Animated Solution for Chemistry - Organic Chemistry: Which one of the following pairs of isomers is an example of metamerism ?

Understanding Metamerism

Welcome to a fascinating puzzle from organic chemistry! Today, we are diving into the world of structural isomerism to identify a pair of metamers.

Before we look at the options, let's build a strong conceptual foundation. What exactly is metamerism?

Imagine a bridge connecting two islands. If you change the size of the islands while keeping the bridge exactly the same, you have created metamers! In the language of chemistry, the "bridge" is a polyvalent functional group—a group that has two or more bonds available to attach to carbon chains. Common examples include ethers ($-O-$), thioethers ($-S-$), amines ($-NH-$), and ketones ($-CO-$).

The "islands" are the alkyl groups attached to this bridge. If two molecules have the same molecular formula and the same polyvalent functional group, but the alkyl groups on either side are different, they are metamers.

Let's systematically analyze the given options to find our metamers.

Option (a): Alkanes Here, we have n-pentane and isopentane. These molecules are simply alkanes. They do not possess any functional group, let alone a polyvalent one! Since their carbon skeletons are different, they are classified as chain isomers, not metamers.

Option (b): Aryl vs. Alkyl Ketones This option presents propiophenone ($C_6{H}_5-CO-C{H}_{2}C{H}_3$) and phenylacetone ($C_6{H}_5-C{H}_2-CO-C{H}_3$). While both contain a carbonyl group, the nature of the attachment is fundamentally different. In the first molecule, the carbonyl is directly attached to the aromatic benzene ring. In the second, it is attached to an aliphatic carbon. Because the core nature of the functional group's environment changes, these are generally considered functional isomers in this context.

Option (c): Alcohols We are given two alcohols. An alcohol is characterized by the $-OH$ group. Notice that the oxygen is bonded to a hydrogen atom, leaving only one bond available to attach to a carbon chain. It is a monovalent functional group! It only has one "hand" to hold an alkyl group, so it is impossible to have different alkyl groups on "either side." Therefore, alcohols can never exhibit metamerism. These are simply position isomers.

Now, let's focus on the final option. Both molecules are ketones, meaning they share the same polyvalent functional group: the carbonyl bridge ($>C=O$).

Let's examine the "islands" attached to this bridge in 3-heptanone: On the left side, we have a two-carbon ethyl group ($-C_2{H}_5$). On the right side, we have a four-carbon butyl group ($-C_4{H}_9$).

Now, let's look at 4-heptanone: The carbonyl bridge is identical. However, on the left side, we now have a three-carbon propyl group ($-C_3{H}_7$). On the right side, we also have a three-carbon propyl group ($-C_3{H}_7$).

The functional group is exactly the same, but the distribution of carbon atoms on either side has changed. The alkyl chains are different! This is the perfect, textbook definition of metamerism.

By simply shifting the position of the carbonyl group along the carbon chain, we altered the nature of the alkyl groups attached to it, successfully creating metamers.

Therefore, the correct pair of metamers is 3-heptanone and 4-heptanone. The correct answer is Option (d).

Always remember the golden rule: when hunting for metamers, first ensure the functional group is polyvalent!