Monosaccharides are carbohydrates which cannot be hydrolyzed to simple molecules. They may contain 3-7 carbon atoms but monosaccharides containing five and six carbon atoms are more abundant in nature. Those containing an aldehyde group are called aldoses while those containing a keto group are called ketoses. The aldehyde group is always 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. Pentoses ands hexoses have cyclic structures, furanose (five membered) and pyranose (six membered). Ribose in RNA and fructose in sucrose exist in the furanose form while glucose, mannose, galactose, etc. all exist in the pyranose form. During ring formation C₁ in aldohexoses and C₂ in fructose becomes chiral and hence all these monosaccharides exist in two stereoisomeric forms called the  α-anomer and the β-anomer while C₁ and C₂ are called glycosidic or anomeric carbon atoms and the bonds connected to glycosidic carbon are called glycosidic linkages. In contrast, stereoisomers, which differ in configuration at any other chiral carbon are called epimers. When two molecule of the same or different monosaccharide combine together through glycosidic linkage, a disaccharide is formed. All monosaccharides and reducing disaccharides react with excess of phenyl hydrazine to form osazones which are oftenly used for their characterization. 

Which of the following pairs give positive Tollen's test? 

Glucose, fructose 

The correct answer is option 2. Glucose, fructose.

Tollen's Test:

Purpose: Tollen's test is used to identify aldehydes and some alpha-hydroxy ketones.

Reagent: Tollen's reagent consists of ammoniacal silver nitrate \((Ag(NH_3)_2^+)\).

Reaction: Aldehydes are oxidized to carboxylic acids, and the Tollen's reagent is reduced to metallic silver, forming a characteristic silver mirror.

Analysis of Each Pair:

1. Glucose and Sucrose:

Glucose: Glucose is an aldohexose, meaning it has an aldehyde group (\(-CHO\)) at one end. The aldehyde group of glucose reacts with Tollen's reagent, reducing it to metallic silver, thus giving a positive Tollen's test.

Sucrose: Sucrose is a disaccharide composed of glucose and fructose linked by a glycosidic bond. The glycosidic bond in sucrose involves the anomeric carbon of glucose and the ketone group of fructose, which "locks" these groups and prevents them from reacting. Sucrose does not have a free aldehyde or alpha-hydroxy ketone group, so it does not give a positive Tollen's test.

2. Glucose and Fructose:

Glucose: As mentioned, glucose has a free aldehyde group and gives a positive Tollen's test.

Fructose: Fructose is a ketohexose, meaning it has a ketone group (\(C=O\)) at the second carbon. Under basic conditions (such as those present in Tollen's reagent), fructose can tautomerize to form glucose and mannose, which have free aldehyde groups. Due to this tautomerization, fructose indirectly gives a positive Tollen's test by converting to glucose or mannose, which can then reduce the Tollen's reagent.

3. Hexanal and Acetophenone:

Hexanal: Hexanal is an aldehyde. It reacts directly with Tollen's reagent, reducing it to metallic silver and giving a positive test.

Acetophenone: Acetophenone is a ketone and does not have an aldehyde group. Ketones do not typically react with Tollen's reagent unless they are alpha-hydroxy ketones, which acetophenone is not. Therefore, acetophenone does not give a positive Tollen's test.

4. Fructose and Sucrose:

Fructose: As explained, fructose can isomerize to glucose or mannose under the test conditions, giving a positive result.

Sucrose: Sucrose does not have a free aldehyde or ketone group due to its glycosidic bond, so it does not give a positive Tollen's test.

Summary:

Only the pair of glucose and fructose both give a positive Tollen's test. Glucose directly reacts with Tollen's reagent due to its free aldehyde group, and fructose can tautomerize to form glucose or mannose under basic conditions, leading to a positive test.

So the correct answer is: Glucose and Fructose