Question

State whether the following statements are true or false? (i) Glucose is the only aldose that shows mutarotation. (ii) The smallest aldose to form a hemiacetal is a tetrose. (iii) a-D-glucose \(\rightleftharpoons\) aldehyde form \(\rightleftharpoons\) b-D- glucose. This conversion is known as mutarotation. (iv) The anomers of D-glucose have specific rotations of same magnititude but opposite sign. (v) Sucrose is a 1,4 '-glycoside. (vi) In alkaline medium, fructose is a reducing sugar. (vii) Cellobiose is a polysaccharide. (viii) Natural sugar has D-configuration.

Short answer

A. Statement 1: Glucose is the only aldose that shows mutarotation B. Statement 4: The anomers of D-glucose have specific rotations of the same magnitude but opposite sign C. Statement 7: Cellobiose is a polysaccharide D. Statement 8: Natural sugar has D-configuration

Step-by-step solution

1. Glucose is the only aldose that shows mutarotation

False. Mutarotation is the change in the specific rotation of a compound as it interconverts between its alpha and beta anomers in an equilibrium. Several aldoses, including glucose, show mutarotation. Other aldoses such as galactose and mannose also exhibit mutarotation.

2. The smallest aldose to form a hemiacetal is a tetrose

True. Hemiacetals form when an aldehyde reacts with an alcohol. In the case of aldoses, this occurs when the carbonyl group reacts with the hydroxyl group on a different carbon. The smallest aldose to form a hemiacetal is a tetrose (four-carbon sugar) since it has at least two carbons apart to allow the formation of a hemiacetal ring structure.

3. a-D-glucose \(\rightleftharpoons\) aldehyde form \(\rightleftharpoons\) b-D-glucose. This conversion is known as mutarotation

True. Mutarotation is the interconversion between alpha and beta anomers of a compound. In this case, the statement correctly states that alpha-D-glucose can interconvert to its open-chain aldehyde form, which can then interconvert to its beta-D-glucose form.

4. The anomers of D-glucose have specific rotations of the same magnitude but opposite sign

True. Specific rotation is a measure of the optical rotation of a compound when polarized light is passed through it. For D-glucose anomers, the specific rotations are indeed of the same magnitude but opposite sign: +112.2° for alpha-D-glucose and -18.7° for beta-D-glucose.

5. Sucrose is a 1,4 '-glycoside

True. Sucrose is a disaccharide composed of glucose and fructose units linked by an alpha-1,2-glycosidic bond, also known as a 1,4 '-glycoside.

6. In alkaline medium, fructose is a reducing sugar

True. A reducing sugar is a sugar that can reduce other compounds and be oxidized itself. Fructose, an isomer of glucose, can act as a reducing sugar in an alkaline medium as it has an open chain aldehyde or ketone group that can be oxidized.

7. Cellobiose is a polysaccharide

False. Cellobiose is a disaccharide (two sugar units) consisting of two glucose molecules linked by a beta-1,4-glycosidic bond. A polysaccharide consists of many sugar units. Examples of polysaccharides include starch, cellulose, and glycogen.

8. Natural sugar has D-configuration

True. Most natural sugars exist in the D-configuration. This means that the hydroxyl group on the chiral carbon furthest from the carbonyl group is on the right side. Examples include D-glucose, D-galactose, and D-fructose.

Key concepts

Mutarotation

Mutarotation is a fascinating process that occurs in the world of carbohydrate chemistry. It involves the change in the optical rotation of a sugar solution as the sugar interconverts between its alpha and beta anomers. This phenomenon is especially noticeable in the case of glucose.

When you dissolve glucose in water, you may start with predominantly one form, either alpha-D-glucose or beta-D-glucose. However, over time, as the solution reaches equilibrium, these forms will convert back and forth between each other through an open-chain aldehyde intermediate.

This dynamic equilibrium results in a change in the optical rotation observed, which is exactly what we refer to as mutarotation. It's important to note that this isn't unique to glucose. Many other aldoses, like galactose and mannose, also exhibit mutarotation. They too can switch between their anomeric forms in solution, contributing to their specific rotations. This makes mutarotation a fundamental concept in understanding how certain carbohydrates behave in solutions.

Hemiacetal Formation

Hemiacetal formation is a crucial concept in understanding the structure of carbohydrates. It occurs when an aldehyde group reacts with an alcohol group within the same molecule. This gives rise to a new functional group known as a hemiacetal.

Aldoses, such as glucose, can form hemiacetals by reacting the carbonyl group on one carbon with a hydroxyl group on another carbon. The smallest aldose capable of doing this is a tetrose, which contains four carbon atoms. This structure allows for enough spatial separation between the aldehyde and alcohol groups to promote hemiacetal formation.

The formation of a cyclic hemiacetal significantly impacts the sugar's structural properties. For example, when glucose forms a cyclic hemiacetal, it creates the familiar ring structure found in many sugars. This cyclic form is often more stable than its open-chain form. Thus, hemiacetal formation is key to the ring structures seen in sugars, which then plays into mutarotation.

Anomeric Effect

The anomeric effect is an interesting phenomenon observed in carbohydrate chemistry, particularly in sugars like glucose. It describes the tendency of certain substituents on the anomeric carbon – the carbon that is the former carbonyl carbon – to prefer an axial configuration in a cyclohexane ring.

This preference has implications for the stability and formation of certain sugar forms. For glucose, the axial position is typically less favorable due to steric hindrance. However, due to the anomeric effect, sometimes, it is stabilized and becomes favored.

• The axial configuration allows for better electron delocalization through what is known as the endo anomeric effect.
• It contributes to the overall stability of the molecule due to a combination of electronic and steric factors.

Understanding the anomeric effect is key in predicting the properties and behavior of different sugar forms. It helps explain why certain anomers (for example, alpha anomers) may be favored over others in solution, which is essential when studying mutarotation and equilibrium in carbohydrate chemistry.

This knowledge is valuable for anyone delving into organic synthesis or carbohydrate biochemistry, as it impacts reactions, stability, and reactivity of sugars.