Question

Choose the incorrect statement about ephedrine and its enantiomer (A) The molecular weights of both enantiomers are the same. (B) The boiling point of both enantiomers is different. (C) Both enantiomers rotate the PPL by the same number of degree but in opposite direction. (D) Absolute configuration of shown enantiomer is ( \(1 \mathrm{R}, 2 \mathrm{~S}\) ).

Short answer

The incorrect statement about ephedrine and its enantiomer is (B) The boiling point of both enantiomers is different. Enantiomers have the same boiling points in racemic mixtures or achiral environments.

Step-by-step solution

Option A: Molecular weights of both enantiomers are the same.

The molecular weights of enantiomers are identical because they have the same chemical formula and the same connectivity of atoms. Therefore, this statement is correct.

Option B: The boiling point of both enantiomers is different.

The boiling points of enantiomers are typically the same, as they have the same intermolecular forces and molecular weight. The only case where the boiling points of enantiomers could be different would be in a chiral environment, where one enantiomer interacts differently with the surrounding molecules. However, in a racemic mixture or an achiral environment, the boiling points of enantiomers are the same. Thus, this statement is incorrect.

Option C: Both enantiomers rotate the PPL by the same number of degrees but in opposite directions.

Enantiomers are optically active and interact differently with plane-polarized light (PPL). They will rotate PPL by the same number of degrees but in opposite directions. One enantiomer, (R)-ephedrine, will rotate it in a clockwise direction, while the other enantiomer, (S)-ephedrine, will rotate it in a counterclockwise direction. Hence, this statement is correct.

Option D: Absolute configuration of the shown enantiomer is (\(1 \mathrm{R}, 2 \mathrm{~S}\)).

Without a specific enantiomer structure given in this exercise, it's quite impossible to confirm this statement. However, if we are referring to the (R,S)-ephedrine configuration (also known as pseudoephedrine), then this statement would be correct, since the enantiomer of ephedrine with (1R,2S)-configuration is called pseudoephedrine. Out of the given options, the incorrect statement that needs to be chosen in this exercise is: (B) The boiling point of both enantiomers is different.

Key concepts

Optical Isomerism

Optical isomerism is a fascinating feature of chirality in chemistry. It occurs when molecules have the same molecular formula and structure but differ in the way they rotate plane-polarized light (PPL). These molecules, also known as enantiomers, exhibit optical activity.
The degree of rotation of plane-polarized light is a key characteristic. One of the two enantiomers will rotate it in the clockwise direction, often referred to as dextrorotatory, while the other enantiomer will rotate it counterclockwise, known as levorotatory.

• These rotations are equal in magnitude but opposite in direction.
• This characteristic helps to distinguish between the two enantiomers.

Understanding optical isomerism is critical in the study of how molecules behave in different environments, such as in pharmacology or racemic mixtures.

Chirality

Chirality is an essential concept in organic chemistry. A molecule is said to be chiral if it cannot be superimposed on its mirror image, much like how your left hand cannot be perfectly superimposed on your right hand.

• Chirality arises due to the presence of an asymmetric carbon atom, also known as a chiral center.
• This carbon atom is bonded to four different groups.

The idea of chirality is crucial for understanding how molecules interact with their environment and with other compounds. In biological systems, for example, only one enantiomer of a chiral molecule might be active, or they might have vastly different effects.
Overall, chirality provides insight into the geometric and stereochemical properties of molecules.

Molecular Properties

Molecular properties of enantiomers, despite having identical molecular formulas and arrangements, can vary in unique environments. However, in a non-chiral or "achiral" environment, two enantiomers have the same physical properties, such as:

• Molecular weight
• Boiling point
• Melting point

These properties may alter if the molecules interact with a chiral environment or are mixed with other chiral molecules.
For example, the boiling points of ephedrine enantiomers do not differ in an achiral environment, but differences may arise in a chiral matrix. This aspect underlines the significance of molecular surroundings on the properties of enantiomers.

Ephedrine Enantiomers

Ephedrine, a well-known pharmaceutical compound, has two enantiomers, which demonstrates the concept of optical isomerism in a real-world context. Each enantiomer of ephedrine reacts differently with plane-polarized light and can have distinct pharmacological effects.
In the context of absolute configuration, the enantiomers of ephedrine are labeled based on the R/S nomenclature.

• (1R,2S)-ephedrine is known as pseudoephedrine, commonly used as a decongestant.
• Both enantiomers, though they convert similarly light polarization, interact distinctly with biological systems.

This distinction in optical activity and biological behavior highlights the importance of understanding stereochemistry in drug development and action.