Problem 25 - Chemistry in 30 days

Question

The change in the optical rotatine (with time) of freshly propared solution of wugar is knewn as (a) Specitic rotation (b) Inversaan (c) Rotatery motion (d) Mutarotation

Step-by-Step Solution

Answer

The correct answer is (d) Mutarotation.

Step 1: Understanding the Question

The exercise asks about the change in optical rotation of a freshly prepared sugar solution over time. Optical rotation is the measurement of the rotation of plane-polarized light by a chiral compound.

Step 2: Identify the Concept

The specific phenomena is related to sugars and the change in their rotation with time due to interconversion between different forms. This change happens notably in solutions of sugars like glucose.

Step 3: Definition of Terms

1. Specific Rotation is a measure of a compound's optical activity. 2. Inversion refers to a change in the order of elements. 3. Rotary motion is a mechanical concept describing an object that turns around an axis. 4. Mutarotation is the change in optical rotation due to the change in equilibrium between two or more anomers.

Step 4: Choosing the Correct Option

Based on the definitions and the specific context here, mutarotation refers to the change in optical rotation over time. Thus, the correct answer is (d) Mutarotation.

Key Concepts

Optical Rotation

Optical rotation is an intriguing phenomenon that occurs when polarized light passes through a chiral substance, such as certain sugars. These substances cause the plane of light to rotate, and this effect is measured as the optical rotation. If you think of light as a wave, polarizing it means allowing only waves vibrating in a single plane to pass through. When this plane-polarized light moves through a chiral solution, it emerges turned to the left or right.

The extent of this rotation is specific to each chiral compound and is influenced by factors such as temperature, wavelength of light, concentration, and the path length of the solution through which the light travels. The measurement is done using an instrument called a polarimeter. The degree of rotation observed is represented as \[ \left[ \alpha \right] \]. This symbol, known as specific rotation, is crucial in identifying and characterizing chiral compounds.

Chiral Compounds

Chiral compounds are essential players in the world of chemistry, especially when discussing optical rotation. Chirality is a property wherein a molecule is not superimposable on its mirror image, much like a person's left and right hands. This non-superimposability arises from the presence of an asymmetric carbon atom, typically called a chiral center, around which different atoms or groups are bonded. As a result, each of these mirror-image forms, called enantiomers, can rotate plane-polarized light, but in opposite directions.

Chiral compounds are found abundantly in nature and are vital in biological systems. Many biomolecules, such as amino acids and sugars, are chiral. The study of these molecules is not just an academic interest—they are crucial for drug development and understandings of biochemical processes. With sugars, the rotation of light they cause can change over time, a process known as mutarotation, demonstrating the dynamic nature of these compounds in solution.

Anomers

Anomers are a specific type of stereoisomer, found primarily in carbohydrates, which differ only at the anomeric carbon. The anomeric carbon is the former carbonyl carbon (the C=O part) of the straight-chain form of a sugar when it cyclizes to form a ring. Upon forming the ring, the hydroxyl group attached to the anomeric carbon can be oriented in different ways, leading to two distinct forms: the alpha (α) and beta (β) anomers.

This difference may seem minor, but it has significant implications for the properties and reactivity of the sugar. In solution, sugars like glucose can interconvert between these anomers through a process called mutarotation. This involves breaking of the ring structure to revert to the flexible open chain form and subsequently closing back into a ring, resulting in a different anomer being formed. This process is why freshly prepared sugar solutions change their optical rotation over time, as the equilibrium between these different forms adjusts.