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Question: enter image description here

My Approach:

Since both the compounds have the same principal functional group, they can't be functional isomers. Obviously they are different compounds. Now options A and C are ruled out. Further the substituent ketone in first compound and aldehyde in the second compound have the same locant, so the given compounds cannot be positional isomers. So the compounds must be Chain Isomers. Further, we can just say, as there is difference in the chain length, the compounds are chain isomers. But the answer is incorrect.

The solution in my book, for this problem states, since compounds have different functional groups ketone in first compound and aldehyde in the second compound, they are functional isomers.

Now, I am totally confused. Please explain how to attend this question. Is the solution incorrect?

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    $\begingroup$ The solution is correct: if you have the same molecular formula but different functional groups, then you have functional isomers $\endgroup$ – blu potatos Aug 11 at 16:45
  • $\begingroup$ @blupotatos, But here we have the same principal functional group - carboxylic acid, but different substituents. There is also difference in chain length. Can it be both chain and functional isomers? $\endgroup$ – Intellex Aug 11 at 16:49
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Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. Constitutional or structural isomers are compounds with the same molecular formula but their structural formulas are different. They also called chain isomers because of the possibility of branching in carbon chains (e.g., n-butane and 2-methylpropane). Note that just the twisted versions of the original molecule can be mistaken by novices as structural or chain isomers (e.g., conformers).

Positional isomers: Positional isomers are constitutional isomers that have the same carbon skeleton and the same functional groups. They differ from each other in the location of the functional groups on or in the carbon chain. In other words, in position isomerism, the basic carbon skeleton of isomers remains unchanged, but important groups on the skeleton are moved around on it (e.g., 1-bromopentane, 2-bromopentane, and 3-bromopentane).

Functional isomers: Functional isomers are constitutional isomers bearing different functional groups (e.g., propanoic acid, methyl ethanoate, and 3-hydroxypropanal).

Accordingly, the answer given is correct: These are functional isomers, since each have at least one different functional group changed (keto vs aldehyde).

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  • $\begingroup$ Thank you for your answer. Here both the compounds have the same principal functional group, carboxylic acid. Aldehyde and ketone groups are just substituents on the basis of seniority order. Further I see difference in the principal chain length. So should I call it more like a chain isomer and less like a functional isomer. $\endgroup$ – Intellex Aug 14 at 9:08
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See, you must use the concept of principal functional group only for nomenclature purposes. Here the compounds you mentioned in your question have two different groups in addition to carboxylic group, i.e., ketone and aldehyde. These compounds are chemically different. They show the traits of ketone as well as aldehyde in the respective compounds. So it is better to call them functional isomers. The answer in your book is correct. No need to worry about that.

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  • $\begingroup$ If you could choose multiple correct answers then yes. Else you must go with functional isomers only. Otherwise both are correct. $\endgroup$ – Sumathi Aug 15 at 8:21
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The two compounds aren't identical by simple observation.

Now as far as chain isomers are concerned it cannot be the case as the functional groups are different. Chain isomers have same functional groups at same positions

They can be functional isomers as they do have different functional groups.

They can't be position isomers as well as the nature of functional group shouldn't change.

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