$2\text{Cu}^{2+} + 4\text{OH}^- + \text{RCHO} \rightarrow \text{Cu}_2\text{O} + \text{RCOOH} + 2\text{H}_2\text{O}$

Reduction of Cu$^{2+}$ to Cu$_2$O by the aldehyde group of the sugar

Question 1

Which of the following sugars would be predicted to be the LEAST reactive in a Fehling's test for reducing sugars?

Accepted Answer

β-D-Glucose

Question 2

Fehling's test detects the presence of which functional group in reducing sugars?

Accepted Answer

Aldehyde group

Question 3

Which of the following statements about Benedict's test is FALSE?

Accepted Answer

It is less sensitive than Fehling's test.

Question 4

In Tollen's test, what is the final product formed if a reducing sugar is present?

Accepted Answer

Ag (metallic silver)

Question 5

Which of the following best explains why sucrose does NOT give a positive result in Fehling's test?

Accepted Answer

Sucrose is a non-reducing sugar because its anomeric carbons are involved in a glycosidic bond.

Question 6

Which of the following is the correct sequence of color changes observed in Fehling's test for a positive result?

Accepted Answer

Blue to red

Question 7

A student adds an unknown sugar to Benedict's reagent and observes a brick-red precipitate. What does this indicate about the sugar?

Accepted Answer

The sugar contains a free aldehyde or ketone group.

Question 8

Which of the following reactions correctly represents the reduction of Cu$^{2+}$ ions in Fehling's test?

Accepted Answer

$2	ext{Cu}^{2+} + 4	ext{OH}^- + 	ext{RCHO} ightarrow 	ext{Cu}_2	ext{O} + 	ext{RCOOH} + 2	ext{H}_2	ext{O}$

Question 9

Which test would you use to distinguish between a reducing and a non-reducing sugar, and what observation would confirm a reducing sugar?

Accepted Answer

Tollen's test; formation of a silver mirror

Question 10

Which of the following best describes the chemical principle behind the color change in Benedict's and Fehling's tests?

Accepted Answer

Reduction of Cu$^{2+}$ to Cu$_2$O by the aldehyde group of the sugar

Question 11

A solution of lactose is tested with Fehling's, Benedict's, and Tollen's reagents. Which of the following results would you expect?

Accepted Answer

Positive result with all three tests

Question 12

Which of the following statements is TRUE regarding the reactivity of ketose sugars in Fehling's and Benedict's tests?

Accepted Answer

Ketose sugars can give a positive result after tautomerization to aldoses under basic conditions.

Question 13

Which of the following best explains why a sugar with a free anomeric carbon is a reducing sugar?

Accepted Answer

The free anomeric carbon allows the sugar to exist in an open-chain form with a reactive carbonyl group.

Question 14

A student tests a sugar solution with Tollen's reagent and observes no silver mirror. What is the most likely explanation?

Accepted Answer

The sugar is a non-reducing sugar.

Question 15

Which of the following is a common misconception about Benedict's and Fehling's tests?

Accepted Answer

They can detect all sugars, including non-reducing sugars.

Question 16

Which of the following equations represents the reaction occurring in Tollen's test?

Accepted Answer

$	ext{RCHO} + 2	ext{Ag}^+ + 2	ext{OH}^- ightarrow 	ext{RCOO}^- + 2	ext{Ag} + 2	ext{H}_2	ext{O}$

Tests for Reducing Sugars: Fehling’s, Benedict’s, and Tollen’s Tests

Study Guide - Practice Questions