Problem 162 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of geometrical isomers of the following alkene \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{Cl}\) is

Step-by-Step Solution

Answer

The alkene has 4 geometrical isomers.

Step 1: Identify Double Bonds

The given molecule is an alkene with alternating single and double bonds. There are three carbon-carbon double bonds present in the structure.

Step 2: Determine the Number of Isomeric Centers

Each double bond in the structure can potentially create geometrical isomers due to the presence of different substituents on the carbons involved. However, terminal double bonds, which include terminal carbons, can't form isomers. Thus, in this case, focus on the internal double bonds for cis/trans isomerism.

Step 3: Analyze Internal Double Bonds

For each internal double bond, examine the groups attached to each carbon atom. Internal double bonds between the 2nd and 3rd as well as 4th and 5th carbons can form cis/trans isomers due to the different groups on either side of the bond.

Step 4: Calculate Total Geometrical Isomers

Each internal double bond can form two isomers, cis or trans. If both internal bonds can isomerize, the total number of geometrical isomers is given by 2 ways (for the 2nd-3rd bond) x 2 ways (for the 4th-5th bond), resulting in a total of 4 geometrical isomers.

Key Concepts

Alkene Isomerism

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. This double bond is crucial as it can lead to a phenomenon called **alkene isomerism**. Just as the name implies, alkene isomerism occurs when compounds with the same molecular formula exhibit different structural arrangements. An important characteristic of these isomers is that they have different spatial orientations, resulting in distinct physical and chemical properties.

Two key types of isomerism are typically found in alkenes: structural isomerism, where the connectivity of atoms varies, and **geometrical or stereoisomerism**, where the spatial arrangement around the double bond is different. The presence of alkene isomerism is significant because it influences molecular stability and reactivity.

• It arises due to the restricted rotation around the double bond.
• This can lead to different arrangements that are non-superimposable.
• Isomers can have different boiling points, melting points, and densities.

Cis/Trans Isomerism

In the realm of geometrical isomerism, **cis/trans isomerism** is often the most noted. This type of isomerism is observed when two substituents on each end of a double-bonded carbon atom differ in their positioning. In simpler terms, it's like a mirror image on either side of the double bond.

Here's how it typically works:

• **Cis isomer**: The substituents are on the same side of the double bond.
• **Trans isomer**: The substituents are on opposite sides of the double bond.

These isomers exhibit differing properties; for instance, the cis isomer usually has a higher boiling point due to polar attractions. In contrast, trans isomers often possess a more stable and compact structure leading to higher melting points.

Cis/trans isomerism is essential for understanding compounds like natural rubber, which is a cis polymer of isoprene, leading to its elastic properties. If the polymer were in a trans configuration, it would become a tough, solid material.

Carbon-Carbon Double Bonds

The **carbon-carbon double bond** is a hallmark of alkenes and plays a pivotal role in defining their chemical characteristics. These bonds are comprised of one sigma (σ) bond and one pi (π) bond.

• The sigma bond forms due to the end-to-end overlap of orbitals, providing strength and stability.
• The pi bond, on the other hand, is due to the side-to-side overlap and is relatively weak, which restricts free rotation around the bond.

The strength of a double bond largely attributes to its several influences:

• The double bond's restricted rotation creates rigid molecular structures, making the formation of isomers possible.
• This leads to distinct configurations, particularly concerning cis/trans arrangements.

Furthermore, the presence of carbon-carbon double bonds increases the reactivity of alkenes compared to alkanes. They serve as the sites of chemical reactions, such as addition reactions, where atoms or groups can add across the double bond, modifying the molecule's structure and its properties.