P.6
Fehiling's testst: 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 sugar molecules with Fehling's reagent.
Fehling's reagent has two separate solutions:Fehling's A and Fehling's B
Fehling's A : Is a blue aqueous solution of copper (2) sulphate.
Fehling's B : Clear and colourless solution of potassium sodium tartrate and sodium hydroxide.
When both mixtures are mixed a deep blue colour solution can be seen.When a sugar has reducing ability, the mixture turns from deep blue colour to green colour suspension with a red precipitate
Some sugars are capable of reducing copper 2 ions to cooper 1 ions .This reducing ability is useful in classifying sugars .When the sugars 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 this electrons ( reduced).
Objectives :
1 - Identify reducing sugars .
2 -Comprehend redox reactions.
3 - Understand the relation between structure and reducing abiliy of some sugars.
Materials:
Test tube rack
10ml Pipet
Distilled water
5 test tubes
5 spatula
Lactose
Maltose
Glucose
Sucrose
Starch
Fehling's A and B solutions
HCL
Procedure:
In this experiment you will first determine with sugars give a positive test with Fehling's reagent and then by , testing the reactions of some organic molecules containing only a single functional group, you should be able to deduce which funcional group of sugar is reacting with Fehling's reagents.
1 -Take 5 test tubes and label:G,M,S,ST
2 -Put 2ml of distilled water inside each tube
3 -With different spatulas put a small amount of each sugar.Dissolve the sugar.
4 -Add 2ml of Fehling's A solution and the Fehling's B
5 -Place each test-tube in a boiling water bath (250ml beaker on 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 alfa-D-glucose covalenty linked togerther
7- Place 2ml of 1% starch in a test tube and add 0.5 ml of3M HCL.Mix and place this mixture in a boiling water bath for 10minutes
8- After 10 minutes , remove the tubew from the water bath and let it cool.Neutralize this solution with 1M NaOH and mix well
9 -Transfer 8-10 drops of this solution to a small test tube .
10 -Add 1ml of Fehling's B
11-Heat for a few minutes in a boiling water bath
12-Record you observations.Compare the results of this test with your results for unhydrolzed starch in the step 1 of this expereriment.
13- You can test the absence of starch with iodine solution too!
Results:
G ,M , lactouse: have reacted with Fehling reagent.The solution turs from ,blue to starch and sucre didn't reduce.
Fehling reagent.They are not reducing sugars.
Questions :
1 -From your observations and the structures of the sugars given above ,indicate which functional group in the sugars molecules reacts with Fehling's reagent
-Totes han reaccionat menys la sacarosa perque el seu enllac es fa entre el-OH del carboni enumeric del primer monosacarid i el -OH del segon monosacarid, acausa d'aixo es l'unic disacarid que no te poder reductor sobre el reactiu de Fehling
2 -Compare the results you obtained for the fehling's test of starch and Fehling test of hydrolyzed starch.Explain your resuls.
-El mido hidrolitzat era de color vermell i el que no estaba hidrolitzat era de un color mes fosc que hauria de haver sigut lila.
3 - Would you have obtained a Fehling's positive test if you had hydrolyzed the sucrose (as you have done with starch)? Why?
- Si s'hidrolitza es te una barreja de D-Glucosa i D-Fructosa ,i sobte levogira una substancia que desvia el pla de la llum polaritzada cap a l'esquerra.
4 -What does "reducing sugars" term mean?
-cuantsevol sucre que tingui un gup aldehi , o que pugui formarne un ,i que pot oxidar-se i passar electrons a un altre element o compost
