Problem 45 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will show geometrical isomerism? (a) 2-methyl butene (b) propene (c) vinyl chloride (d) but-2-ene

Step-by-Step Solution

Answer

But-2-ene shows geometrical isomerism.

Step 1: Understanding Geometrical Isomerism

Geometrical isomerism, also known as cis-trans isomerism, occurs in compounds where there is restricted rotation around a double bond or in cyclic structures. For geometrical isomerism to be possible, each carbon atom of the double bond must have two different substituents.

Step 2: Analyzing Compound (a) 2-Methyl Butene

2-Methyl butene can exist in different forms, such as 2-methyl-1-butene and 2-methyl-2-butene. Both of these have different groups attached to the double-bonded carbon atoms. However, only 2-methyl-2-butene can display geometrical isomerism due to different groups at each carbon of the double bond.

Step 3: Analyzing Compound (b) Propene

Propene has the structural formula: CH₃-CH=CH₂. The carbon atoms involved in the double bond do not each have two different substituents. Therefore, propene cannot exhibit geometrical isomerism.

Step 4: Analyzing Compound (c) Vinyl Chloride

Vinyl chloride has the structural formula: CH₂=CHCl. One of the carbons in the double bond, the CH₂ carbon, has two identical hydrogen atoms, which means it does not have two different substituents. Thus, vinyl chloride cannot show geometrical isomerism.

Step 5: Analyzing Compound (d) But-2-ene

But-2-ene has the structural formula: CH₃-CH=CH-CH₃. It has two different groups on each of the carbon atoms involved in the double bond (a hydrogen atom and a methyl group on each side). This allows but-2-ene to show geometrical isomerism, with potential cis and trans forms.

Key Concepts

Cis-Trans Isomerism Explained

Cis-trans isomerism is a fascinating topic in organic chemistry that deals with the arrangement of atoms around a double bond. This type of isomerism arises because of the restriction of rotation around the double bond, unlike single bonds which freely rotate. To have cis-trans isomerism, a compound must possess a double bond between two carbon atoms, where each carbon has two different substituents attached.

• Cis Isomer: This form occurs when similar or identical substituents are on the same side of the double bond.
• Trans Isomer: This form appears when similar or identical substituents are on opposite sides of the double bond.

The importance of cis-trans isomers lies in their physical and chemical properties, which can be notably different even though they share the same molecular formula. Such differences are essential for applications in pharmaceuticals and materials science.
One classic example is but-2-ene, which can exist both as cis-but-2-ene and trans-but-2-ene, manifesting distinctly different characteristics because of this isomerism.

Double Bonds in Organic Chemistry

Double bonds are a specific kind of chemical bond crucial in organic chemistry, found predominantly in alkenes. They consist of one sigma bond and one pi bond, which collectively create a more rigid and stable bond than a single bond. This rigidity is significant because it prevents rotation around the double-bonded carbon atoms, essentially creating a "fixed" configuration.

• Sigma Bond (σ): The sigma bond is the first bond between two atoms and is the strongest of the two in a double bond.
• Pi Bond (π): The pi bond is formed from the side-to-side overlap of p orbitals, providing additional stability to the molecule.

The presence of double bonds in a molecule contributes to its chemical reactivity. For instance, alkenes typically exhibit reactions such as hydrogenation, addition of halogens, and oxidation, which serve as ways for the double bonds to interact with other atoms or compounds. Understanding double bonds is vital for predicting how organic molecules react and behave under different conditions.

Structural Analysis of Alkenes

Alkenes are hydrocarbons characterized by having at least one carbon-carbon double bond. The simplest alkene is ethene (ethylene), but they can become increasingly complex. Analyzing the structure of alkenes is essential for comprehending their physical properties and chemical behavior.
In structural analysis, focus is usually on:

• Chain Lengths: The number of carbon atoms in the longest continuous chain.
• Position of the Double Bond: This helps differentiate between isomers. For example, but-1-ene and but-2-ene differ due to the location of their double bond.
• Substituents: Groups attached to the carbon chain that impact reactivity and isomerism.

This analysis enables chemists to predict the compound's stability, reactivity, and the likelihood of exhibiting isomerism. As seen with but-2-ene in the original problem, the presence of different substituents on the carbon atoms in the double bond allows it to show geometrical isomerism, a concept critical for biological and synthetic chemistry applications.