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 geometric isomerism.

Step 1: Understanding Geometric Isomerism

Geometric isomerism occurs in compounds that have restricted rotation around a bond. This typically involves alkenes where there is a double bond between carbon atoms, and different groups are attached to these carbons.

Step 2: Analyze 2-methyl butene

2-methyl butene contains a double bond and has potential for geometric isomerism if there are different groups on the carbons involved in the double bond. However, in some forms, like 2-methyl but-1-ene, there are not two different groups allowing for isomerism.

Step 3: Analyze Propene

Propene has the formula CH extsubscript{3}CH=CH extsubscript{2}. The carbons in the double bond have identical hydrogen atoms, and so it cannot exhibit geometric isomerism.

Step 4: Analyze Vinyl Chloride

Vinyl chloride has the structure CH extsubscript{2}=CHCl. The double-bonded carbons have different groups (H and Cl on one, but H and H on the other), limiting it from showing geometric isomerism, as one carbon is not bonded to two different groups.

Step 5: Analyze But-2-ene

But-2-ene has a double bond in the middle of the chain (CH extsubscript{3}CH=CHCH extsubscript{3}). Both carbons involved in the double bond have two different groups attached (H and CH extsubscript{3}), allowing for cis-trans isomerism.

Key Concepts

Alkenes

Alkenes are a class of hydrocarbons that contain at least one carbon-carbon double bond (\( C=C \)). This double bond is a key structural feature that influences the chemical behavior of alkenes compared to their saturated counterparts, alkanes. The ability of these double bonds to act as sites for chemical reactions is what makes alkenes quite reactive.

Regarding their structure, the presence of the double bond prevents rotation around that bond axis, leading to the possibility of different spatial arrangements of atoms or groups attached to the double-bonded carbons.

• Each carbon atom in a double bond is typically \( sp^2 \) hybridized, resulting in a planar structure around the double bond.
• This \( sp^2 \) hybridization causes each carbon to form sigma bonds with adjacent atoms and a pi bond with each other.

These structural features are what give rise to potential geometric isomerism in alkenes.

Cis-Trans Isomerism

Cis-trans isomerism is a type of geometric isomerism specific to alkenes and molecules with similar structures. This type of isomerism arises when each carbon atom of a double-bonded pair has two different groups attached, creating two distinct configurations.

• Cis-isomer: In this arrangement, similar or identical groups are positioned on the same side of the double bond.
• Trans-isomer: In the trans configuration, similar groups are situated on opposite sides of the double bond.

Cis-trans isomerism is significant because the spatial arrangement of atoms affects the physical and chemical properties of the molecules. For example, cis isomers can differ in boiling points, melting points, and reactivity when compared to their trans counterparts.

Such isomerism is possible only when each carbon in the double bond is attached to two different substituents, as seen in but-2-ene, which clearly exhibits this phenomenon.

Molecular Structure Analysis

The analysis of molecular structures is a critical aspect in determining if a compound can exhibit geometric isomerism. To perform this analysis, one must examine the groups attached to the carbons in a double bond.

• Check for the presence of a carbon-carbon double bond, which is essential for geometric isomerism.
• Identify the substituents on each double-bonded carbon to ensure they are different. Without this difference, cis-trans isomerism cannot occur.

For instance, in but-2-ene, both carbons involved in the double bond each have two different groups: a hydrogen atom and a methyl group, allowing for distinctive cis and trans configurations.

In contrast, molecules like propene and vinyl chloride lack the necessary substituent differences on both double-bonded carbons, thus inhibiting geometric isomerism. Through careful analysis, we can predict the behavior and properties of these molecules based on their structural attributes.