carbohydrate and colour reaction

Reading File

Finding Sources

The library doesn't have a dedicated biochemistry chapter on carbohydrate colour reactions. I have strong knowledge on this topic - let me provide a thorough answer from my own knowledge base.

Carbohydrate Colour Reactions

Colour reactions are used to detect, identify, and differentiate carbohydrates based on their chemical properties. Here is a complete overview:

1. Molisch's Test (General Test for All Carbohydrates)

Feature	Detail
Reagent	Alpha-naphthol in alcohol + conc. H2SO4
Result	Violet/purple ring at the junction of two liquids
Principle	H2SO4 dehydrates pentoses to furfural and hexoses to hydroxymethylfurfural, which then condenses with alpha-naphthol to give a purple colour
Detects	ALL carbohydrates (most sensitive general test)
Note	A negative Molisch's test rules out carbohydrates

2. Benedict's Test (Reducing Sugars)

Feature	Detail
Reagent	Sodium citrate + sodium carbonate + CuSO4 (alkaline)
Result	Green → Yellow → Orange → Brick red precipitate
Principle	Free aldehyde or ketone groups reduce Cu²+ to Cu+ (Cu2O), forming a coloured precipitate
Detects	All reducing sugars: glucose, fructose, galactose, maltose, lactose
Does NOT detect	Sucrose (non-reducing), starch
Sensitivity	Semi-quantitative - brick red = high sugar, green = trace

3. Fehling's Test (Reducing Sugars)

Feature	Detail
Reagent	Fehling's A (CuSO4) + Fehling's B (KOH + sodium potassium tartrate) mixed equally
Result	Brick red/orange precipitate (Cu2O)
Principle	Same as Benedict's - Cu²+ reduced to Cu+ by reducing sugars in alkaline conditions
Detects	Reducing sugars
Note	More specific than Benedict's; aldehydes give positive, ketones (like fructose) may be weaker

4. Tollens' Test (Silver Mirror Test)

Feature	Detail
Reagent	Ammoniacal silver nitrate (Ag(NH3)2+)
Result	Silver mirror on the inner wall of tube
Principle	Aldehyde groups reduce Ag+ to metallic silver
Detects	Aldoses (not ketoses in most conditions)

5. Seliwanoff's Test (Ketoses vs Aldoses)

Feature	Detail
Reagent	Resorcinol + conc. HCl, heated
Result	Cherry red/deep red colour
Principle	Ketoses are dehydrated faster than aldoses. Fructose → hydroxymethylfurfural → condenses with resorcinol → cherry red
Detects	Ketoses (fructose, sucrose on hydrolysis)
Aldoses	Give a faint pink or no colour
Key use	Differentiates fructose from glucose; sucrose gives a positive test due to fructose moiety

6. Bial's (Orcinol) Test (Pentoses)

Feature	Detail
Reagent	Orcinol + conc. HCl + FeCl3, heated
Result	Green/blue-green colour
Principle	Pentoses dehydrated by HCl to furfural, which reacts with orcinol in presence of Fe³+ to give a green colour
Detects	Pentoses (ribose, xylose, arabinose)
Hexoses	Give a muddy brown colour (not green)
Key use	Distinguishes pentoses from hexoses

7. Barfoed's Test (Monosaccharides vs Disaccharides)

Feature	Detail
Reagent	Cupric acetate in acetic acid (weakly acidic)
Result	Red/brick red precipitate (Cu2O)
Principle	Monosaccharides reduce Cu²+ under acidic conditions faster than disaccharides
Detects	Monosaccharides only (in short time); disaccharides may give positive result after prolonged heating
Key use	Differentiates mono- from disaccharides

8. Iodine Test (Starch)

Feature	Detail
Reagent	Iodine in potassium iodide (I2/KI)
Result	Deep blue-black colour
Principle	Iodine molecules are trapped within the helical coil of amylose to form a starch-iodine complex
Detects	Starch (specifically amylose)
Glycogen	Gives brownish-red colour
Dextrins	Give red, purple, or no colour
Simple sugars	No colour change
Reversible	Colour disappears on heating (helix uncoils), returns on cooling

9. Mucic Acid Test (Galactose)

Feature	Detail
Reagent	Conc. HNO3, heated
Result	White crystalline precipitate (mucic acid/galactaric acid)
Principle	HNO3 oxidises both terminal -OH groups of galactose to form mucic acid, which is insoluble
Detects	Galactose and galactose-containing sugars (lactose gives mucic acid after hydrolysis)
Glucose	Forms saccharic acid (soluble) - no precipitate

10. Osazone Test (Crystal Shape for Identification)

Feature	Detail
Reagent	Phenylhydrazine + sodium acetate + acetic acid, heated
Result	Yellow crystalline precipitate (osazone) - different crystal shapes for different sugars
Crystal shapes	Glucose/fructose/mannose: needle-shaped; lactose: cotton ball/mushroom; maltose: sunflower petals
Principle	Phenylhydrazine reacts with C1 and C2 of reducing sugars to form osazones
Key use	Identifies specific sugars by their crystal morphology and melting point
Note	Glucose, fructose, and mannose form the same osazone (glucosazone) because they differ only at C1 and C2

Summary Table

Test	Reagent	Positive Result	Detects
Molisch's	α-naphthol + conc. H2SO4	Purple ring	All carbohydrates
Benedict's	Cu²+ alkaline	Brick red ppt	Reducing sugars
Fehling's	Cu²+ alkaline tartrate	Brick red ppt	Reducing sugars
Tollens'	Ammoniacal AgNO3	Silver mirror	Aldoses
Seliwanoff's	Resorcinol + HCl	Cherry red	Ketoses (fructose)
Bial's	Orcinol + HCl + FeCl3	Blue-green	Pentoses
Barfoed's	Cu acetate (acidic)	Red ppt (fast)	Monosaccharides
Iodine	I2/KI	Blue-black	Starch (amylose)
Mucic acid	Conc. HNO3	White crystals	Galactose
Osazone	Phenylhydrazine	Yellow crystals	Reducing sugars (shape identifies type)

Clinical Relevance

Benedict's test is used clinically to detect glucosuria (e.g., in diabetes mellitus). Certain cephalosporin antibiotics can cause false-positive Benedict's and Fehling's tests.
Reducing sugars in urine of neonates (e.g., galactose in galactosemia, fructose in hereditary fructose intolerance) are detected using these tests.
Iodine test is used in histochemistry (PAS staining principle) to detect polysaccharides in tissues.

This is a shared conversation. Sign in to Orris to start your own chat.