1.6 Fehling's test: reducing sugars

Introduction

Fehling's solution is a chemical test used to differentiate between reducing and non-reducing sugars. This test is based on the reaction of a functional group of a sugar molecules with fehling's reagent.

Fehling's reagent has two separate solutions: Fehling's A is a blue aqueous soluion of copper (II) sulphate; and Fehling's B clear and colourless solution of potassium sodium tartrate and sodium hydroxide.

Materials

-Test tube rack

-10 mL pipet

-Distilled water

-5 test tubes

-5 spatula

-Lactose
-Maltose
-Glucose
-Sucrose
-Starch
-Fehling's A and B solutions
-HCL

Objectives

1.Identify reducing sugars.
2.Comprehend redox reactions.
3.Understend the relation between structure and reducing ability of some sugars.

Procedure

In this experiment you will first determine with sugars give a positive test with Fehling's reagents and then, by testing the reaction of some organic molecules containing only a single functional group, you should de able to deduce with functional group of sugars is reacting with Fehling's reagent.

          1.Take 5 test tubes and label: G, M, S, L, ST.
          2.Put 2mL of distilled water inside each tube. 
          3.With different spatula put a small amound of each sugar. Dissolve the sugar.
          4.Add 2mL of Fehling's A solution and then Fehling's B.

          5.Place each test tube in a boiling water dath (250mL beaker on a hotplate stirrer).
          6.Observe what is happening.

Starch Hydrolysis:
Hydrolysis is the reaction of a compound with water. As you know, starch is a polymer, consisting of many units of -D-glucose covalent linked together.
          7.Place 2mL of 1% starch in a test tube and add 0,5mL of 3M HCL. Mix and place this           mixture in a boiling water bath for 10 minutes.
          8.After 10 minutes, remove the tube fromthe water bath and let it cool. Neutralize this           solution with 1 M NaOH and mix well.
          9.Transfer 8-10 drops of this solution to a small test tube.
          10.Add 1mL of Fehling's Ac solution and 1mL of Fehling's B.
          11.Heat for a few minutes in a boiling water bath.
          12.Record your observations. Compare the results of this test with your results for                 unhydrolyzed starch in the steo 1 of this experiment.
          13. You can test the absence of starch with iodine solution too!

Observations

We can see that the differents test tube have an orange colour.

Questions
1.From your observation and the structures of the sugars given above, indicate whith functional group in the sugar molecules reactys with Fehling's reagent.
2.Compare the results you obtained for the Fehling's test of starch and Fehling's test of hydrolyzed starch. Explain your results.
3.Would have you obtained a Fehling's positive test if you had hydrolyzed the sucrose (as you have done with starch)? why?
4.What does "reducing sugars" term means?

1.5 Saccharides properties

Introduction 
Saccharides are organic molecules consisting of C, H and O atoms; and the empirical formula is Cn H2n On.

Are divided into thte goups:
-Monosaccharides: formed by a linear carbon chain, the oligo and polysaccharides are formed by these.
They have a one funcional group: cetone (C=O) or aldehyde (COH).
Pentoses and hexoses in dissolution can cyclic forming shapes called furanoses and pyranosres.
-Oligosaccharides: small polymer containing between 2 and 10 monosaccharides.
-Polysaccharides: big polymers with more than 10 monosaccharides.

These molecules perform numerous roles in living organisms like storage if energy, structural components .

Saccharides yield 4,2 Kcal/g. They are a common source of eneregy in living organisms.


Materials
-Test tube rack
-10 mL pipet
-Distilled water
-5 test tube
-1 dropper
-5 spatula
-Lactose
-Maltose
-Glucose
-Sucrose
-Starch
-Lugol's iodine


Objectives
1.Identify differents sugars from it's properties.
2.Differentiate monoand disaccharides.
3.Understand the relation bettween structure and some properties.


Procedure
1.In the firspartof the experiment we are going to test some physcal properties of the saccharides you have in the lab: flavour, crysta structure and colour. How?
          a. Flavour:Put a small amount of each saccharides in your hand and test it! Is it sweet            or not sweet?
          b. Crystals: Observe a small amount of each saccharide on a clock glass under                      magnification.
          c. Colour: White, transparent or creamy.

The propierties of 5 saccharides:



To test solubility:
2. Clean and dry 5 test tubes and label them "G, M, L, SU, S" 
3. Put 5 mL of water in each test tube.
4. With the aid of a spatula, put a small amount of each saccharide inside the labelles test tube and test if they are soluble or insoluble.
5. Observe if each saccharide forms a mixture called dissolution or a colloidal suspension.

Lugol's iodine Test:
6. Finally, add 2 drops of Lugol's iodine to each test tube and test if the reaction is positive or negative. Lugol's is a solution of elemental iodine (I) and potassium iodine (KI) in water that is use to test a saccharide. The reaction is positive when iodine reacts by turning from yellow to a purple, dark-blue/dark colour.

Finally we are going to test if some foods contain starch. Try with potato!
1. Add 2 or 3 drops of Lugol's solution to one piece of potato observe what is happening.


Observations
We can see that different test tube we put lugol's iodine, and one is in the different colour, is ti the strach (positive).
Questions
1.Write the empirical formula of each saccharide thet you have use. Show structures ofthe five saccharides.Classify each one in one group: mono, oligo, or polysaccharide.
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2.Which of the monosaccharide are aldoses and wich are cetoses?
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3.Which bond links monosaccharide?
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4.Which saccharide/s is/are sweet? Is this property related to the structure of the molecules?
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5.Whichsaccharide/s is/are insoluble? Is this property related to the structure of the molecules?
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6.Which saccharide has reacted whith Lugol's iodine solution?
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7.Which kind of food contains starch?
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8.Calculate the energy from the nutrition facts label from a cereals:
   a.Calculate the energy that comes from the saccharides.
   b.Which % of the total energy comes from the saccharides?