Color Reactions of Carbohydrates

Test

Rx

Expected Result

Component Responsible forVR

Sugars which gives (+) result

General Test

Molisch

Detect compounds other than CHO

α- Naphthol

• Reacts w/ cyclic aldehydes

Conc. H2SO4

[pic 1]

Furfural                     Naphthol

strong acid(conc. H2SO4)

• 2 layers will form

sugar hydrolyze to become monosaccharide:  removed H2O  → formedfurfural or Hydroxymethylfurfural

• →condensed with (αnephthol )to form a violet ring(Furfuryldiphenylmethane orFurfurylDinaphthyl

Furfural

• produced from pentoses/pentosans

Hydroxymethylfurfural

• produced from hexoses and hexosans

Arabinose ,Cellulose, Fructose,Galactose,Glucose, Inulin ,Latose

Maltose, starch ,Sucrose

Differentiating Test for aldoses  and Ketoses

Seliwanoff

RESORCINOL

ConcHCl:dehydrate sugars[pic 2]

Cherry red (ketoses)& yellow to faint pink color(aldohexoses)

Ketoses( react faster)

• dehydrated:furfural derivatives

(Hydroxymethylfurfural)

→resorcinol: redcomplex

• Aldoses

Fructose- Ketose

Glucose- aldose

Inulin (red)-ketose

Sucrose-both

Pentoses (Green color)-presence:resorcinoleg. Arabinose, xylose

Bials

Qualitative test for pentoses& nucleotides w/ pentoses

Orcinol

Fe3+ ions

Blu-green Color

Furfural  (pentoses)

• Heated with conc. HCl
• Reacts with orcinol

Arabinose, Xylose

Barfoed’s test

Cupric acetate Glacial acetic acid

brick  red ppt

Cuprous Oxide

Glucose

Test for Reducing Sugar ( R – CHO  + 2Cu2+ +4OH→R – COOH + Cu2O + 2H2O)

Benedict Test

Sodium Citrate

Na3C6H5O7

Insoluble brick  red ppt

Cuprous Oxide (Cu2O)

Fructose ,Glucose , Maltose &Lactose

Fehilngs Test

Blusol’n indicates no reducing property

A: CuSO4

B: Rochelle salt( Sodium Potassium tartrate)

Insoluble brick  red ppt

Cuprous Oxide (Cu2O)

Fructose ,Glucose , Maltose &Lactose“

Tollen’s Silver Mirror Test

Ammonium Hydroxide (ammonia silver Nitrate)

1% Silver Nitrate

Silver Metal (Ag)

Glucose

Ferric Cyanide Test

Yellow color

Aldoses or ketoses

Phenylhydrazine Test (Osazone Formation)

Orange or yellow orange (2,4) Dinitrophenylhydrazine

Crystallize :seen under low power objective microscope

Glucose(needle)  , fructose, (needle) maltose (sunflower), lactose (powdered puff )

Specific Test for Sugars

Indole3-acetic acid

• Presence of acid catalyst

Intence purple color (40 C- 50C)

Fructose& large amount of glucose

Iodine Test for starch

•        This is a test for polysaccharides

• Dextrins- Amylo, Eryhthro and Achrodextrins, formed as intermediates during hydrolysis of starch give violet, red and no color with iodine respectively.

Deep blue black color →Reddish brown

Yellow-orange - negative. Purple-black -positive.

Amylose – deep blue starch-iodo complex Amylopectin/glycogen – blue violet

↑amt :Reddish violet

Glucose in the form of starch

Mucic Acid for Galactose

• Monosaccarides by nitric acid yields sol. Dicarboxylic acids.

InsolMucic acid: due to hydrolysis of glycosidic linkage b/w glucose &galactose

galactose