Question

Select the incorrect statement: (A) Molecules of different compound with same absolute configuration must rotate plane of polarised lightin same direction. (B) One of the enantiomer of aspartic acid when dissolved in water has \([\alpha]_{\mathrm{D}}^{20^{\circ} \mathrm{C}}=+4.36^{\circ}\) and \([\alpha]_{\mathrm{D}}^{140^{\circ} \mathrm{C}}=-4.36^{\circ} .\) It proves that on increasing temperature compound changes into its mirror image. (C) A diastereomeric pair may have optically pure and optically impure stereoisomers. (D) Enantiomeric mixture of trans-cyclooctene can be resolved by using platinum complex with a chiral amine.

Short answer

The incorrect statement is (B) because changing the temperature may alter the specific rotation of an enantiomer, but it doesn't transform it into its mirror image. Enantiomers are separate compounds and cannot interconvert just by changing the temperature.

Step-by-step solution

Statement A

Molecules of different compounds with the same absolute configuration will indeed rotate the plane of polarized light in the same direction. This is because the absolute configuration determines the optical activity of the compound. If two compounds have the same absolute configuration, they will have the same effect on polarized light, regardless of their overall molecular structure. Therefore, statement A is correct.

Statement B

The statement implies that, upon increasing the temperature, the enantiomer of aspartic acid changes into its mirror image. However, this conclusion is incorrect simply based on the given information. Changing temperature may alter the specific rotation of an enantiomer but doesn't transform it into its enantiomer (mirror image). Enantiomers are separate compounds and cannot interconvert just by changing the temperature. Therefore, statement B is incorrect.

Statement C

A diastereomeric pair can indeed have optically pure and optically impure stereoisomers. Diastereomers are stereoisomers with multiple chiral centers, and they are not mirror images of each other. Due to differing configurations of their chiral centers, some diastereomers may be optically pure (having optical isomers), and some may be optically impure (having no optical isomers or having racemic mixtures with equal amounts of enantiomers). Thus, statement C is correct.

Statement D

It is possible to separate enantiomeric mixtures of trans-cyclooctene using a platinum complex with a chiral amine. This process makes use of chiral ligands to selectively coordinate with a specific enantiomer, allowing for separation from its enantiomeric counterpart. Since the resolution of enantiomeric mixtures can be achieved using such a method, statement D is correct. In conclusion, the incorrect statement is: (B) One of the enantiomer of aspartic acid when dissolved in water has \([\alpha]_{\mathrm{D}}^{20^{\circ} \mathrm{C}}=+4.36^{\circ}\) and \([\alpha]_{\mathrm{D}}^{140^{\circ} \mathrm{C}}=-4.36^{\circ} .\) It proves that on increasing temperature compound changes into its mirror image.

Key concepts

Plane-Polarized Light

Plane-polarized light is a special kind of light where the waves vibrate in only one plane. Generally, light waves move in all directions, but plane-polarized light is restricted to oscillate in a single plane after passing through a polarizer. This property is key to studying optical isomerism, which is the focus of many questions in organic chemistry.
Optical isomers, or enantiomers, have the fascinating ability to rotate this light. Depending on their structure, these isomers can rotate plane-polarized light to the right (dextrorotary) or to the left (levorotary). This characteristic is important because it helps distinguish between different enantiomers and understand their optical activity.
The angle by which an enantiomer rotates plane-polarized light is termed as the specific rotation. It provides a definite way to identify the optical isomer and enhances our understanding of the compound's properties in solutions.

Absolute Configuration

The absolute configuration of a molecule determines the 3D arrangement of atoms around a chiral center. It's like a molecule's fingerprint and is denoted using the R/S nomenclature system. With this system, each chiral center is assigned an R (rectus) or S (sinister) configuration based on the Cahn-Ingold-Prelog priority rules.
To determine this, you assign priority to substituents based on atomic numbers. The atom with the highest atomic number gets the highest priority. By looking from a specific angle and arranging them in increasing priority, you can assign an R or S configuration. If the order is clockwise, it’s R, and if counterclockwise, it’s S.
This configuration plays a crucial role in defining the compound's properties, including how it interacts with plane-polarized light. Thus, molecules with the same absolute configuration will often show similar behaviors in optical activity.

Stereoisomers

Stereoisomers are molecules with the same structural formula, but different arrangements of atoms in space. This group includes both enantiomers and diastereomers, two major classes of optical isomers.
The importance of stereoisomers lies in their different physical and chemical properties, despite having the same connectivity of atoms. They can exhibit vastly different biological activities and interactions with other chemicals. For example, one stereoisomer of a drug might be therapeutic while another may be inactive or even harmful.
These differences underscore the importance of stereoisomers in fields like pharmacology and can have significant implications for drug development and testing.

Enantiomers

Enantiomers are a specific type of stereoisomers that are non-superimposable mirror images of each other. They have identical physical properties except for the direction in which they rotate plane-polarized light. One enantiomer will rotate the light in a clockwise direction, while the other will rotate it similarly in a counterclockwise way.
Despite their close resemblance, enantiomers can have very different effects in chiral environments. This is because they interact distinctively with other chiral molecules, which is crucial in biological systems. The specificity is pivotal in drug development, food chemistry, and other areas of applied chemistry.
When a mixture contains equal parts of both enantiomers, it becomes "racemic," and typically, such mixtures are optically inactive because the rotations cancel each other out.

Diastereomers

Unlike enantiomers, diastereomers are stereoisomers that are not mirror images of each other. They arise when a compound has two or more stereocenters. While enantiomers differ only by their chiral centers, diastereomers have differences in the configuration of at least one stereocenter, but not all.
Diastereomers often have distinct physical and chemical properties such as boiling points or solubility, which make them easier to separate compared to enantiomers.
Some diastereomers can be optically active, while others might be inactive, which provides further complexity. They are highly significant in the synthesis and development of organic compounds because of their varying properties, influencing how molecules like drugs function in the body.