L6. Fehling's Test: Reducing sugars

On Monday 27th of October after doing our last experiment we had a little bit of time left so we decided to do another experiment realted with the sacharides. 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 sugar molecules with Fehling's regent. The objectives were to identify reducing sugars, comprehend redox reactions and understand the relation between structure and reducing ability of some sugars.

MATERIALS:
-Test tube rack
-10 mL pipet
-Distilled water
-5 test tubes
-Spatula

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

PROCEDURE:
1-We took 5 test tubes and labeled them: G, M, S, L, SU
2-We put 2 mL of distilled water inside each tube.
3- Then, with differents spatulas we put a small amount of each sugar. 
4-After that we added 2 mL of Fehling's A solution and then Fehling's B.
5-We placed each test tube in a boiling water bath (250 mL beaker on a hotplate stirrer)
6- Then we observed what was happening.

Results, observations and conclusions:  Glucose, maltose and lactose have reducing ability because they turned from deep blue to a red colour.  However, sucrose, in which the anomeric carbons of the two units are linked together, is a non-reducing disaccharide since neither of the rings is capable of opening. Starch is not a reducing sugar either, because the first ring cannot open up, there's no hydrogen on the circled oxygen to allow for ring opening. Similarly the next ring, and the next ring, et cetera, cannot open up.

QUESTIONS

1-From your observations and the structures of the sugars given above, indicate which functional group in the sugar molecules reacts with Fehling's reagent.

The OH group is the one that reacts with Fehling's reagent because when it is free the sacharide will have the reducing power.

If the bond is monocarbonilic the sacharide will have reducing power, that's why all monosacharides have it (the OH from the C1 is always free).

 

2-Compare the results you obtained for the Fehling's test of starch and Fehling's test of hydrolyzed starch. Explain your results.

I haven't done the Fehling's test of hydrolized starch but i can deduce it:

In the Fehling's experiment the starch doesn't have a reducing power because the OH is not free but when the starch is hydrolyzed it turns into glucose and glucose has a free OH because it's a monosacharide. Also the last glucose of the starch chain will have a reducing power (the ones that are in the ends).

The starch components are alfa D glucoses: amylose alfa (1 -> 4) linear chain, and amylopectine alfa (1->6) ramifications.

 

3-Would have you obtained a Fehling's positive test if you had hydrolyzed the sucrose? Why? 

Yes i would. Hydro meaning "water", and lysis, meaning "separation" usually means the cleavage of chemical bonds by the addition of water. So this means if we hydrolyze sucrose we will obtain glucose and fructose and they both will have a free OH because they are monosacharides. 

4-What does "reducing sugars" mean? 

A reducing sugar is the one that reacts positive to the Fehling's test. This means that they are capable of reducing coper II ions to copeer I ions. When the sugar to be tested is added to the Fehling's solution and the mixture is heated, some sugars can be oxidized (to lose electrons) and the Fehling's mixture can obtain the electrons (reduced). 

 

L5. Saccharides properties

On Monday 27th of October we did our weekly experiment in the laboratory. Saccharides are organic molecules consting of C, H and O atoms. The main objectives of this experiment were to identify the different sugars from its properties, differentiate mono and disaccharides and understand the relation between structure and some properties.

MATERIALS:
-Test tube track
-19 mL pipet
-Water
-5 test tubes
-1 dropper
-A spatula

-Lactose
-Maltose
-Glucose
-Sucrose
-Starch
-Lugol's iodine

PROCEDURE: 

In the first part of the experiment we had to test some physical properties of the saccharides we had in the lab: flavour, structure and colour. To know the flavour we put a small amount of each saccharide in our hand and tasted it. To see if they had crystals or not we observed a small amount of each saccharide on a clock glass under magnification. And to determine the colour we chose between white, transparent or creamy. 

Then we proceed to do the next part of the experiment, the test of solubility. 
1. We cleaned and dried 5 test tubes and labeled them "G, M, L, SU, S"
2. Then we put 5 mL of water in each test tube.
3. With the aid of a spatula, we put a small amount of each saccharide inside the labelled test tube and observed if they were soluble or insoluble. 

Lugol's iodine test
To each test tube we added two drops of Lugol's iodine (it acts as a starch detector) and tested if the reaction was positive or negative. Lugol's is a solution of elemental iodine (I) and potassium iodine (KI) in water that is used to test a saccharide. The reaction is positive when iodine reacts by turning from yellow to a purple, dark-blue colour. 

Results, observations and conclusions:
Lastly we completed our chart with the information. 

QUESTIONS:

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

Glucose: monosaccharide

 

C₆H₁₂O₆

Maltose: disaccharide

 C₁₂H₂₂O_{11  ·    H2O}     



                  

Sucrose: disaccharide 

C₁₂H₂₂O₁₁





Lactose: disaccharide

C₁₂H₁₂O_{11  ·}    H₂O 

Starch: polysaccharide

C₆H₁₂O₅ 








2-Which of the monosaccharides are aldoses and which are ketoses? 
Glucose: aldose // Maltose: aldose // Sucrose: ketose // Lactose: aldose // Starch: aldose

3-Which bond links monosaccharides?
O-glycosidic bond.

4-Which saccharides are sweet? Is this property related to the structure og the molecule?
Glucose, maltose and sucrose are sweet. Polysaccharides aren't sweet because of the size of the molecule, the human tongue can detect the short one- and two-molecules of the short chains. The long chains just slide on by.
 
5- Which saccharides are insoluble? Is this property related to the structure og the molecule?
 Starch because it's a big molecule (polysaccharide) with big molecular weight. 

6-Which saccharide has reacted with Lugol's iodine solution? 
 Starch.

7-Which kind of foods contain starch? 
Carbohydrates, which include pastas, breads, rice, cereals, flour, and beans. Starchy vegetables, such as potatoes, peas and corn.

8- Calculate the energy that comes from the nutrition facts label from a cereal:  
Calculate the energy that comes from the saccharides.

1 g of saccharides ----------> 4,2 Kcal
23 g of saccharides ---------> x

23 · 4,2 / 1 = 96,6 Kcal