The carbohydrates in which the following properties are found are called reducing sugars. The reduction of Tollen’s reagent, Fehling’s solution, Benedict’s solution. All monosaccharides are reducing sugars. Contrary to this the carbohydrates in which the above properties are not found are called non-reducing sugars. For example, sucrose, starch, cellulose, etc. The disaccharides, in which carbonyl group of any one monosaccharide from both the monosaccharides, is present as hemiacetal or ketal and does not involve in glycosidic linkage formation, are reducing. For example, maltose and lactose. Two diastereomeric monosaccharides are called epimers when their configuration differs only at stereogenic centre. Monosaccharides form glycoside with alcohol in the presence of an acid. All monosaccharides show mutarotation.

Fructose reduces Fehling’s solution due to the presence of

α-hydroxyketone group

The correct answer is option 4. \(\alpha\)-hydroxyketone.

Fructose, a monosaccharide and a type of sugar found in fruits, possesses a unique functional group called the \(\alpha \)-hydroxyketone group, which distinguishes it from other sugars like glucose and sucrose. This group is crucial for its reducing properties, making it capable of reducing certain chemical reagents like Fehling's solution and Tollens' reagent.

The \(\alpha \)-hydroxyketone group consists of a ketone group (C=O) and a hydroxyl group (-OH) attached to the same carbon atom. The presence of both these functional groups in close proximity allows fructose to act as a reducing sugar. During a chemical reaction, the ketone group can undergo oxidation, while the hydroxyl group may participate in nucleophilic addition reactions.

When fructose is subjected to Fehling's solution, which contains copper(II) ions complexed with tartrate ions, the \(\alpha \)-hydroxyketone group of fructose gets oxidized. This oxidation leads to the reduction of copper(II) ions to copper(I) ions. The reduction of copper ions is accompanied by a color change in the solution, typically from blue to brick-red, indicating the formation of copper(I) oxide precipitate.

In summary, fructose reduces Fehling's solution due to the presence of the \(\alpha \)-hydroxyketone group, which undergoes oxidation, resulting in the reduction of copper(II) ions to copper(I) ions. This reaction serves as a qualitative test for the presence of fructose as a reducing sugar.