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

Option (c): Both are included in stereo isomerism.

Step 1: Understanding Isomerism

Isomerism refers to compounds with the same molecular formula that have different structures or arrangements of atoms. Optical isomerism occurs when compounds have chiral centers and exist as enantiomers (mirror-image structures). Geometrical isomerism, a subset of stereoisomerism like optical isomerism, refers to the spatial arrangement around a double bond or ring structure, leading to cis and trans forms.

Step 2: Analyzing Similarity Options

Evaluate the given options to find similarities: (a) suggests co-occurrence; however, optical and geometrical isomerism doesn't necessitate coexistence. (b) implies equal number of isomers, which is context-dependent and not a rule. (c) involves both being stereo isomers, which is true as both depend on spatial orientation.

Step 3: Conclusion on Similarity

The correct option is (c) because both optical and geometrical isomerism are included under stereoisomerism. This means they both involve isomerism based on the orientation of atoms in space, not on the connectivity of atoms as in structural isomers.

Key Concepts

Optical Isomerism

Optical isomerism is a fascinating aspect of stereochemistry that plays a crucial role in many biological processes. It arises when a compound has what is known as a chiral center. A chiral center is typically a carbon atom bonded to four different groups. This unique arrangement means that the compound can exist in two forms that are non-superimposable mirror images of each other, much like your left and right hands. These two forms are called enantiomers.

Enantiomers have identical physical properties except when it comes to their interaction with plane-polarized light. Each enantiomer will rotate plane-polarized light to an equal degree but in opposite directions. One will rotate light to the right (")+" direction, known as "dextrorotatory," while the other rotates it to the left ("-" direction), known as "levorotatory." This unique behavior is why optical isomerism is so named.

• Occurs due to chiral centers.
• Generates enantiomers - mirror-image isomers.
• Involves the rotation of plane-polarized light.

Geometrical Isomerism

Geometrical isomerism, also known as cis-trans isomerism, is another facet of stereochemistry and is pertinent when dealing with compounds that have restricted rotation around a bond. This often occurs in compounds with double bonds or within cyclic structures since the atoms can't freely rotate around these bonds.

In cis-trans isomerism, the term "cis" means that similar groups or atoms are on the same side of the double bond or plane of symmetry, while "trans" means that they are on opposite sides. The different arrangements lead to variations in physical and chemical properties, although the molecular formula remains the same. A classic example is the difference in boiling points and melting points between the cis and trans forms of many organic compounds.

• Occurs in compounds with double bonds or rings.
• Involves cis (same side) and trans (opposite side) arrangements.
• Affects physical and chemical stability.

Chiral Center

A chiral center, the focal point of optical isomerism, is often a carbon atom bonded to four different substituents. This creates a scenario where there is no plane of symmetry within the molecule, making it impossible to superimpose it on its mirror image. This lack of symmetry is what gives rise to the unique properties associated with chirality.

Molecules with chiral centers can have multiple enantiomers depending on the number of chiral centers it contains. When a molecule has more than one chiral center, it can lead to a variety of stereoisomers, including diastereomers which are not mirror images of each other. Determining the arrangement around each chiral center is essential to understanding a compound's reactivity and interaction with other molecules, especially in biological systems.

• Defined by carbon with four different groups.
• Creates potential for non-superimposable mirror images.
• Leads to complex stereochemistry as more chiral centers are present.