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

Question

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

Options (a) and (d) show metamerism.

Step 1: Understanding Metamerism

Metamerism is a type of isomerism where compounds have the same molecular formula but differ in the arrangement of alkyl groups around a multivalent atom or functional group, such as oxygen or sulfur. It is typically observed in ethers, secondary amines, thioethers, etc.

Step 2: Analysis for Option (a)

For the compound \( \mathrm{CH}_{3}-\mathrm{O}-\mathrm{C}_{2} \mathrm{H}_{5} \), which is an ether, we observe that it can have metameres depending on the distribution of alkyl groups around the oxygen atom.

Step 3: Analysis for Option (b)

The compound \( \mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3} \) is also an ether. However, it cannot show metamerism since both alkyl groups (methyl groups) are identical and there is no different structural arrangement possible.

Step 4: Analysis for Option (c)

The compound \( \mathrm{CH}_{3}-\mathrm{CO}-\mathrm{C}_{2} \mathrm{H}_{5} \) is a ketone and generally, ketones do not exhibit metamerism.

Step 5: Analysis for Option (d)

The compound \( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{S}-\mathrm{C}_{2} \mathrm{H}_{5} \) is a thioether. It can show metamerism by changing the arrangement of ethyl groups around the sulfur atom, similar to ethers with oxygen.

Step 6: Conclusion and Final Answer

After analyzing all options, the compound in option (a) and option (d) will show metamerism owing to the possible rearrangement of alkyl groups around the oxygen and sulfur atoms respectively.

Key Concepts

Isomerism

Isomerism is a fascinating concept in organic chemistry, involving compounds that share the same molecular formula but have different structures or arrangements of atoms. This concept plays a critical role in understanding the diversity of organic molecules. Isomers can be broadly categorized into two types: structural isomers and stereoisomers. Metamerism falls under structural isomerism, which involves differences in connectivity of atoms.

• Structural Isomers: These isomers differ in how their atoms are arranged or bonded. Metamerism is a specific type where the alkyl group distribution around functional groups like ethers or thioethers varies.
• Stereoisomers: These have the same bonds but differ in spatial orientation. This type is not directly related to metamerism but is an important part of the isomerism family.

Ethers

Ethers are a class of organic compounds where an oxygen atom is connected to two alkyl or aryl groups. This structural feature is represented in the general formula, \{ R - O - R' \}, where R and R' are alkyl groups. Ethers are known for their relative stability and their role in exhibiting metamerism.
In ether molecules, metamerism arises when the alkyl groups around the oxygen differ in arrangement. Consider the compound \( \mathrm{CH}_{3}-\mathrm{O}-\mathrm{C}_{2} \mathrm{H}_{5} \.\) This ether shows metamerism because its alkyl groups can be rearranged to form different structures with the same molecular formula.

• Metamerism in ethers occurs when changing the distribution of carbon chains around the oxygen, like shifting the methyl group into an ethyl position or vice versa.
• Ethers such as \( \mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3} \) do not exhibit metamerism because the alkyl groups are identical, restricting alternate arrangements.

Thioethers

Thioethers, also referred to as sulfides, are similar to ethers but have a sulfur atom in place of the oxygen. The general structure is \{ R - S - R' \} with R and R' being alkyl groups. This class of compounds is interesting due to its ability to engage in metamerism.
A good example of a thioether is \( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{S}-\mathrm{C}_{2} \mathrm{H}_{5} \.\) While it may seem simple, this compound can exhibit metamerism. This is because the arrangement of alkyl groups around the sulfur can be varied.

• Metamerism occurs when either ethyl group is rearranged in thioethers, leading to the formation of different but related structures.
• The ability of the sulfur atom to connect to varied carbon chains enhances the metameric possibilities.

Alkyl group arrangement

Alkyl groups, being versatile chains of carbon and hydrogen, play a crucial role in organic chemistry's structural diversity. Arrangement of these groups around central functional atoms can lead to interesting phenomena like metamerism.
In compounds, especially those with multivalent atoms like oxygen or sulfur, different arrangements of alkyl groups can lead to distinct structural properties.

• Alkyl arrangements can alter the connectivity in compounds, giving rise to new isomers.
• This rearrangement impacts properties such as boiling point, solubility, and chemical reactivity, highlighting why metamerism is essential.

Understanding the impact of alkyl group arrangement helps chemists manipulate molecular structures to create desired chemical properties and functionalities.