Seliwanoff's Test: Ketose vs. Aldose
Seliwanoff's Test is a chemical test used to distinguish between Aldose and Ketose sugars. While both are reducing sugars, ketoses (like Fructose) dehydrate much faster than aldoses (like Glucose) in the presence of concentrated acid, leading to a specific color change.
1. Reagents and Procedure
The reagent consists of Resorcinol dissolved in concentrated Hydrochloric Acid ($HCl$).
- Reagent: 0.5% Resorcinol in 3N or 6N HCl.
- Procedure: Add reagent to the sugar solution and boil for a short time (1-2 minutes).
Observation (Positive Test)
Ketoses: Rapid formation of a Cherry Red complex within 1-2 minutes.
Aldoses: React very slowly. May produce a faint pink color only after prolonged boiling (false positive).
2. Principle & Mechanism
The test relies on the difference in the rate of dehydration.
Step 1: Acid-Catalyzed Dehydration
In the presence of hot concentrated HCl, ketoses (hexoses) undergo rapid dehydration to yield 5-Hydroxymethylfurfural (5-HMF). Aldoses dehydrate much slower because the aldehyde group is less conducive to ring formation/dehydration under these conditions.
Step 2: Condensation
The 5-HMF formed reacts with Resorcinol (1,3-dihydroxybenzene) via an electrophilic substitution/condensation reaction to form a red-colored condensation product (Xanthenone derivative).
3. Comparison Table
| Sugar Type | Examples | Observation (1-2 mins boiling) |
|---|---|---|
| Ketose | Fructose, Sucrose* | Cherry Red Color |
| Aldose | Glucose, Galactose, Mannose | No change (or very faint pink later) |
*Sucrose gives a positive test because the acid hydrolyzes it into Glucose + Fructose. The liberated Fructose reacts.
4. Key Considerations
- Time Sensitivity: The test is time-sensitive. Prolonged boiling must be avoided, as aldoses will eventually dehydrate and give a false positive result.
- Concentration: The HCl concentration is critical (usually 6M) to ensure rapid dehydration of ketoses but slow dehydration of aldoses.
- Reaction with Pentoses: Pentose ketoses (like Ribulose) yield Furfural (not HMF) and give a blue-green color with Orcinol (Bial's Test), but may give varying colors with Resorcinol. Seliwanoff is specific for Hexoses.
Seliwanoff's Quiz
Test your concepts on Sugar differentiation. 10 MCQs with explanations.
No comments:
Post a Comment