lab activity 2 reducing & non-reducing sugars iug, 2015 dr. tarek zaida iug, 2015 dr. tarek...
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Lab Activity 2Reducing & Non-reducing Sugars
IUG, 2015Dr. Tarek Zaida
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Reducing- & Non-reducing Sugars
Sugars
Open-chain C=O
Closed-ringCarbonyl C
attached to O of ring & -OH
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Reducing sugars• Oxidized by mild oxidizing agents
Non-reducing sugars• Not oxidized by mild oxidizing
agents
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• All monosaccharides• Maltose, LactoseReducing
sugars
• SucroseNon-reducing
sugars
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Common Oxidizing Agents
Benedict’s sol. Fehling’s sol. Tollen’s reagent
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Oxidizing Reagent Benedict's Solution Fehling's Solution Tollen's Reagent
Composition copper sulfate in
alkaline citrate
copper sulfate in alkaline
tartrate
silver nitrate in aqueous
ammonia
Color of Solution deep blue deep blue colorless
Color After Reaction with a
Reducing Sugar
brick red precipitate
Cu2O(s)
brick red precipitate
Cu2O(s)
silver mirror forms
Ag(s)
Species Being Reduced
(the oxidant)
Cu2+
Cu2+ + e ---> Cu+
Cu2+
Cu2+ + e ---> Cu+
Ag+
Ag+ + e ---> Ag(s)
Species Being Oxidized
(the reductant)
reducing sugar
oxidized to carboxylate
reducing sugar
oxidized to carboxylate
reducing sugar
oxidized to carboxylate
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In-lab ExperimentsBe
nedi
ct’s
Test
For
red
ucin
g su
gars
Barf
oed’
s Te
st fo
r mon
osac
char
ides
Picr
ic A
cid
Test
For r
educ
ing
suga
rs
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1. Benedict's Test(positive for reducing sugars)
• Principle:Benedict's reagent contains cupric ions, which in an alkaline environment, oxidize the aldehyde group to a carboxylic acid. Cupric ions are reduced to cuprous oxide, which forms a red precipitateRCHO + 2Cu2+ + 4OH- ----> RCOOH + Cu2O + 2H2O
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Reagents
• Set up 1 % solutions of: glucose, sucrose, starch, maltose, fructose, lactose.• Benedict's reagent (ready to use)
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Procedure
1. Place 15 drops of the following 1% carbohydrate solutions in separate, labeled test tubes: glucose, fructose, sucrose, lactose, maltose, and starch.
2. Also place 1 ml of distilled water in another tube to serve as a control.
3. To each tube, add 1 ml of Benedict's reagent and heat the tubes in a boiling water bath for 5 minutes.
4. Remove the tubes from water bath. Note and record the results.
In the presence of a reducing sugar a precipitate which may be red, yellow or green will form.
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2. Barfoed's Test(Used to distinguish between mono- & di-saccharides)
• PrincipleBarfoed's reagent reacts with mono-saccharides to produce cuprous oxide at a faster rate than disaccharides do:
• RCHO + 2Cu2+ + 2H2O -----> RCOOH + Cu2O + 4H+
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Reagents
Set up 1 % solutions of the following solutions separately:
• glucose, • maltose, • fructose, • lactose, • sucrose• Barfoed's reagent.
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Procedure
• Procedure: 1. Place 15 drops of the following 1% carbohydrate
solutions in separate, labeled test tubes: glucose, fructose, sucrose, lactose, and maltose.
2. To each tube, add 1 ml of Barfoed's reagent, and heat in a boiling water bath for 10 minutes.
3. Remove the tubes from water bath. Note and record your observations.
A red precipitate will form if the test is positive.
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3. Picric Acid Test(for reducing sugars)
Principle• Picric acid (2,4,6-trinitrophenol) or TNP reacts
with reducing sugars to give a red colored picramic acid C6H2.OH.NH2(NO2)2
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Reagents
• Set up 1 % solution of: • maltose, • sucrose• Saturated solution of picric acid• 1N NaOH solution
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Procedure
1. Into a test tube add 1 ml of maltose solution, into the second tube, 1ml of sucrose solution.
2. Add into each tube 1 ml of a saturated solution of picric acid, and then add into each tube 0.5 ml of sodium hydroxide solution.
3. Heat both samples in a boiling water bath. In the presence of reducing sugars, the
solution stains red; a sodium salt of picric acid is formed.
4. Tollen’s Test (Silver Mirror)
• Is used to test for the presence of aldehydes. In this reaction, an aldehyde is oxidized to a carboxylic acid while the Ag+ is reduced to silver metal, which deposits as a thin film on the inner surface of the glass. The reaction is as follows:
Materials:0.1 M AgNO3 0.8 M KOH
0.5 M Dextrose Large test tube/rubber stopper
Conc. HNO3 Beaker
Conc. NH4OH Deionized water
Prepare Tollen’s reagent as follows: Add 50 mL of 0.1 M AgNO3 to the beaker and add NH4OH to this. A brown precipitate will form. Continue adding NH4OH until the
solution becomes clear. To this, add 25 mL of 0.8M KOH. Again, add NH4OH until solution becomes clear.
Procedure
1. Clean the test tube to be used by rinsing with concentrated nitric acid and washing well with hot water.
2. Add 1 mL of sample solution to the test tube3. To this add 5 mL of Tollen’s reagent.
The solution will turn yellow and brown then become cloudy and dark before silver begins to form on the inside of the test tube. This should take a couple of minutes.
4. Remove the contents from the test tube and rinse the tube with water.The tube with a “silver mirror” can now be observed.