Problem 18 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following will not show mutarotation? [Main Online April 12, 2014] (a) Maltose (b) Lactose (c) Glucose (d) Sucrose

Step-by-Step Solution

Answer

Sucrose does not show mutarotation because it's a non-reducing sugar.

Step 1: Understanding Mutarotation

Mutarotation is the change in the optical rotation that occurs by the interconversion between different anomers of a sugar in solution. Anomers are different forms of the same sugar differing at the anomeric carbon (the stereocenter created from the carbonyl carbon in cyclic forms).

Step 2: Identify Sugars Showing Mutarotation

Sugars that have a free anomeric carbon capable of opening to an aldehyde or ketone form can undergo mutarotation. This is characteristic of reducing sugars. Maltose, lactose, and glucose have free anomeric carbons and are reducing sugars, thus they can show mutarotation.

Step 3: Identify Sugars Not Showing Mutarotation

Sucrose is a non-reducing sugar; it does not have a free anomeric carbon because the anomeric carbons of glucose and fructose are involved in the glycosidic linkage. Therefore, it does not undergo mutarotation.

Key Concepts

Anomers

Anomers are a type of stereoisomer found in carbohydrates that are critical in understanding mutarotation. When a monosaccharide forms a cyclic structure, it creates a new chiral center at the anomeric carbon. This carbon is derived from the carbonyl group (either an aldehyde or ketone) of the sugar.

• The two possible configurations at the anomeric carbon are known as alpha (α) and beta (β) anomers.
• These configurations differ in the spatial arrangement of atoms, specifically the position of the hydroxyl group (-OH) attached to the anomeric carbon.
• In an α-anomer, the hydroxyl group is positioned down, opposite the -CH₂OH group, while in a β-anomer, it is up, on the same side as the -CH₂OH group.

Mutarotation involves the equilibrium between these two forms as they interconvert in aqueous solution. It results in a mixture with specific optical properties. Mutarotation is an indication of the sugar's ability to exist in more than one anomeric form, but only sugars with a free anomeric carbon can show this behavior.

Reducing Sugars

Reducing sugars play a key role in the concept of mutarotation and are defined by their ability to donate electrons. This is possible through the free aldehyde or ketone group that forms when the cyclic sugar opens in solution.

• All monosaccharides, like glucose and galactose, are reducing sugars because they have free anomeric carbons.
• When disaccharides like maltose and lactose have a free anomeric carbon, they too can act as reducing sugars.
• The presence of the free anomeric carbon allows these sugars to potentially reduce other substances, like the Benedict’s or Fehling’s solution, during oxidation.

Reducing sugars exhibit mutarotation because they have an anomeric carbon that can freely switch between open and closed forms. As these sugars interconvert between different structural forms, changes in optical rotation can be observed.

Glycosidic Linkage

A glycosidic linkage is a covalent bond that joins a carbohydrate molecule to another group, which may also be a carbohydrate. This bond is mediated through the anomeric carbon of one sugar and often determines the sugar's classification as reducing or non-reducing.

• In sucrose, the glycosidic linkage involves the anomeric carbons of glucose and fructose, locking them in place and preventing them from opening up into free aldehyde or ketone forms.
• This linkage is an α(1→2)β bond, where one sugar is connected from its α-anomeric position to the β-anomeric position of the other.
• The absence of a free anomeric carbon in sucrose is what makes it a non-reducing sugar.

In contrast, maltose and lactose have glycosidic bonds that preserve a free anomeric carbon on one of their monosaccharide units, allowing them to participate in mutarotation and act as reducing sugars. Understanding the specific nature of these linkages helps in recognizing why some sugars can or cannot undergo mutarotation.