Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of isomeric alkyl halides possible for \(\mathrm{C}_{5} \mathrm{H}_{11} \mathrm{Cl}\) is (a) 5 (b) 6 (c) 4 (d) 8

Step-by-Step Solution

Answer

The number of isomeric alkyl halides possible for \(\mathrm{C}_5\mathrm{H}_{11}\mathrm{Cl}\) is 7, so none of the given options are correct.

Step 1: Understand the Molecular Formula

To determine the number of isomeric alkyl halides, we first analyze the molecular formula given: \(\mathrm{C}_5\mathrm{H}_{11}\mathrm{Cl}\). This compound has five carbon atoms, eleven hydrogen atoms, and one chlorine atom.

Step 2: Consider Possible Carbon Skeletons

For \(\mathrm{C}_5\mathrm{H}_{11}\mathrm{Cl}\), we need to consider possible carbon skeleton arrangements: straight-chain, branched-chain, and more complex isomers. The possible carbon structures include: pentane, isopentane, and neopentane.

Step 3: Identify Attachment Points for Chlorine

For each carbon skeleton, determine the possible positions where a chlorine atom can attach. This involves changing each hydrogen position on different carbon atoms to chlorine.

Step 4: Calculate Isomers for Each Structure

1. **Normal pentane (n-pentane):** Attach \(\text{Cl}\) to any of the 4 middle carbons leading to 3 isomers: 1-chloropentane, 2-chloropentane, and 3-chloropentane.2. **Isopentane (methylbutane):** Attach \(\text{Cl}\) to the primary carbon at the end or any secondary carbon leading to 3 isomers: 1-chloro-2-methylbutane, 2-chloro-2-methylbutane, and 3-chloro-2-methylbutane.3. **Neopentane (dimethylpropane):** Attach \(\text{Cl}\) only on the tertiary carbon, leading to 1 isomer: 1-chloro-2,2-dimethylpropane.

Step 5: Total the Number of Isomers

Add up the possible isomers from all structures: 3 isomers from n-pentane, 3 from isopentane, and 1 from neopentane, which totals to 7 isomers.

Key Concepts

Molecular Formula Analysis

Understanding the molecular formula is crucial when analyzing compounds, especially in organic chemistry. The formula for a compound provides essential information about its composition. In this case, the molecular formula is \( \text{C}_5\text{H}_{11}\text{Cl} \). This tells us the compound contains:

• Five carbon atoms (C)
• Eleven hydrogen atoms (H)
• One chlorine atom (Cl)

Each element in the formula gives clues about the possible structural arrangements, which are essential for determining isomers. By analyzing the formula, you can determine how atoms are bonded together and how they influence the overall shape and properties of the molecule. This step lays the foundation for exploring the different isomers.

Carbon Skeleton Arrangements

Once the molecular formula is understood, the next task is to visualize the carbon skeletons. These skeletons define the backbone of the molecule. For \( \text{C}_5\text{H}_{11}\text{Cl} \), there are multiple ways the carbon atoms can be arranged:

• **Straight-chain configuration**: Here, the carbons are arranged in a linear fashion, like in n-pentane.
• **Branched-chain configuration**: This involves a structure where one or more carbon branches off the main chain, such as in isopentane and neopentane.

Analyzing these carbon skeletons allows us to explore different possibilities for attaching the remaining atoms, particularly chlorine in this case. Each arrangement can potentially lead to different physical and chemical properties, which are essential when identifying isomers.

Chloroalkane Isomers

Chloroalkanes, such as \( \text{C}_5\text{H}_{11}\text{Cl} \), exhibit fascinating isomeric variations due to the position of the chlorine atom. Once the carbon skeleton is set, the chlorine can be attached at different positions:- **In straight-chain isomers (n-pentane)**, chlorine can be attached to different carbon atoms, leading to isomers such as 1-chloropentane, 2-chloropentane, and 3-chloropentane.- **In branched structures like isopentane**, the positions for chlorine attachment vary, resulting in isomers like 1-chloro-2-methylbutane, 2-chloro-2-methylbutane, and 3-chloro-2-methylbutane.- **With neopentane**, chlorine attachment is more limited, offering only a single isomer: 1-chloro-2,2-dimethylpropane.By examining these attachment points, one can see how slight changes in structure can lead to distinct isomers with unique properties.

Organic Chemistry Reactions

Understanding the reactions involving organic compounds, especially isomers, is an integral part of studying organic chemistry. Chloroalkanes undergo various reactions, primarily influenced by the structural arrangement and the position of halides: - **Substitution reactions**: Chloroalkanes can participate in nucleophilic substitution reactions where the chlorine atom is replaced by another nucleophile. - **Elimination reactions**: These occur when a hydrogen and a chlorine atom are removed from adjacent carbon atoms, forming alkenes. - **Reactivity variances**: The type of carbon to which chlorine is attached (primary, secondary, or tertiary) significantly affects the reactivity and preferred reaction pathway. Exploring these reactions helps reveal how isomeric variations can lead to distinct and sometimes surprising products, highlighting the intricate nature of organic chemistry.