Fructose (a 2-ketohexose): C-1 and C-6 are CH₂OH, C-2 is C=O, C-3 to C-5 are CHOH. Again five bonds are cut, but only the three internal CHOH carbons (C-3, C-4, C-5) give HCOOH: 3 HCOOH. C-1 and C-6 give 2 HCHO, and the keto carbon C-2, cleaved on both sides, gives CO₂. C₆H₁₂O₆ + 5 HIO₄ → 3 HCOOH + 2 HCHO + CO₂ + 5 HIO₃ + H₂O
3. Both sugars consume 5 mol HIO₄; the difference in HCOOH (5 against 3) comes from the position of the carbonyl group: aldose (C-1) or ketose (C-2). Titration of the HCOOH formed and estimation of HCHO therefore distinguish the two sugars.

Conclusion. Glucose, with CHO at C-1 and four CHOH carbons, gives 5 HCOOH + 1 HCHO; fructose, with C=O at C-2 flanked by two CH₂OH groups, gives 3 HCOOH + 2 HCHO + CO₂. Both use 5 HIO₄.

Application. Periodate oxidation (Malaprade reaction) is used to find ring size of glycosides, the number of adjacent OH groups in carbohydrates, and to make dialdehyde starch and cellulose.