EXPERIMENT 2 Flashcards
molisch’s test
P: dehydration of carbohydrate by
sulfuric acid producing an aldehyde
• pentose → furfural
• hexose → hydroxymethylfurfural
• these aldehyde products
condense with α-naphthol to form
a purple product
• Molisch’s reagent: α-naphthol
dissolved in ethanol; + H2SO4
FEHLING’S TEST
P: cupric oxide is reduced by the
aldehyde portion of an aldose forming
a carboxylic acid and cuprous oxide
(aldehyde oxidizes to RCOOH)
Experimental Correlation:
• glu, xyl, fru, lac, mal: (+); reducing sugars
• suc: (-); non-reducing sugar
BENEDICT’S TEST
P: Benedict’s reagent contains blue
copper(II) ions (Cu2+) which are
reduced to copper(I) (Cu+). These are
precipitated as red copper(I) oxide
which is insoluble in water
• A&R:
• reducing sugar: orange - brick red
precipitate
• non-reducing sugar: absence of
orange – brick red precipitate
Experimental Correlation:
• all samples (yield positive result except
sucrose
• Sucrose can produce a positive result if
heated with dilute HCl prior to the test.
• The acidic conditions and heat break
the glycosidic bond in sucrose through
hydrolysis releasing glucose and
fructose (which are reductants).
clinical correlation of Benedict’s test
• tests reducing sugars in urine
• presumptive test for diabetes mellitus
• requires specific tests for glucose
(indicative of DM)
BARFOED’S TEST
• P: reduction of copper(II) acetate to
copper(I) oxide (Cu2O), which forms a
brick-red precipitate
• A: tests reducing monosaccharides
• disaccharides may also react, but the
reaction is much slower (heating for >2
min causes hydrolysis of glycosidic
linkage resulting to monosaccharides)
• a number of other substances,
(including NaCl), may interfere
• similar to Benedict’s test but lower in
pH and shorter heating time
Experimental Correlation:
• monosaccharides (+): glu, fru, xyl
• disaccharides (-): lac, mal, suc
NYLANDER’S TEST
• P: bismuth hydroxide is reduced to
black metallic bismuth
• mixture will darken if reducing sugar is
present and a black precipitate forms
after long standing
Experimental Correlation:
• all samples (glu, fru, xyl, lac, mal) yield
positive result
TOLLEN’S TEST
P: the diaminesilver(I) complex is an
oxidizing agent which in the presence
of the aldehyde of a sugar, is reduced
to silver metal, and further which in a
clean glass reaction vessel forms a
“silver mirror”.
• the aldehyde is oxidized to carboxylic
acids
Experimental Correlation:
• all samples yield positive result
- detection for ketoses
Principle: - ketohexoses (fructose) and
disaccharides with ketohexoses (sucrose) - cherry red condensation product
- other sugars ( aldoses)
- yellow to faint pink color
SELIWANOFFS TEST
- Detection for pentoses
• Principle
– Pentoses are converted to furfural by this
reagent, which forms a blue green color with
orcinol
– A blue-green color indicates a positive result.
Prolonged heating of some hexoses yields
hydroxymethyl furfural which also reacts with
orcinol to give colored complexes
BIAL’S ORCINOL TEST
IODINE TEST
• PRINCIPLE
• The amylose, or straight chain portion of starch, forms
helices where iodine molecules assemble, forming a
dark blue/black color.
• The amylopectin, or branched portion of starch, forms
much shorter helices and iodine molecules are unable to
assemble, leading the colour to be of an orange/yellow
hue. As starch is broken down or hydrolyzed into smaller
carbohydrate units, the blue-black color is not produced.
IODINE TEST
- formation of deep blue –
black color - presence of starch
- dark blue-black color –
starch and glucose - brown to blue – glycogen
- yellow or clear color –
negative test - (amylose +; amylopectin -)
• PRINCIPLE
• Starch – long chain glucose monomer, a polysaccharide
–poor solubility in solvents
• Glycogen is a branched polymer of glucose, and like
glucose, it has several hydroxyl groups per monomer. As
a result, it has lots of opportunities for hydrogen bonding
with water.
• Alcohols are non-polar and are equally charged on all
sides. The charged ends of the sucrose have nothing to
be attracted to other SUCROSE molecules,
therefore they will not dissolve.
ALCOHOL TEST
