Question

What is the results when the following are subjected to the feeling’s test and why?

Answer 1

Answer:

B, C, D, and E give positive tests.

Explanation:

Fehling's test is a test for reducing sugars, i.e., those that have a hemiacetal or potential aldehyde group (Fig. 1)

If the test is positive, a deep blue solution of complexed copper(II) ion is converted to a brick-fed precipitate of copper(I) oxide.

The equation for the reaction is

$\rm\underbrace{\hbox{RCHO}}_{\hbox{aldehyde}} +\underbrace{\hbox{2Cu^{2+}}}_{\hbox{deep blue}} + 5OH^{-} \longrightarrow \, RCOO^{-} +\underbrace{\hbox{Cu_{2}\text{O}}}_{\hbox{brick-red ppt}} + 3H _{2} O$

Positive Tests

Lactose (Fig. 2) is a disaccharide of galactose and glucose. It is a reducing sugar because it has a hemiacetal unit, indicated by the red arrow.

Maltose (Fig.3) is a disaccharide of glucose. It is a reducing sugar because it has a hemiacetal unit, indicated by the red arrow.

Fructose (Fig. 4) is a ketose, but it tautomerizes in basic media to aldoses glucose and mannose. thus, it gives a positive Fehling's test.

Glucose (Fig. 1) gives a positive test, because it has a potential aldehyde group.

Negative tests

Sucrose (Fig. 5) is a disaccharide of glucose and fructose. It is not a reducing sugar, because it lacks a hemiacetal group. It has only acetal groups (blue arrows).

Answer 2

Answer:

See below.

Explanation:

Sucrose - negative because this is not a reducing sugar.

Lactose, Maltose, Fructose and Glucose - positive as they are reducing sugars..

Reducing sugars have a free aldehyde or ketone group   whereas non-reducing sugars do not.