Question

To determine the size of ring of Aldohexoside, a scientist uses periodic acid on sample of glycoside. He obtained one mole of compound which reduces Tollen's reagent (unique behaviour in its category). This suggests (A) Pyranoside form in glycoside (B) Furanoside form in glycoside (C) Open chain structure (D) Insufficient data

Short answer

The correct answer is (D) Insufficient data. The given information on the reaction of the glycoside with periodic acid and the unique behavior of reducing Tollen's reagent is not enough to conclusively determine whether the glycoside is a pyranoside, furanoside, or an open-chain structure.

Step-by-step solution

Understanding the periodic acid test

A periodic acid test is a chemical test used to determine the amount of glycoside rings in a compound. The test is performed by oxidizing the glycoside with periodic acid, and then the amount of the oxidation products will be analyzed to give information about the ring structure of the glycoside.

Determining the meaning of the results

The fact that the scientist obtained one mole of a compound with the unique behavior of reducing Tollen's reagent suggests that the glycoside may have a specific ring structure that allows it to reduce Tollen's reagent. Tollen's reagent (Ammoniacal silver nitrate) reduces aldehyde groups, so we can conclude that the compound contains an aldehyde group.

Analyzing each option

(A) Pyranoside form in glycoside: Pyranosides are six-membered ring structures (five carbon atoms and one oxygen atom). The aldehyde group is usually part of the ring structure, making it less likely for it to react with Tollen's reagent, so this option is not likely to be the correct one. (B) Furanoside form in glycoside: Furanosides have five-membered ring structures (four carbon atoms and one oxygen atom). These rings are more susceptible to cleavage and oxidation reactions than pyranosides due to the strained ring structure. However, the aldehyde group obtained from furanoside should not have the unique ability to reduce Tollen's reagent since a regular furanoside does not possess this property. So, we don't have enough information to determine if this is the correct option. (C) Open chain structure: In the open-chain structure, the glycoside is no longer part of a ring, and the aldehyde group is exposed. This could explain the ability of the compound to reduce Tollen's reagent. However, the fact that we got only one mole of the reactive compound indicates that not all molecules in the sample were open-chained. Again, we don't have enough information to determine if this is the correct option. (D) Insufficient data: Since we cannot determine with certainty whether the glycoside is a pyranoside, furanoside, or open-chain structure based on the given data, the correct answer is (D) Insufficient data.

Key concepts

Periodic Acid Test

The periodic acid test is a valuable chemical procedure used to gain insights into the structure of carbohydrates and glycosides. This test involves treating the compound with periodic acid (\(HIO_4\)), which oxidizes specific carbons in the molecule. During this oxidation, glycosidic linkages are broken at particular points, leading to the formation of smaller fragments. By analyzing these fragments, chemists can deduce the type and arrangement of rings in the original glycoside.

One highlight of the periodic acid test is its ability to distinguish between five-membered and six-membered sugar rings. Here’s how it typically works:

• Furanosides, or five-membered rings, will more readily undergo cleavage because of the higher ring strain.
• Pyranosides, with six-membered rings, may show different cleavage patterns due to less ring strain.

This capacity to break glycosidic bonds and highlight the structure makes periodic acid a powerful tool in structural analysis.

Tollen's Reagent

Tollen's reagent, known formally as ammoniacal silver nitrate, is a classic chemical reagent used to detect aldehyde groups. It is colorless and contains silver ions in an aqueous ammonia solution. This reagent is famous for its ability to distinguish aldehydes from other functional groups due to the following properties:

• Aldehydes reduce Tollen's reagent, converting the silver ions to metallic silver. This results in the characteristic "silver mirror" coating on the reaction vessel.
• Ketones, in contrast, do not typically react with Tollen’s reagent, making it a selective test for aldehydes.

In the context of glycosides, if a glycoside reacts with Tollen’s reagent, this indicates the presence of a free or tautomeric aldehyde group within or generated by the structure upon cleavage. Hence, this reactivity provides insights into the structural possibilities of the glycoside being tested.

Ring Strain in Glycosides

Ring strain is a concept in organic chemistry that refers to the instability that arises from differences between the natural angles in a ring and the angles the atoms are forced into. Glycosides, which are ring-shaped carbohydrates, can exhibit varying degrees of ring strain.

Furanosides, for instance, are five-membered rings that tend to have more ring strain compared to their six-membered counterparts, pyranosides. This strain is because the bond angles in a furanoside are narrower than the ideal tetrahedral angle, leading to greater instability.

• This instability makes furanosides more reactive, particularly under conditions where bonds may be cleaved, such as the periodic acid test.
• Pyranosides, with their six-membered rings, are closer to the ideal angle and thus more stable, exhibiting less reactivity in similar conditions.

Understanding ring strain helps chemists predict the reactivity of these molecules and infer structural information from reactions.