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 The correct matching is: \(\begin{array}{llll}1 & 2 & 3 & 4\end{array}\) (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

Option (b) is correct.

Step 1: Identify Metamers

Metamers are isomers of the same functional group but with different alkyl groups attached on either side of the functional group. In this case, none of the given pairs represent metamers. Thus, option (i) is not applicable.

Step 2: Identify Position Isomers

Position isomers have the same molecular formula but differ in the position of a functional group. For example, both \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{CH}\) and \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\) have the same molecular formula but differ in the position of the triple bond, making them position isomers. Therefore, match (2) with (ii).

Step 3: Identify Tautomers

Tautomers are constitutional isomers of organic compounds that readily interconvert. They often differ in the position of a proton and a double bond. The compounds \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHNH}_{2}\) and \(\mathrm{CH}_{3}-\mathrm{CH}(\mathrm{NH}_{2})-\mathrm{CH}_{3}\) are such examples. Thus, match (3) with (iii).

Step 4: Identify Functional Isomers

Functional isomers have the same molecular formula but belong to different functional groups. An example is \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (an alcohol) and \((\mathrm{CH}_{3})_{2} \mathrm{O}\) (an ether). These two belong to different functional groups but have the same number of atoms, confirming they are functional isomers. Thus, match (4) with (iv).

Step 5: Conclusion from the Matching

Given the matches: (1) has no match as metamers, (2) is matched with (ii), (3) is matched with (iii), and (4) is matched with (iv). Therefore, the correct option based on the matching description is option (b): (i), (ii), (iii), (iv).

Key Concepts

Functional Isomers

Functional isomers are molecules that share the same molecular formula but differ in the types of functional groups they contain. This means that while the structure of the molecule may change, the number and types of atoms remain the same. For example, the compounds \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) and \((\mathrm{CH}_{3})_{2}\mathrm{O}\) are functional isomers. The former is an alcohol, and the latter is an ether. Despite having the same composition of carbon (C), hydrogen (H), and oxygen (O), they exhibit different chemical properties due to their functional groups.

Functional isomerism is significant because different functional groups can dramatically change how a compound behaves in chemical reactions. For this reason, identifying functional isomers is crucial in fields such as medicinal chemistry, where the activity of a drug can be significantly affected by the nature of its functional groups.

Position Isomers

Position isomers are compounds that have the same molecular formula but differ in the position of a specific functional group or a substituent. This shifts the location of chemical bonds, leading to different chemical properties. For instance, the compounds \(\mathrm{CH}_3\!\text{-}\!\mathrm{CH}_2\!\text{-}\!\mathrm{C}\equiv\mathrm{CH}\) and \(\mathrm{CH}_3\!\text{-}\!\mathrm{C}\equiv\mathrm{C}\!\text{-}\!\mathrm{CH}_3\) are position isomers. Both share the same molecular formula, yet they differ because the triple bond is located between different carbon atoms.

Understanding position isomerism is essential because the location of a functional group can influence the boiling point, solubility, and reactivity of the molecule. In crafting synthetic routes in chemistry, being aware of position isomers helps chemists predict and control the outcome of reactions effectively.

Metamers

Metamers are a subtype of structural isomers where compounds have the same molecular formula and the same functional group but differ in the arrangement of alkyl groups on either side of the functional group. This difference in structural arrangement leads to variations in physical and chemical properties. However, it is important to note that metamers are only applicable when dealing with compounds that have bivalent functional groups, such as ethers, thioethers, and certain amides.

In the exercise provided, none of the given pairs represented metamers, indicating that the recognition of metamers requires an understanding of both the functional group and the spatial configuration of the molecules. Recognition of metamerism is particularly important in the design of new molecules in pharmaceutical research, as changing the length or branching of alkyl chains can influence the biological activity of a compound.

Tautomers

Tautomers are unique isomers known for their ability to interconvert through a chemical reaction, typically involving the migration of a hydrogen atom along with the switch of a single and double bond. This process is known as tautomerism and most commonly occurs between keto and enol forms, among other types. An excellent example is seen in the compounds \(\mathrm{CH}_3\!\text{-}\!\mathrm{CH}_2\!\text{-}\!\mathrm{CHNH}_2\) and \(\mathrm{CH}_3\!\text{-}\!\mathrm{CH}(\mathrm{NH}_2)\!\text{-}\!\mathrm{CH}_3\). Here, the shifting location of the double bond and a hydrogen atom creates two distinct forms, which are in dynamic equilibrium.

Understanding tautomers is crucial in organic chemistry because they can impact the stability and reactivity of compounds. In biochemical systems, the ability of a molecule to exist in tautomeric form can affect enzyme activity and metabolic pathways. As such, comprehending tautomerism is integral to predicting biological function and designing pharmaceuticals that interact specifically with target molecules.