FOURTH EXPERIMENT: Saccharides Properties

• Introduction:

Saccharides are organic molecules consisting of C, H and O atoms. Usually the empirical formula is C_nH_2nO_n . Are divided into three groups:

-Monosaccharides: formed by a linear carbon chain, are the building blocks of oligo and polysaccharides.
-Oligosaccharides: small polymer containing between 2 and 10 monosaccharides.
-Ploysaccharides: big polymers with more than 10 monosaccharides.
Saccharides yield 4,2 Kcal/gr.

• Materials:

1. Test tube rack.
2. 10 ml pipet.
3. Water.
4. 5 test tubes.
5. 1 dropper.
6. 1 Spatula.
7. Lactose.
8. Maltose.
9. Glucose.
10. Sucrose.
11. Starch.
12. Lugol's iodine.
13. Distilled Water.

• Procedure:

Complete the properties' table that you have below from the 5 saccharides you have at the lab:

	Glucose (G)	Maltose (M)	Sucrose (SU)	Lactose (L)	Starch (S)
Flavour (sweet/no sweet)	sweet	sweet	Very sweet	No sweet	No sweet
Cystalline from	crystalline	no cystalline	cystalline	no crystalline	no crystalline
Colour (white/cream)	white	white	white	white	cream
Solubility (soluble/insoluble)	soluble	soluble	soluble	soluble	Insoluble
Lugol's iodine (+/-)	-	-	-	-	+

                                            

1.      In the first part of the experiment we are going to test some physical propierties of the saccharides you have in the lab: flavour, crystal structure and colour. How?

a)      Flavour: put  a small amount of each saccharide in your han and taste it! Is it sweet or not?

b)      Crystals: Observe a small amount of each  saccharide on a clock glass under magnification.

c)      Colour: White, trasparent or creamy.

To test solubility:

2.      Clean and  dry 5 test tubes and label them  "G, M, L, SU, S".

3.      Put 5ml of water in each test tube.

4.      With the aid of a spatula, put a small amount of eavh saccharide inside the labelled test tube and test tube 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 solutions of elemental iodine (I) and potassium iodine (KI) in waterr that is use to test a saccharide. The reaction is positive when iodine reacts by turning from yellow to a purple, dark-blue/black 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 and observe what is happening.

• Questions:

1. Write the empirical formula of each saccharide that you have use. Show structures of the five saccharides. Classify each in one group: mono, oligo or polysaccharide.
   

   Glucose (mono): C₆H₁₂O₆

   Maltose (disaccharide): C₁₂H₂₂O₁₁ · H₂O

   Sucrose (disaccharide): C₁₂H₂₂O₁₁ · H₂O

   Lactose (disaccharide): C₁₂H₂₂O₁₁ · H₂O

   Starch (polysaccharide): (C₆H₁₀O₅)_n
2. Which of the monosaccharides are aldoses and wich ketoses? Aldoses: glucose, maltose, lactose, starch. Ketoses: sucrose.
3. Which bond links monosaccharides? Oglicocidic.
4. Which saccharide/s is/are insoluble? Is this property related to the structure of the molecule? Glucose, Maltose and Sucrose. Polysaccharides.
5. Which saccharide/s is/are insoluble? Is this property related to the structure of the molecule? Starch, high molecular density.
6. Wich saccharide has reacted with Lugol's iodine solution? Starch.
7. Which kind of food contains starch? Flour, cereal, pasta, rice, legums, etc.
8. Caculate the energy that comes from the saccharides. 23gr saccharides · 4,2 Kcal/gr = 96,6 Kcal.

   
   

Fheling's test: reducing sugar.

• Introduction:

Fehling's solution is a chemical test used to different between reducing and non-reducing sugars. This test is based on the reaction of a functional group of sugar molecules with Fehling's reagent.
Fehling's A: is a blue aqueous solution of copper (II) sulphate.
Fehling's B: clear and colourless solution of potassium sodium tartrate and sodium hydroxide.

• Procedure:

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

1. Take 5 test tubes and label: G, M, S, L, ST.
2. Put 2 mL of distilled water inside each tuve.
3. With different spatulas put a small amount of each sugar. Dissolve the sugar.
4. Add 2mL of Fehling's A solution and then Fehling's B.
5. Place each test-tuve in a boiling water bath (250mLbeaker 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 α-D-glucose covalently linked together.

1. Place 2mL of 1% starch in a test tuve and add 0.5mL of 3M HCl. Mix and place this mixture in a boiling water bath for 10 minutes.
2. After 10 minutes, remove the tuve from the water bath and let it cool. Neutralize this solution with 1M NaOH and mix well.
3. Transfer 8-10 drops of this solution to a small test tuve.
4. Add 1mL of Fehling's A solution and 1mL of Fehling's B.
5. Heat for a few minutes in a boiling water bath.
6. Record your observations. Compare the results of this test with your results for unhydrolyzed starch in the step 1 of this experiment.
7. You can test the absence of starch with iodine solution too!!

•  Questions:

1. From your observations and the structures of the sguras given above, indicate wich functional group in the sugar molecules reacts with Fehling's reagent. Glucose, Fructose, Maltose, Lactose
2.  Compare the results you obtained for the Fehling's test of the starch and Fehling's test hidrolyzed starch. Explain results. The starch react and becomes blue, because it react with lugol's iodine.
3. Would have you obyined a Fehling's positive test if you had hydrolyzed the sucrose (as you have done with starch9? Why? The sucrose has no free OH.
4. What does "reducing suars" term mean? Are reacting positively reduces teh Fehling's test. Because it has a fre OH.