Experiment 02Carbohydrate Chemistry • B.Sc. / M.Sc.
Qualitative Identification & Differentiation of Unknown Carbohydrates
๐ฏ Aim: To perform systematic qualitative biochemical tests on an unknown carbohydrate sample to differentiate between polysaccharides, reducing disaccharides, non-reducing disaccharides, aldohexoses, and ketohexoses.
๐ฌ Theoretical Principle & Chemical Mechanisms
Carbohydrates are polyhydroxy aldehydes or ketones classified by degree of polymerization and reducing capacity:
- Molisch Test (General Carbohydrate Test): Concentrated sulfuric acid dehydrates pentoses to furfural and hexoses to 5-hydroxymethylfurfural (5-HMF). These cyclic aldehydes condense with $alpha$-naphthol to form a characteristic purple/violet ring at the liquid-liquid interface.
- Iodine Test (Polysaccharides): Triiodide ions ($text{I}_3^-$) slip into the interior helices of amylose to form an intense deep-blue charge-transfer inclusion complex. Highly branched glycogen and amylopectin yield reddish-brown to purple complexes.
- Benedict’s Test (Reducing Sugars): Sugars with a free hemiacetal or hemiketal group tautomerize into enediols in mild alkaline medium ($text{Na}_2text{CO}_3/text{sodium citrate}$). Enediols reduce cupric ions ($text{Cu}^{2+}$) to insoluble cuprous oxide ($text{Cu}_2text{O}$), creating a green $to$ yellow $to$ brick-red precipitate.
- Barfoed’s Test (Monosaccharide vs Disaccharide): Copper acetate in weak acetic acid ($text{pH} approx 4.6$). Because of the acidic conditions, only rapidly reducing monosaccharides reduce $text{Cu}^{2+}$ within 2–3 minutes of boiling, whereas reducing disaccharides require prolonged boiling (>8–10 min).
- Seliwanoff’s Test (Aldose vs Ketose): Resorcinol in $3,text{M} text{HCl}$. Ketohexoses (fructose) dehydrate much more rapidly than aldohexoses to 5-HMF, which condenses with resorcinol to form a cherry-red complex within 60 seconds.
Figure 2.1: Systematic Carbohydrate Qualitative Identification Decision Flowchart
๐งช Reagents, Materials & Solution Recipes
Reagents & Test Solutions
- Molisch Reagent: $5% (text{w/v}) alpha$-naphthol in $95%$ ethanol.
- Concentrated $text{H}_2text{SO}_4$ (Analytical Grade).
- Lugol’s Iodine: $1% text{I}_2 + 2% text{KI}$ in deionized water.
- Benedict’s Qualitative Reagent: $173,text{g}$ sodium citrate $+ 100,text{g} text{Na}_2text{CO}_3$ anhydrous $+ 17.3,text{g} text{CuSO}_4cdot 5text{H}_2text{O}$ per liter.
- Barfoed’s Reagent: $66.5,text{g}$ copper acetate $+ 9,text{mL}$ glacial acetic acid per liter.
- Seliwanoff’s Reagent: $0.05,text{g}$ resorcinol in $100,text{mL}$ of $3,text{M} text{HCl}$.
Samples & Apparatus
- Standard $1% (text{w/v})$ solutions: Glucose, Fructose, Maltose, Lactose, Sucrose, Starch.
- Unknown test carbohydrate solutions (A, B, C).
- Boiling water bath with tube racks.
- Pyrex test tubes ($15 times 125,text{mm}$), test tube holders, timers.
๐ Systematic Experimental Protocol & Observation Matrix
| Test Protocol | Experimental Procedure | Observation | Inference / Chemical Meaning |
|---|---|---|---|
| 1. Molisch Test | $2,text{mL}$ sample $+ 2$ drops Molisch reagent. Incline tube, gently layer $2,text{mL}$ conc. $text{H}_2text{SO}_4$ down the wall. | Purple / violet ring at the junction of two layers | Carbohydrate confirmed (furfural-naphthol condensation) |
| 2. Iodine Test | $2,text{mL}$ sample $+ 2$ drops Lugol’s iodine. Shake gently. | Deep blue color (disappears on heating, reappears on cooling) | Polysaccharide (Amylose/Starch) confirmed |
| 3. Benedict’s Test | $2,text{mL}$ Benedict’s $+ 8$ drops sample. Boil vigorously in water bath for 3 min. | Brick-red precipitate ($text{Cu}_2text{O}$) | Reducing sugar present (free hemiacetal group) |
| 4. Barfoed’s Test | $2,text{mL}$ Barfoed’s $+ 1,text{mL}$ sample. Heat in boiling water bath for exactly 3 min. | Red precipitate at the bottom of the tube within 3 min | Reducing Monosaccharide confirmed (acidic reduction) |
| 5. Seliwanoff’s Test | $2,text{mL}$ Seliwanoff’s $+ 0.5,text{mL}$ sample. Heat in boiling water bath for 60 seconds. | Intense cherry-red color within 1 minute | Ketohexose confirmed (Fructose rapid dehydration) |
๐ฌ Viva-Voce Questions & Model Answers
Q1: Why is sucrose a non-reducing sugar despite being composed of glucose and fructose?
Sucrose is linked via an $alpha(1leftrightarrow 2)beta$-glycosidic bond between the anomeric C-1 of $alpha$-D-glucose and the anomeric C-2 of $beta$-D-fructose. Because both potential aldehyde and ketone carbonyl groups are locked in the glycosidic bond, sucrose lacks a free anomeric hemiacetal/hemiketal and cannot mutarotate or form reducing enediols.
Q2: Why does Barfoed’s reagent differentiate monosaccharides from reducing disaccharides?
Barfoed’s reagent operates in a weakly acidic environment ($text{pH} approx 4.6$) where enediol formation is severely suppressed. Monosaccharides, possessing a higher reducing power per mole, reduce cupric ions rapidly within 2–3 minutes. Disaccharides reduce copper much more slowly and require >8–10 minutes of boiling to form cuprous oxide.
Q3: Why does the blue color of the starch-iodine complex disappear upon heating?
The blue color results from polyiodide chains ($text{I}_3^-$, $text{I}_5^-$) trapped inside the hydrophobic cavity of the single-stranded helical amylose coil. Heating disrupts the hydrogen bonds maintaining the amylose helix, causing the coil to unwind and release the iodine molecules. Upon cooling, the helices reform and the blue color returns.
