Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Fructose is ketose sugar, even then it gives red precipitate with Fehling solution because (a) keto sugars undergo transformation into aldose sugars in the presence of Fehling's solution (b) ketones are oxidized by Fehling solution (c) both (a) and (b) (d) none of these

Step-by-Step Solution

Answer

(a) keto sugars undergo transformation into aldose sugars in the presence of Fehling's solution.

Step 1: Understand Fehling's Solution

Fehling's solution is a chemical reagent used to differentiate between water-soluble aldehyde and ketone functional groups. When aldehydes are present, they are oxidized to carboxylic acids, resulting in the formation of a red precipitate of copper(I) oxide.

Step 2: Identify Fructose as a Ketose

Fructose is a sugar molecule that contains a ketone group, specifically at the second carbon atom, which classifies it as a ketose. Normally, ketoses are not directly oxidizable by Fehling's solution.

Step 3: Keto-Enol Tautomerism

In aqueous solutions, fructose can undergo tautomerism (a chemical equilibrium between structural isomers) to convert from a ketose to an aldose form. This involves an enediol intermediate that rearranges the carbonyl group to the first carbon atom, temporarily forming an aldose like glucose.

Step 4: Reaction with Fehling's Solution

Once fructose is converted into its aldose form through tautomerism, it can react with Fehling’s solution. The aldehyde group in the newly formed aldose is oxidized to a carboxylic acid, producing the red precipitate.

Step 5: Evaluate the Answer Choices

Given that fructose undergoes keto-enol tautomerism to transform into an aldose capable of reacting with Fehling's solution, option (a) is correct. Option (b) is incorrect because ketones themselves are not oxidized by Fehling's solution without this transformation.

Key Concepts

Keto-Enol Tautomerism

Keto-enol tautomerism is an interesting concept in chemistry that involves the reversible transformation between two structural isomers, known as tautomers. In the case of fructose, which is a ketose sugar, this involves a shift from a keto form to an enol form. In aqueous solutions, ketoses like fructose can rearrange their structure to form enol tautomers. This transformation eventually results in what is known as aldehyde forms (aldoses). This shift from ketose to aldose is catalyzed by the presence of acids or bases, which facilitate the rearrangement process by enabling the formation of an intermediate known as an enediol. This enediol is crucial as it allows the migration of the carbonyl group, leading to the conversion into an aldose like glucose. Keto-enol tautomerism is essential in understanding how fructose can react with certain reagents, even though it is primarily a ketose under normal conditions.

Fehling's Solution

Fehling's solution is a classic chemical reagent used to detect the presence of aldehydes, but not ketones, in carbohydrates. The reagent is composed of two separate solutions: one containing copper(II) sulfate and the other containing potassium sodium tartrate mixed with a strong base, such as sodium hydroxide. When these two solutions are combined, a deep blue solution is formed that is able to oxidize aldehydes to carboxylic acids. This oxidation process subsequently results in the reduction of copper(II) ions to copper(I) ions, creating a characteristic red precipitate of copper(I) oxide (Cu2O). For a sugar like fructose, which initially doesn’t have an aldehyde group, it may seem that it wouldn't react with Fehling's solution. However, through keto-enol tautomerism, fructose converts to an aldose structure, thereby acquiring an aldehyde group capable of reacting with Fehling's solution, producing the red precipitate. This behavior underscores the importance of understanding the chemical transformations that certain sugars can undergo.

Aldose-Ketose Transformation

Aldose-ketose transformation is another fascinating aspect of carbohydrate chemistry. It occurs when a sugar with an aldehyde functional group (aldose) can convert into a sugar with a ketone functional group (ketose), and vice versa. This conversion is influenced by the presence of hydroxide ions and occurs through an intermediary known as an enediol. The enediol form acts as a bridge, allowing the interchange between aldoses and ketoses.

• In aqueous solutions, an aldose, such as glucose, can transform into a ketose, such as fructose, through this equilibrium process.
• This means that under the right conditions, sugars can extensively interconvert among different forms.

Understanding the interconversion between aldose and ketose forms is important because it allows for the flexibility of sugars to react differently depending on the environment. This adaptability is what enables fructose, typically a ketose, to convert into an aldose in solution. This flexibility also demonstrates how fructose can react with Fehling’s solution, as discussed above. These chemical transformations are crucial for many biochemical processes, making this a key area of study in organic chemistry.