Problem 24 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

\(\alpha\)-D-glucose and \(\beta-D\)-glucose have a specific rotation of \(+112^{\circ}\) and \(+19^{\circ}\) respectively. In aqueous solution the rotation becomes \(+52^{\circ} .\) This is known as: [AFMC 1996] (a) racemisation (b) mutarotation (c) inversion (d) enolisation

Step-by-Step Solution

Answer

The phenomenon observed is mutarotation.

Step 1: Understand the concept of specific rotation

The specific rotation is a property of chiral compounds that describes the degree to which they rotate plane-polarized light. Different isomers, like \(\alpha\)-D-glucose and \(\beta-D\)-glucose, have different specific rotations. \(\alpha\)-D-glucose has a specific rotation of \(+112^{\circ}\), while \(\beta-D\)-glucose has \(+19^{\circ}\).

Step 2: Analyze the observed specific rotation in aqueous solution

In aqueous solution, the specific rotation reaches \(+52^{\circ}\). This observed rotation indicates that both forms of glucose are present in equilibrium, causing an average rotation value between \(\alpha\)-D-glucose and \(\beta-D\)-glucose.

Step 3: Define mutarotation

Mutarotation is the change in optical rotation due to the epimerization of a sugar. In the case of glucose, \(\alpha\)-D-glucose and \(\beta-D\)-glucose interconvert through an open-chain form, leading to a change in the optical activity of the solution.

Step 4: Conclude that the phenomenon observed is mutarotation

Since the specific rotation of the solution has changed and stabilized at a value between those of \(\alpha\)-D-glucose and \(\beta-D\)-glucose, the phenomenon is known as mutarotation, where both anomers equilibrate in solution.

Key Concepts

Specific Rotation

Specific rotation is a fundamental concept in understanding how chiral compounds, such as sugars, interact with light. It refers to the degree to which a chiral compound can rotate plane-polarized light. This property is crucial in identifying and characterizing different optical isomers of a chemical compound. A chiral compound, due to its asymmetric carbon atoms, can exist in multiple forms, which have the ability to rotate light either clockwise or counterclockwise. The value of specific rotation is expressed in degrees and is unique to each enantiomer of a compound. For example, in the case of glucose, specific rotations differ between the two anomers:

• \(\alpha\)-D-Glucose: Exhibits a specific rotation of \(+112^{\circ}\).
• \(\beta\)-D-Glucose: Shows a specific rotation of \(+19^{\circ}\).

Understanding specific rotation helps in determining the proportion of different isomers in a solution by measuring the observed optical activity.

Optical Activity

Optical activity is the phenomenon by which certain substances rotate the plane of polarized light passing through them. This property is a hallmark of chiral compounds, which lack an internal plane of symmetry, allowing them to exist in non-superimposable mirror images known as enantiomers. When plane-polarized light encounters an optically active substance, the direction and angle of rotation depend on:

• The nature of the substance.
• The concentration and path length of the solution.
• The wavelength of light used in the measurement.

In aqueous solutions, the optical activity observed can indicate the presence and proportion of different isomeric forms. As seen in the mutarotation of glucose, the optical activity changes until it reaches equilibrium. The resultant optical rotation, in this case \(+52^{\circ}\), informs us about the ratio of \(\alpha\)-D-glucose and \(\beta\)-D-glucose in the solution.

Chiral Compounds

Chiral compounds are molecules that have a specific three-dimensional arrangement, leading to their ability to exist as mirror-image isomers, termed enantiomers. These compounds contain chiral centers, commonly characterized by tetrahedral carbon atoms bonded to four different groups. The unique spatial arrangement of these groups results in molecules that are non-superimposable on their mirror images, similar to how left and right hands are mirror images yet not identical. Chiral compounds are pivotal in numerous fields, including pharmaceuticals and food science, as different enantiomers can possess drastically different properties and biological effects. In glucose, the concept of chirality is demonstrated through its ability to form different cyclic structures with distinct chiral centers, known as \(\alpha\)- and \(\beta\)-anomers. This contributes to its behavior in solutions, where such chiral forms interconvert, leading to phenomena like mutarotation, which results from the equilibration of these stereoisomeric forms in solution.