Problem 118 - A Complete Resource Book in Chemistry for JEE Main

Question

A compound gives a yellow ppt. on warming with the aqueous solution of \(\mathrm{NaOH}\). Its vapour density is 29 . The compound is (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) (b) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CHOHCH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)

Step-by-Step Solution

Answer

The compound is \(\text{CH}_3\text{COCH}_3\) (option b).

Step 1: Identify the Test

The question mentions a yellow precipitate is formed with an aqueous solution of NaOH. This is a typical test for identifying the presence of iodine and methyl ketones, known as the iodoform test.

Step 2: Understand the Iodoform Test

In the iodoform test, when a methyl ketone is treated with NaOH and iodine (or its equivalent), it produces a yellow precipitate of iodoform (I2). For this to occur, the compound must contain a \(CH_3CO\) group.

Step 3: Calculate the Molecular Weight from Vapour Density

Given the vapour density of the compound is 29, we use the relation between vapour density and molecular weight: \(\text{Molecular Weight} = 2 \times \text{Vapour Density}\). Thus, \(\text{Molecular Weight} = 2 \times 29 = 58\).

Step 4: Check Molecular Weight of Options

Calculate the molecular weights for each option:- \(\text{CH}_3\text{CH}_2\text{CHO}\): 58- \(\text{CH}_3\text{COCH}_3\): 58- \(\text{CH}_3\text{CHOHCH}_3\): 60- \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\): 60

Step 5: Identify Correct Structure

Both \(\text{CH}_3\text{CH}_2\text{CHO}\) and \(\text{CH}_3\text{COCH}_3\) have a molecular weight of 58, but only \(\text{CH}_3\text{COCH}_3\) contains the \(\text{CH}_3\text{CO}\) group necessary to give a positive iodoform test and form a yellow precipitate. Thus, it must be \(\text{CH}_3\text{COCH}_3\) that fits the description.

Key Concepts

Vapour Density

Vapour density is a useful concept in chemistry that helps determine the molecular weight of gaseous substances. It is defined as half the molecular weight of a gas relative to the mass of an equal volume of hydrogen. The formula for vapour density (VD) can be written as:\[ \text{Molecular Weight} = 2 \times \text{Vapour Density} \]Using this formula, if we have the vapour density of a compound, we can easily calculate its molecular weight by multiplying the vapour density by 2. This simple relation comes in handy in identifying compounds, especially in multiple-choice scenarios. In the given exercise, we see the vapour density is 29, which means the molecular weight is \( 2 \times 29 = 58 \). Breaking this down step-by-step aids in understanding why only certain compounds fit the criteria, particularly in relation to other chemical characteristics.

Methyl Ketone

A methyl ketone is a type of organic compound that possesses a specific functional group. This functional group is characterized by the carbonyl group (\(C=O\)) bonded to at least one methyl group (\(CH_3\)). The general formula for methyl ketones is \( RC(O)CH_3 \), where \( R \) is any hydrocarbon chain.One key feature of methyl ketones is their reactivity in the iodoform test. When they react with iodine and base, usually NaOH, they yield a yellow precipitate called iodoform. This reaction is useful for the identification of methyl ketones in an unknown compound. Testing for Methyl Ketone:

• The presence of a \(CH_3CO\) group is necessary for a positive iodoform test.
• Both the formation of the yellow precipitate and the distinct smell of iodoform help in confirming the test.

In the exercise, the compound \(\text{CH}_3\text{COCH}_3\) (acetone) was identified as a methyl ketone, meaning it would respond positively to the iodoform test given its structure.

Molecular Weight Calculation

Molecular weight, also known as molecular mass, refers to the mass of a single molecule of a chemical compound. It is calculated by summing up the atomic weights of all atoms in a molecule. Understanding how to calculate molecular weight is crucial when predicting which compounds fit certain chemical properties, like in the iodoform test.Steps for Molecular Weight Calculation:

• Identify all atoms present in the molecule.
• Use the periodic table to find the atomic weight of each atom.
• Multiply the atomic weight by the number of each type of atom present in the molecule.
• Add all the values to get the total molecular weight.

In the exercise, calculating the molecular weight of potential candidates was essential to confirming the correct structure. For instance, the molecular weight of \(\text{CH}_3\text{COCH}_3\) (acetone) was calculated to be 58, matching the vapour density-derived molecular weight of the unknown compound. This alignment helps identify the compound accurately.