Question
Glucose () and fructose are both reducing sugars and both give the same osazone with phenylhydrazine. Why? Also, identify the difference between the open-chain structures of glucose and fructose.
Solution — Step by Step
A reducing sugar is one that has a free aldehyde or α-hydroxy ketone group capable of being oxidised. Glucose is an aldohexose — has free CHO group. Fructose is a ketohexose — has a -hydroxy ketone group. Both reduce Tollens’ and Fehling’s reagents.
Glucose: CHO-CHOH-CHOH-CHOH-CHOH-CH₂OH (aldehyde at C1).
Fructose: CH₂OH-CO-CHOH-CHOH-CHOH-CH₂OH (ketone at C2).
The difference is the position of the carbonyl: C1 in glucose, C2 in fructose. Carbons C3, C4, C5, C6 have identical configurations.
Phenylhydrazine reacts with the carbonyl group at C1 (glucose) or C2 (fructose), then a second molecule attacks the adjacent carbon. After three molecules of phenylhydrazine react, both glucose and fructose end up with the same structure at C1 and C2 — both become bis-phenylhydrazones at C1, C2. C3 onwards is unchanged. Since both share C3–C6 stereochemistry, the resulting osazone is identical.
Both give the same osazone because the osazone-forming reaction “erases” the difference at C1 and C2 — exactly where glucose and fructose differ. Mannose also gives the same osazone for the same reason (mannose differs from glucose only at C2).
Why This Works
Osazone formation involves three molecules of phenylhydrazine. The first replaces the carbonyl oxygen with . The second oxidises the adjacent CHOH to a carbonyl, which the third condenses with. The result is a bis-phenylhydrazone covering C1 and C2 — and any sugar with the same C3-C6 configuration gives the same osazone.
This is a classic “structural detective” problem in NEET — testing whether students understand both the open-chain structures and the mechanism of osazone formation.
Alternative Method
Memorise the trio: glucose, fructose, mannose all give the same osazone (called “glucosazone”). Mannose is a C2 epimer of glucose; fructose is a C2 ketose isomer. Whatever happens at C1 and C2 is washed out by osazone formation, so anything matching at C3-C6 lands at the same product.
Common Mistake
Students claim “both have CHO group, so same osazone” — fructose does not have a CHO group in its open-chain form. It has a C=O at C2 (ketone). The reason both give the same osazone is more subtle: it’s the “erasing” effect of triple phenylhydrazine attack. Stating it correctly earns a full mark in NEET.