Problem 148 - A Complete Resource Book in Chemistry for JEE Main

Question

A similarity between optical and geometrical isomerism is that (a) if in a compound one is present then the other will also be present (b) each gives equal number of isomers for a given compound (c) both are included in stereo isomerism (d) they have no similarity

Step-by-Step Solution

Answer

(c) both are included in stereo isomerism.

Step 1: Understand Isomerism Types

Optical isomerism is specifically due to the arrangement of atoms in space, leading to non-superimposable mirror images called enantiomers. Geometrical isomerism, often found in alkenes, involves differences in the arrangement around a double bond or a central metal atom, leading to cis and trans forms.

Step 2: Define Stereo Isomerism

Stereo isomerism is a type of isomerism that involves atoms having the same connectivity but differing in spatial orientation. Both optical and geometrical isomerisms fall under stereo isomerism because both involve spatial arrangement without changing connectivity.

Step 3: Evaluate Each Option

Evaluate each option in the context of stereo isomerism: (a) Incorrect, as both isomers can occur independently. (b) Incorrect, they do not necessarily yield the same number of isomers. (c) Correct, both optical and geometrical isomerism fall under stereochemistry. (d) Incorrect, they have the similarity of being stereo isomers.

Step 4: Select the Correct Answer

Based on the analysis, the similarity between optical and geometrical isomerism is that both are included in stereo isomerism. Thus, option (c) is the correct answer.

Key Concepts

Optical Isomerism

Optical isomerism is a fascinating concept in chemistry that deals with molecules that are mirror images of each other but cannot be superimposed. Think of your left and right hands. They mirror each other, but you can't perfectly line them up on top of each other. These molecules are called enantiomers.

In optical isomerism, the presence of a chiral center, usually a carbon atom bonded to four different groups, leads to these unique arrangements. Chiral molecules have the distinct ability to rotate plane-polarized light in different directions—either clockwise or counterclockwise. This rotation property is key to determining the specific type of optical isomerism.

• Chiral molecules have no internal plane of symmetry.
• Rotating light is a signature characteristic of their behavior.
• Enantiomers can have different effects in chemical reactions, especially in biological systems.

Understanding optical isomerism helps in various fields like pharmacology, where one enantiomer of a drug could be beneficial, and the other might be harmful.

Geometrical Isomerism

Geometrical isomerism, also known as cis-trans isomerism, is another type of stereo isomerism. It occurs mainly in compounds containing double bonds or in coordination complexes with restricted rotation. The difference between isomers lies in the positions of substituents relative to a reference plane, often dictated by a double bond or a central metal atom.

This form of isomerism results in distinguishable molecules such as cis and trans isomers. **Cis** isomers have substituents on the same side, while **trans** isomers place them on opposite sides. This simple difference can have significant implications for the chemical and physical properties of the molecules.

• Cis isomers tend to have higher boiling points due to the polarity.
• Trans isomers typically have higher symmetry, leading to different solubility and melting points.

Geometrical isomerism is crucial in understanding the behavior and interaction of compounds in solutions and reactions.

Enantiomers

Enantiomers are a pair of optical isomers that are non-superimposable mirror images of each other. These molecules often arise due to a single chiral center, which means they cannot be overlapped perfectly. A classic example of enantiomers is the amino acids that make up proteins.

They exhibit identical physical properties in a non-chiral environment but differ in how they interact with other chiral entities. This includes how they rotate plane-polarized light, with one enantiomer rotating light clockwise (dextrorotatory) and the other counterclockwise (levorotatory).

• They often have sweet or bitter distinctions in taste, important for flavors.
• They can exhibit differing activities in drug formulations.
• Their separation is vital in manufacturing for correctly targeted applications.

Understanding enantiomers is key in industries like pharmaceuticals, where only one form might be active in a biological context, reducing side effects and enhancing effectiveness.

Cis-Trans Isomerism

Cis-trans isomerism is a special case of geometrical isomerism that aptly illustrates how the positioning of atoms can influence the characteristics of a molecule. This isomerism occurs when there is a difference in the spatial arrangement of atoms across a double bond or around a cyclic structure.

The terms **cis** and **trans** help to specify the relative positions of groups attached to double-bonded carbons. In cis isomers, similar atoms or groups are on the same side of the double bond. In contrast, trans isomers have them on opposite sides. This difference is not just academic; it affects properties like:

• Boiling and melting points due to the change in molecular symmetry and polarity.
• Biological interactions, as seen with fatty acids where cis and trans forms can affect nutritional properties.

This subtle yet crucial difference explains why understanding cis-trans isomers is essential in areas ranging from food science to materials chemistry.