ArtsyArtist25

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What is the total number of isomers with the formula C3H6O that are either cyclic or chiral?

The answer was 6 isomers on Khan Academy, and the hints are provided below. But I don't seem to understand why the cis/trans non cyclic isomers can be considered chiral. I mean is there an explanation I am missing?

Hint #11 / 4
Let’s consider possible cyclic compounds. The oxygen will be single bonded either as an ether and part of the ring structure or as an alcohol and outside any ring structure. Whether the nomenclature of the following structures is apparent or not is not important, but there are three possible structural isomers: cyclopropanol, oxetane, and methyl oxirane.

08248dfd5fa773f76e5f944e6fbcc25e4321d2f3.png

Hint #22 / 4
Two isomers, propen-1-ol and methyl oxirane, have a stereogenic center, but from methyl oxirane there will be a set of enantiomers:

fc81c28a6bafb49415e657fa415999fea91e0e43.png


a71269e2670863562327600d912d12ebce5b1b9a.png

Hint #33 / 4
Propen-1-ol’s geometric isomers are diastereomeric, and the count for stereoisomers is four:

f07b8b840ff5c4b773a358d820ee34b7ccc8fa90.png

Hint #44 / 4
Since there is overlap of methyl oxirane in both categories, the total number of isomers is 6.
 

JimKimSlim

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By definition, you can’t have chirality for double bonds (you need four different substituents to be chiral). Thus, cis and trans double bonds are not chiral. they’re just different isomers.
 
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ArtsyArtist25

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By definition, you can’t have chirality for double bonds (you need four different substituents to be chiral). Thus, cis and trans double bonds are not chiral. they’re just different isomers.

That is what I thought as well but Khan Academy said they were chiral, at least in the question section. They didn't really go over chirality in this context so I was confused.
 

JimKimSlim

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Yeah, that’s confusing. IMO, it looks like they weren’t thorough when they were formulating the question. Chirality exists only when there are four substituents (Orgo 101). You will definitely not see this kind of mistake on AAMC contents.
 
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Geometrical isomers are technically a subset of stereoisomers. E and Z have the same bonds as one another, but they are not superimposable. While it is uncommon to describe them as chiral isomers, KA is not wrong here. The E and Z compounds you drew are in fact geometrical isomers as well as diastereomers. Be careful to not fall into the trap of thinking that chiral means there are four unique substituents. This is a common misconception, because it's an easy way to think about the vast majority of compounds presented in organic chemistry classes.
 
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ArtsyArtist25

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Geometrical isomers are technically a subset of stereoisomers. E and Z have the same bonds as one another, but they are not superimposable. While it is uncommon to describe them as chiral isomers, KA is not wrong here. The E and Z compounds you drew are in fact geometrical isomers as well as diastereomers. Be careful to not fall into the trap of thinking that chiral means there are four unique substituents. This is a common misconception, because it's an easy way to think about the vast majority of compounds presented in organic chemistry classes.

Thank you so much! I will make sure to keep an eye out for it on the MCAT!
 
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