Question

Define the term reducing sugar.

Short answer

A reducing sugar is a sugar that can act as a reducing agent due to having a free aldehyde or ketone group.

Step-by-step solution

Understanding Reducing Sugar

A reducing sugar is a type of sugar that has the ability to act as a reducing agent. This means it can donate electrons to another molecule, often in the presence of an alkaline solution.

Structure of Reducing Sugars

Reducing sugars typically have a free aldehyde or ketone group. These functional groups are reactive and can participate in redox reactions.

Examples of Reducing Sugars

Common examples of reducing sugars include glucose, fructose, and lactose. These sugars can reduce copper(II) ions to copper(I) ions in solutions such as Benedict's reagent.

Test for Reducing Sugars

To test for reducing sugars, one can use Benedict's test. When a reducing sugar is present, the solution changes color to indicate the reduction reaction.

Key concepts

reducing agent

In chemistry, a reducing agent is a substance that donates electrons to another substance. This process is known as reduction because the receiving substance gains electrons and, as such, its oxidation state is reduced. Reducing sugars have certain chemical properties that make them act as reducing agents. For example, glucose and fructose can donate electrons to other molecules due to their specific functional groups.

free aldehyde group

One of the critical structures in reducing sugars is the free aldehyde group or ketone group. The presence of these groups is what allows the sugar to act as a reducing agent. An aldehyde group has the chemical structure R-CHO, where the carbon atom is bonded to a hydrogen atom and a double bond oxygen atom. When placed in an alkaline solution, these groups are prone to oxidation, meaning they can lose electrons and reduce other substances in the process.

Benedict's test

Benedict's test is a simple way to determine if a sugar can act as a reducing agent. This test involves adding Benedict's reagent, which contains copper(II) sulfate, to a solution of the sugar. If the sugar is a reducing sugar, it will donate electrons to the copper(II) ions in the reagent, reducing them to copper(I) oxide, which forms a reddish precipitate. The color change in Benedict's test typically ranges from blue to green to yellow to brick-red, depending on the amount of reducing sugar present. This visual indication helps confirm the presence of reducing sugars in the tested solution.