Problem 5 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following shows geometrical isomerism? (a) but-1-ene (b) but-2-ene (c) 2,3 -dichlorobutane (d) ethene

Step-by-Step Solution

Answer

But-2-ene shows geometrical isomerism.

Step 1: Understand Geometrical Isomerism

Geometrical isomerism occurs in alkenes and is characterized by different spatial arrangements of groups or atoms around a double bond. This happens when there is restricted rotation around the double bond and different groups are attached to the carbon atoms of the double bond.

Step 2: Analyze Option (a) But-1-ene

But-1-ene has the structure CH₃-CH₂-CH=CH₂. The double bond is terminal, meaning one of the carbons in the double bond is attached to two hydrogens, not allowing for different spatial arrangements of different groups.

Step 3: Analyze Option (b) But-2-ene

But-2-ene has the structure CH₃-CH=CH-CH₃. The double bond is internal, and each carbon atom involved in the double bond is attached to two different groups (a methyl group and a hydrogen atom), allowing for the existence of geometrical isomers (cis-trans isomerism).

Step 4: Analyze Option (c) 2,3-Dichlorobutane

2,3-Dichlorobutane is a saturated compound, not containing any double bonds in the main chain. Geometrical isomerism requires the presence of a double bond, so this molecule cannot exhibit geometrical isomerism.

Step 5: Analyze Option (d) Ethene

Ethene (C₂H₄) only has a single type of group (hydrogen atoms) attached to each of the carbon atoms in the double bond, which doesn't allow for different spatial arrangements or geometrical isomerism.

Key Concepts

Alkenes

Alkenes are a class of hydrocarbons that contain at least one carbon-carbon double bond. This double bond is a defining feature because it imparts unique properties to alkenes compared to other hydrocarbons such as alkanes, which only have single bonds.

One of the critical characteristics of alkenes due to the presence of the double bond is their ability to undergo addition reactions. In these reactions, the double bond can be broken to add new atoms or groups to the molecule. Alkenes are known to be more reactive than alkanes because the double bond is electron-rich and more accessible to reactants.

Additionally, the presence of a double bond allows for the possibility of geometrical isomerism, which is not possible in alkanes. Therefore, when analyzing alkenes, it is crucial to consider the arrangement around the double bond to identify any isomeric forms.

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometrical isomerism, is a type of stereoisomerism that occurs due to the restricted rotation around a carbon-carbon double bond. This restricted rotation creates a situation where the special spatial arrangement of substituent groups can lead to different isomers with distinct properties.

In cis isomers, the substituent groups are on the same side of the double bond. In trans isomers, they are on opposite sides. This difference in position can affect the physical properties such as boiling point, melting point, and solubility.

For example, in the case of but-2-ene, if the two methyl groups (CH₃) are on the same side, it is a cis isomer. If they are on opposite sides, it is a trans isomer. It's important to note that if any carbon atom in the double bond has identical groups, cis-trans isomerism is not possible, as seen in but-1-ene and ethene.

Double Bond

A double bond between two carbon atoms is a region where two electron pairs are shared between the carbons, making it a more robust bond than a single bond. It consists of one sigma ( σ) bond and one pi ( π) bond. The sigma bond is formed by the direct overlap of orbitals, while the pi bond is a result of the sideways overlap of p-orbitals.

The presence of the pi bond is critical because it restricts the rotation around the double bond. This restricted rotation is what makes geometrical isomerism possible in alkenes.

The double bond also significantly impacts the geometry of the molecule. The carbons involved in the double bond assume a planar arrangement, giving the molecules roughly a trigonal planar shape. This shape is crucial when considering their reactivity and how they interact with other molecules in chemical reactions. Understanding these nuances of double bonds forms the cornerstone of analyzing geometrical isomerism in alkenes.