Question

Which of the following statements is/are correct (a) The \(\mathrm{C}_{1}\) carbon atom in aldoses and \(\mathrm{C}_{2}\) carbon atom in ketoses around which the configuration of epimers differ are called glycosidic carbon (b) All monosacchorides and disaccharides are reducing sugars. (c) All reducing carbonhydrates undergo mutarotation in aqueous solutions. (d) Glucose and fructose give the same osazone.

Short answer

Statements (c) and (d) are correct.

Step-by-step solution

Evaluate Statement (a)

Consider the definition of epimers and glycosidic carbon. Epimers are sugars that differ only at one chiral center. The carbon atom around which the configuration changes in epimers in aldoses is usually the carbon after the carbonyl group (like C-2 in aldoses). **Glycosidic carbons**, however, are involved in bond formations for glycosidic bonds, not the difference in epimers. Therefore, statement (a) is incorrect.

Evaluate Statement (b)

Monosaccharides and some disaccharides can be reducing sugars if they have a free aldehyde or ketone group that can be oxidized. However, not all disaccharides are reducing sugars, as some may have their reducing ends involved in glycosidic bonds. Hence, statement (b) is incorrect.

Evaluate Statement (c)

Reducing carbohydrates generally have a hemiacetal or hemiketal group that allows them to undergo mutarotation, changing the optical rotation as they open and close in solution. Therefore, all reducing carbohydrates do undergo mutarotation, making statement (c) correct.

Evaluate Statement (d)

Both glucose and fructose can form the same osazone because the reaction to form an osazone involves the C-1 and C-2 carbon atoms. The structural difference between glucose and fructose at these positions does not affect the formation of identical osazone crystals, thus verifying statement (d) as correct.

Key concepts

Glycosidic bonds

In carbohydrate chemistry, glycosidic bonds are essential connections that join two sugar molecules together. These bonds form when the hydroxyl group (-OH) of one monosaccharide interacts with the anomeric carbon of another. This occurs through a dehydration reaction, where water is expelled.
Typically, the anomeric carbon refers to the C-1 position in aldoses and the C-2 position in ketoses. Once a glycosidic bond is formed, the sugars are linked, and a disaccharide or polysaccharide is created.
Some important characteristics of glycosidic bonds include:

• They can involve different carbon atoms on the sugars, leading to various structural forms.
• The position and orientation of this bond influence the properties and digestibility of the polysaccharide.
• Breaking a glycosidic bond usually requires enzymatic activity since it is covalent.

Understanding these bonds is crucial for distinguishing how sugars will behave in biological systems. Once bonded, the sugars involved in a glycosidic bond may not participate in reactions typical of reducing sugars.

Reducing sugars

The term 'reducing sugars' refers to sugars capable of acting as reducing agents. This occurs when they possess a free aldehyde or ketone group in solution.
Such sugars can donate electrons (hence, the term 'reducing') and are prone to oxidation.

• Monosaccharides, like glucose and fructose, function as reducing sugars because they have such available groups.
• Some disaccharides may also act as reducing sugars if their glycosidic bond leaves a hemiacetal or hemiketal structure accessible.
• An example of a non-reducing sugar is sucrose, where its glycosidic bond ties up both reducing ends, preventing their reactivity.

Reducing sugars hold significant relevance in food chemistry and biological systems. For instance, they contribute to the Maillard reaction, which is responsible for browning and flavor in cooked foods.

Mutarotation

Mutarotation represents the change in optical rotation observed when a solution of a sugar undergoes equilibration between different anomeric forms. This behavior is prominent in reducing sugars that have either a hemiacetal or hemiketal group.
These sugars can exist in two different forms: alpha (α) and beta (β), differing at the anomeric carbon. As the sugar dissolves in water, it fluctuates between these forms, resulting in a gradual change in optical rotation until equilibrium is reached.

• For glucose, the rotation changes as it interconverts between α-glucose and β-glucose.
• Mutarotation is specific to sugars capable of opening their ring structure to form a straight-chain form containing the carbonyl group.
• The rate and extent of mutarotation can be affected by factors such as temperature and pH.

This property is an essential consideration, especially in analytical chemistry, as it impacts the precise measurement of sugar concentrations in solutions.

Osazone formation

Osazone formation is a classic chemical test used to identify reducing sugars based on their behavior with phenylhydrazine. In this reaction, the sugar reacts at specific carbon atoms to form a solid crystallized derivative known as an osazone.
This test is useful because it highlights structural similarities between sugars.

• The reaction typically targets the C-1 and C-2 carbons, transforming them into a specific crystalline structure.
• Remarkably, different sugars, like glucose and fructose, can give rise to the same osazone crystals due to their similarity at these reaction sites.
• This feature aids in distinguishing sugars with different structures but similar reaction properties.

Hence, osazone formation is not just a method for identifying sugars but also understanding sugar chemistry's subtleties at the molecular level.