Fehling's test is used to detect reducing sugars; which option lists sugars that are not reducing?

Fehling's test is used to detect reducing sugars; which option lists sugars that are not reducing?

• A. Trehalose
• B. Maltose
• C. Sucrose
• D. Two of the above

Answer: (D)

Fehling's test works on reducing sugars—those that have a free carbonyl form (aldehyde or ketone) capable of reducing copper. For disaccharides, this depends on whether the anomeric carbon(s) are involved in the glycosidic bond. Trehalose has a 1↔1 linkage between the anomeric carbons of two glucose units, so neither anomeric carbon is free; it cannot open to form a carbonyl, making it non-reducing. Sucrose links glucose C1 to fructose C2, involving both anomeric carbons as well, so it is also non-reducing. Maltose, however, has the glycosidic bond from the anomeric carbon of one glucose to C4 of the other, leaving the anomeric carbon of the second glucose free to form a carbonyl in solution, so maltose is reducing. Therefore, the sugars that are not reducing are trehalose and sucrose. The option reflecting two non-reducing sugars is the correct one.

Fehling's test works on reducing sugars—those that have a free carbonyl form (aldehyde or ketone) capable of reducing copper. For disaccharides, this depends on whether the anomeric carbon(s) are involved in the glycosidic bond. Trehalose has a 1↔1 linkage between the anomeric carbons of two glucose units, so neither anomeric carbon is free; it cannot open to form a carbonyl, making it non-reducing. Sucrose links glucose C1 to fructose C2, involving both anomeric carbons as well, so it is also non-reducing. Maltose, however, has the glycosidic bond from the anomeric carbon of one glucose to C4 of the other, leaving the anomeric carbon of the second glucose free to form a carbonyl in solution, so maltose is reducing. Therefore, the sugars that are not reducing are trehalose and sucrose. The option reflecting two non-reducing sugars is the correct one.