Problem 39 - A Complete Resource Book in Chemistry for JEE Main

Question

But-2-ene exhibits cis-transisomerism due to (a) rotation around \(\mathrm{C}_{3}-\mathrm{C}_{4}\) sigma bond (b) rotation around \(\mathrm{C}_{1}-\mathrm{C}_{2}\) bond (c) restricted rotation around \(\mathrm{C}=\mathrm{C}\) bond (d) rotation around \(\mathrm{C}_{2}-\mathrm{C}_{3}\) double bond

Step-by-Step Solution

Answer

(c) Restricted rotation around C=C bond.

Step 1: Understanding Cis-Trans Isomerism

Cis-trans isomerism occurs in compounds with double bonds (C=C) because these bonds restrict rotation. Rotation can happen freely around single (sigma) bonds but is restricted around double bonds, leading to different spatial arrangements of substituents linked to the double-bonded carbon atoms.

Step 2: Analyze Bonds Capable of Rotation

Identify which bonds in but-2-ene would potentially allow or restrict rotation. In but-2-ene, the critical bond for this analysis is the carbon-carbon double bond, which prevents rotation, unlike single bonds (sigma bonds) which allow for free rotation.

Step 3: Focusing on Double Bond Behavior

But-2-ene has a double bond between the second and third carbons (C_2=C_3), and this double bond restricts rotation. This restriction is what leads to the formation of cis and trans isomers—substances identical except for orientation around the double bond.

Step 4: Verifying Isomerism Cause

Given that only double bonds restrict rotation enough to result in cis-trans isomers, recognize that the rotation around a sigma bond cannot cause such isomerism. Instead, the lack of rotation around the carbon-carbon double bond is the reason for cis-trans isomerism in but-2-ene.

Key Concepts

Double Bond Rotation

Double bonds play a critical role in the concept of rotation within molecules. Unlike single bonds, which are known as sigma bonds and allow for free rotation, double bonds are composed of both sigma and pi bonds. This combination creates a stronger bond that restricts the movement of atoms attached to the bonded carbons.

In organic chemistry, when you have a carbon-carbon double bond, the pi bond is particularly crucial in preventing rotation. The pi bond results from the side-to-side overlap of p orbitals above and below the plane of the atoms. This setup "locks" the bonded atoms in place, meaning they can no longer rotate freely as they would around a single bond.

This restriction is what allows for the existence of different spatial arrangements, known as isomers, around the double bond. This phenomenon is especially important in cases of cis-trans isomerism, where double bond rotation, or rather the lack of it, dictates the structural variation.

But-2-ene

But-2-ene is a simple organic compound that serves as an excellent example of cis-trans isomerism. Understanding its structure is key. But-2-ene contains a double bond between the second and third carbons, expressed as \(C_2=C_3\).

This position of the double bond is crucial because it restricts the free rotation of the atoms attached to these carbon atoms. As a result, but-2-ene can exist in two isomeric forms: cis-but-2-ene and trans-but-2-ene.

The term "cis" indicates that the similar substituents are on the same side of the double bond, while "trans" denotes they are on opposite sides. This difference in spatial orientation around the double bond can affect the physical properties like boiling point and melting point of the isomers, despite having identical molecular formulas.

When studying but-2-ene, it's essential to remember that it exemplifies how restricted rotation leads to distinct isomers in compounds with double bonds.

Isomerism in Chemistry

Isomerism is a fascinating concept in chemistry where molecules with the same chemical formula can have different structures and properties. This diversity is primarily due to the arrangement of atoms within a molecule.

One of the most common types of isomerism is stereoisomerism, which includes cis-trans isomerism. This type of isomerism occurs frequently in compounds containing carbon-carbon double bonds. The restricted rotation around these double bonds allows different spatial arrangements of atoms to exist.

The significance of isomerism extends beyond mere structural differences. Isomers can have vastly different chemical and physical properties. For instance, in the case of cis-trans isomerism, the spatial arrangement of atoms can lead to differences in melting points, boiling points, and even reactivity.

Understanding isomerism is crucial for chemists, as it opens the door to discovering and utilizing various chemical compounds with unique properties. It exemplifies the incredible complexity and versatility found within molecular structures.