Problem 205 - A Complete Resource Book in Chemistry for JEE Main

Question

The alkene that exhibits geometrical isomerism is: (a) 2 -methyl propene (b) 2 -butene (c) 2 -methyl-2-butene (d) propene

Step-by-Step Solution

Answer

Option (b): 2-butene exhibits geometrical isomerism.

Step 1: Understanding Geometrical Isomerism

Geometrical isomerism, also known as cis-trans isomerism, occurs when compounds have the same molecular formula but different spatial arrangements. This typically occurs in alkenes due to restricted rotation around the double bond, which requires that each carbon of the double bond have two different substituents.

Step 2: Analyzing Option (a): 2-Methyl Propene

2-Methyl propene has the structure: CH3-C(CH3)=CH2. Here, the double-bonded carbon on the right has two identical hydrogen substituents, which makes geometrical isomerism impossible.

Step 3: Analyzing Option (b): 2-Butene

2-Butene has the structure: CH3-CH=CH-CH3. The two carbons in the double bond both have different substituents (methyl and hydrogen), allowing for both cis and trans formations: cis-2-butene and trans-2-butene.

Step 4: Analyzing Option (c): 2-Methyl-2-Butene

2-Methyl-2-butene has the structure: C(CH3)2=CH-CH3. The central carbon has two identical methyl groups, which precludes any geometrical isomerism.

Step 5: Analyzing Option (d): Propene

Propene has the structure: CH2=CH-CH3. One of the double-bonded carbons has two hydrogens, which means it cannot exhibit geometrical isomerism.

Step 6: Identifying the Correct Answer

From the analysis, only 2-butene has the necessary conditions to exhibit geometrical isomerism, as it can form both cis and trans isomers.

Key Concepts

alkenes

Alkenes are a fascinating class of hydrocarbons characterized by the presence of at least one carbon-carbon double bond. This feature makes them unsaturated, which means they have the potential to form additional bonds by the addition of atoms to two carbon atoms in the double bond.

A typical example of an alkene is ethene, also known as ethylene (C2H4), which has a single carbon-carbon double bond. The chemistry of alkenes is largely influenced by this double bond due to its higher reactivity compared to single bonds.

One of the unique traits of alkenes is that there is no free rotation around the carbon-carbon double bond. This is unlike the free rotation seen in carbon-carbon single bonds. This restricted rotation leads to interesting phenomena such as geometric isomerism, where compounds have the same structural formula but different spatial arrangements of atoms, giving rise to unique properties.

Alkenes are versatile in organic chemistry due to their ability to undergo a variety of reactions, including hydrogenation, polymerization, and halogenation. These reactions are foundational in the production of everything from plastics to pharmaceuticals.

cis-trans isomerism

Cis-trans isomerism, also known as geometric isomerism, is a special type of stereoisomerism seen in compounds with restricted rotation around a bond, such as the double bond in alkenes. This isomerism occurs when there are different groups attached to the carbon atoms involved in the double bond.

To visualize this, imagine a molecule like 2-butene, which can exist in two distinct forms: cis-2-butene, where similar groups are on the same side of the double bond, and trans-2-butene, where they are on opposite sides. These different spatial arrangements lead to different physical properties, such as boiling points and stability.

For a molecule to show cis-trans isomerism, the following criteria must be met:

• There must be a carbon-carbon double bond.
• Each carbon atom of the double bond must have two different groups attached to it.

Notably, if one of the double-bonded carbons has two identical groups, the molecule cannot exhibit this kind of isomerism. Hence, options like 2-methyl propene and propene do not show geometrical isomerism due to the identical groups attached to the double-bonded carbons.

molecular structure in chemistry

Understanding molecular structure in chemistry is crucial because it provides insight into the arrangement of atoms within a molecule. This arrangement dictates the molecule's properties and behavior in chemical reactions.

In the context of alkenes and cis-trans isomerism, molecular structure helps explain why some alkenes can form geometric isomers while others cannot. The specific positioning and types of atoms bonded to the double-bonded carbons in alkenes are what determine the potential for geometrical isomerism.

Molecular structures can be represented in several ways:

• Structural formulas that show the arrangement of atoms and the bonds between them.
• Lewis structures that provide a more visual depiction of the bonding between atoms, including lone pairs.

The spatial arrangement of atoms, as seen in isomers like cis-2-butene and trans-2-butene, highlights how a simple rotation, which is not possible around double bonds, can drastically change a compound's characteristics.

Ultimately, the study of molecular structure is foundational in understanding not only isomerism but also reactivity, polarity, and the macroscopic properties of molecules.