Problem 160 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Fructose reduces Fehling's solution due to the presence of (a) hydroxy group (b) ketone group (c) aldehde group (d) \(\alpha\) -hydroxyketone group

Step-by-Step Solution

Answer

Fructose reduces Fehling's solution due to the presence of an \(\alpha\)-hydroxyketone group.

Step 1: Understand the Chemical Properties

Fructose is a type of sugar, specifically a ketohexose, meaning it has six carbon atoms and a ketone group.

Step 2: Identify Functional Groups in Fructose

Fructose contains a ketone group and multiple hydroxy groups. However, it does not have an aldehyde group.

Step 3: Concept of Tautomerization

Fructose can tautomerize (rearrange its atoms) in aqueous solution. This allows the ketone group in fructose to convert into an aldehyde form through an intermediate enediol, which can further reduce Fehling's solution.

Step 4: Explanation of Reduction of Fehling's Solution

Fehling's solution is used to differentiate between water-soluble carbohydrates and ketone functional groups that can undergo enediol rearrangement. Fructose reduces Fehling's solution because it has an \(\alpha\)-hydroxyketone group. The presence of this group allows it to undergo tautomerization, temporarily forming an aldehyde during the reaction.

Key Concepts

Fructose

Fructose is one of the simplest sugars found naturally in fruits and honey. It is a ketohexose because it contains six carbon atoms and a ketone functional group. This sugar is important in food because it is sweeter than glucose, making it a popular choice for sweetening foods and drinks. In its structure, fructose features multiple hydroxy groups which contribute to its solubility in water. These hydroxy groups are alcohol-like, enabling various chemical reactions, including reductions in certain tests such as Fehling's solution test.

Tautomerization

Tautomerization is a chemical process where a compound can switch between two forms by rearranging its atoms. In the case of fructose, this process allows it to change its structure. This flexibility is due to the presence of an 1-hydroxyketone group in fructose. When in aqueous solution, fructose can undergo tautomerization to form an enediol intermediate which can rearrange into an aldehyde form. This is a reversible change and is central to why certain sugars can reduce Fehling's solution, as it involves temporary formation of an aldehyde.

α-Hydroxyketone

The b1-hydroxyketone group is a functional group where a hydroxy group is attached to the carbon next to the ketone group. This specific arrangement plays a crucial role in tautomerization. In fructose, the b1-hydroxyketone can quickly convert into different structures like an enediol form. This rearrangement capability is necessary for certain reactions, including the reduction observed with Fehling's solution. The ability to rearrange into an aldehyde form is crucial in testing for reducing sugars.

Ketohexose

Ketohexose refers to any sugar that contains six carbon atoms and a ketone group. Fructose is a perfect example of a ketohexose. Its classification as a ketohexose is important because it determines how it behaves in chemical reactions. The presence of both the ketone group and hydroxy groups allows ketohexoses to undergo tautomerization. This makes them active in chemical tests like the Fehling’s solution test. It's through this property that ketohexoses can temporarily form aldehydes, thus giving the characteristic reaction.