Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds will show metamerism? (a) \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{C}_{2} \mathrm{H}_{5}\) (b) \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3}-\mathrm{CO}-\mathrm{C}_{2} \mathrm{H}_{5}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{S}-\mathrm{C}_{2} \mathrm{H}_{5}\)

Step-by-Step Solution

Answer

Compounds (a) and (c) show metamerism.

Step 1: Understand Metamerism

Metamerism is a type of isomerism where the compounds have the same molecular formula but differ in the nature of the alkyl groups attached to the same functional group. It is commonly displayed by compounds like ethers, ketones, and sulfides.

Step 2: Analyze Compound (a)

The structure \( \mathrm{CH}_{3}-\mathrm{O}-\mathrm{C}_{2} \mathrm{H}_{5} \) is an ether. Ethers can exhibit metamerism due to different alkyl groups on either side of the oxygen atom. Thus, \( \mathrm{CH}_{3}-\mathrm{O}-\mathrm{C}_{2} \mathrm{H}_{5} \) can have a metameric isomer like \( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{O}-\mathrm{CH}_{3} \).

Step 3: Analyze Compound (b)

The structure \( \mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3} \) is also an ether, but it has identical alkyl groups (both are methyl groups). Therefore, it cannot exhibit metamerism.

Step 4: Analyze Compound (c)

The structure \( \mathrm{CH}_{3}-\mathrm{CO}-\mathrm{C}_{2} \mathrm{H}_{5} \) is a ketone. Ketones can show metamerism as they have potential for different alkyl groups on either side of the keto group (CO). This structure can have metamers such as \( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{CO}-\mathrm{CH}_{3} \).

Step 5: Analyze Compound (d)

The structure \( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{S}-\mathrm{C}_{2} \mathrm{H}_{5} \) is a sulfide. Similar to ethers, sulfides can show metamerism due to the possibility of different alkyl groups on either side of the sulfur atom. However, this specific structure has identical ethyl groups, so it does not exhibit metamerism.

Step 6: Conclusion: Identify Metameric Compounds

Based on the above analysis, compounds (a) and (c) exhibit metamerism, as they can have different alkyl groups attached to their functional groups.

Key Concepts

Isomerism

Isomerism is a fascinating chemical phenomenon where compounds with the same molecular formula have a different arrangement of atoms. This results in distinct structures and properties even though their compositional formula is the same. Isomerism is a broad topic and can be categorized into different types such as structural isomerism and stereoisomerism.
In structural isomerism, the connectivity of atoms varies, leading to various subtypes like chain isomerism, position isomerism, and metamerism. Metamerism, in particular, involves isomers that have identical functional groups but different alkyl groups attached to them. These isomers can arise in organic compounds such as ethers, ketones, and sulfides. Understanding isomerism is essential as it plays a critical role in drug development, material science, and more.

Ethers

Ethers are a class of organic compounds characterized by an oxygen atom connected to two alkyl or aryl groups. They have a general formula of \( R-O-R' \). The oxygen atom serves as a bridge connecting two chains which can include a variety of groups comprising carbon and hydrogen.
One of the interesting aspects of ethers is their ability to exhibit metamerism. This occurs when these two groups are different. For instance, in the ether \( \mathrm{CH}_3-\mathrm{O}-\mathrm{C}_2 \mathrm{H}_5 \), different alkyl groups, methyl, and ethyl, are connected to the oxygen. This configuration allows for the formation of metamers, such as the transformation into \( \mathrm{C}_2 \mathrm{H}_5-\mathrm{O}-\mathrm{CH}_3 \), illustrating different connectivity with the same formula. Metamerism in ethers can significantly affect their properties like boiling points and solubility.

Ketones

Ketones are another group of organic compounds which contain a carbonyl group (a carbon-oxygen double bond) flanked by two carbon atoms. The general structural formula for ketones is \( R_2C=O \). This carbonyl center is the functional point around which metamerism can occur.
In ketones, metamerism arises when different alkyl groups attach on either side of the carbonyl group. For example, the compound \( \mathrm{CH}_3-\mathrm{CO}-\mathrm{C}_2 \mathrm{H}_5 \) can have metameric isomers such as \( \mathrm{C}_2 \mathrm{H}_5-\mathrm{CO}-\mathrm{CH}_3 \). This slight change in structure can alter the physical and chemical properties of the ketone. Ketones are widely used in industrial applications including as solvents and in perfume production due to their distinct aromas.

Sulfides

Sulfides are organic compounds where sulfur is bonded to two carbon-containing groups. Similar to ethers, they have a structural formula of \( R-S-R' \). Sulfides can exhibit metamerism if the alkyl groups connected to the sulfur atom are different.
However, when sulfides like \( \mathrm{C}_2 \mathrm{H}_5-\mathrm{S}-\mathrm{C}_2 \mathrm{H}_5 \) have identical groups, they do not show metamerism. This is because metamerism requires variation between the connected groups. Metamerism in sulfides can be crucial, especially when looking at their various applications including in agriculture as fungicides and insecticides, and as additives in the petrochemical industry for enhancing lubricants.