Monosaccharides are carbohydrates which cannot be hydrolysed into simpler molecules. Those containing an aldehyde group are called aldoses while those containing a keto group are called ketoses. The aldehyde group is present at C1 while the keto group is usually present at C2. All monosaccharides reduce Tollens’ reagent as well as Fehling’s solution and hence are called reducing sugars. Glucose has a six-membered cyclic structure called the pyranose structure, while fructose in sucrose has a five-membered cyclic structure called the furanose structure. During ring formation, C1 of glucose becomes chiral, and hence glucose exists in two cyclic forms called the α-anomer and the β-anomer. When two molecules of the same or different monosaccharides combine through a glycosidic linkage, a disaccharide is formed.
In which of the following pairs do both compounds give a positive Tollens’ test?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Glucose, fructose
The passage states that all monosaccharides reduce Tollens’ reagent as well as Fehling’s solution, and so are reducing sugars. Glucose (an aldose) and fructose (a ketose) are both monosaccharides, so both give a silver mirror with Tollens’ reagent.
Why the other options are incorrect
Option (1) Glucose, sucrose — Incorrect. Glucose gives the test, but sucrose is a non-reducing sugar: the reducing groups of its glucose and fructose units are tied up in the glycosidic linkage.
Option (3) Hexanal, acetophenone — Incorrect. Hexanal, an aldehyde, gives the test, but acetophenone is a ketone; ketones are not oxidised by mild oxidising agents such as Tollens’ reagent.
Option (4) Fructose, sucrose — Incorrect. Fructose gives the test, but sucrose does not, as it is a non-reducing sugar.