After a series of Kiliani–Fischer syntheses on an unknown sugar is isolated from the reaction mixture

/ Exams & Certification / By

After a series of Kiliani–Fischer syntheses on an unknown sugar is isolated from the reaction mixture. The following experimental information is obtained:

(1) Molecular formula C6H12O6

(2) Undergoes mutarotation.

(3) Reacts with bromine water to give an aldonic acid.

(4) Reacts with phenylhydrazine to give an osazone, mp 178 °C.

(5) Reacts with to give an optically active aldaric acid.

(6) Ruff degradation followed by oxidation gives an optically inactive aldaric acid.

(7) Two Ruff degradations followed by oxidation give meso-tartaric acid.

(8) Formation of the methyl glycoside (using and HCl), followed CH3OH wed by periodic acid oxidation, gives a

of products that includes(+)glyceraldehyde.

(a) Draw a Fischer projection for the open-chain form of this unknown sugar. Use Figure 23-3 to name the sugar.

(b) Draw the most stable conformation of the most stable cyclic hemiacetal form of this sugar, and give the structure a complete systematic name.

The Correct Answer and Explanation is :

Let’s break down the information systematically and use it to identify the unknown sugar.

Information Analysis:

  1. Molecular Formula C6H12O6: This suggests a hexose sugar (a 6-carbon sugar).
  2. Mutarotation: This indicates that the sugar is a cyclic hemiacetal, meaning it can switch between two anomeric forms (α and β).
  3. Reacts with bromine water to give an aldonic acid: This means the sugar has an aldehyde group, which can be oxidized to a carboxylic acid.
  4. Reacts with phenylhydrazine to give an osazone, mp 178°C: This suggests that the sugar is an aldose (as osazone formation occurs with aldoses).
  5. Reacts with a solution to give an optically active aldaric acid: Aldaric acids are produced when the aldehyde is oxidized to a carboxylic acid at both ends.
  6. Ruff degradation followed by oxidation gives an optically inactive aldaric acid: This suggests that the sugar has a symmetric structure (because the degradation of the molecule produces an optically inactive compound).
  7. Two Ruff degradations followed by oxidation give meso-tartaric acid: Meso-tartaric acid has a symmetric structure and two hydroxyl groups. This indicates the presence of chirality centers in the sugar that result in a meso form.
  8. Formation of the methyl glycoside (using CH3OH and HCl), followed by periodic acid oxidation, gives products that include (+)-glyceraldehyde: This indicates the presence of a free aldehyde group in the open-chain form of the sugar.

Step (a): Fischer Projection of the Open-Chain Form

Based on the clues, the sugar is likely D-glucose. Here’s why:

  • Glucose is a six-carbon aldose sugar with the formula C6H12O6.
  • It undergoes mutarotation because it can form α and β anomers in its cyclic form.
  • It reacts with bromine water to form an aldonic acid, which is consistent with an aldose.
  • It forms an osazone with phenylhydrazine and gives an aldaric acid upon oxidation.
  • The Ruff degradation steps leading to meso-tartaric acid indicate that glucose has a symmetric structure with two chiral centers.

Here is the Fischer projection for D-glucose:

        CHO
         |
    HO–C–H
         |
    HO–C–H
         |
    HO–C–H
         |
        CH2OH

Step (b): Cyclic Hemiacetal Form of D-Glucose

In solution, D-glucose predominantly forms a pyranose ring (six-membered ring) because it can form a stable six-membered cyclic structure through the reaction of the hydroxyl group on C5 with the aldehyde group on C1.

The most stable conformation of the pyranose form of D-glucose is the chair form, with the bulky groups in the equatorial positions. In this conformation, the hydroxymethyl group at C6 is axial, but the hydroxyl groups at C2, C4, and C5 are in the equatorial position, minimizing steric strain.

Here is the structure of D-glucose in its most stable cyclic form (α-D-glucopyranose):

        O
       / \
  HO-C   C-OH
     |     |
  HO-C     C-H
     |     |
  HO-C     CH2OH
     |
    CH2OH

Systematic Name: α-D-glucopyranose.

Explanation (300 words):

The unknown sugar described in the problem is most likely D-glucose, a hexose aldose sugar. Let’s go through the reasoning for this identification:

  • The molecular formula C6H12O6 indicates a hexose, which is a six-carbon sugar. Given that it undergoes mutarotation and reacts with bromine water to form an aldonic acid, it points to an aldose (a sugar with an aldehyde group).
  • The fact that the sugar forms an osazone with phenylhydrazine suggests it is an aldose, which is characteristic of sugars that have a free aldehyde group. In addition, the sugar reacts with periodic acid to form glyceraldehyde, which further supports the idea of an aldose.
  • The mention of Ruff degradation and the formation of meso-tartaric acid after two degradations suggests the sugar is symmetric. D-glucose has two chiral centers, and its degradation leads to symmetric products like meso-tartaric acid.
  • The formation of an optically active aldaric acid and the fact that it can be oxidized to produce symmetrical products like meso-tartaric acid help confirm the structure of glucose as the sugar in question.

Thus, the sugar is D-glucose, and its open-chain form is a D-aldose with the structure provided above. Its most stable cyclic form is α-D-glucopyranose, a six-membered ring formed by the reaction of the hydroxyl group at C5 with the aldehyde at C1.

Related Posts

Scroll to Top