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.

Which of the following statements is incorrect?

Fehling solution and Tollen's reagent can reduce non-reducing sugars

The correct answer is option 3. Fehling solution and Tollen's reagent can reduce non-reducing sugars.

Let us explain each statement in detail:

1. All monosaccharides, whether aldose or ketose, are reducing sugars:

Monosaccharides are simple sugars that cannot be hydrolyzed into simpler sugars. They are classified into two main categories based on the presence of a carbonyl functional group: aldoses and ketoses. Aldoses are monosaccharides that contain an aldehyde functional group \((-CHO)\), while ketoses are monosaccharides that contain a ketone functional group \((C=O)\). Both aldoses and ketoses possess a carbonyl group that can undergo oxidation-reduction reactions, making them capable of reducing other substances. Therefore, all monosaccharides, whether they are aldoses (e.g., glucose, galactose) or ketoses (e.g., fructose), are considered reducing sugars.

2. Lactose is present in milk:

Lactose is a disaccharide sugar composed of one molecule of glucose and one molecule of galactose linked by a β-glycosidic bond. Lactose is commonly found in milk and dairy products, where it serves as the primary carbohydrate source for mammalian milk.

3. Fehling solution and Tollens' reagent can reduce non-reducing sugars:

Fehling's solution and Tollens' reagent are chemical tests used to detect the presence of reducing sugars. These reagents contain metal ions (copper ions in Fehling's solution and silver ions in Tollens' reagent) that are reduced by the aldehyde or ketone groups present in reducing sugars. Non-reducing sugars, such as sucrose, do not contain a free aldehyde or ketone group and therefore cannot undergo oxidation-reduction reactions with Fehling's solution or Tollens' reagent. Only reducing sugars can reduce these reagents.

4. Glucose is an aldohexose:

Glucose is a monosaccharide sugar consisting of six carbon atoms (hexose) and is classified as an aldohexose. It contains an aldehyde functional group \((-CHO)\) at the first carbon atom, making it an aldose. Therefore, glucose is specifically categorized as an aldohexose.

In summary, while the first three statements are correct, the third statement is incorrect. Fehling's solution and Tollens' reagent specifically react with and reduce reducing sugars, not non-reducing sugars. Non-reducing sugars lack a free aldehyde or ketone group necessary for the oxidation-reduction reactions with these reagents.