Problem 154 - A Complete Resource Book in Chemistry for JEE Main

Question

A compound with molecular mass 180 its acrylated with \(\mathrm{CH}_{3} \mathrm{COCl}\) to get a compound with molecular mass 390 . The number of amino groups present per molecular of the former compound is (a) 4 (b) 6 (c) 2 (d) 5

Step-by-Step Solution

Answer

The initial compound has 5 amino groups.

Step 1: Understanding the Initial Problem

We have a compound with a molecular mass of 180 that is acrylated with \( \mathrm{CH}_3\mathrm{COCl} \), forming a new compound with a molecular mass of 390. We need to find out how many amino groups are present in the initial compound.

Step 2: Calculate the Mass Increase

The new compound has a molecular mass of 390. To find the mass increase, subtract the original mass from the new mass: \[ 390 - 180 = 210 \] So, the molecular mass has increased by 210.

Step 3: Understand the Reaction Involved

During the acrylation process, an acetyl group (\( \mathrm{CH}_3\mathrm{CO} \)), with a molecular mass of 43, is added. Therefore, each acrylation adds 43 units of mass due to the attachment of one \( \mathrm{CH}_3\mathrm{CO} \) group (from \( \mathrm{CH}_3\mathrm{COCl} \)).

Step 4: Determine the Number of Amino Groups

To determine how many acetyl groups (equivalent to the number of amino groups the molecule originally had) were added to increase the mass by 210, divide the total mass increase (210) by the mass of one acetyl group (43):\[ \frac{210}{43} \approx 4.88 \] Since this number must be whole, rounding gives approximately 5 acetyl groups, implying 5 amino groups in the original compound.

Key Concepts

Molecular Mass Calculation

When you start working with chemical equations and reactions, calculating molecular mass is a critical step. It forms the basis for understanding how substances interact and transform. To find the molecular mass of a compound, you add up the atomic masses of all the atoms present in a molecule. These atomic masses are typically found on the periodic table and are measured in atomic mass units (amu). In our exercise, we dealt with an initial compound with a molecular mass of 180 and a new acrylated compound with a mass of 390. By subtracting these two values, we discovered the mass increased by 210 due to the chemical reaction. It's a straightforward process:

• Identify the molecular formula of the compound.
• Refer to the periodic table for atomic masses.
• Add all atomic masses in the formula together.

Doing this practice regularly will help you become more adept at recognizing how mass is redistributed and calculated in chemical reactions.

Acrylation Reaction

An acrylation reaction involves adding an acyl group (typically derived from acyl chlorides) to a molecule, which adds specific properties or changes its structure. In the context of our exercise, an acetyl group (\( \mathrm{CH}_3\mathrm{CO} \)) is added to our original compound.The compound used for acrylation is \( \mathrm{CH}_3\mathrm{COCl} \), which is also known as acetyl chloride. The molecular mass of the acetyl group alone is 43 amu, meaning every acrylation reaction at an amino group increases the mass of the original compound by this amount.Understanding acrylation involves these steps:

• Recognize the functional groups present in the compounds.
• Use the mass of the added acyl group to determine how many such reactions have occurred.

In our specific problem, the acrylation reaction resulted in a total mass increase of 210, indicating that 5 acetyl groups were added, which corresponds to 5 amino groups originally present in the substance.

Chemistry Problem Solving

Solving chemistry problems, like determining the number of amino groups in a compound, requires a systematic approach. It involves understanding both the chemical reaction and the stoichiometry involved.Here's a simplified method for approaching such problems:

• Identify what information is provided and what is needed.
• Use simple arithmetic to separate the changes that occur in the reaction.
• Apply the knowledge of molecular masses and chemical formulas to interpret these changes.

In our exercise:

• We knew initial and final molecular masses.
• Understood that the mass increase was due to acrylation.
• Calculated the mass contributed by each acrylation (\( 43 \ ext{amu each} \)).
• Divided the total mass increase by the mass per acrylation to find the number of occurrences (5 in this case).

By practicing this structured problem-solving approach, you can make complex chemistry problems much easier to navigate and solve. Each step helps build a clearer picture of the chemical transformation, leading to the correct answer.