Problem 112 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: List I List II 1\. \(\mathrm{CH}_{3} \mathrm{COOH}\) and \(\mathrm{HCOOCH}_{3}\) (i) metamers 2\. \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{CH}\) and (ii) position isomers \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\) 3\. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) and (iii) tautomers \(\mathrm{CH}_{3}-\mathrm{CH}\left(\mathrm{NH}_{2}\right)-\mathrm{CH}_{3}\) 4\. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) and \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{O}\) (iv) functional isomer 1 2 3 4 (a) (iii) (iv) (i) (iv) (b) (i) (ii) (iii) (iv) (c) (iii) (ii) (i) (iv) (d) (iv) (ii) (ii) (iv)

Step-by-Step Solution

Answer

The correct matching is option (b).

Step 1: Analyze Molecule Pair 1

Examine the first pair: \( \mathrm{CH}_{3} \mathrm{COOH} \) and \( \mathrm{HCOOCH}_{3} \). \( \mathrm{CH}_{3} \mathrm{COOH} \) is acetic acid, a carboxylic acid, while \( \mathrm{HCOOCH}_{3} \) is methyl formate, an ester. This pair involves different functional groups, so they are functional isomers.

Step 2: Analyze Molecule Pair 2

Consider the second pair: \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{CH} \) and \( \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3} \). Both are alkynes, but the position of their triple bonds is different. Thus, they are position isomers.

Step 3: Analyze Molecule Pair 3

Examine the third pair: \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2} \) and \( \mathrm{CH}_{3}-\mathrm{CH}\left(\mathrm{NH}_{2}\right)-\mathrm{CH}_{3} \). The first is a primary amine and the second is a secondary amine. These are structural isomers, which also qualify as metamers due to different alkyl groups attached on either side of the amine functional group.

Step 4: Analyze Molecule Pair 4

Consider the fourth pair: \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \) and \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{O} \). The first is an alcohol, while the second is an ether. These are functional isomers.

Step 5: Match the pairs

From the analysis: 1 - (iv), 2 - (ii), 3 - (i), 4 - (iv). Thus, the answer matches option (b).

Key Concepts

Functional Isomerism

Functional isomerism occurs when compounds have the same molecular formula but different functional groups. This means that, despite having the same number of each type of atom, the atoms are connected in different ways that change their chemical properties. An example of functional isomerism is the molecule pair from Step 1: \( \mathrm{CH}_{3} \mathrm{COOH} \) (acetic acid) and \( \mathrm{HCOOCH}_{3} \) (methyl formate). Both share the molecular formula \( \mathrm{C}_{2}\mathrm{H}_{4}\mathrm{O}_{2} \), but acetic acid is a carboxylic acid, while methyl formate is an ester. Thus, they demonstrate functional isomerism because the change in functional group gives the molecules different chemical behaviors.

Position Isomerism

Position isomerism is observed when there is a change in the position of a functional group or a structural feature within a molecule, resulting in isomers. The molecular formula remains the same, but the connectivity of the atoms differs. In the molecule pair from Step 2: \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{CH} \) and \( \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3} \), both compounds are alkynes with the molecular formula \( \mathrm{C}_{4}\mathrm{H}_{6} \). However, the triple bond is at a different location in each molecule. In the first, it is between the second and third carbon atoms, while in the second, it is between the first and second carbon. This variation in the position of the triple bond leads to different properties, despite having the same basic structure.

Metamers

Metamers are a specific type of isomer that show variation in their alkyl groups, differing on either side of a functional group, often involving heteroatoms like oxygen or nitrogen. For example, the pair in Step 3: \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2} \) and \( \mathrm{CH}_{3}-\mathrm{CH}\left(\mathrm{NH}_{2}\right)-\mathrm{CH}_{3} \) illustrate metamers. Both molecules have the general formula of an amine, but the first is a primary amine (amine group connected to one alkyl group), and the second is a secondary amine (amine group connected to two alkyl groups). Here, the difference specifically involves how the alkyl groups are arranged around the amine group. This affects physical properties like boiling points and solubility, while both remain amines at a fundamental level.

Primary and Secondary Amines

Primary and secondary amines are sub-classes of amines distinguished by the number of organic substituents attached to the nitrogen atom. A primary amine has one alkyl or aryl group, while a secondary amine has two. For instance, in Step 3, \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2} \) is a primary amine with one alkyl group attached to nitrogen, featuring a single hydrogen atom also bonded to the nitrogen. In contrast, \( \mathrm{CH}_{3}-\mathrm{CH}\left(\mathrm{NH}_{2}\right)-\mathrm{CH}_{3} \) is a secondary amine where two alkyl groups are bonded to the nitrogen. This structural change impacts the reactivity and characteristics of the amines, such as basicity and solubility.