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 number of amino groups is 5.

Step 1: Identify Changes in Molecular Weight

The molecular weight of the original compound is 180. After acetylation, the molecular weight becomes 390. This increase in molecular weight is due to the addition of acetyl groups.

Step 2: Calculate the Increase in Molecular Weight

Subtract the original molecular weight from the final molecular weight: \[ 390 - 180 = 210 \] Thus, the increase in molecular weight is 210.

Step 3: Determine Molecular Weight of One Acetyl Group

The molecular weight of one acetyl group (\(CH_3CO\)) is 43. Therefore, each acetyl group adds 43 to the molecular weight.

Step 4: Calculate the Number of Acetyl Groups

Divide the total increase in molecular weight by the molecular weight of one acetyl group to find the number of groups:\[ \frac{210}{43} \approx 4.88 \] Since the number of groups must be an integer, the closest integer is 5.

Step 5: Relate Acetyl Groups to Amino Groups

Each acetyl group attaches to an amino group. Thus, the number of acetyl groups corresponds to the number of amino groups in the original compound.

Key Concepts

Acetylation

Acetylation is a chemical process where an acetyl group, usually derived from acetic acid, is introduced into an organic compound. This process is significant in biochemistry and organic chemistry, as it frequently modifies molecules to change their function or activity.
During acetylation, an acetyl group, represented as \(CH_3CO\), is added to a molecule. This reaction often involves replacing a hydrogen atom, giving the molecule new properties.

• This modification can make the compound more lipophilic (fat-loving) and less polar, often altering its solubility and reactivity.
• For proteins, acetylation can influence factors such as enzyme activity, DNA binding capabilities, and protein-protein interactions.

In our exercise, the compound of molecular weight 180 undergoes acetylation, leading to an increase in molecular weight. This increase is solely attributed to the numerous addition of acetyl groups, calculated from the data given.

Amino Groups

Amino groups are fundamental parts of molecules that contain the functional group \(-NH_2\). These groups are pivotal in biology and chemistry, present in amino acids, which are the building blocks of proteins.
They can act as basic sites in molecules, meaning they can accept a hydrogen ion (proton). This characteristic is crucial in many reactions, particularly in forming peptide bonds in proteins.

• Amino groups can form bonds with acetyl groups, as they provide a site for acetylation.
• They often determine the polarity and reactivity of the molecules they are part of.

In the exercise, amino groups are the sites where acetyl groups are attached. The number of acetyl groups added indicates the number of amino groups initially present in the compound.

Chemical Calculations

Chemical calculations are a vital skill in chemistry, enabling us to quantify aspects of chemical reactions and compounds. They involve stoichiometry, molecular weights, and concentration calculations.
In the given problem, understanding and calculating molecular weight changes are crucial steps.

• The molecular weight of a compound is the sum of the atomic weights of all atoms in its formula. Here, it helped in identifying how many acetyl groups were added.
• By subtracting the original molecular weight from the final product's molecular weight after acetylation, we determined the total weight change.
• The division of this weight change by the weight of a singular acetyl group gave us the number of acetylations, thus revealing the number of amino groups originally present.

This exercise demonstrates the power of chemical calculations in deducing structural changes in a compound based on its molecular weight.