Problem 71 - A Complete Resource Book in Chemistry for JEE Main

Question

A compound of molecular weight 180 is acetylated to give a compound of molecular weight \(390 .\) The number of amino groups in the compound is (a) 6 (b) 5 (c) 4 (d) 3

Step-by-Step Solution

Answer

The compound has 5 amino groups.

Step 1: Determine the Weight Difference

First, calculate the molecular weight increase upon acetylation. Subtract the original molecular weight from the acetylated molecular weight: \( 390 - 180 = 210 \). This is the increase in weight due to acetylation.

Step 2: Calculate the Weight Per Amino Group

An acetyl group has the chemical formula \( CH_3CO \) and a molecular weight of 43. Therefore, each amino group being acetylated adds 43 units to the molecular weight.

Step 3: Compute the Number of Amino Groups

Divide the total increase in molecular weight by the weight added per amino group: \( \frac{210}{43} \approx 4.88 \). Since the number of amino groups must be an integer, round this to the closest whole number, which is 5.

Key Concepts

Molecular Weight Calculation

Understanding how to calculate molecular weight is key to many areas of chemistry. The molecular weight is essentially the sum of the atomic masses of all atoms in a molecule. For example, if you have a molecule composed of two hydrogen atoms and one oxygen atom, you sum their atomic masses to get the molecular weight.
To calculate this, you would look at the periodic table for the atomic masses:

• Hydrogen (H) has an atomic mass of about 1.01 u (atomic mass units).
• Oxygen (O) has an atomic mass of about 16.00 u.

Adding these together for water (H₂O) gives: \[ (2 \times 1.01) + 16.00 = 18.02 \text{ u} \] This process applies to any molecule, regardless of complexity, enabling us to understand and predict its behavior in chemical reactions.

Acetylation Process

The acetylation process is a chemical reaction that introduces an acetyl group into a molecule. An acetyl group is characterized by the formula \(CH_3CO\). This process is especially relevant in biochemistry and organic chemistry.
Acetylation can modify the properties of molecules significantly. When applied to amino groups in a compound, it can change solubility, stability, and reactivity.
During acetylation, every amino group that is modified increases the molecular weight by 43 due to the addition of the acetyl group. This is crucial when trying to determine the number of amino groups, as seen in the exercise.

Chemical Formula Analysis

Chemical formula analysis involves interpreting the symbols in a chemical formula to understand the composition of the compound. It tells us which elements are present and in what proportions.
For instance, in the formula \(CH_3CO\), we observe:

• One carbon atom (C)
• Three hydrogen atoms (H) combined into a methyl group (CH₃)
• Another carbon atom (C) linked to an oxygen atom (O) representing the carbonyl group (C=O)

Understanding chemical formulas allows you to do more than just calculate molecular weight; it helps you predict reactions and interactions with other compounds. These reactions and interactions are vital for designing experiments and interpreting results accurately.