Two sugars, A and B, are known to be glucose and galactose, but it is not certain which one is which. On treatment with nitric acid, A gives an optically inactive aldaric acid, while B gives an optically active aldaric acid. Which sugar is glucose, and which is galactose?

Verified step by step guidance

1. Step 1: Understand that glucose and galactose are both monosaccharides, and they can be differentiated by their reactions with nitric acid. Nitric acid oxidizes the aldehyde group at the reducing end of the sugar to a carboxylic acid, forming an aldaric acid.
2. Step 2: Recall that glucose forms an optically inactive aldaric acid upon oxidation with nitric acid. This is because the aldaric acid formed from glucose is meso-gluconic acid, which is optically inactive due to the presence of a plane of symmetry.
3. Step 3: On the other hand, galactose forms an optically active aldaric acid upon oxidation with nitric acid. This is because the aldaric acid formed from galactose is not symmetrical and thus does not have a plane of symmetry, making it optically active.
4. Step 4: Based on the information given in the problem, sugar A forms an optically inactive aldaric acid and sugar B forms an optically active aldaric acid.
5. Step 5: Therefore, sugar A is glucose and sugar B is galactose.

Key Concepts

Aldaric Acids

Aldaric acids are dicarboxylic acids derived from aldoses, formed when an aldose is oxidized at both the aldehyde and alcohol functional groups. The optical activity of these acids can vary based on the structure of the original sugar. In this case, the formation of an optically inactive aldaric acid indicates that the sugar has a symmetrical structure, while an optically active aldaric acid suggests asymmetry.

Optical Activity

Optical activity refers to the ability of a compound to rotate the plane of polarized light, which is a characteristic of chiral molecules. A sugar that is chiral will produce an optically active aldaric acid, while a sugar that is achiral will yield an optically inactive product. This property is crucial for distinguishing between glucose and galactose based on their oxidation products.

Glucose vs. Galactose

Glucose and galactose are both hexose sugars but differ in the arrangement of hydroxyl groups around their carbon atoms, making them stereoisomers. Glucose is more symmetrical, leading to the formation of an optically inactive aldaric acid upon oxidation, while galactose, being less symmetrical, results in an optically active aldaric acid. This distinction is key to identifying which sugar corresponds to A and B in the question.