1.
POST-LAB QUESTIONS Fructose is a ketose. Ketones usually do not give a positive reaction
with Fehling's reagent. Why is fructose oxidized by Fehling's reagent? (Consult your textbook)
2. What chemical test is performed to indicate the completion of the hydrolysis of starch by acid?
3. Which hydrolysis of starch is faster: the acid or the enzyme catalyzed reaction? 4. In your
textbook the structure of amylose is depicted. Amylose gives a positive ?ndine Ist and a negative
tt wt Fehling's reagent. On the basis of the structure of amylose explain the results of the two
tests.
Solution
Ans 1 :
Fehling's test is given positive only by aldehydes , so it becomes a differentiating test between
aldehydes and ketones.
But fructose being a ketose sugar also gives positive fehling's test because the base present in
the reagent shifts the double bonded carbon over the terminal position , due to which keto group
is converted into aldehydic group now. So the sugar then gives a positive test to the reagent.