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Today, I was confronted with this question -

Is 2-methylpenten-3-one an isomer of 5-hexen-1-ol?

Some people say yes; while some say otherwise.

I'll put forward the logic on both sides.


The side of people who say 'yes':

IUPAC, in its gold book has told that:

One of several species (or molecular entities) that have the same atomic composition (molecular formula) but different line formulae or different stereochemical formulae and hence different physical and/or chemical properties.

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This flowchart of LibreTexts from the beginning itself tells that 'same molecular formula = isomers'.

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Here's 5-hexen-1-ol, its condensed structure is $\ce{CH2=CH-CH2-CH2-CH2-CH2-OH}$ making its molecular formula $\ce{C6H12O}$.

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This is 2-methylpentan-3-one, whose condensed structure is $\ce{(CH3)2-CH-CO-CH2-CH3}$ making its molecular formula $\ce{C6H12O}$.

So, we've got two compounds with the same molecular formula, but different arrangements, which is what IUPAC wants! So, we have isomers in hand!


The side of people who say no:

If they differ in multiple ways you can't call them isomers.
If they only differ by functional group then yes.
If they only differ by position then yes.
These two are not related to each other, thus, it can't be isomers.


Then how exactly are these two related?

If these two are not isomers, what is the problem with IUPAC's definition of isomers?

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    $\begingroup$ An isomer of my isomer is my isomer. $\endgroup$ Commented May 2 at 14:58
  • $\begingroup$ To be honest I am not sure whether the two are isomers or not. It can't be categorized as a certain type of isomer. The alternate answer was the best option, therefore for the original question this would not be answer. $\endgroup$
    – Kacst
    Commented May 2 at 17:40
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    $\begingroup$ I am upvoting because the question prompted a poem (or something) by Ivan. $\endgroup$
    – Karsten
    Commented May 2 at 19:56

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It's about how you define isomerism. And as you have quoted IUPAC Gold Book in your answer, so there's no debate at all on the topic. Since IUPAC is the final body that defines definitions in chemistry.

Hence Both the compounds are indeed isomers since IUPAC has defined isomerism as such.

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According to the widely understood understanding, isomerism refers to molecules with the same molecular formula (same number of each type of atom) but different arrangements of those atoms. There can be stereoisomerism and structural isomerism.

In this case(sticking to what is said by the IUPAC):

Both molecules have the same molecular formula (C6H12O). However, they differ in both functional groups and the carbon chain arrangement. 2-methylpenten-3-one has a ketone (C=O) and a double bond, while 5-hexen-1-ol has an alcohol (OH) group and a double bond at a different position. Therefore, 2-methylpenten-3-one and 5-hexen-1-ol are indeed isomers. They are specifically structural isomers, with differences in both functional groups and classes.

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