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

When it comes to understanding the reactivity of sugars with reagents like Fehling’s solution, it’s crucial to distinguish between different types of sugars. Fructose is a common example of a ketose sugar, which means it has a ketone functional group.
Fructose participates in a specific reaction when treated with Fehling's solution, despite being a ketose and not an aldose, which typically reacts with this reagent. In simple terms, the ketone group of fructose gets chemically transformed into an aldehyde group through a reaction known as tautomerization, a process that enables ketoses to react with Fehling's solution just like aldoses.

The Role of Fehling's Solution

The presence of Fehling's solution, which contains a complexed copper(II) ion, acts as an oxidizing agent. It is commonly used to test for the presence of reducing sugars, which have free aldehyde or ketone groups capable of reducing the copper(II) ions to copper(I) ions, resulting in the characteristic red precipitate of copper(I) oxide.

The concept of tautomerization is particularly important in carbohydrate chemistry and is a fine example of structural isomerism. In the case of sugars like fructose, tautomerization refers to the chemical equilibrium between two isomers, where the keto form (ketose sugar) can reversibly convert to the enol form.
This conversion is facilitated by the presence of a catalytic amount of base or acid, and in the context of the Fehling’s test, the alkaline conditions promote this transformation. Once the ketose changes to an enol, it doesn't just stop there; it quickly rearranges to form an aldose sugar—a process called enolization.

Practical Relevance

In practical terms, tautomerization explains why a ketose sugar like fructose can give a positive Fehling's test, which would otherwise only be expected for aldose sugars. The transformation allows the otherwise non-reactive ketone group to become an aldehyde group, which is reactive in Fehling's solution.

Enolization is an essential concept when discussing the structure and reactivity of sugars in aqueous solutions. It is the process through which a keto form of a sugar molecule, such as fructose, is converted into its enol form. This interconversion takes place when a proton is transferred from a carbon atom to an oxygen atom within the molecule, resulting in the formation of a double bond with the adjacent carbon (this double-bonded carbon with an OH group defines an enol).
Keto-enol tautomerism is dynamic and depends largely on the conditions, such as temperature and pH. In the case of Fehling's test, the alkaline medium promotes this enolization, enabling the reaction to proceed.

Importance in Chemical Reactions

Enols are generally less stable than their keto counterparts, but the dynamic equilibrium allows for the temporary existence of the enol form, enabling certain chemical reactions that the stable keto form couldn't undergo. The enol form readily reacts with oxidizing agents like those present in Fehling's solution to yield a positive test for reducing sugars. This tautomerization is a fascinating example of chemical adaptability and reactivity.