Problem 16 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of dibromo derivatives of propane are (a) 5 (b) 4 (c) 3 (d) 2

Step-by-Step Solution

Answer

The number of dibromo derivatives of propane is 3.

Step 1: Understanding the Problem

The problem asks for the number of distinct dibromo derivatives possible for propane. Propane is a hydrocarbon with the formula C₃H₈ which has 3 carbon atoms in a chain.

Step 2: Structure Analysis of Propane

In propane, the carbon atoms can be numbered as C-1, C-2, and C-3. The symmetry of the molecule allows for equivalent positions of the carbon chains.

Step 3: Identifying Possible Dibromo Derivatives

Substituting bromine atoms can occur at any of these carbon atoms. A dibromo derivative implies substitution of bromine at two different positions in the propane molecule.

Step 4: Analyzing Substitution Combinations

1. Substitution on the same carbon atom (e.g., both on C-1) doesn’t change its isomer nature, so it results in only one compound. 2. Substitution on adjacent carbon atoms such as C-1 and C-2 results in one distinctive isomer. 3. Substitution on non-adjacent carbon atoms C-1 and C-3 results in a different isomer.

Step 5: Determining the Number of Isomers

Check for distinct possible dibromo structures: 1. 1,1-Dibromopropane (both bromines on the same carbon, C-1). 2. 1,2-Dibromopropane (bromines on neighboring carbons, C-1 and C-2). 3. 1,3-Dibromopropane (bromines on opposite ends, C-1 and C-3). Each structure is a distinct compound.

Key Concepts

Hydrocarbon Structure Analysis

Understanding the structure of hydrocarbons is fundamental in organic chemistry. Propane, a simple alkane, is composed of three carbon atoms and eight hydrogen atoms, arranged in a chain. The molecular formula is \( \text{C}_3\text{H}_8 \). This is a saturated hydrocarbon, meaning it has single bonds between all of its carbon atoms.

In propane, the carbon atoms are linearly connected, which can be identified as C-1, C-2, and C-3 starting from any end of the molecule due to its symmetrical arrangement. This symmetry plays a crucial role in how different atoms or groups can be substituted while still maintaining the same molecular framework.

Analyzing this structure helps us predict how propane will react and the possible derivatives that can form when other atoms or molecules, like bromine in this case, are introduced.

Isomer Identification

Isomers are compounds with the same molecular formula but different structural arrangements. Understanding isomerism is essential in distinguishing between possible compounds and predicting their chemical behavior.

When dealing with dibromo derivatives of propane, we are interested in how bromine atoms can be substituted at different positions in the hydrocarbon chain. Since propane is a three-carbon molecule, multiple isomer formations are possible.

Let's consider propane with two bromine substitutions:

• **1,1-Dibromopropane**: Both bromine atoms replace hydrogens on the same carbon atom, specifically on C-1. This is the only way to have dibromine on the same carbon, leading to a single isomer.
• **1,2-Dibromopropane**: Bromines are substituted on adjacent carbon atoms, C-1 and C-2. This configuration gives a distinctive isomer different from the previously mentioned one.
• **1,3-Dibromopropane**: In this case, bromines are placed on non-adjacent atoms, C-1 and C-3, creating another unique isomer.

Each of these arrangements represents a distinct structural isomer with specific physical and chemical properties.

Bromine Substitution Patterns

When analyzing bromine substitution patterns in hydrocarbons like propane, it’s important to consider the positions that bromine atoms can occupy. This impacts the resulting compound's characteristics, such as boiling point, melting point, and reactivity.

Substitution patterns are determined by replacing hydrogen atoms in the propane structure with bromine atoms. The combination leads to different structural outcomes:

• **Both on the Same Carbon (1,1)**: This pattern results in the simplest form of dibromide, where both bromine atoms are bonded to a single carbon. It's uniform but doesn’t add much diversity in terms of isomer types.
• **On Adjacent Carbons (1,2)**: With bromines on neighboring carbons, the molecule becomes more complex. This substitution typically leads to significant changes in the physical attributes compared to the 1,1-dibromide.
• **On Non-Adjacent Carbons (1,3)**: Bromines substituted on the carbon atoms at the ends of the chain without being close to each other introduce a completely different isomer, showcasing propane's versatility in forming varied structural compounds.

Recognizing these patterns is crucial for chemistry students to predict reactivity and understand the properties of each resulting dibromo derivative.